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ECC99 TUBE - ThaiHdBox

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1. 10 1K Frequency Hz FIGURE 4 Open loop frequency response The driver a high voltage pnp device is used in the common emitter mode stage bias is obtained by simply adjust ing P1 To ensure high open loop gain T6 s collector is loaded by the constant current source comprising T5 and R9 Obtaining a large open loop gain for the amplifier is fairly straightforward More difficult is to use this gain effec stability the gain of the amplifier needs to fall below unity before 180 phase feedback is applied unless measures to control open loop gain are applied Normally the loop gain would be rolled off at 6dB octave by connecting a small A as previously discussed Negative po larity output current is handled by T9 and T10 The impedance comprises the parallel combination of R10 R12 R15 and D1 which force the output stage to operate in Class A R11 and C8 com pensate for inductive speaker leads While on the subject of stability C2 and C3 are essential Power supply ca bling is prone to rf and noise pickup which can potentially cause oscillation The final part of the circuit is the feed back loop comprising R5 C6 and R6 The amplifier s gain is set by the ratio of R5 to R6 at R5 R6 1 C6 looks like an open circuit at DC while appearing as a short circuit at audio frequencies This ensures that the amplifier s gain is unity at
2. duced was mainly second harmonic just like a conventional class A stage It also sounded the same as the traditional cir cuit After some more work on the circuit quiescent The fly in the ointment was _ I took out a patent on it GB2374220 low efficiency speakers I had at the time that the circuit was rather touchy and dif Every year since I ve sworn that I However the sound set a benchmark ficult to adjust so I abandoned the idea would get out and exploit it but every for me for what a good amp should sound _ year I ve paid my patent continuation like and set me off on a quest for an ef SHELTER FROM THE STORM fees and put it on the back burner This ficient class A amp Now I know that this There matters stood until one rainy year after another hefty payment I de sounds like an oxymoron but for most of day in 2000 while trying to avoid the cided enough was enough and so I ve the time class A amplifiers especially sin weather in a shop doorway I suddenly finally got around to writing it up gle ended ones simply dissipate unneces had a brainstorm On one of those rare rather too limited to cope with the fairly FIGURE 1 The author s original class A amp 16 audioXpress 8 11 www audioxpress com FIGURE 2 Class B push pull output CIRCUIT OPERATION To understand the genesis of the circuit you must return to the basics The sim plest class A output stage is the single
3. read 1 3 1 35V E Certainly the use of batteries eliminates completely any hum from the preamplifier However with an audio grade transform er and great care in the layout and ground ing system you can reach an acceptable level of hum which in practice disappears masked when a record is played at any 48 Vcc with 10kQ 1W resistor by volume level Have fun aX PHOTO 7 Detailed view of DIP switch board Its position close to the input sockets prevents hum pickup Tl toroidal transformer Batteries plug 6 volt filament regulator and 48 voll anode regulalor point lo point wiring Top view other connections on the other side Externa bal tery TB3 48 Volt anode vollage 6 Volt Fila ment voltage FIGURE 5 Power supply layout For a more professional and reliable solution DIYers capable of doing it can develop a PC board to host all power supply compo nents instead of a multi holes board as did Ready willing offering an extensive range of ready to go toroidal transformers to please the ear but won t take you for a ride O Avel Lindberg Inc 47 South End Plaza New Milford CT 06776 tel 860 355 4711 fax 860 354 8597 sales avellindberg com www avellindberg com SUBSCRIBE AND SAVE TUBES VS TRANSISTORS A CALL FOR MORE TESTING Building A True RMS Voltmeter ie tee e SAVE NEARLY 50 OFF THE COVER PRICE Call 888 924
4. samples sec and digital input word widths from 16 to 24 bits Balanced and single ended analog outputs e onsen Sa shes pr CO ier otan HE This unit s performance and features are unique for Its price point While it has the look of a CD player in the 600 price range Photo 1 the back of the unit with Its XLR balanced outputs reveals something very different In ad dition there are extra sets of coax and optical S PDIF plugs marked as input Inout from what Perhaps a computer or other electronic component that can stream high resolution music files or stream data directly from an Internet radio station You can activate the digi tal inputs with a source switch on the remote control Why use these instead of the sound card on your computer Because the electronics of the HD 990 are state of the art as will discuss shortly The S PDIF inputs are connected to a Cirrus Logic 192k Sa s Digital Audio Interface Receiver but the inputs only accept 96k Sa s inputs on the HD 990 The unit mutes if a digital signal is sampled at a rate of 192k Sa s as is the case for other units have tested with S PDIF receivers limited to locking to only a 96k Sa s maximum signal The audibility of a sampled signal at 192k Sa s over one at 96k Sa s Is de batable and most computers or other sources capable of playing a 192k Sa s sampled file should have an option to limit the sampling rate to 96k Sa s so it can be played on the HD
5. EFFICIENT 100W CLASS A AMPLIFIER 8 3 E S E E One author s circuit solution to smooth class A operation By Jeff Macaulay audio pr RESS THE STAFF Editor and Publisher Edward T Dell Jr Vice President Karen Hebert Assistant Publisher Editorial Assistant Graphics Director Graphics Assistant Marketing Director Customer Service Dennis Brisson Richard Surrette Jason Hanaford Jay Sennott Laurel Humphrey Sharon LeClair Regular Contributors Chuck Hansen G R Koonce Tom Lyle Erno Borbely Richard Campbell Dennis Colin Joseph D Appolito Vance Dickason Jan Didden Bill Fitzmaurice James Moriyasu Nelson Pass Richard Pierce David A Rich Paul Stamler David Weinberg James T Frane Gary Galo Advertising Department Strategic Media Marketing 1187 Washington St Gloucester MA 01930 Peter Wostrel Phone 978 281 7708 Fax 978 281 7706 E mail peter smmarketing us Jamie Costa Advertising Account Coordinator audioXpress US ISSN 1548 6028 is published monthly at 50 00 per year Canada add 12 per year overseas rates 85 00 per year by Segment LLC at 4 Park St Vernon CT 06066 Periodicals postage paid at Peterborough NH and additional mailing offices POSTMASTER Send address changes to WEBSITES YOU SHOULD KNOW www audioamateurinc com HAVE WE GOT A DEAL FOR YOU audioXpress 4 Park St Vernon CT 06066 Old Colony Sound Lab is now offering a Deal of the Day Great
6. Static Loadline 16K5 Gain 19 6 Hana m x 0 4 Rudy Godmaire February 2011 Voltage Amplifier Total Gain 17 8 Hz max 0 5 Phase Splitter grid g1 To EL34 grid g1 YAW resistor 2W resistor Matched resistors FIGURE 7 The final circuit impedance at the anode will approxi mate Ry while the output impedance Ri will help mitigate this imbalance _ might be true in some circumstances it is not a problem in my VDV amplifiers because the voltage amplifier stage pre ceding the split load phase splitter pro vides plenty of gain to drive the EL34s to clipping Going backward again brings me to the input stage namely the voltage ampli fier In Table 1 I already established that _ the output swing of the voltage amplifier AV would be 29 7V As mentioned previously you should try to design the circuit so that the signal current in the 2 voltage amplifier is equal and opposite to that in the phase splitter Provided that both stages use the same type of triodes as is the case here this condi tion will be met when 16 4840 iR You can obtain this value by paral leling two 33kQ resistors which gives a value of 16k5Q Figure 6 shows the loadline of the voltage amplifier based on such load resistance Note that be TABLE 1 DETERMINING POINT B BY INTERPOLATION ly Vio Var mA V V 9 2 138 0 137 0 74 111 0 107 3 5
7. a low torque motor fewer poles so less cogging results in much better perfor mance The Bardo Brinkmann Audio Ach berg Germany is highly rated but it costs about 8000 without a tonearm or other accessories For more details you can read the entire review which is posted online at www stereophile com Ron Tipton rtiptonOzianet com of Stereophile magazine in which Mi upa LK Di PP sm te FIGURE 1 The Grommes model 205A PA preamp circuit AF AMP 2 6SN7GT OUTPUT E DC Voltage measurements are al 20 000 ohms per volt AC Voltages measured af 1000 ohms per volt Sockel connections are shown as boltom views Meagured values are from socket pin to common negative Line voltage maintained al 117 volte for voltage readings Nominal tolerance im component values makes possible a variation of 19h in voltage and resistance readings All controls at minimum proper out eut load connected audioXpress August 2011 29 T Classified VENDORS Amplifier building is easy with ezChassis Pre punched holes and slots www designbuildlisten com Electronic and Electro mechanical Devices Parts and Supplies Many unique items www allelectronics com Free 96 page catalog 1 800 826 5432 C IE PRODUCT REVIEW Audience s Au24e interconnects and speaker cables perform as well as any cables I ve heard What s more they compare favorably with other
8. 250V 1 250V 1 25V Metal Metal Metal Metal Metal Metal Metal Metal Metal Cermet Electrolytic Polypropylene Polypropylene Polypropylene Electrolytic 2N3904 RF coil 66M8 T1 Main Transformer Terminal boards Switches fuses etc www audioxpress com B80C5000 LED LED LED 24V TIP 102 BFX40 LM338K 6800yF 1000yF 1 0yuF 100yF 100yF 12V Toroidal 400V 1A 1A 1000V 80V 5A 1W Darlington PNP Adj Regulator 35V 35V 100V 25V 100V 3A contacts 30VA Or equivalent Or equivalent Or equivalent Red Green Blue Zener BDX53C BSS68 Or equivalent Electrolytic Electrolytic Polypropylene Electrolytic Electrolytic See schematic See schematic See schematic V5 cathode follower if connection to a line preamplifier is relatively short To do this remove V5 R21 R22 R23 C7 and install C9 R5 by reusing R24 and C8 ADJUSTMENTS You should check out the power sup plies before making any connection to the preamplifier sections to make sure all voltages are properly set It would be a good idea to test them by loading each output as follows 6Vcc heater PS with 400 10W re sistor 1 5A then adjust P1 6800 trim mer for 6 3V output adjusting P2 20kQ potentiometer for proper voltage TR1 bias adjustments P1 on the preamplifier must be carried out as fol _ lows with no input signal measure TR1 collector DC voltage and adjust P1 to
9. But because my amplifiers use a push pull output _ at cathode will be below 1kQ It then becomes evident that lower values of from class A to class B over a certain Because the low r of the ECC99 makes it possible to select a lower load resis VOLTAGE AMPLIFIER stage the EL34s will operate in class AB which means that they will switch threshold Under class B loading the output tance than in the classic designs using the ECC82 the phase splitter will bene _ fit from an improved performance under all circumstances From that perspective the 6H30pi makes an exceptional con tender for the Concertina Another way to further mitigate the eventuality of an imbalance is to design _ the output stage in a way that maximizes its class A operation Using a moder ately high tension for the output tubes as in my VDV amplifiers V 365V promotes class A operation because the plate current in any specific valve flows most of the input cycle Higher values of high tension would increase the output power of the amplifier but at the ex _ pense of reduced class A operation _ The absence of amplification of the Concertina is another aspect that is often considered a weakness While this hi id sCaecd i The Sts Posible Leads The Wired Battery Drive Selector 1s Paced lnn Preserving The Sounds Of A Lifetime www kabusa com 10 audioXpress 8 11 ECC 99
10. above or below 1 the range of center frequency adjustability can be controlled by capacitor value selection the filter op amp s non in verting input can be referenced to ground rather than Vin to produce a traditional bandstop or bandpass filter and the center frequency can be controlled by a processor programmable logic device or a manually operated encoder aX 28 audioXpress 8 11 Description N iN HA l Manufacturer A A E f J J my Y y iiai ill WIM FIGURE 4 SPICE simulation results from the adjustable notch filter shown in Fig 3 Transfer function magnitude dB is plotted vs frequency showing the filter s characteristic at every tenth setpoint Yageo 1 00k resistor 1 14W metal film through hole Yageo 51 1k resistor 1 14W metal film through hole Nichicon Capacitor 0 047uF 10 high stability metalized polypropylene through hole Intersil 1k 100 tap digitally controlled potentiometer 8 pin DIP Intersil 100k 100 tap digitally controlled 8 pin DIP Texas Instruments Quad op amp 8 pin DIP Contributors Edward T Dell Jr Editorial p 5 editor publisher of audioXpress bids farewell after over 40 years of audio related publishing Rudy Godmaire Designing Amps for Maximum Linearity p 6 works as a senior sales consultant for Bell Canada Since 1998 DIY audio has been his passion which he deeply enjoys sharing with other DlYers and e
11. 9 88 5 82 5 5 6 84 1 716 4 6 69 0 61 0 Note Vio i X Rio and Var Varas nE Vio www audioxpress com AV V 27 0 0 22 5 26 9 42 0 AV V 29 7 0 24 8 29 7 46 3 Operating Point A U B B B Interpolation Factor cause the input impedance of a phase splitter is very high I only rely on the PERFORMANCE 2 Overall both stages will theoretically exhibit a global second order harmonic distortion of approximately 0 5 for input voltages of 1 5V which will be sufficient to drive the EL34s to clipping If you compare Figs 4 and 6 you will also notice that the current variations in the voltage amplifier are equal and op DC conditions to make all my evalua tions for the voltage amplifier Next you must determine the qui escent point point Q of the voltage amplifier and the corresponding value of Ry Looking at Table 1 again you know that V must be at 107 3V Pro jecting this voltage on the loadline shows that point Q will be at 2 5V which leads to a quiescent current of 13 8mA From there it becomes easy to calculate the cathode resistor Ry the usual way by simply dividing the bias voltage by the quiescent current The resulting value of 181Q can be closely matched with a 180Q resistor Applying a voltage swing of 29 7V from the quiescent point determines point A on the graph 77 6V Luckily enough point A ha
12. 9465 or subscribe on line at www audioXpress com audioXpress August 2011 25 EI EAL 8y Bill Reeve Analog Notch Filter Digital potentiometers enable an adjustable analog notch filter minil L TTT Do 204000000000 L gt PHOTO 1 Notch circuit on PCB Red parts are polypropylene capacitors 8 pin SOIC ICs are digital potentiometers and 14 pin ICs are quad op amps ombining modern digital po analog circuit enables a simple processor controlled adjustable frequency gain and quality factor Q over an adjustable frequency span Audio feedback suppression open loop harmonic noise elimination and closed loop controller performance enhancement are a few applications for this adjustable narrowband filter Specifically controllers may need gain peaks below their crossover frequency to suppress harmonic distur bances or mechanical bending modes or closed loop performance may benefit from gain notches above the controller s cross over frequency to avoid residual controller gain amplifying higher frequency harmon ic signals This circuit s advantage in these applications is that its out of band gain and phase distortion are small so phase loss does not threaten controller stability CIRCUIT DESCRIPTION Figure 1 is a simplified schematic The 26 audioXpress 8 11 circuit is composed of gain o and a fil tentiometers with a classic divider to ob
13. Park St Vernon CT 06066 E mail editorial audioXpress com No respon sibility is assumed for unsolicited manuscripts Include a self addressed envelope with return postage The staff will not answer technical queries by telephone www audioxpress com CLASSIFIEDS amp WEB LISTINGS Contact Jamie Costa Advertising Department audioXpress PO Box 876 Peterborough NH 03458 603 924 7292 FAX 603 924 9467 E mail advertising audioXpress com Printed in the USA Copyright 2011 by Audio Amateur Corporation All rights reserved Editorial Farewell Dear Friends Audio Amateur Inc is in a wonderful transition By the time you read this all of our assets will have been acquired by the Elektor group which publishes Circuit Cellar and the North American version of Elektor This ensures a more firm well funded publishing or ganization behind our periodical and book publishing activities The buyers have major plans to strengthen and expand audioXpress Voice Coil and our two direc tories the annual Loudspeaker Industry Sourcebook and the biennial World Tube Directory as well as our list of books and CDs On a personal note some of you will realize that I founded the company in 1970 and that we are now in our 41st year of operation Birthdays pile up Next February I will be 89 I am in fine health and still ac tive But it is time to hand the reins to others I have been a publisher for a long time and my
14. art and I still do There are numerous topologies that can achieve high fidelity some rather complex and others quite simple The circuitry I use to drive the push pull output stage of my VDV amplifiers cer tainly belongs to the latter category Figure 1 shows a generic view of the topology designed by Menno van der Veen and explained in his book Modern High End Valve Amplifiers I like the minimalism of this circuit very much _ because it enables excellent musicality In this article I will thoroughly ex plain the methodology I developed to design and optimize the input circuitry of such amplifiers using an ECC99 in place of the ECC82 I will use the case of my own VDV amplifiers as an ex _ ample I believe even experienced de _ signers will be pleased with this protocol _ because it reduces the drawing process _ with loadlines to a maximum of three _ iterations Note that the proposed meth _ odology works also very well with other tubes having low plate resistance such as the ECC88 6DJ8 and the 6H30pi OVERVIEW The input circuitry of my amplifiers is _ designed around the ECC992 manufac tured by JJ Electronic in Slovakia www _ jj electronic com This medium mu double triode tube offers some very nice features with its relatively high trans conductance S 9 5mA V and its low plate resistance Lr 2300Q The am plification factor u 22 of this tube _ constitutes a
15. bias of the phase splitter Vp 5 Coordinating _ blit and van der Veen the same supply voltage V for both THE OUTPUT STAGE Because the following stage always de termines the requirements of the preced SPLIT LOAD PHASE SPLITTER CONCERTINA It is usually accepted that the higher the supply voltage of the phase splitter the better As you can see in Fig 3 while loadlines 1 and 3 have the same load resistance it appears obvious that loadline The output stage of my amplifiers uses EL34 power tubes With a plate voltage of 365V you can normally set the quiescent current to a maximum of 68mA 25W 365V However because _ they are connected to the output trans _ formers in the triode mode the EL34s can be biased higher up to a maximum these two voltages while maintaining triodes constitutes the main design chal lenge of this circuit On another front when you look ing stage the design of a power ampli deeper into the way this circuit func tions you realize that when i increases i decreases and vice versa As stated by Morgan Jones in his book Valve Ampli _ the grid voltage swing that is required to fiers Ideally we would juggle with the signal current in the input stage to be equal and opposite to that in the Con certina Doing so makes ir remain constant all the time This way of de signing the circuit is easier
16. called con tact potential is not mentioned in the ECC99 datasheet nor in many other 8 audioXpress 8 11 ECC 99 Static Loadline 30K Gain 0 91 Hing wax 0 1 From Voltage Amplifier Rudy Godmaire February 2011 TOEL34 grid ToEL34 grid Note Ra and Ry are matched resistors FIGURE 5 Phase splitter circuitry small tube datasheets This is why it is value of grid voltage This determines a phase splitter a voltage amplifier or any other topology that appears on the graph is therefore a straight line that represents all the operat ing points that can be assumed with the plate supply voltage of my circuit V the grid of the phase splitter From there I first set the 1V upper across Ry know that the voltage across Vi must to drive the EL34s to full power Divid ing 27V by Ry converts this voltage into the corresponding variation of cur current to 7 4mA and the voltage drop jecting this value on the DC loadline also determines the bias of the phase www audioxpress com 335V when there is no signal applied to _ splitter which will be 3 7V wise to consider 1V as the maximum Determining the lower limit of the grid voltage swing point B requires _ the upper limit of the grid voltage swing of all my designs with small tubes be it high supply voltage while significantly some calculation If the characteris tic curves were p
17. dard practice of adding dither before the 16 bit ADC ensures the quantiza tion residuals are noise not correlated harmonics Digital filters can be designed in the time domain the frequency domain or a combination of both Harman Kardon indicates the filter uses a reconstruc tion process called Real Time Linear Smoothing RLS Ill but provides no details Most CD players lack an ASRC and leave the digital filtering to the DAC chip which has internal digital filters The rejection of the stop band images from the sampling process would not be as high if the filtering is done by the DAC chip because there is only limited silicon space for DSP filtering on the DAC chip compared to the space on a dedicated DSP chip The DACs on the HD 990 are top flight Analog Devices AD 195bA chips Although the AD1955 has two DACs on each chip for stereo the HD 990 uses one per channel in a balanced configuration for reduced noise and dis tortion The balanced digital inputs are developed on the Blackfin DSP In mono www audioxpress com mode the AD1955A has an equivalent worst case noise floor of 19 bits A weighted Current to voltage conver sion and analog reconstruction filtering respectively are performed by an Ana log Devices OP27 5A and a Texas Instru ments OPA2134A dual operational am plifier The output stage is fully discrete to drive low impedance balanced loads Two discrete circuits for each channel are on the board t
18. great advantage for the to _ pology presented in Fig 1 Figure 2 focuses on the circuitry sur rounding the input tube The first half of the ECC99 is a voltage amplifier stage whose function is to amplify the signal when I design a circuit This means I conscientiously endeavor to set the pa rameters in the most linear zone of the tube usually at the expense of voltage gain Personally I find that high fidelity VDV amplifiers I set the bias to 70mA coming from the source With its cath ode bypassed by a capacitor the gain is perfect to drive power tubes such as the EL34 and KT77 This stage is direct coupled to the second half of the tube that acts as a split load phase splitter _ circuits and use them well within their limits Concertina and driver stage for the power tubes Direct coupling requires that the far are addressed in the methodology I developed and that I will now expose _ phase splitter must provide a voltage swing of at least 27V with minimum anode of the voltage amplifier must have a DC potential V slightly lower than the DC potential of the cathode of the have learned while studying the works of experts such as Chaffee Crowhurst is no need to add a safety margin now to _ this voltage swing evaluation because the way I design my circuits leaves plenty of headroom phase splitter V a The difference be tween these potentials sets the
19. love for the profession continues undiminished I am deeply pleased about the plans and enthusiasm Elektor is bringing to our enterprise One of the privileges I will miss the most is my ongoing relationship with the more than 900 authors who have made our publications possible Of these over a third are from countries outside of the United States From the beginning over 40 years ago authors have offered us outstanding fare Just yesterday I re ceived a new manuscript from our star performer Nel son Pass I take this as a good omen that the quality of these periodicals will continue I am pleased to report that Vance Dickason whose talents have been a very significant factor in our suc cess will continue as editor of Voice Coil We are all working hard in this transition hoping that all will go smoothly We ask for your patience during this period Please accept my thanks for the pleasure of being publisher for an audience who have responded in all sorts of wonderful ways over our years together E T D JANTZEN AUDIO DENMARK C COIL POWER INDUCTOR with up to 2 000 W capacity Where other coils get overheated you just order Jantzen C Coils Designed for bass subwoofers and am plifiers CROSS COIL high end induction coil Absolutely the closest to the ideal inductor in the World WAX COIL wound of copper foil and paper insulation Wax impregnated Hard as a rock Second to none For more info Superio
20. nor mal listening environment does not de teriorate sound quality Last great care must be taken on the grounding scheme by separating signal 2 ground from the power ground wires SS NOC COOLOS 0999 L 9090002900 2306 e a 0 5 JOCES E wre EZ THE WINGS OF MUSIC The Airborne line of drivers was developed to fill the gaps left by the industry in the use of exotic and unconventional material to reproduce sound For example the use of real wood as a cone material gives you very natural and worm sound the use of Bamboo which is very light and strong which translate in very fast reacting driver and the use of Carbon Fiber which is extremely light and strong gives you a very analytic and very detailed sound 12cm Midrange ch GOZ A C T is 900 R F 5 ZE C pS S y 00000 tee o 2 i j L C j REOEOOOOF POOL L L 006 OL O0DODO er C Lb gi x TND A Pigs ota 20900 M z OLOOOO 0000 7 Si RE d Z SE 8 ia 0 ooo ES T a d TE E SOSO D 20066 eg JE P a A AA E Wood Cone 88db HR124B8 10L 31 62 15cm MidBass a li a SG OOO OO 13000000 La G T KA ac 1040 5000000MH_d 0 EEE 004 10000000 a y PHOTO 3 Power supply top view Note the LM338K heatsink and the toroidal trans former positioning PHOTO 4 Top view of the preamplifier Box measures 200mm x 280mm x 110mm DxLxH O Wood Cone 86db FR148B6 11L 59 84 O Wood Cone 85db FR148B6 a 59
21. of ax www audioxpress com SR en eae oy Ge CVR2 T 7 E R 23 www audioxpress com PARTS GNN EI G N Www partsconnexXion com order partsconnexion com lt ass info partsconnexion com lt 905 631 5777 Visa Mastercard Amex PayPal Money Order amp Bank Draft 5109 Harvester Rd Unit B2 O SUS UA UNS iuas TAKING SOUND TO THE NEAT 4 LEVEL HiWave exciter technology is designed to transform ordinary surfaces and materials into amazing high quality loudspeakers using patented Distributed Mode Loudspeaker technology From low power exciters small enough to add audio to small scale devices to the robust weatherproof Extreme Exciter designed to go anywhere while withstanding the rigors of the great outdoors HiWave offers you infinite audio possibilities HiWave exciters are the latest sound sertsation you ll soon be discovering endless ways to transform everyday objects and surfaces into great speakers Contact Parts Express today and take your sound to the next level Weatherproof Extreme Exciter Standard Range of High Power Standard Housed High Power Quadpod Self Amplified Moving Coil Exciters Balanced Exciter Exciter Housed Exciter Sound Exciter Distributed By U de my 729 Pleasant Valley Dr NS PAR E Springboro OH 45066 LM WE BESS Tel 800 338 0531 YOUR A CONNECTION parts express co m axm sensation innovation
22. reference quality cables not only as an overall package of strengths and weaknesses but line by line in any sort of audiophile accounting Brian Damkroger June 2010 aucience av con ley 30 audioXpress 8 11 Ad Index ADVERTISER ACO Pacific Inc Antique Radio Classified Audience Avel Lindberg Hammond Manufacturing Jantzen Audio Denmark KAB Electro Acoustics Madisound Loudspeakers Michigan Antique Phonograph Society OPPO Digital ardor ena eee Parts ConneXion Parts Express Int l Inc SS STG T eT TE TET H CLASSIFIEDS All Electronics Design Build Listen Yard Sale Free Ads For Subscribers Yardsale Guidelines 1 Submissions accepted from subscribers to audioXpress magazine only You must include your subsciption account number with each submission Submit your ad to Yard Sale PO Box 876 Peterborough NH 03458 Or fax to 603 924 9467 or e mail to editorial audioXpress com Please be sure your submission is legible We are not responsible for changing obvious mistakes or misspellings or other errors contained in ads We will not handle submissions over the phone Please do not call to verify acceptance or inquire about the status of your submission Each submission will be used until the next issue of the magazine subscribed to is published Maximum 50 words No accompanying diagrams or illustrations or logos will be used Yard Sale is published in each issue
23. values on an assortment of products You can t afford to miss a single day So check it out at www audioxpress com or call 1 888 924 9465 to order amateur incorporated LEGAL NOTICE Each design published in audioXpress is the intellectual property of its author and is offered to readers for their personal use only Any commercial use of such ideas or designs without prior written permission is an infringement of the copyright protection of the work of each contributing author 4 audioXpress 8 11 SUBSCRIPTION CUSTOMER SERVICE INQUIRIES A one year subscription to the printed edition is 50 for 12 issues and includes access to the digital edi tion Canada please add 12 per year Overseas rate is 85 00 for one year A one year subscription to the digital edition is 25 for 12 issues worldwide All sub scriptions begin with the current issue To subscribe renew or change address write to the Customer Ser vice Department PO Box 876 Peterborough NH 03458 0876 or telephone toll free US Canada only 888 924 9465 or 603 924 9464 or FAX 603 924 9467 E mail is required for the digital edition www audioxpress com www voicecoilmagazine com E mail custserv audioXpress com Or online at www audioXpress com For gift subscriptions please include gift recipient s name and your own with remittance A gift card will be sent EDITORIAL INQUIRIES Send editorial correspondence and manuscripts to audioXpress Editorial Dept 4
24. 2PIZ X9C 102PIZ ND U2 X9C104PIZ X9C104PIZ ND U3 OPA4277PA OPA4277PA ND Author s first resistor value 1s RZ it could be R nitude and bandwidth The higher and broader the center frequency gain the larger the phase variation at the center frequency and the longer the phase takes to return to zero Digital potentiometers usually have equally spaced resistance increments so the filter s resolution is better in the lower region of its adjustable span Also as the potentiometer resistances become small at higher center frequencies ca pacitor ESR becomes significant and diminishes the center frequency gain Figure 3 is the schematic of an adjust able notch filter with good frequency resolution from about 340Hz to 1 1kHz using non volatile digital potentiom eters For this circuit the gain ap proximately equals 1 02 Digital signals SET ADJUST DIR and INC control the potentiometer values and thus the notch frequency Figure 4 shows SPICE simulation results of this filter s transfer function magnitude plotted every tenth poten tiometer step In other words there are eight settable positions between each of the notches shown in Fig 4 Note the good resolution over the lowest 70 of the frequency span approximately 340Hz and 1 1kHz This architecture is flexible you can configure the circuit as a notch or pass gain filter by setting the gain
25. 84 Bamboo Cone 87db FR148B6 18L 45 90 e Carbon Fiber 87db FR148B8 10M 59 50 17cm MidBass Wood Cone 89db FR173B6 16L 79 90 E Carbon Fiber 90db FR173B8 10M 79 90 SOLEN lectronique Inc 4470 Av Thibault St Hubert QC J3Y 7T9 E Canada Tel 450 656 2759 Fax 450 443 4949 Email solen solen ca http www solen ca audioXpress August 2011 23 The detailed schematic shows how this must be done _ Finally two blue LEDs positioned in front of the tubes produce a nice visual _ effect when looking through the small _ front window Photo 5 CONSTRUCTION _ I have reused an old Fisher tape cassette deck as a container and built a small chassis to host all preamp components _ All wiring has been made in a point to point mode as seen in Photo 2 I took great care to keep high impedance _ paths as short as possible to avoid the use of screened wires Signal and power grounding wires were kept separately _ and joined together at the RCA input parne socket point Similar grounding arrange CI ments are required in the power supply Wak ER ated as properly shown in the schematic _ Batteries because of their size are _kept outside the box they feed the preamp through four plug connectors mounted on the back panel Photo 6 Cartridge load can be selected by po sitioning a DIP switch on the required _ value DIYers who own an MC cartridge _ may elect to install solely the l
26. 990 When moving from a CD rate sampling rate of 44 1k Sa s to 96k Sa s an audible difference is much more likely and is clearly measurable Many computers do not have S PDIF outputs and instead send digital music down the USB output Relatively inexpensive interface units between USB and S PDIF are available to handle sampled signals up to the 96k Sa s rate did not use these units with the HD 990 With its S PDIF inputs acqui sition occurred without any clicks or pops The digital outputs have a maximum sampling rate of 44 1k Sa s meaning the higher rate sampled signals are con By David Rich verted to 44 1k Sa s Harman reports the outputs are designed to interface with recording devices which are typi cally limited to 44 1k Sa s lt makes little sense to take audio off the unit digitally given all the sophisticated elec tronics in this unit used to convert the digital signal to analog The unit is equipped with the Har man proprietary HRS Link output that works with some Harman AVRs and the matching HK 990 stereo integrated amplifier with DSP It is bidirectional four wires with the system clock for the DACs in the amplifier sent to the HD 990 This forces the rate of data transmission from the HD 990 across the HRS link to the integrated ampli fier or AVR to match exactly eliminat ing the need for an asynchronous data converter on the amplifier side The HD 990 is the lowest priced CD player in the Harma
27. FINDING Z While sheltering in the shop doorway I realized that all I needed to do to pro sign The spec requires a quiescent cur rent of 50mA I You can calculate the ended type Here a single device con ducts through the entire signal cycle To duce such a circuit was to include a single do this the quiescent current must be equal to the peak load current required vices as shown in Fig 2A What happens then Well if the impedance is properly chosen the output stage is unbalanced to such an extent that T1 in this case decades of current x from the relation ship 10 Imax 1_ hence x log Imax As a result the amplifier has a maxi mum efficiency of 25 and in practice seldom manages more than 12 Compare this to the class B push pull output in Fig 2 Here in its simplest form it comprises two complementary ventional single ended output stage if _ T2 were replaced with either a resistor or 0 119V transistors R1 R2 and R3 bias the tran sistors just into conduction in the ab sence of an input signal causing a small tortion is generated but even harmonic distortion becomes prevalent in the out put signal again exactly what you would H DI 1 r t ag Paradoxically single ended class A op eration generates more measurable distor _ Although I have used this output stage quiescent current to flow through T1 and T2 Th
28. FINE TUNING CLASS A SOUND ASUS USTA 1 al all g y US 7 00 Canada 10 00 Tube Solid State Loudspeaker Technology y _ d mut a hes 1 Pe oe E KW L qa ES y wel ve E c ne B y AN o ae e C DESIGNING AMPS WITH THE ECC99 TUBE wage g l 0 rd ina SREVIEW HARMAN KARDON HD 990 CD PLAYER TE 95461 74470 84697 Se GO m O m I EA gt E ssis 1 AAA 0 BDP 95 OPPO Universal Blu ray Disc Player with 3D amp Network Streaming e A universal player for audiophile users e Toroidal Power Supply e SABRE Reference Audio DAC e Dedicated Stereo Output e Blu ray DVD SACD and DVD Audio e XLR Balanced Output e FLAC amp WAV playback from USB network e Latest Odeo video processing e Dual HDMI RS232 eSATA Wireless N e 2U Rack Mountable Three ways to play high resolution audio on the OPPO BDP 95 1 Directly play common high resolution audio discs such as Blu ray music DVD Audio SACD and HDCD Purchase and download studio master quality audio tracks in FLAC and WAV formats from sites such as itrax com hdtracks com and www 2L no Save the files to a USB thumb drive or hard disk The BDP 95 can play from the USB drive or hard disk directly DVD R or DVD ROM recorded with high resolution WAV files such as the HRx discs released by Reference Recordings and Hi Rez DVD
29. Overload 30 db RIAA H 1 db Al V c Regulated Output impedance I kOhm piate supply Batteries 48 Voc plate voltage 6 V heaters 56 db La m ve i d L DD Topohybode da i i IM pe Gnd gt ME Consumption 48 Vec 10 mA max 6 Vo 1 6 Amp AC Switch FA Signal Gnd FIGURE 1 Michele MC preamplifier block diagram The graph at the bottom shows the gain loss of each stage me pre michele Seri i PHOTO 1 The Michele preamplifier 20 audioXpress 8 11 www audioxpress com y purpose in building a bat tery powered tube based preamplifier was to show the benefits of this type of power compared to a traditional AC power sup _ ply with rectification by diodes This project required the use of a tube ca pable of working with low plate volt _ ages on the order of 20 30V I chose a tube I have used in the past for high fre quency amplifiers in cascode circuits the ECC86 6GM8 whose main features are Va Anode voltage 25V la Anode current 20mA maximum Vf Heater voltage current 6 3V 0 3A Amplification factor 14 My aim with this project was to achieve the following A the use of tubes except in the moving coil input stage B zero feedback C passive RIAA equalization D 48V DC maximum anode DC 2 voltage in a totem pole configuration E hi fi standard performance CIRCUIT OPERATION The block diagram in Fig 1 shows only one channel A high gain input s
30. ROM by M A Recordings can be played back directly on the N UJ BDP 95 e e BLOCKBUSTER A Dual Q RE TM z Hers ON DEMAND O HDMI D EO For product information and availability please visit www oppodigital com OPPO Digital Inc 2629 Terminal Blvd Suite B www oppodigital com 650 961 1118 Mountain View CA 94043 HAMMOND MANUFACTURING nm USA UK Tel 716 630 7030 Tel 01256 812812 Fax 716 630 7042 Fax 01256 332249 Canada Australia Tel 519 822 2960 Tel 8 8240 2244 Fax 519 822 0715 Fax 8 8240 2255 Tube Output Enclosed Plate amp Filament Open Transformers Chokes Transfomers Chokes IN STOCK Tra 18 formers amp Enclosures an ADT MO LTS LLL LIE MMO CHM S LYE ihe Ws 1H Hey H YE ie E C d dd f j 74 WH e S 7 H Ventilated Bottom Chassis Rubber Chassis Covers Plates Feet Contact us for free catalogs amp a list of stocking distributors www hammondmfg com CONTENTS VOLUME 42 NUMBER 8 AUGUST 2011 DESIGNING AMPS FOR MAXIMUM LINEARITY How to get the very best from your tube amp designs By Rudy Godmaire HARMAN KARDON HD 990 THE MICHELE PREAMP How does a battery powered tube preamp stack up against conventional AC power units By Atto Rinaldo ANALOG NOTCH FILTER Here s an interesting circuit application you can build into your designs By Bill Reeve A closeup look at this high performance CD player DAC By David Rich
31. ample 44 1k samples sec or 705 6k bits per second often abbreviated BPS For ste reo the CD data rate is two times Os maca ls SES This stream of data per second is often called the data rate A signal sampled at 96k Sa s with a quantization of 16 7 7216 levels 24 bits has a bit rate in stereo of 2 96k Sa s 24 bits which is 4608k BPS A digital data rate coming off a CD may be too fast to stream over the Internet or produce a file too big to store The data rate can be compressed using a pro cessor such as the MP3 encoder For example an MP3 file may stream at 128k BPS Compar ing the data rate of a CD to the MP3 rate you see the signal is compressed by 1411 2k BPS 128 KBS of a factor of 11 1 which is the compression ratio having no dimension DAR M audioXpress August 2011 15 solid state By Jeff Macaulay Efficient 100W Class A Amplifier Experience good class A sound with this author s circuit design y first introduction to class A a rule done with an amplifier deliver ing full output In the nature of things most musical information is at a low level amplifiers occurred in 1974 I had read an article by G A French in the now defunct sary power Listening to music is not as occasions I saw the circuit complete in my mind Luckily I had a pen handy and hastily drew the circuit on the back of a till receipt punctuated with sudden and unpredict Frankly I look
32. available in ather 4 ohm or 8 ohm is a fantastic performer in ather home or autosound systems 4 or 8 ohm impedance Unique dual cone design 3 diameter voice coil 71 5mm deep 2 82 12mm peak excursion 290mm cast frame 11 4 The 8 ohm version will work well in 1 cubic foot sealed and with theaid of a plant amplifier with 25Hz boost your F3 is about 27Hz The 4 ohm version will work in 1 2 cubic foot sealed and have an F3 of 35Hz on its own or an F3 of 24Hz with aoe of a plate amplifier and 25Hz For Autosound the 8 ohm woofer in 1 2 cubic foot sealed and 6dB of cabin gain should provide an F3 of 22Hz Use two wired in paralla in a 1 cubic foot box for a 4 ohm load Madisound Speaker Components Inc P O Box 44283 Madison WI 53744 4283 U S A Tel 608 831 3433 Fax 608 831 3771 info madisound com madisound com audioXpress August 2011 21 FIGURE 3 Point to point wiring arrangement On my project always draw by hand a sketch of all connections This helps reduce errors and improves circuit layout BOX S30 bol vee LISIK Bod View Battery powered when closed DC input socket Back panel view 48 de Fil 8 TB3 5 TB3 1 AR Grd Fi Grd TB3 3 TB3 gnd FIGURE 4 Power supply schematic LM338K must be mounted on a 5 x 10 cm or larger heatsink 48V and 6V batteries are located outside preamplifier chassis PHOTO 2 Point to point wiring The aluminium chassis was housed in an old Fisher
33. can be utilized as is in the VDV 6040 and the VDV 3070 amplifiers proposed by Menno van der Veen in his book Using the aforemen tioned methodology you may design an optimized circuit for any amplifier having a different voltage swing require ment for the output tubes For those 7052PH Phantom Powered Measurement Mic System NOW 4Hz to 25 kHz lt 18dBA gt 135dBSPL 1dB div T A SPE LEU RS G E EPIA PUE ENS AR Z CH audioXpress August 2011 nu A Diaphragm www acopacific com 11 using EL84 output tubes I suggest you try the 6H30pi In this particular appli cation I believe this super triode will be an exceptional choice Whatever type of input tube you use I strongly recommend you purchase a matched pair with matched sections for this particular design This is the best way to ensure excellent balance between both channels even as tubes age I have presented my VDV amplifiers as if they were using a single push pull output stage In fact these are VDV 3070 amplifiers that use a double push pull topology Should you use a parallel configuration as I do you simply double the value of R and halve that of C in Fig 7 Aside fon this the rest of de circuitry is identical In both cases the suggested values will give a 3dB cutoff Amateur Press First Reprint Edition 1995 1941 available on the Old Colony online catalog frequenc
34. capacitance across the T6 s collector base circuit This works fine but better per formance can be gained by using a more complex network Here double pole com pensation is used C5 R4 and C4 form a second order filter at low frequencies so that the gain rolls off more quickly initial ly than a single capacitor would provide At yet higher frequencies C5 looks like low impedance to the signal which causes the filter to revert to first order mode The advantage is that the turn over frequency of the network can be pushed up while retaining stability This means that more feedback can be safely applied at the critical mid to high fre quencies lowering the overall distortion of the amp Figure 4 shows the open loop frequency response of the amp with first and second order compensation circuitry for comparison purposes Using the latter 17dB extra feedback can be applied in the midrange with constant current This is the func and T5 with the associated components Having discussed at some length the stage Pll keep the description of this as a Darlington pair and operate in class DC and thus a low output offset voltage As indicated earlier the amplifier is versatile enough to build on strip board although a suitable PCB layout would definitely be better Whatever method of T1 2 and T6 all need to be supplied construction you use the power devices nee
35. cept perhaps the use of double pole compensation which I will discuss shortly As an overview the amplifier can be split into four major sections a long tailed pair fed by a current mirror a driver supplies because the diode conducts the output current once the 0 6V threshold is exceeded It also actually reduces the distortion of the stage without altering its efficiency whilst the dissipation of the 3 30 resistor is reduced to about 50mW 18 audioXpress 8 11 More important though normal global feedback will reduce the distor tion generated in the output stage to HOW IT WORKS Input signals are fed into the amplifier via C1 This device performs two sepa 0 004 THD mostly 2nd harmonic at that might be present with the signal minuscule dimensions TINA simula tion suggests the amp generates about 1kHz at any level below clipping The devices chosen for the amplifier are also widely available and inexpensive This is an important consideration if I had a pound for every time I ve used a new highly vaunted device in my de the amp at around 5Hz obtainable or prohibitively expensive after a couple of years I could probably retire on the proceeds Another consideration is the overall sta bility of a design Experience has taught me that the safest way to test this is to build a prototype on strip board because there are so many more reactive stray
36. d to be mounted on a suitable heatsink A single 1 C W is adequate for a stereo pair of amps Dont forget the mounting hardware Construction should bring few problems Be sure that the semiconduc tors and electrolytic are correctly oriented Caution the pin outs of the small signal transistors differ between types Last before you can listen to your new amp you ll need to adjust it Because the design operates from a large number of supply voltages a PSU circuit is not pro vided Nothing special is required just a standard 9V DC to 50V DC supply tion of the circuit block comprising T3 4 for maximum voltage gain Because of the high thermal stability of the output Naturally the output power depends en tirely upon the voltage used For 100W R6 7 8 9 and C7 The basic circuit is _ formed by the directly coupled transistors T3 and T4 T4 controls current through T3 by the base emitter voltage of T4 which appears across R8 R7 completes the circuit providing collector current for _ T4 To maintain stability C7 is shunted across T4 s collector base circuit Because tively To explain in order to maintain output the 50V DC supply will be re quired If you only listen to music and not test tones a 300VA transformer will be more than adequate for stereo use To set up the amp attach the power supply and speaker to the amp making sure it s disconnected Adju
37. e extent a more sophisticated circuit de signed back in the 80s by the Italian engineer Bartolomeo Aloia Having established a set of require ments A through E above it becomes immediately apparent that a convention al cascade of triodes is unable to meet the prerequisite because it is impossible to obtain a distortion of less than 1 unless several dB of feedback are ap Combination of sav d and 5 8 will re sui in parailetng se leztad waluta I dosined ditenent values may be in Stalled 10 sui diie rent carindges To other channel Cartridge load PERZPRAZ 2 gt q 1 ll A el C1 DIP Sw T TRI d BC 149 203904 a E D te hj 5 t i Q O ptt isto TR J conan Gnd Other sechon of Vion the othar channel Select appropriate factory recommended carindge load value LUI Adjusi fer 1 2 1 3 Vee on BC 149 collector or min cutpul distorsion FIGURE 2 Michele MC preamplifier schematic one channel shown Tube pin number ing i e 1 6 shows left right channel pin assignment plied to the circuit This conflicts with requirement B and E which note that zero feedback and high fidelity require ments are design goals Additionally a high performance cir cuit must be fast high slew rate and _ present a wide bandwidth in order to pro _ vide good response to the transients To _ achieve these design requirements I sur _ vey
38. e Ma 3 d a Ne Fal HE THEG A gE H PESK 230 Toi 4 HE act IWA 0 ALL MEASURRMRNTS TAREK WITH MEC PHOMG RAING SWITCH IM MEC PRONO POSITION MEASURED FROM PIN 2 OF POWER PLUG T MEATURED FROM PIN 1 F POWER PLUG an electrostatic loudspeaker s ESL It s been a good while since we ve had an octal base tube phono preamp so I two areas of misbehavior the high impedance at low frequencies and the _ low impedance at high frequencies matic for the Grommes model 205 PA Any help or advice would be greatly preamp Fig 1 The 205PA got its B appreciated and heater voltage from the Grommes 2 Angel Rivera Speakerman excite com Readers with information on this topic are encouraged to respond directly to this ment with it will make a great project letter writer at the address provided eds Neal A Haight TURNTABLE TYPES Tracy CA Routh s question about direct drive ver sus belt drive turntables in the June aX there is a review of the Bardo direct drive turntable in the May 2011 issue As a follow up to my response to Mr chael Fremer discusses this question The problem is many commercial DD turntables are built for DJ use and have high torque motors and lightweight turntables ITs for quick starts and stops The lightweight TT s do not smooth out the cogging steps from the DD motors and thus have moderate to high wow and flutter A heavier TT and
39. e output at the emitters of T1 and T2 is directly connected to the load The bias voltage A B is the sum of the bias voltages for T1 and T2 CA and CB respectively On positive signal excursions tion than class AB stages the open loop THD is on the order of a few percent with this circuit However you can easily reduce this to minute levels with a dash of negative feedback Much more important is that the basic class A sound is there minimum today But more power out put was only one aspect of the design of this amplifier I wanted to produce a de T1 turns on increasing the CA bias volt age while reducing the CB bias for T2 which then turns off On negative going excursions the position is reversed with T2 turning on and thus turning off T1 The result push pull operation produces large amounts of odd order distortion which sounds harsh crossover distortion as the signal forces the transistors to turn pect It is a characteristic of transistor _ junctions that the base emitter voltage increases by about 60mV for every ten fold increase in current flowing through it This relationship holds true over six _ supplies without the need of adjustment on and off Even order harmonics are largely canceled out by the circuit Some thought will show that if ei ther T1 or T2 could be prevented from turning off during the signal cycle you would have instead a sing
40. ed alternative circuits and ultimately focused on the SRPP Shunt Regulated Push Pull configuration better known as totem pole It has been largely used in the _ past in instrumentation and professional fields as high frequency line driver and voltage amplifier Its operation has been described several times in aX and you can also find an exhaustive description at www tubecad com may2000 page2 html Finally as stated earlier in order to operate with low plate voltage I had to select a suitable valve Among limited alternatives the ECC 86 6GM8 was a good choice because it is still available at a reasonable price The totem pole circuit with the ap propriate components values provides a gain of about 20dB 10 times with a distortion on the order of lt 0 1 at 1V output without application of a feedback loop a high slew rate and a bandwidth reaching a MHz Low level signals such as those coming from the MC cartridges are in the range of 200 5004 V at 1kHz and require an am _ plification factor up to 1000 times 60dB to reach at least 200 500mV output level needed to drive a line amplifier A vacuum ACOUSTICS Danish engineered speakers built in Indonesia The SB Acoustics line is a fantastic value for any speaker project This shallow profile 10 subwoofer provide fantastic bass in small or Shallow enclosures This versatile woofer
41. ed at my circuit in dis able peaks The result is that class A amps belief It was just too simple to work just generate unnecessary heat because although I couldn t fault my own logic sadly Radio Constructor magazine that described how you could produce a class A output stage by combining a con stant current source with an emitter follower This was just within my de sign skills at the time and so 1 produced my own version Fig 1 Although primitive the sound was a revelation So smooth with huge bass extension and no sense of listener fa tigue Unfortunately there was a down side You could have fried eggs on the heatsinks or the mains transformer come to that and power output was the amps quiescent current is fixed at the peak current required In fact the inefficiency assigned to class A amps in general and single ended designs in particular is better character ized as a design fault rather than an in evitable feature It took me only a few months to work out a way to dynami cally alter the constant current source of my amp so that it stayed in class A but responded to signal levels and had a low 2 As soon as I got home I simulated it on Electronic Workbench To my con siderable surprise it worked as I had supposed In short it operated in single ended class A without requiring a high 2 quiescent current More important the distortion pro
42. er by its input swing Vkomax Vk2MIN _ 2 A2 0 91 VAIMAX l VatMIN You can approximate the overall dis tortion at maximum excursion by using formula y l e N Vienn E 2 Ana harmonic van pnp i 2 X 100 1 V e The excellent linearity of the ECC99 Under AC conditions when a signal is o applied to the grid of the phase splitter the grid resistors R of the power tubes act as a load on the phase splitter and _ become coupled in parallel with Ry and RC While the quiescent point remains the same the loadline rotates around point Q giving a somewhat steeper slope 14 However when the grid resis tors are more than ten times the value of the load resistors the difference between the DC and AC conditions is small and the optimization of the circuit are the first contributors to this very low level _ of distortion Being a special case of the cathode degenerative triode the Concer tina also benefits from current feedback which also greatly improves the linearity Once this ratio is known you simply FURTHER THOUGHTS ON use it to interpolate the values of Vp and i at point B which will allow the DESIGN E So far I have calculated the gain and distortion of the phase splitter based on the static loadline which only re flects the DC conditions of the circuit Normally this exercise should be done using the dynamic loadline to obtain t
43. erfectly linear point B would be at 6 4V Unfortunately that is not the case because as the grid voltage Figure 4 shows the plate diagram of _ the phase splitter I have set Ry and Rx to 15kQ because this value provides the best optimization in my circuit Note that it is highly recommended that these resis tors be matched to ensure perfect balance _ of both output legs The static loadline becomes more negative the grid lines be come closer to each other In reality point B will be slightly more negative and you must find its value by interpolation You will see that the split load phase splitter is a special case of the cathode degenerative triode in which the cath ode and anode resistors exhibit the same value Some methods enable the calcula tion of the gain and distortion of such total DC load resistance of 30kQ and the based on Krauss method a circuit The following development is Figure 5 shows a different view of the phase splitter From this schematic it becomes quite obvious that limit on the slope point A Projecting this point on the current axis indicates that the circuit will draw 9 2mA which will result in a voltage drop of 138V also implies that you consider another 3 form of very objectionable distortion the 1 Vas Vaasa Ve Using a spreadsheet I first computed the values for Vp v i Vko Vap at points A and Q Table 1 The
44. et to 1 5V the THD reaches a mere 0 3 This is especially good considering the fact this result is based on THD instead of solely second order harmonic distor tion Examination of the harmonic con tent through a FFT analyzer revealed that the harmonics are mainly of the second order for all input levels up to 1 5V However pushing the circuit into the positive grid current area completely changed the pattern of harmonics caus ing a very significant increase of third that it will be at 136 5V a variation of 2 29 2V relative to point Q Comparing this result obtained from the graph to and higher order harmonics I stated earlier that my designs benefit from plenty of headroom In my VDV amplifiers the EL34s clip when the ex citation of their control grids exceeds 27V It is therefore unlikely that the input circuitry will be asked to provide more amplification Should this happen the input tube could easily swing well above points A and B I know however that this will lead to higher levels of dis tortion mainly because of the positive grid current that will progressively occur above 1V But whether the amplifier distorts because of this phenomenon or because the EL34s clip does not make much difference it distorts Figure 7 shows the final design of the _ input circuitry developed in this article This optimized circuit
45. he utmost accuracy enough to be neglected In my design I set R to 220kQ 15 times which de creases the total dynamic load to 28 1kQ as compared to 30kQ The incidence on the slope of the loadline is so negligible that it leads to a rotation of less than 0 5 making AC evaluation futile The Concertina is sometimes dis liked by DIYers because of the inherent discrepancy that is found between the output impedance of the anode and the cathode Though the output impedance is different for the two output legs the balance will remain unaffected at either low or high frequency as long as the total effective impedance of the anode leg is equal to that of the cathode leg To meet this condition fully the stage _ following the phase splitter must oper ate in class A at all times as you find TX Y J i U mn H 5 E 1 e R ha de be Rediscover historic recordings and the ingenious machines that played them subscribe to In The Groove magazine by becoming a member of Michigan Antique Phonograph Society Subscribe amp Become a Member for only y E EA m A i Y E PHONO U AE ann ELA e e MICHIGAN gt o 30 US Residents 35 Canada 45 rest of the world N N IID SADA A A AAA A A a audioXpress August 2011 9 Rudy Godmaire February 2011 FIGURE 6 Voltage amplifier loadline in single ended topologies
46. l noise reduction and overall quieter circuits The power supply schematic Fig 4 4 shows the hi voltage 48Vcc power _ supply is provided by a simple voltage _ regulator while the 6 Vcc is made up of an LM338K a three terminal integrated circuit You can adjust the output volt ages on both circuits by means of a dedi _ cated potentiometer Battery supplies are activated when mains is off and switch 2 is turned on _ Diode 1N4007 on TB3 3 prevents an accident polarity reversal of 48 Vcc ex ternal battery to protect all electrolytic caps Polarity reversal of 6 Vcc battery _ does not have any adverse effect You can load the MC input with a 2 selected resistor to better suit cartridges _ specifications A DIP switch allows se _ lection of load values from 12Q through 1K in 15 steps V1 output feeds a passive RIAA net work made up of R1 R2 C1 C2 The stage for an additional 20dB amplifica tion V5 functions as cathode follower to provide the lowest possible output im pedance it has no gain DIYers may o elect to eliminate this stage if the con nection to a line preamp is kept short less than a meter My implementation uses a high grade toroidal power transformer The pream signal is then applied to V3 V4 buffer plifier is exceptionally quiet when fed with batteries when powered through an AC outlet it introduces a certain amount of hum this however in a
47. le ended class A stage combined with a compliant current V FIGURE 2A Impedance between the output devices source In other words an efficient class A all you need to know is the change in Vbe over the current range of the output stage _ The impedance is then equal to that volt output stage without the giant heatsinks impedance Z between the output de wont turn off until T2 reaches its maxi mum negative excursion just like a con constant current stage No crossover dis expect of a single ended output stage both when used open or closed loop It is not such a problem to calculate the value of Z as you might first ex orders of magnitude You can use this to calculate the value of the impedance required In principle age divided by the quiescent current For example take the current de peak current Imax from the maximum power P and load impedance Ry Imax 1 41 PR R 7 4 994 Similarly you can find the number of 1 log 4 99 0 05 1 99 decades Then the change in Vbe V 0 06x And finally you can calculate Z Z V I 0 119 0 05 2 390 Or Z 0 06 log 1 41 PR PRL DESIGN CONSIDERATIONS over the years I don t as a rule design high power amps Although I realize with the passage of time the definition of high powered has drifted up from a 10W output in the early 70s to 100W sign that will reliably operate over a large range
48. led with this signal process ing block If the Incoming signal has an input clock with jitter the ASRC will re clock the signal to a low jitter clock the output clock from the crys tal oscillator on the main board The presence of an ASRC Is especially im portant if the unit operates with an ex ternal S PDIF signal The next operation the Blackfin DSP performs is digitally filtering the signal after it performs the sample rate con version The process of passing through the digital filter produces a digital datastream from the DSP chip at 384k Sa s ata 24 bit depth The DSP has not 14 audioXpress 8 11 produced the same amount of informa tion avallable from an analog music sig nal actually recorded at 384k Sa s rate and a 24 bit density Instead the DSP has suppressed part of the spectrum of the sampled signal For a band limited 20kHz analog signal sampled at a rate of 44 1k sam ples sec the spectra appear around OHz desired signal as well as from 24 1kHz to 64 1kHz and then repeat around multiples of 44 1k The repeat ing spectra are the stop band images that result from the sampling process The suppression of the stop band images allows the sampled signal to better represent an analog signal sampled at 44 1kHz Information lost as a result of band limiting the signal before the sampling process cannot be restored The signal to noise SNR ratio of the 16 bit quantization also cannot be lowered though the stan
49. lf to design with transistors In 1975 he had his first technical article published and soon he was working as a freelance writer In the mid 1980s he ran a firm providing valve amplifier kits in the UK More recently he has written for E ectronics World Atto Rinaldo The Michele Preamp p 20 is a retired 32 year IBM employee who has a keen interest in tubes He spends some of his time with his grandson Michael who was the inspiration for this project Visit his website at www nuvistor it Bill Reeve Adjustable Analog Notch Filter p 26 is a Director at Lockheed Martin Corporation in Palo Alto Calif He has graduate engineering degrees from the Colorado School of Mines and the University of California Berkeley as well as a Masters in Electrical Engineering from Santa Clara University www audioxpress com XPRESSMail VINTAGE OCTAL TUBE PREAMP thought I would pass along this sche model 117PS power supply Figure 1 indicates that 275 325V DC is required e for the B and of course 6 3V for the tube heaters Perhaps someone willing to experi and share it with the rest of us HELP WANTED Does anyone have an idea how to de sign a zobel for capacitive reactance I would like to have a means of taming RADIO INPUT 100 R i or G CRYSTAL OR REOK o Tauf mam Tobe wi v 7 A PHOTOFACT STANDARD HOTATIOM SCHEMATIC G Howord W Somi A Co Inc 1953 I LHT A l Mia m
50. listen to a CD 1 Determine the high tension you will use for your input tube For better results V1 should range between 325V and 400V the higher the tension the better 2 Design the entire circuit as de scribed in the article Start with a load resistance RK2 that is approximately seven times the value of rp so 16k 3 Answer the following question when your design is completed What is the grid voltage Vg1 of the voltage amplifier at point A If it is 1 0V you are finished If it is lower than 1 0V say 1 4V www audioxpress com 11 For further details see reference 6 chapter 7 section 5 v 12 It is possible to calculate the distortion up to the fourth harmonics by using Espley s method However because third and higher harmonics are usually negligible with audio tubes it is usually ac tent to figure out the distortion of the circuit See reference 7 section 4 9 for the description of Esp ley s method 13 It is recommended to use a grid resistor R that is at least four times the value of the load re sistance of the preceding stage This maximizes the output voltage while minimizing the intermodulation distortion However this value must not exceed the maximum stated in the power tube datasheet See 14 Because of the reactive components of the circuit the real dynamic path of operation would be an ellipse whose center would be point Q Realizing s
51. n I calcu At the quiescent point point Q you Va around the quiescent point which decrease by 27V as compared to point A lines might suggest excellent linearity when the grid voltage is between OV and 1V you must avoid using the circuit in this area because this is where grid current starts to occur The exact point through R to 111 0V Carefully pro _ lie somewhere between the 6V and 8V grid voltage lines in Fig 4 By comput lated the absolute variation of Vio and appears as AV and AV in the table If you make the hypothesis that the input signal is undistorted you know 2 that AV will be identical either side of rent of 1 8mA This sets the quiescent 29 7V point B will need to be at 29 7V point Q Consequently point A being at You can also deduce that point B will ing the values of i for these operating the following second order harmonics points in the spreadsheet it becomes easy to calculate point B by interpola tion This is done by first determining an interpolation ratio this way AV a4 at Point B 24 8V AV a4 at Point B 29 7V 4 9V 21 5V AVA1 at Point B 46 3V Interpolation Ratio 4 9V 21 5V 22 8 calculation of Vy AV va and Va Table 1 provides all the information you need to calculate the gain of the phase splitter and also its level of sec ond order harmonics 1 You find the gain by dividing the output swing of the phase splitt
52. n between 340Hz and 1 1kHz schematic used quad op amp which could be a dual SETADJUST R2 1 1 or a Vout Vi APPLICATION Note that the maximum center fre A r ANTIQUE RADIO CLASSIFIED depth is limited by the filter capacitors ESR Use polypropylene or ceramic ca o e Vintage Radio and TV pacitors for a relatively deep notch Elec trolytic capacitors can perform poorly e Vintage Tube Audio If amp is equal to 1 the filter produces e Restorations no gain 0dB When 0 becomes slightly different than 1 gain appears at fre quency 0 As amp moves away from unity e Test Equipment the center frequency gain grows in mag e Articles e Classified Ads e Free 50 Word Ad Each Month for Subscribers e Radio Audio Flea Market Listings Subscriptions 36 for 1 year 48 for 1st Class Mail ANTIQUE RADIO CLASSIFIED PO Box 1558 Port Washington NY 11050 Toll Free 866 371 0512 Fax 516 883 1077 Web www antiqueradio com Email ARCO antiqueradio com ms i VISA Paral e History E 3 y 5 S 5 3 Crcuit Gain Linear Scale FIGURE 2 Detail of center frequency gain as a function of the circuit gain 0 with a slightly below unity audioXpress August 2011 27 TABLE 1 Reference Manufacturer s designator part number R2 MFR 25FBF 1K00 Digi key part number 1 00KXBK ND R3 MFR 25FBF 51K1 51 1KXBK ND OXP2E473KRPT 493 3629 ND U1 X9C10
53. n line with the HRS link a digital connection with HRS link may make sense to take advantage of func tionality such as room correction that operates in the digital domain of the audioXpress August 2011 13 Harman AVRs and HK990 integrated amplifier Inside the Unit Without a service manual can only provide direct observations of the two PC boards on the chassis One is the power supply the other is the main board with the electronics The power Supply is a switching unit increasingly common in mid and low priced CD and DVD players The board is 3 5 x 7 Analog power supplies are 14V at the output of the switcher Linear regulation for the DACs and analog operational amplifiers is on the main board which contains the electronics for the CD drive no electronics under the drive the DSP chip and associated memory the DACs analog electronics and digital audio interface chips Digital data from the CD or the digi tal inputs of the HD 990 are passed to the DSP chip which is a 32 bit Analog Devices Blackfin DSP that Suppresses jitter and does digital re construction filtering This DSP chip is absent from most CD players and is rather expensive by itself not to men tion the higher build price when the cost of the memory chips for the DSP is included The DSP suppresses jitter using an Asynchronous Sample Rate Converter ASRC As the name implies signals with different input and output clocks are reconci
54. nline quote shopping cart ordering system China competitive prices free electrically test web http www pcbcart com E mail sales pcbcart com Tel 86 571 87013819 Fax 86 571 87036705 Add No 76 GuCui Road Hangzhou China WWW PCBCART COM audioXpress August 2011 7 February 2011 FIGURE 4 Phase splitter loadline 1 provides better linearity With a high tension of 400V and a load resistance of 10kQ the grid voltage lines show fairly even spacing from OV to 10V On the other hand maintaining a increasing the value of the load resis tance flattens the loadline to a point where it compromises linearity see loadline 4 Designing an efficient phase splitter in the most linear zone thus implies aiming at the highest volt age possible while using an optimum load resistance that will not draw un necessary current such as in loadline 2 My experience with the ECC99 and also with the ECC88 6DJ8 and the 6 H30pi reveals that excellent linearity and efficiency are obtained when the value of the anode load R and cathode load Ryo is set between five to ten times the value of the plate resistance r of the tube Knowing that r_ is around 23000 for the ECC99 the value of Ry and Ry will range between 11 5kQ and 23kQ Designing for maximum linearity graph does not reveal positive grid cur rent distortion Even though the load where this begins which is
55. ns are the same color black and identically sized offering no distinction by function To make matters worse lettering is small and with little contrast The remote has larger buttons of varying sizes for the main control features but the small letters and poor contrast re main Secondary controls are all the same sized round buttons The critical source select button is especially hard to identify The display shows the sampling rate for S PDIF inputs CD text information or ID3 tag information for MP3 files The graphics are large with good con trast A DVD player used as a CD player replacement would display none of this information Instead the text would be shown on the TV connected to the DVD player in a stereo music setup where no TV would be found this in formation would be lost Unfortunately the instruction manual which you can download from the company s website www harmankardon com is only 16 pages thin and does not highlight how the display varies across the unit s oper ating modes The unit had a problem with the wider gaps in the Pierre Verany error correction test disc but it played CDs without any click or skips that tripped up players that performed better with the Verany disc This left me question ing the value of the test disc not the performance of the player heard no degradation in sound qual ity from the unit Given the parts qual ity and measured performance this comes as no s
56. nsfers data at the ex ternal S PDIF input with a 24 bit depth to the DSP Harman personnel could not confirm whether the HD 990 sent 24 bit data to the DSP or truncated it to 16bits Conclusion Short and simple the quality of the electronics in this CD player and the resulting analog performance are nearly impossible to beat at this price The state of the art electronics with ex ternal S PDIF data makes an already highly desirable unit even more so by combining what are normally two dif ferent components into one box was bothered by the uniformity of the front panel buttons and the minimal con trast of the lettering on the front panel and remote though through repetitive use you soon develop spatial aware ness of the functions rr Notes on Notation The sampling rate defines the num ber of samples per second taken from a continuous time wave to produce a discrete signal used the notation of the sampling rate as samples per second or in the abbreviated form Sa s It is typical to specify the sam pling rate in Hz but then you may become confused by what is the frequency of a continuous time sig nal cycles per second and a sam pled signal samples per second A CD player data is quantized to 1 of 65536 levels which can be represented by a 16 bit word The data rate coming from a quantized and sampled signal is the bit rate and is measured in bits per second For a CD per channel the bit rate is 16 bits s
57. o drive the balanced outputs saw no signs of a DC servo to reduce the number of inter stage DC blocking capacitors cannot measure a unit with the HD 990 s performance refer you to the Australian site avhub com au for a complete set of measurements as a CD player Steven Holding s results are exemplary as you would expect from Analog Devices highest grade DAC Surrounded by quality op amps The noise floor is remarkably low with no signs of any power supply noise You can see the presence of the Blackfin DSP in the measurement of the worst case stop band image attenu ation for a 44 1k sample sec signal slightly above 24kHz for a 20kHz sign wave sample at the 44 1k Sa s rate The stop band image is suppressed by over 100dqB normally would not re port jitter results because believe its significance on audio quality is over Stated However Mr Holding reports they are as low as he has measured This verifies the algorithms for jitter at tenuation have been well chosen for the Blackfin DSP chip Ergonomics and Performance The styling of the unit is reminiscent of the Harman Nocturne receiver intro duced 45 years ago When the power is off nothing Is visible The downside of the ultra high tech look is the tedious front panel controls The switches to operate the unit are pencil thin at the center left of the unit with the excep tion of the open close button at the right side of the CD loading tray The butto
58. oad re quired avoiding installation of a DIP _ switch and related small board Photo 7 Finally it is possible to eliminate the x E Y e aks a A N TA a Pat Pst oe KESAS PHOTO 5 Front view Two bright blue LE e the tubes to provide a pleas ant effect when seen through the small front window PHOTO 6 Back view of preamplifier On the left input output RCA connectors and on the right a four plug receptacle for external batteries Please note the recycled Fisher cassette deck box TABLE 2 POWER SUPPLY PARTS LIST COMPONENT RATING R1 11 fuse 1 4W 5 1W 5 1 2W 5 1 2W 5 1 2W 5 1 4W 5 1 2W 5 1W 5 1 2W 1 2W TABLE 1 PREAMPLIFIER SINGLE CHANNEL PARTS IN GRAY FOR TWO CHANNELS COMPONENT VALUE RATING 47002 1 4W 5 10002 1 4W 5 330 1 4W 5 150 1 4W 5 100002 1 4W 5 4 7kQ 1 4W 5 1kQ 1 4W 5 1002 1 4W 5 22002 1 4W 5 NOTE Metal Metal Metal Metal Metal Metal Metal Metal Cermet Cermet NOTE Metal Metal Metal Metal Metal Metal Metal Metal R9 11 17 20 26 Metal R10 R12 19 R13 R14 R15 R16 18 R21 R22 R24 25 see text P1 C1 7 10 C2 5 8 9 see text V1 V2 V3 V4 V5 DIP Switch 24 audioXpress 8 11 1 8k0 470kQ2 82kQ2 10 5kQ IMQ 3 9kQ 6 8kQ2 39kQ 100k lt 2 20k02 22uF 1uF 10nF 28 4nF 220uF BC 149 6 8uH ECC86 Eight ways 1 4W 5 1 4W 5 1 4W 1 1 4W 1 1 4W 5 1 2W 5 1 4W 5 1 2W 5 1 4W 5 1 4W 20 100V 250V 20
59. of supply voltages without modi _ fication Here the balanced design ap _ proach pays dividends I have successfully used this design from 9 to t50V power Also when it comes to the output audioXpress August 2011 17 FIGURE 3 One channel of the amplifier stage a compromise must be reached between open loop distortion and quies cent current as determined by the series impedance This is a matter of judgment and I have come down on the side of high impedance 3 3Q and open loop distortion Why Well one useful result of using a higher impedance is a lack of sensitivity to thermal runaway In short you can set the quiescent current by a _ simple preset thermal feedback is not required So no difficulty setting up In deed in my previous designs of this type I have used a fixed resistor for this duty but I decided to allow you something to fiddle with should you feel inclined If you calculate the power rating re quired of a 3 3Q resistor in series with an 8Q load dissipating 100W of audio you end up with the rather startling re sult of 82 2W peak Obviously this is not a satisfactory result and so you must take measures to shrink this to a reason able level This is achieved by the use of a shunt diode wired in parallel with the 3 3Q resistor This allows high out put power from sensible sized power nel is identical There aren t many surpris es here ex
60. ppens to be at a grid voltage of 1V exactly where I wished it would be In reality this did not happen by chance Three iterations were neces sary to reach this optimized value see sidebar Because the input signal coming from the source is undistorted I know that point B will be situated at 4 0V Pro jecting point B on the voltage axis shows Table 1 reveals that the voltage ampli fier has some distortion which you can calculate using formula 3 V Aimax Via V l ss N ussi 0 E Alquiescent 2 wv Ae Hong harmonic A ro X 100 A You only need to determine the gain of the voltage amplifier Referring to Fig 6 you find the gain by dividing the out put swing of the voltage amplifier by its input swing Therefore Almas Vaimin V 5 Al y 19 6 gimax gimin TABLE 2 MEASURED THD VIN THD RMS dB 0 35 0 5V 60 0 0 71 1 0V 53 5 1 06 1 5V 50 5 1 42 2 0V 48 0 1 77 2 5V 44 5 FFT analyzer reveals very significant high order har monic content due to positive grid current GLOBAL CIRCUIT posite to those in the phase splitter Out of curiosity I measured the total harmonic distortion THD at the anode of the phase splitter using my HP 339A distortion analyzer for various input voltages The results are presented in Table 21 I was very pleased to find that the circuit performs better than I anticipated When the input signal is s
61. pplications or by using a voltage notch or alternately a bandpass filter that maintains a constant shape center Vi s s 2E0 00 Vout _ E sQS 0 Wo The filter center frequency in radians per second is 0 1 VR1R2C1C2 The numerator s damping ratio or the zeros damping is E R1 C1 C2 R2C1 1 ot 2VR1R2C1C2 The denominator s damping ratio Sd or the poles damping is E RI C1 C2 2VR1R2C1C2 LOGIC LEVEL Z TG ALS INC Setting the Laplace variable s to jo cl and looking at the case where the filters center frequency the trans fer function magnitude simplifies to En Lp Sd Oz To obtain a reasonable center fre quency gain use a 100K digital pot for R2 and a 1K digital pot for R1 These are commonly available values By send ing them identical digital commands the ratio R2 R1 is fixed at 100 The de sired frequency range is set by choosing the value of the capacitors C1 C2 The center frequency gain and more important whether the filter pro vides a notch or a bandpass is now con trolled by the value of the gain Figure 2 is a plot of the filter s center frequency gain as a function of G over a range of O values between 0 8 and 1 02 assuming o l l the ratio R2 R1 100 FIGURE 3 Schematic of a digitally adjustable notch filter with good frequency resolu tio
62. r Cap Even the finest nuances can be heard The sound never gets over edged really superb naturalness with a somewhat bright top end Silver Cap Super smooth cap without any harsh additions to the sound ON neutral tonal balance A truly outstanding audio part Silver Gold Cap More resolution more sound SE Lots of dynamics Fast reaction life feeling and natural sound 1 6 amp 1 8 mm baked wire coil Long expected amp wanted New round wire coil made of 1 6 amp 1 8 mm baked wire Available as air cored coil and with non ferrite core JA 8008 Jantzen Audio 8 driver 8 ohm 95 dB designed by Troels Gravesen and made by SEAS Norway Ideally mated with high efficiency 34 mm dome Audax tweeter with JA waveguide www jantzen audio com Parts available at www parts express com amp other Jantzen Audio dealers audioXpress August 2011 Designing Amps tor Maximum Linearity with the ECC99 The author shares his design methodology for a simple tube amp Source Modern High End Valve Amplifiers There Reproduced with kind permission of ir Merno van der Veen FIGURE 1 The VDV amplifier concept Voltage Phase Amplifier Splitter To EL34 grid g1 To EL34 grid g1 Rudy Godmaire Note R and R are matched resistors February 2011 FIGURE 2 The input tube circuitry 6 audioXpress 8 11 www audioXpress com always thought that designing a great tube amplifier was a mixture of science and
63. s on a strip board than any other method of construction This design passed this test with flying colors as I write I am listening signs only to find it becomes either un to my Vero board prototype channel of the amplifier The other chan www audioxpress com and finally the output stage rate functions First it blocks any DC and that might upset the balance of the long tailed pair T1 and T2 Such an im balance would lead to unwanted output offset voltage across the speaker Second C1 defines the low frequency limit of At the heart of most modern power amps is the long tailed pair differential amplifier in this design it comprises T1 and T2 T s function is to compare the amp s input signal with the output of the amplifier via the feedback loop The output of the stage is the difference be tween input and output which becomes the feedback signal for the amp Here the usual collector resistors are replaced 3 with a current mirror comprising T11 and T12 A small but significant amount Figure 3 shows the schematic of one and R3 which increases the output im pedance of the circuit The use of a cur of negative feedback is applied by R2 rent mirror greatly improves the linearity of this part of the circuit The driver circuit consists of T6 and its constant current source T5 T6 s base is fed directly from the collector of T1 A Y G E 3 L gt 4
64. specially with friends Francois and Pierre Above all listening to music and attending classical concerts and opera with his wife Elaine remains among the most fulfilling activities he cherishes David A Rich Harman Kardon HD 990 review p 13 received his MSEE from Columbia University and his Ph D from Polytechnic University of NYU He specializes in the design of analog and mixed signal integrated circuits and has taught graduate and undergraduate courses in integrated electronics and electroacoustics Student work under his guidance including a novel high efficiency mixed signal integrated power amplifier has won numerous awards His industrial positions include Technical Manager at Bell Laboratories His portfolio has spanned the design of audio ICs for Air Force One to RF ICs for wireless cell phones and his innovations have earned 14 patents He is a Senior Member of the IEEE and has frequently served as chairperson for technical and panel sessions at IEEE conferences He has been a member of the AES signal processing technical committee and has been Technical Editor for Audio Critic He is the head of the music committee of the Bethlehem Chamber Music Society Jeff Macaulay Efficient Class A Amp p 16 first became interested in hi fi in 1970 when he heard the sound of Jimi Hendrix through a pair of Cambridge R50 transmission line speakers During a long period of illness which left him with lots of time he taught himse
65. st PR1 to short out T7 and T9 bases Check for this on a the voltage on T4 s collector is constant this is also used for base biasing T5 pro viding constant current for T6 3 shift occurs Due to the inevitable para sitic inductance and capacitance of the circuit oscillation is guaranteed when part of the circuit short T7 8 9 and T10 form a basic quasi complementary output stage T7 and T8 are configured multimeter set to indicate low resistance Now the quiescent current needs set ting up Monitor the voltage across D1 with a multimeter set to clearly show 1V operating principles behind the output on the PSU Nothing should happen DC With no input signal applied switch If everything is OK slowly adjust RV1 for 200mV Switch off and remove your multimeter That s it job done All that remains is to apply an input signal sit back and enjoy true class A sound aX AMPLIFIER PARTS LIST FOR FIG 3 RESISTORS 100nF 100V WKG CERAMIC 270pF Ceramic 1nF Ceramic 100uF 25V Electrolytic BC550C MPSA42 MPSA92 TIP3055 BC557C MISCELLANEOUS TO3P Insulating kits Heatsink PCB M3 Screws audioXpress August 2011 19 The Michele MC Preamp Discover how this battery powered low output moving coil tube preamplifier stacks up against conventional AC power units PERFORMANCE DC Switch F l Total Gain 56 db 4x 12 Vee Noise 70 db Distorsion 0 1 500mV output
66. table to evaluate only the second harmonic con catalog www audioXpress com 4 E L Chaffee Theory of Thermonic Vacuum Tubes McGraw Hill Book Company Inc First Edition 1933 5 Norman Crowhurst the Audio Classroom se ries available for free download at www audioXpress com resources 6 F Langford Smith Radiotron Designers Hand book Audio Amateur Publications Inc 1996 1952 CD ROM available on the Old Colony online cata reference 6 chapter 12 section 2 i log www audioXpress com 7 Herbert J Reich Principles of Electron Tubes Understanding and Designing Simple Circuits Audio audioXpress com 8 Bruce Rozenblit Beginners Guide to Tube Audio Design Audio Amateur Press First Edition Second Printing 1997 2001 available on the Old Colony 2 ence 7 section 4 6 online catalog www audioXpress com 9 Mullard EL34 datasheet available at http 2 chapter 7 section 2 ii B reference 3 chapter 6 pp 405 407 lated from the pentode characteristics when the triode www mif pg gda pl homepages frank 10 The grid bias voltage Var may also be extrapo mode plate characteristics are not available See my ar ticle Va Correlations audioXpress March 2009 p 32 1 2 3 FINISHED mplementing the methodology I propose is truly easy The following hints will further help you optimize your designs in less time than it takes to
67. tage pro vides a 56dB gain required for low output MC cartridges 200 500uV This stage must also have a high signal to noise ratio in order to minimize hum and hiss Its output feeds a passive RIAA equaliza tion network which has a negative gain of 20dB at 1kHz immediately restored by the next buffer stage From these the sig nal through a cathode follower is applied to the RCA output connectors The power supplies section is made up of voltage regulated circuits both for heater and plate voltages A relay provides proper voltages to the circuit either from an internal power supply or external batteries When the pre is fed through an AC outlet relay RY1 is automatically energized and feeds all circuits by means of internal power sup plies Under this condition the AC DC switch does not have any effect If mains is turned off and you set the switch to DC the preamplifier is then fed by the DC voltages supplied by the external rechargeable batteries 12V 1 3 A h x 4 48 Vcc for plate voltages plus 6V 12A h for heaters The diagram at the bottom of Fig 1 shows graphically the gain loss of various stages The construction of this preamplifier requires a thorough knowledge of audio frequency electronics assembly tech niques and the problems inherent in high sensitivity circuits It is not recom mended for inexperienced DIYers DESIGN CHOICES The whole design resembles to som
68. tain a bandpass The circuit Photo 1 is a slight adap tation of the classic Delyiannis Friend filter It applies gain at a selectable fre quency while minimizing gain and phase variation at other out of band frequen cies The filter op amp s non inverting terminal is referenced to the input signal to provide unity OdB out of band gain GAIN hz s PE R1 FIGURE 1 Simplified filter schematic www audioXpress com so the filter can be applied in series with traditional proportional integral and de 2 rivative PID type controllers The filter contains two poles and two zeros all at the filter s center frequency _ and the difference in damping between these poles and zeros produces gain at that frequency The fact that the poles and zeros are all at the same frequen cy minimizes gain and phase variation away from the filter s center frequency A unique advantage of this formu lation is that by setting C1 C2 and _ keeping the ratio of R1 and R2 constant which can be accomplished by electron ically ganging two digitally controlled potentiometers of different maximum values the filter s center frequency can be moved in frequency without transfer function distortion The filter transfer function output _ voltage input voltage Vout Vin is ter op amp The gain amp is implemented with either a non inverting op amp in notch a
69. tape cassette deck 22 audioXpress 8 11 ri www audioXpress com tube circuit can hardly respond to these requirements considering also the intrinsic noise problem introduced by them A bril _ liant solution was identified back in the 80s by the designer Aloia who called this particular circuit TOPOHYBODE an acronym for TOtem POle HYBrid cas CODE It consists of a totem pole in a cascode configuration with an NPN low noise transistor Fig 1 V1 The performance of this circuit is outstanding it exploits all advantages _ of the vacuum tube and the transistor With its simplicity and over 30dB over load capability it surpasses by far any _ other solution Finally a passive RIAA equalization network completes the hard part of this circuit CIRCUIT IMPLEMENTATION Low level input stages must be rather quiet in order to maximize the signal to noise ratio necessary for good perfor mance All possible technical arrange ments have been implemented into the _ design and the construction to reach this _ goal This has been achieved by selecting a quiet tube such as the ECC86 6GM8 as previously stated In addition all the tube heaters are _ fed by direct current while the plate voltages are supplied by a dedicated voltage regulator to stabilize its output and remove any possible ripple You have the option of using external rechargeable batteries for additiona
70. than it ap pears and if you are aiming at the very best design possible I believe this extra care is really worth it Before launching into the main devel Loadilne 1 Loadilne 2 Loadilne 3 10 Loadiine 4 Rudy Godmaire February 2011 FIGURE 3 Linearity of various loadlines opment of this article I must mention that fidelity is always my main leitmotif sipation of the plate and screen grid to gether P Pa is 30W However bi asing the tubes near their absolute maxi mum reduces their lifespan which is why is better achieved when you favor simple All the considerations discussed so It is derived from the best practices I Langford Smith Jones Reich Rozen fier is realized going backward from the output to the input As far as the phase splitter is concerned what interests me is drive the output tubes to full power Loadiines 1 amp 3 10K Loadiline 2 16K Loadline 4 48K of 82mA This can be explained by the fact that the maximum permissible dis I prefer a more conservative value In my Inspection of the Mullard EL34 data sheet reveals that the grid bias voltage V is 27V for a plate voltage of 365V 8 and a quiescent current of 70mA Be cause I have a push pull topology the distortion As you will realize later there GENERAL CIRCUITS CO LTD QUALITY PCB amp SERVICE PROTOTYPE TO PRODUCTION instant o
71. uch a graphical analysis would be very complicated www and this is why it is customary to assume that the load is purely resistive which enables you to use a straight line to approximate the behavior of the circuit under AC conditions For further details see reference 6 chapter 2 section 4 vi reference 5 and also refer 15 For a detailed explanation see reference 6 16 The measurements were made without the EL34 output tubes to avoid any distortion coming from their clipping and from capacitor blocking redo the design of the circuit using a lower load If it is higher than 1 0V say 0 6V redo the design of the circuit using a higher load Hint For each deviation of 0 1V of grid voltage adjust the load by a factor of 0 2rp If a third iteration is required extrapolate the value from the first two results For your convenience I have pre pared a Microsoft Excel spreadsheet that will assist you when you design your own circuits with the ECC99 This tool which requires only minimal skills with MS Excel proposes a Design Protocol that summarizes my meth odology in 24 simple steps The file is available for free download at www audioxpress com magsdirx ax addenda media ecc99designer xls Mi reviews PA Reliable Reviews Harman Kardon HD 990 CD Player and S PDIF Input DAC _ Dimensions 2 5 x 17 5 x 13 _ Weight 8 6 Ibs E List Price 699 H Street price 600
72. urprise Others might hear some difference between the HD990 and higher priced units but would be very surprised if they found fault with this unit A Comparison The Cambridge Audio Azur 840C offers a good comparison It shares the same Blackfin DSP and a pair of AD1955A DACs lt also has S PDIF inputs and balanced outputs At 1800 it is three times the price and has significant build quality differences The power supply is traditional with the transformer at 60Hz The unit is twice as heavy owing to more substan tial sheet metal and the larger power transformer The disc transport is per haps more robust and the electronics associated operating the transport are different The ergonomics at the front panel and the remote are substantially better Cambridge supplies a 14 page white paper on the novel jitter reduction and digital filtering algorithms encoded in the Blackfin DSP As with a computer hardware performance is often second ary to the software loaded on it Indeed the distinguishing feature of four and fivefigure CD players and DACs is of ten the algorithms in the DSP Harman offers no information on the signal processing though the measurements cited above leave little doubt the perfor mance is improved compared to a CD player that does not have an external DSP chip on the board The Cambridge Audio Azur 840C S PDIF input will accept a 192k Sa s sig nal that the Harman HD 990 cannot The 840C also tra
73. y of 7Hz CONCLUSION Simple tube circuits often benefit from excellent musicality When these are optimized with care and utilized well within their limits high fidelity rewards the diligent designer In this respect the _ methodology presented in this article should help you get the very best from your designs Should you wish to design your own circuits with the ECC99 using my methodology I invite you to download my spreadsheet from the audioXpress website as mentioned in the sidebar aX DRAWING LOADLINES he old fashioned way of work ing with loadlines means that you print a few copies of the plate characteristics take a ruler and a pencil and draw your loadlines I prefer to use drawing software such as Microsoft Visio 1 simply cut the chart from the PDF document and paste it in my drawing software From there I can easily draw any loadline I want over the image and save them separately One significant advantage with this method is that you can zoom in to make more ac curate evaluations Alternatively you may also use the drawing features provided by MS Excel Mi 12 audioXpress 8 11 REFERENCES AND NOTES 1 Ir Menno van der Veen Modern High End Valve Amplifiers Elektor Electronics First Edition 1999 2 ECC99 datasheet available at www jj elec tronic sk 3 Morgan Jones Valve Amplifiers Newnes Third Edition 2003 available at the Old Colony online cep

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