Tube Stages for DACs..
Contents
1. a very low voltage gain and notable RF Filtering to protect following equipment from high levels of ultrasonic noise On the surface it also seems to require a form of balanced to single ended conversion It is the latter requirement that has thrown many people off track have seen one solution by Chris Found which uses differential SRPP valve stages but ends up with far too much gain excessive output voltage and a substantially unnecessary complexity Another rather ingenious solution Stephano Perugini was to use a transformer which of course works great offers galvanic isolation from all the noise in the dac and allows bandwidth limiting to eliminate ultrasonic noise in the most simple and direct way While each solution has its merits one is too complex and has too many glowing bottles while the other is really ingenious but has no glowing bottles and relies upon a very high specification 40k 40k Transformer neither a readily nor cheaply available part A pair of good quality wideband 40k _line level transformer can easily exceed 60 and the best from Jensen cost several hundred dollars each The use of just a transformer as output stage for differential voltage output dac s has since been also popularized by Jensen a maker of fine audio transformers and a fewothers Recently the Neko dac 100 has featured such a just one Jensen transformer output stage And in England my friend Simon Ashton of Audiosmile modifie2. designs On the contrary they are technologically much less capable of high fidelity reproduction than Multibit dac s and as they operate on the basis of clock cycle modulation any instabilities of the master clock jitter have a much larger impact on sound as with the old multibit Designs Progress has been made on all these fronts and more recent hybrid dac s do quite vell all considered yet most still have some wey to go to match a good 16 Bit 44 1KHz dac such as Philips TDA1541 and Burr Brom PCM56 when measured using methods that do not rely on heavy averaging and of course when listened to Yet the presence or absence of multibit dac s or time slicing dac s should not be seen as an absolute indicator of quality As usual much depends upon the implementation This is shown by the fact that several digital processors or CD players using time slicing dac s sound very good Timbre Dac Acoustic Precision EIKOS CD Player and certainly a lot better than the worst multibit dac sporting machines did feel that the discussion of the different technologies was necessary as too often even experienced designers tend to view dac s as a Black Box with little concern about how they really work Together with mistaken assumptions based on the datasheet of the dac issued primarily not as design guide but as thinly disguised marketing material one often sees a lot of effort and care being lavished on designs based on chip s that shoul
3. filter that is differential just as wth the 1999 circuit Unlike the 1999 circuit it only uses a first order filter This is because the following active circuit offers another inherent low pass so in combination a second order lowpass wth a 3db point of 60KHz is generated This 60KHz filter is sufficient for modern hybrid dac s which have in essence replaced the pure time slicing bit stream designs by noweven in very lowcost designs For those modifying cd players or dac s using very early bit stream dacs the original LC RC filter can be used and applied instead in a cut and paste fashion This first order lowpass is followed by a circuit that combines the SRPP circuit applicable to the multibit dac s wth the local feedback loop taken from the so called anode follower circuit The combination gives the correct gain for voltage output dac s to produce 2V RMS output and provides lowoutput impedance 4 7K 1w 48 Ecces 2 a out from OAC 15K 1M O FUF aw ea KP1 72 ik 100K 100 Gnd 6 15K 330K ovi from DAC 22u AGNO Ov Sonically feel that this design is somewhat more interesting and alive sounding than the super linear cathode follower presented in 1999 It can work vell wth all the differential and single ended output dac s that have their output overlaid wth around 2 5V DC and produce 1V RMS output at full scale which includes practically any current design that is not covered by the multibit d
4. filters an all use multibit dac s Many of these are even available wth valve output stages True for every one such dac there are probably ten or twenty dac s wth time slicing dac s yet like other technologies long claimed to be obsolete and dead multibit dacs just as Valves and Vinyl cling to life wth the tenacity of those unjustly said to be dead Why Its the Music stupid With Multibit dac s featuring Valve outputs from Scott Anodyne Nixon s super low cost offerings and the kits mentioned above to such extreme units as the Audio Note UK s top of the range offerings and Zanden s dac s and cd players we find valved multibit offerings are found at any level of cost and performance a marked change from a decade ago wien it seemed that apart from Audio Note UK non such existed 3 Timeslicing or Delta Sigma dac The device call time slicing dac is known by many names Technics calls it MASH pronounce like mush Pioneer calls it Pulseflow pronounce as painflow and Philips calls it Bit stream pronounce as bitscream and so on These dac s are often also referred to as low bit or single bit dac s delta sigma is also frequently used They are in many ways ingenious Devices Instead of attempting to make precision analogue silicon always expensive they circumvent the issue by using only one on off switch and by toggling this switch very very fast The result is a square wave of very h
5. s are still the order of the day Multibit dac Holdouts in the better budget consumer CD Player Market are Rotel and Denon mell they have since thrown the towel All but their really bottom feeding range uses multibit dac s The top of the line Teac and Pioneer cd players Pioneer only in their latest model also use multibit dac s not any longer they stopped using the rather expensive multibit dacs completely around the time of writing the original article Linn and Naim have been long standing advocates of multibit chips though their more recent lower priced models no longer use multibit dac s Well at least Naim stuck to their guns and still use multibit dac s in their upper models specifically the previously mentioned PCM1704 Almost all pre 1990 cd players are of the multibit type However before trying to hunt such a device down to modify it consider that they have seen a lot of service in the last 10 to 16 Years spares tends to be unavailable and breakdowns are likely Yet even a decade later surprisingly quite a fewof these older machines have proven to be by far more a bunch of ruggers than considered at the time am still modifying late 1980 s CD Players using multibit chipset a decade later and they keep trucking on while many of the late 90 s and 2k CD Players have since died One possible option would be to buy very cheaply one of the earlier Philips or Marantz cd players and remove the SAA7220 digital filte
6. the current generation of Crystal dac s by far the worst sounding time slicing dac s yet closely followed by the latest generation of low cost dac s from Burr Brown A decade later all vendors budget dac s are hybrid and sonically a lot better than their earlier efforts however there is still a wide range of sonics and not all currently manufactured dacs are created equal A number of recent Time slicing dac have gone further and even incorporated the rest of the analogue stage onto the chip Philips TDA1305 and Burr Brown PCM 1710 16 28 32 come to mind Just as it is the habit of large corporations these op amps s used on chip are not your audiophile favourites On the contrary with Philips it is of course the equivalent of a NE5532 or not wed actually wsh it was that good Burr Brown did not even spring for something like their well respected OPA2604 or OPA2132 2134 they used a really nasty CMOS Op Amp The latest trend is in fact dac s that incorporate a small switched mode supply on board so they can produce 2V RMS from a single 5V supply have not had the chance to test such units hovever while clearly nice from a viewpoint of simplifying product design you need a 5V supply a fewsmall value capacitors and the dac chip all else is catered for suspect that the sonic merit for these devices will be less than one might wish for Of course these op amp s you cannot see Nor can you bypass them anymore They are al
7. to the Grid of the follower valve This allows about 4 to 5V to develop on the cathode more than enough voltage headroom to allow the about 1V RMS output to be buffered with low measured and perceived distortion It is possible to use a simple classic cathode follower in the above circuit This will certainly result in a much more valvey and possibly subjectively preferable sound In this case the ECC82 6CG7 6FQ7 or 6SN7 seem ideally suited with a 2 2kOhm resistor in the cathode An ECC83 might be employed with a 3 3kOhm cathode resistor The sound will be very different to the circuit shown above Mostly transparency will be substantially compromised however the subjective result might be considered pleasing Perhaps it is worthwhile trying designs the classic and the Super Linear Cathode Follower In 1999 I was rather prejudiced against using local loop feedback around a single valve stage Hence at the time did not include this option have since had ample time to work wth this circuit and to cure myself of this prejudice By all indications the circuit above has been build many times and has its followers however I do not use it or its like any more The following circuit is the one that should have perhaps been included in the original article at the time instead of what was published above It is designed to operate direct coupled to dacs wth a voltage output and wth 2 5V DC offset The circuit contains a first order RC input
8. 72 4 appx 8 10V 6CG7 6SN7 0 47uF H 470k 2 4 7m 22k 620p 25K170 Gnd ps ps Idss 4m Toko Fon out from DAC 4 7mH 1k 10k KP72 Foil amp Film Polypropylen Capacitors 1000 DC Rating Arcotranic KP72 Series RS Components OR LCR PC HY S Series FARNELL Universal DS DAC Output PS Polystyren Capacitors all rights reserved A SS Ansar Supersound metalised polypropylen Capacitor 400 DC Rating or equivalent 2000 Thunderstone Audios All Resistors Holco H4 Vishay EE10 or better Thorsten Loesch The circuit seems very novel but it is not at all so The differential filter at the input of the circuit is used in order to present an identical load to either output of the dac and to implement a well damped 3 order low pass filter Again a small loss about 0 8db is allowed at 20 kHz The positive output of this filter is then buffered by an ECC88 cathode follower This cathode follower has been subjected to some improvements over the classic version By bootstrapping or cascoding the anode of the follower valve the circuit is linearised The use of a J Fet connected as current source in the cathode of the follower valve serves the same purpose More on this circuit can be found in Allen Wrights Valve Preamp Cook Book and hence it will be not discussed much further The usual 2 5V DC offset on the dac Output is put to good use in order to provide additional bias
9. Thermionic Valve Analogue Stages for Digital Audio A short overview of the Subject by Thorsten Loesch Updated from the Article originally written for the newsletter of the London Live DIY Hifi Circle in 1999 0 Preface and update The bulk of this article is retained from the original witten in 1999 and published worldwde first in valve 2 2000 It is perhaps to the credit of the early revival of DIY Valve Audio as embodied by Valve and Sound Practices that this article which owes so much to both magazines and the communities that sprang up around them in the early days of the internet has remained in demand and is still occasionally referenced Reading ones om writings wth a decade distance is often an interesting experience Yet feel that overall this article has stood the test of time vell and remains relevant in today s world In the days wote the original article was working by day on doomsday scenarios where all computers would crash on the first of Jan 2000 and destroy civilisation as we knowit or worse and howto stop it from happening that one went wth such a fizzle expected more after all that build up The article was witten quite aggressive as it was originally published in an extremely limited circulation privately published newsletter let it stand as wes for publication nearly a year later vorldwde and decided to do so again a decade later On occasion a spade must be called a spade not in misguid
10. a hybrid Mu Follower using a self adjusting current source as load for the ECC88 6DJ8 6922 Valve Jumpers are used to select different values of current voltage conversion resistors as well different configurations of the anode follower configurations diyhifisuooly Universol Tube Stoge Principle Diogrom The power supply is derived from the simple design however it substitutes the large and heavy chokes wth an electronic version called a zener follower and integrates the resistance for the snubber into the custom transformer This circuit is a zero feedback supply design also found for example in the top of range Conrad Johnson preamplifiers The UTS is also fitted wth a mute circuit that can be triggered by the dacs or CD Players circuitry and offers a turn on delay to remove swtch on and switch off thumbs Fitted wth screwterminals which can be removed to allowdirect soldered connections instead for those sufficiently possessed of audiophilia nervosa to care about that kind of stuff the whole makes a very compact and quite easy to installed module It can fit easily inside most digital players and dac s wth a size of 140mm X 80mm X 45mm size It normally comes wth the matching lownoise torrid mains transformer pictured If you get my favourite NOS Tesla ECC88 Valves and tinfoil amp polypropylene output coupling capacitors included the total comes to 230 US Dollar plus shipping which while not exactly beer money or small chang
11. ally if combined wth modern minimum phase often called slowroll off filters or indeed the so called Craven apodising digital filters Let s look at the technology 2 Multibit Digital to Analogue Converters The multibit dac is the classic technology used from the beginning and has a very long history outside digital audio In most simple terms a multibit dac contains a number of switches and a resistor network and some auxiliary circuitry Each of the switches corresponds to a Bit and will switch in a resistor allowing a certain current to be presented on the output of the dac chip Multi level D A conversion mz _ mz 1s las VOLEN 32 768 am 16 Bit Multi bit D A conversion employed 16 switches corresponding to the 16 bits of the CD sample Each switch produced a different level of current according to the significance of the bit If take the classic Philips TDA 1541 multibit dac as example the MSB most significant Bit will toggle on or off a Switch which will switch on or off a current of 2mA With all Bits at Zero all switches will be off and the output will be at zero Switching the MSB on will produce 2mA current and represents basically digital silence Switching all other 15 Switches on will add another 2mA to the total now bringing the output to 4mA So switching from all bits off to all bits on will change the Output Current from OmA to 4mA Many more modern dac s use a separate offset current
12. athode follower seems to answer the purpose admirably My own original solution omitted the cathode follower entirely and later introduced a J Fet based buffer This works and sounds very good indeed All this said under certain conditions amplifiers or preamplifiers with an input impedance of 47kOhm or higher we can completely omit ANY analogue stage for Time slicing dac s and use only a simple passive circuit avoiding any additional active components This is especially true for recent devices from Burr Brown PCM 1710 1716 1728 1732 Series Analogue Devices insert list of devices and Philips TDA1305 It is interesting to see that several modifications for the ever popular due to exceptional cheapness combined wth decent performance range of Behringer Pro Audio digital products both of the DIY kind and in the form of various commercial packages implement exactly this approach described above a brutally simple circuit using a handful of components and using only one side of the balanced output for single ended applications The individuals publishing these very likely independently came to their applications but for those who did not and read it from my original article kindly send a small portion of your profits to Doctors wthout Borders or give some to any local charity caring for children that have been ill treated or even better volunteer some hours a month to work wth children you can instil your love of music and tink
13. circuits will lead to severe levels of inter modulation distortion This is easily observable with noise load testing as much as 1 with leading high end converters and worse for much consumer equipment So having defined the almost absolute need for zero feedback circuits then if our goal is to offer high fidelity without recourse to repeated design iterations and use of expensive superfast high resolution FFT analysers and noise load testing the only readily available devices with an ability to offer low enough distortion levels in zero feedback mode are thermionic valves most notabene triodes have since had the chance to use such high resolution test gear developing a lot of circuitry and still prefer to use triodes Now how do we interface triode valve circuits to the kind of dac chip s commonly found in consumer digital equipment To answer this we must first understand that currently two fundamentally different dac architectures are being used and that the two different architectures require a fundamentally different approach to analogue stage design The two dac architectures in question are multibit and timeslicing dac s Since this was written the standard approach to making dacs has become to combine 4 to 6 bits worth of multibit dac wth a timeslicing core providing the rest In the comments these wil be called hybrid dacs If well implemented these dacs can offer a considerable advance over traditional delta sigma types especi
14. d be buried quietly in a dark spot around midnight and not used in quality audio equipment With all this bleak view have of the CD digital format it s recordings and the competence of the designers of much of the equipment used to play CD s one is tempted to ask Why bother And must say that have almost taken this view With a huge and still fast growing record collection and excellent analogue replay equipment personally can do and mostly do without CD s However even as founder and currently only Member of The Holy Order of the Latter Day Luddites own a CD Player a DVD Player actually and own a number of CD s Indeed as type this m listening to Phil Collins from CD Others have committed themselves entirety to the CD Medium doubt that ANY current audiophile or ultra fi Audio System is without a CD player Love them hate them we have to make do with them and then extracting the last bit of quality becomes essential Well a marriage a divorce and a fewmoves across continents later the record collection is lost and the digital devil has claimed my analogue soul Yet using the cream of the crop of current digital gear I do no longer mind this all that much And this Latter Day Luddite nowuses a PC based Audio Player as sole source of music but wth a multibit dac followed by a valve stage As I write the update I am browsing my albums on the screen wth my remote and decide to listen to the Big Drum Small W
15. due to the non zero impedance of the I V conversion node limited it is essential to reduce the resistors in the circuit accordingly that is to halve their values Further we need to increase the capacitors to double their values Yet we are still not there The PCM 63 is unique in that it allows us to disconnect the output from the pull down current source No such thing exists in the TDA1541 In order to offer a OmA output at digital silence that is NOT all bit s set to zero by the way almost all Multibit dac s use a pull down current source offering a 2 full scale current offset zeroing the output at digital silence The PCM63 allows us to eliminate this current source a bunch of transistors resistors and Jah knows what else simply by disconnecting pin 5 offset form pin 6 output Having done that we are left with a dac that will swing now from 2mA for digital silence to 0 and 4mA peaks for negative or positive full scale signals respectively It is in this mode that the highest degree of sound quality from the PCM63 is realised at the penalty of a full scale offset In many circuits the measures required to eliminate this offset would be worse than the problem so this is rarely done In our own little circuits the offset matters little and hence we can safely employ this scheme The highest value of I V conversion resistor usable before significant distortion becomes notable is 100 Ohm in this case for the PCM63 Simila
16. e also available but pricey have not really tried them much but they are reportedly excellent While it is possible to further increase the capacitance would not recommend doing it in order to drive a lower load impedance For ECC88 6FQ7 and 6SN7 the 20kOhm load possible in this configuration are the lower limit for ECC82 ECC83 and 12AY7 6072 the load impedance should not fall below about 50kOhm and hence in this case even a single 0 47uF coupling capacitor will suffice All of this gives a basic circuit now almost a decade old vell by now two decades TEMPVS FVGIT and well worn but it still delivers the good s Here is what it looks like out from DAC 22 120 270 gt ooz 12uF Deem K mo Aqnd 4 KP72 Foil amp Film Polypropylen Capacitors 1000V DC Rating Arcotronic KP72 Series RS Components OR LCR PC HY S Series FARNELL Universal DAC Outputstage all rights reserved A4 SS Ansar Supersound metalised polypropylen Capaciter 400 DC Rating or equivalent 2000 Thunderstone Audio Thorsten Loesch 1 2 Polystyren Capacitors All Resistors Holco H4 Vishay EE10 or better Now on to the PCM63 and TDA1541 The TDA 1541 is still one of the absolute best sounding dac chip s ever made followed closely by the PCM63 Both chips are also fundamentally different from most other dac s of their kind by delivering O to 4mA output current In order to keep the error
17. e is about the same money the original Musical Fidelity X 10D missing link sold for in 1999 Hovever unlike the missing link it is not a band aid to be applied after a conventional CD Player but a fundamental replacement for the solid state analogue stage in any kind of digital sound producing device from CD and DVD Players through dacs to satellite radio receivers suspect it can even be used in PC cases to upgrade the analogue stages of computer soundcards As a final teaser here a picture of a Marantz CD 80 that was converted from the standard op amp output stage to the UTS had the digital filter bypassed and also had the EZ Ultimate Clock system fitted The sonic results of the over 20 Year old CD Player wth these upgrades are most surprising or perhaps not all the above considered A blowby blowaccount of the upgrades and results of them will hopefully grace the pages of this Journal about as soon as I can manage to type Conclusion Armed with the designs shown above and especially the Universal Valve Stage as well as the background information presented it should be possible to convert any available digital audio replay device to a state with a more transparent less subjectively compromised design Most cd players and dac s will have sufficient internal space to fit both the valve circuit and the psu parts The perhaps best mechanical solution for most cd players would be to obtain a suitable sized alumi
18. ed political correctness a wood and metal digging instrument of limited application Valve Amplifiers were a comparably rare sight at hifi shows and dac s or cd players wth valves could be compared to rocking horse droppings Nowedays you cannot swng a cat or throwa brick in any hifi exhibition wthout it destroying several valve amps and so called valve or valve cd players in the process Of course the valve cd players and dac s rarely are true valve units usually they just contain a valve buffer after a thoroughly conventional and often not so vell designed solid state analogue stage Still one has got to give points for effort have taken the liberty to insert comments to account for a world that has much changed in the intervening decade wth_9 11 and_7 7 al Qaeda the long awaited sequel to Operation Desert Storm the infamous Talivan s prowing the streets of the UK and_the RIAA mass suing its customers the rise and fall of the Piratebay plus a global economical crisis which would not have looked bad as the Y2K disaster that never arrived have cleaned up some spelling mistakes and the odd bit of grammar also append a circuit for voltage output dac s not present in the original article which in my current experience may be a better choice than the one presented originally which is however retained All later additions are presented in italic letters like this section compared to the original text 1 In
19. ering in a newgeneration Back to dacs For Multibit dac s it is also possible to completely avoid active analogue stages if the following amplification chain has enough spare gain available usually one would expect to see about 25 to 100mV RMS Input sensitivity for clipping The cases where one can afford to operate without external analogue stage are comparably rare though I might discuss the dac Direct Output Topology have developed for both types of dac s another time For the time being let s get practical In the decade since wote this article the visible on line DIY audio scene has become much more active lively and varied It pleases me immensely to note the presence of such websites as Lukasz Lampizator Fikus Website mostly dedicated to putting valves into any CD Player and dac he can get his hands on applaud his enthusiasm and dedication Even if you do not intend to use his Lampizator Circuit which does share parallels wth the circuits published herein but mine instead his website is an invaluable resource that helps to select just the right player to modify from the thousands on offer on E bay So please peruse his website as much as you require and can Practical Implementations 1 Pve got the Power a power supply digression Prior to launching into a lengthy discussion of the specific example circuits for analogue stages let s briefly cover a simple fairly inexpensive but high performance
20. esign Thus slight variations of the same basic design are applicable to nearly all DAC s under this sun so one may change the dac chip dramatically but keep the valve circuit mostly unchanged 4 DIYHIFISUPPLY Universal Valve Stage Over a year ago Brian Cherry from wwdiyhifisupply com asked for a simple and compact valve analogue stage fora dac he sells The design eventually came up wth drew heavily on the designs presented in this article It was decided to make the circuit universal enough so that it could work both as straightforward gain stage for current output multibit dacs and as anode follower wth a gain of 2 or 4 for voltage output dacs and fill many other roles in line preamplifiers phono stages and pro audio designs Using a fewjumpers just as on old computer motherboards allows the Universal Valve Stage board to be configured to work wth pretty much any dac under the sun wth re configuration taking a few10 seconds For people more hardcore who dislike jumpers it is of course possible to solder bridge across the jumpers though to be honest the gold plated brass jumpers do not seem to affect the sound quality that unduly Unlike the earlier designs cannot present a full design circuit of the Universal Valve Stage as itis a commercial design paid for and omed by wwudiyhifisupply com What I can present is the principle schematic also included in the manual and a picture of the whole thing The gain stage is
21. h time slicing would drop the speed requirements for a direct conversion system free from noise shaping and filtering to 11 2896MHz a speed eminently achievable using generic logic A recent AES paper called hybrid multibit devices at least theoretically infinitely perfectible while issuing a scathing dismissal of pure single bit systems on very good theoretical and practical grounds Thus using these methods the infinitely perfectible hybrid dac s of recent designs wth a multibit core can attain excellent performance for CD However the reason they do this was not to improve CD Audio but to be able to handle audio wth greater word length than CD Audio which they generally do nowhere nearly as vell Still one is not complaining Perhaps more crucially so called Class D Amplifiers which have in recent times sprouted up like mushrooms after a warm rain continue to use the straight two or three level modulation scheme described above And thus they still require the use of heavy handed noise shaping to attain anything like acceptable 16 Bit Audio performance The clock frequencies for these amplifiers are usually at 300 KHz to 1MHz in the best cases That is 3 000 to 10 000 times lower than what is required to attain 16 Bit 44 1 KHz performance without noise shaping and other forms of signal manipulation And again one is baffled and perplexed by the rave reviews many Class D amplifiers receive as baffled as one wa
22. igh frequency which is however symmetrical So once we pass this square wave through a low pass filter we are left assuming digital silence as input with a voltage of zero l bit D A conversion Current pulse On off PWM PULSE WIDTH MUDULATION The principle of 1 bit D A conversion If we now start taking let s say every 65536 s pulse out we will achieve a tiny deflection of the Voltage Depending if we omit the positive going pulse or the negative going pulse we will get a deflection to the positive or negative side Now for this to really work for CD Style Digital Audio we theoretically require 216 Pulses 65536 pulses for every 44 1 kHz Sample That means we need theoretically a 2890 MHz or about 2 9 GHz Clock to run the dac This is clearly not practical okay today s high end PC CPU s do operate this fast however for a dac it is still not very practical and a number of mathematical tricks noise shaping especially are employed to get a reasonable performance at much lower Clock Rates around 16 9 or 33 8 MHz All of this still holds true and is as applicable to dacs but more crucially to the swtched mode amplifiers aka Class D Amplifiers that have become common since the original article was published Combining a multibit core wth a few real bits and applying time slicing to this device considerably reduced the problems An 8 Bit multibit dac more bits than common in modern hybrid dac s combined wt
23. iyhifisupply directly some of the money you pay for their products goes towards bailing you out men trouble arises Till the next piece have fun and Enjoy The Music About the Author Thorsten Loesch has been the Director of Technology for Abbingdon Music Research AMR since 2003 and design consultant for DIY Hi Fi Supply since 2001 Mr Loesch has for a number of years witten extensively on audio equipment designs and modifications and is also an experienced audio equipment reviever Before joining AMR Mr Loesch vas a financial systems manager for the UK public health service Prior to this he was a Tonmeister Sound Engineer in Germany and gained extensive experience in professional audio design setup and operations He holds degrees in both Electronic Engineering and in Business Information Systems
24. low 39 BYYS6D 39 1k SW B INS 10H 22uFlt 220ufl 240 100mA 1N5822 0 68 Sw LA LA 1N5822 a 2200uF 2200uF 2200uFl 2200uFH 2200uFI 2200uFl x3 Dx3 Dx3 Dx3 Dx3 0x3 1N5822 ices AWA AAN egter 0 1 0 68 5w KP72 Foil amp Film Polypropylen Capacitors 1500 DC Rating Arcetronic KP72 Series RS Components OR LGR PC HV S Series FARNELL DX3 Dubiler low inductance low ESR long live Electrolytics 16 or Higher DC Rating or similo Universal Valve Powersupply all rights reserved cc Carbon Composite Non Inductive Resistors 2W Allen Bradly Morganite Rikken or similar kk adjust these resisters in circuit under load for correct heater tension of 6 3 2000 Thunderstone Audio Thorsten Loesch believe one or two unusual features in this supply need explaining First looking at the HT Supply there are a number of unusual networks around the very standard rectifier bridge These are snagged from the late John Buddha Camille and have two purposes The CRC pi networks ahead and after the rectifier bridge help to reduce any noise The use of carbon composite resistors in this position is mandatory to avoid resonance effects However the chosen resistor values are unusually high for a reason They simulate the anode impedance of an EZ80 in full wave mode The resulting rectifier with added networks behaves very much like a valve rectifier removing the often notable unpleasan
25. nce have shown that both using unusual solutions in traditional engineering as vell as guerrilla engineering can produce a much reformed and improved sound quality from CD At the time of witing the original article had already been experimenting for some time using dacs without digital filtering finding them capable of exceptional performance in some areas yet in need of improvement in others something on mhich I have spend quite some time on wth some success if may say so Before continuing with this Article the august reader is advised to make herself or himself comfortable with the underlying principles of Digital Audio and CD player in special as no further consideration will be given to this topic The article covering CD on the How Stuff Works site is a good choice to get a quick initial overview of the subject and is recommended The Wikipedia Entry for CD Player may also help Instead we shall concern ourselves only with delivering of the however compromised output Signal from the Digital to Analogue Converter to the Output Jacks with the least amount of further perceived degradation Considering this task the use of Zero Feedback circuits seems almost mandatory almost using the right configuration of a local feedback loop can sound very good too This is because with a dac or cd player we deal with a generator of massive levels of wideband RF noise which if used with conventional feedback based
26. nium box chassis from Maplin the 150mm X 114mm X 76mm one looks right This would be attached at the rear of the Player or dac and one would run some flying leads from the Player dac circuitry trough some holes in the rear panel how about the ones previously containing the output jacks into the valve stage All other components and the psu could be placed safely in this separate chassis and again the mains power could be send through another suitably drilled hole making the whole solution neat tidy and professional looking If done well enough it would even allow the Player dac to be returned to stock condition with no major cosmetic blemishes Now suspect that some people would think it having been convenient had listed in detail the relevant Pin s on all noted dac chip s and provided more practical notes on the way these are accessed This was not done for a number of reasons Primarily I feel that anyone should get a service manual for their piece of equipment before starting to work on it This will contain a schematic that has the relevant information on it If such a schematic even if present would not be able to convey the needed information perhaps it would be useful to take someone reasonably experienced with digital audio circuits and their modification as mentor for the specific project And kindly find some one else than me If you need support to implement the_www diyhifisupply com Universal Valve Stage please e mail d
27. orld Album by The Dhol Foundation a British indian Techno New Age Punjabi Fusion outfit whose music combines traditional Punjabi instruments and musical themes wth techno trance inspired electronic music in a way find stunningly beautiful and involving Have a listen wth whatever dac or CD player you om BTW the massed Punjabi Dhol Drums on Track 12 Drummers Reel are a major workout for any Audio System fewsystems lend much realisms but on systems that do the results are unbelievable 4 Analogue Stage Requirements After having discussed the different types of dac we can now sit down and formulate a few basic requirements for analogue stages solid state or thermionic valve based to be used with the respective dac architectures An analogue stage for current output pure multibit dac s requires a low input impedance the lower the better about 100 to 200 Ohm appears to be a sensible maximum a fairly high voltage gain and while it must withstand some notable RF Noise usually no extreme low pass filtering is required This suggests a resistor for V conversion followed by a suitable gain stage with plenty of gain to present a reasonable output signal Schemes for this have been long available and what is presented below for multibit converters is mostly a rehash of other peoples work with the odd little touch from me here and there An analogue stage for time slicing Bit stream Delta Sigma dac s requires
28. ough about the newmedia to make the fight worth fighting High resolution audio that is Audio exceeding the CD sample rates and word length is finally becoming more common as downloads hovever they only play via computers and similar devices not traditional disk players so CD has remained the dominant music format wth LP making a small but surprising comeback And wth all this time past still find the combination of TDA1541 and valves to be the most analogue sounding and satisfying way to reproduce CD Audio and still find that such a combination implemented vell offers a sound quality that at least closely approaches that of the better SACD Players playing SACD and that of vell if not exceptionally implemented DVD Audio read 24 192 3 Thermionic Valve Analogue Circuits for Time slicing Digital to Analogue Converters As noted before the time slicing or delta sigma dac doesn t really need any analogue stage in order to provide enough voltage output However due to its ill conceived modus operandi we require low pass filtering and if we insist on being boring and conservative a balanced to SE converter in case of most high performance devices Let s first split the range of time slicing dac s into three groups The latest most recently introduced group uses extensive on chip analogue stages sounds by far worst of the lot and for some reason receives in its implementations consistently rave reviews These are as mentioned the B
29. p amps are NE5534 or NE5532 or functionally equivalent from JRC NJR All these devices are incredibly ill suited for this purpose and no doubt the often less than ideal sonic character of early cd players can be blamed at these devices In the practical section we will look at options for doing all this with valves and somewhat better However the coverage of classic multibit dac s is somewhat redundant as they have been almost entirely replaced with the other type of dac the one refer to as TIMESLICING dac Well a decade later multibit dac s still are being manufactured but nowvery very few remain so guess vas a little off then however the Burr Brom PCM1704 mentioned is the original article a decade ago is still the top audio targeted multibit dac in production nothing has been introduced that betters it and it was several times about to be discontinued ever since Texas Instruments took over Burr Brown Multibit dac s where and are made chiefly by Burr Brown PCM56 63 67 69 and PCM 1700 1702 1704 Analogue Devices AD1862 AD1865 and Philips TDA1541 1543 and 1545 Others exist and any of the above manufacturers also made custom chips for large OEM Manufacturers like Technics Panasonic Sony and so on The classic multibit dac s often require fairly complex psu Arrangements and are comparably expensive to make In the extreme High End Equipment Spectral Mark Levinson Krell Audio Synthesis and Wadia multibit dac
30. power supply for these circuits For simplicity of procurement it primarily uses components from Maplin Maplin have an inexpensive Valve Mains Transformer in their program as well as inexpensive but adequate chokes Used for this psu is one XP27E Mains Transformer around 14 00 and two ST28F Filter Chokes around 7 00 each Other sources exist but seriously doubt that a 50mA 20H Choke and a suitable mains Transformer can be obtained for under 30 00 the set from any other source The quality is more than adequate Well Maplin in the UK is still around but have shifted their focus from Electronic components for hobbyists and small repair workshops alike over to PC parts accessories super cheap DJ CD Player mixers and light effects for wannabe bedroom DJ s wth electronic parts being cut down into next to nothing Fortunately the post Y2K world features much easier international ordering and e bay And thanks to e bay you can now buy such lovely items as_rare mint snow globes Smurf TV trays some Beanie Babies new with tag from some guy you ve never met in Normey and last but not least many many DIY audio parts including the needed transformers To alloweasy purchase of alternatives the Maplin transformer is rated at secondary voltages of 240V 100MA and 6 3V 1 5A The same exact transformer is still offered by the original manufacturer Danbury Electronics as General Purpose Valve Mains Transformer Code VT342 and the Choke i
31. r Limitations apply to the TDA1541 We hence need to substitute the following values R1 100 Ohm R2 43 Ohm 44 Ohm accurately C1 33nF C2 330nF 320nF accurately By the way I m looking at making a dac using the PCM63 kindly donated by Geoff Mead which will use the above with a Euridice style amplification stage and NO digital filter This project eventually developed into the Adagio TDA1541A based dac project which in turn inspired several TDA1541 based dacs available in Kit and finished form It is also the inspiration for the Satch Kit produced by wwvdiyhifisupply com The circuit wise similar Audio Note UK made dac5 at a whopping 18 500 retail was just mell just a decade ago when the British Pound wes actually worth something noted by Martin Colloms in HiFi News as best from CD so far and uses actually a much inferior chip the AD1865 a dual 18 bit Multibit dac Martin noted a subjective quality approaching that of experimental new high resolution digital systems 96kHz 24bit and SACD Perhaps it s time to go out and buy the last remaining PCM63 and TDA1541 before they walk in order to make THE DEFINITIVE CD format dac With a decade passed SACD has come and is nearly gone and is having a second coming as downloadable files and DVD Audio may not be dead but it certainly smells rather funny The bloody gory format war between SACD and DVD Audio Disks that satirised in 1998 vas a non event no body cared en
32. r and the TDA1541 dac From these one can make a stand alone dac similar to the one offered by Curcio Audio from where one could obtain suitable PCB s or the van Alstine Valve dac A decade later the Curcio Audio dac kit remains available wth a TDA 1541 and has been joined by the www diyhifisupply com Satch dac Kit in whose genesis yours truly had a small part So in this day and age the DIY Enthusiast may choose from at least two dac Kits featuring a TDA1541 and Valve Output Stages They have been joined by several commercial products also offering the TDA1541 and mostly valves As St Paul said in 1 Corinthians 55 O death where is thy sting O grave where is thy victory With budget standalone dac s one tends not to find Multibit dac s at all as they became common only in the 90 s And to buy an older High End dac for modification is often also not such a good ldea as their input circuitry is often jitter ridden and hence will not allow much scope for sound quality The best bet perhaps is to start from a Denon the 825 835 Models and upwards CDP a suitable Rotel CDP or the Pioneer PD SO6 Otherwise the above section is primarily history for in the current days reigns supreme the timeslicing dac Well yes the section is historical to great degree yet a surprising renaissance of the use of multibit dac s took place during the previous decade especially combined wth the use of dacs that avoid the use digital
33. roposed is based around the SRPP circuit This tends to be somewhat coloured sonically however the specific coloration s seem to blend well with digital audio s failings producing a generally subjectively pleasing sound in this context In order to allow easy procurements of valves and reasonable gain as well as reasonably low output impedance the circuit is proposed for use with the ECC88 6922 However a 6CG7 6FQ7 can be used without changes Even a 6SN7 can be employed if the higher heater current and the 8 Pin socket are accommodated know we have a few people in the circle who think the 6SN7 a good sounding and useful valve even if don t If desired the ECC83 or ECC82 can be used In this case the cathode resistors should be increased to 3 3kOhm Depending upon Valve used the output voltage of the circuit will range from a nominal 2V RMS for digital full scale with the 6CG7 ECC82 12AU7 and 6SN7 in circuit 3V RMS if the ECC88 6DJ8 6922 is employed and 8V RMS if the ECC83 is used As a special case the use of the 12AY7 6072A will result in about 3 5V RMS out The circuit includes a low pass filter and I believe uniquely at least fm not aware of any other design that does the de emphasis correction The low pass will produce a mild 0 7db attenuation at 20 kHz which should be innocuous CD s using de emphasis are rare but they exist Many older cd players and dac s use analogue circuitry to carry out de emphasis so belie
34. s Behringer Digital Equalisers Crossovers and SRC Analogue Digital amp Digital Analogue Converters using a direct transformer output wth of course British made transformers and reputedly excellent results an approach of whose taking may not be entirely innocent Picture courtesy of TNT Audio These units use more modern hybrid dac chips that can drive 600 Ohm loads directly from their outputs and hence work fine wth much less expensive 600 Ohm Impedance transformers So for dacs that are rated at 600 Ohm output load compatibility this is a very viable solution The results have heard are rather good though I still prefer to use Valves for my own gear Back in 99 rejected either approach and looked again at the issue decided to try the Microsoft approach and decided to just keep driving because no one will notice anyway Three Microsoft VP s notably a Marketing Manger a Helpline Manager and a Software Engineer drive down the Autobahn in a BMW The Car has a flat Tire The marketing futzi immediately proclaims We need a new Car The helpline guy say Just a minute let me ring up the BMW Helpline maybe they can ship us a new Wheel Finally the Software Engineer says What s all the fuss about guy s Just get in and keep on driving no one will notice So the output was taken from only one side of the balanced pair Some form of low pass filtering is needed together with a buffer stage An LC Filter followed by a c
35. s about the late 90 s reviews of timeslicing dacs The best of breed have auditioned were certainly not bad however in direct comparison to the best available valve and solid state amplifiers they do not produce a very good sound Well at least they offer novelty and the reviewers something to wite about other than another however good sounding 8 Watt valve amp Incidentally the best sounding Class D amps tend to be really lowpower single chip devices putting out little more than the 8 vatt valve amps presumably because they are faster AND because they always work near what one might call full scale if they would be dac s On second thought they of COURSE are DA Converters and where a Class D amplifier accepts analogue input directly it is an A2D converter followed by a power D2A converter What an insight Back to classic audio dac s though Usually the dac chip switches the psu Voltage 5V and in order to make the output more immune from the psu noise a most all time slicing dac s use a differential output This means we normally have a positive and a negative output for each channel These outputs are voltage based and usually carry a 2 5V DC offset and have a swing each of 1 4V peak or 1V RMS This is enough voltage to require no further amplification for line level outputs These two outputs however need to be combined into a single ended output and then further low pass filtering is needed as the signal is overlaid
36. s rated as 10H 150 Ohm DCR and 100mA maximum It too is available from Danbury as 10 Henries Smoothing Choke Code VT495 Prices are notably higher than a decade ago Heck whats newpussycat hear myself say that decade after decade after decade but still within an ordinary DlYers reach Other items wth similar or identical ratings will be perfectly suitable so dont lose sleep just use what you can find Most other components are standard industrial quality ones The electrolytic capacitors in the HT Supply should be of high quality but it is not necessary to spring for Elna Cerafine s or Black Gate s not that you could get them anyway shame about the Cerafine s good riddance to Black Gate The lowest cost suggested devices are Nichicon VX Series axial types available from good electronic stockists Alas the Nichicon VX series beloved by me and Sakuma San are nowvery difficult to obtain please use good grade Japanese types that is Panasonic Sanyo or Nichicon many others on the market now are very low grade types made in China and audibly degrade the sound quality Decent quality long life 105 degrees C rated types are a good choice If you have the space using motor run not motor start film capacitors or indeed audio grade film capacitors are also a good choice however this makes it almost certain that the power supply would have to be external to the CD Player The Circuit for the simple but effective supply is given be
37. sistor Ansar are nowvery difficult to source recent production has also been reported as non too reliable so SCR Solen or Obbligato by wwwdiyhifisupply com are now recommended note that the Obbligato Capacitors are generally thought of as sonically a bit better than Solen but have not compared them myself It should be noted that the above psu Design is hardly ultimate or in any way maxed out Significant improvements can be applied However the deciding criteria s here were simplicity and reasonably low cost to get our project underway More elaborate designs are open to anyone caring to implement them Lastly the very same supply is obviously suitable for any Circuit needing about 210V B with 12mA or less current draw and 6 3V Heaters with 0 6 to 0 7A current draw For the heaters up to about 1 2A can be accommodated if one of the 0 1 Ohm Resistors and one of the 0 68 Ohm resistors is bridged out The HT can also support more current if resistors are reduced in value and capacitors increased 2 Thermionic Valve Analogue Circuits for Multibit Digital to Analogue Converters The circuits discussed in the following are suitable generally for any dac chip having around 1mA output current This includes Analogue Devices AD1862 and AD1865 as well as Burr Brown PCM56 PCM67 PCM69 PCM1700 PCM1702 and PCM1704 The Burr Brown PCM63 and Philips TDA1541 will require somewhat different circuit values Due to a number of factors the circuit p
38. source often inaccessible for the user of the chip to bring the output at digital silence to OmA giving a xx mA Modulation As the signal current is normally derived from a reference voltage via precision trimmed resistors the output needs to be held close to zero volts in order to avoid introducing distortion However it might be noted that the distortion actually introduced is of a type similar to that of a triode and hence if we do not specifically concern ourselves with measured performance we can more or less ignore this requirement within reason However regardless how specifically we convert the output current of the dac into a voltage in the easiest way simply by using a small value Vishay bulk foil resistor with a classic multibit dac we generally require some form of voltage gain and quite a bit of it Commonly used is an op amp chip where the output current of the dac is applied to the inverting input of the Op Amp This due to its normal function presents a virtual ground As a result of the steep slopes of the steps in the dac output current between changes we require a device in this position that is not subject to limitation by its Slew Rate If this is not observed severe transient inter modulation distortion TIM will occur which while very audible as aggressiveness clipped sibilants and similar artefacts that are relatively hard to measure using conventional methods In many existing players and dac s the used o
39. t sharpness when solid state rectifiers are used The diodes used should be soft recovery types but are relatively uncritical Without having tested all available diodes recommend the Telefunken BYV and BYW Series I still personally use BYV96E as High Voltage rectifier of choice a decade later as well as the Motorola MUR4XX Series All these cost less and sound as good as the much vaunted Hexfred s Furthermore the HT Supply circuit operates in choke input mode For load currents of about 12 to 15mA the Maplin chokes can be used for choke input supplies It is however necessary to place a piece of rubber between chassis and core to securely and strongly clamp the choke s core in place as otherwise the core will buzz have myself used these chokes for up to 40mA choke input supplies in the described manner and fashion with no ill effects Using one choke in each leg offers much improved common mode noise rejection and it doubles the inductance compared to using a single one The rest of the circuit is bog standard though the noise fanatics in the audience might want to replace the 1k filter resistor preceding the 220uF main filter capacitor with another Maplin choke In my books the 1K filter resistor is just fine Not shown here is the final filter stage that for each individual channel will be shown in the schematic for the analogue stage In any case these will be a 22uF Ansar Supersound polypropylene capacitor combined with a 4k7 Re
40. troduction In the last decade of the 20 century the whole issues how to interface the silicon nastiness of CD players and digital to analogue converters to valve equipment has received extensive coverage in the audiophile and DIY audio press Much polemic has issued forth and has since died down A number of companies have even introduced add on Valve Stages claiming them to be the missing links and the like and have since disappeared again of the market This brief article shall not concern itself with the polemic surrounding the issue or with specific criticism of many implemented solutions Nor shall we discuss the inherent futility of attempting to reform or improve by introducing valves a digital audio format which not only suffers from enough build in limitation as to make true High Fidelity inherently impossible and for which the vast majority of recordings not even make use of the available quality Nor shall the issues of massively incompetent digital design clock jitter psu decoupling managing of RF Noise and so on in almost all commercially available equipment be discussed here in length Despite being the decade of so called CD Quality lossy compression and the iPod the decade that has passed since the original article has also brought acceptable audiophile sound from CD much much closer for average audiophiles and into the grasp of those who do not mind paying very large amounts Experiments that carried out si
41. urr Brown PCM1710 16 28 32 Philips TDA1305 Cirrus Logic CS4327 and Analogue Devices insert list Either reviewers are blinded or deafened by the 24 Bit claims for those chip s or the fact that finally the last bit of control over analogue performance has been taken away from the CD Player Designer stop s them from screwng up the sound of potentially better sounding devices up Whichever wey Im just not getting it suggest taking the hint from the manufacturer Just take the output pin of the dac chip directly to the output jack with only a good quality coupling capacitor in between Believe me it truly does sound best The on chip filtering is usually sufficiently good to not cause any problems even with solid state equipment A valve buffer cathode follower can be employed just as per the circuit below but again do not feel that it improves anything For a while modifications in exactly this style to lowcost Toshiba DVD Players where very popular and widely debated on the Audio Asylum notice that Lukasz Lampizator Fikus s vebsite also recommends the go direct mod as the most basic and effective one though he and me agree more better wth toobzs must also note that personally find the sound coming from these new devices to be exceedingly poor when compared to really competently implemented multibit dac s or earlier generation timeslicing dac s Well the trend has been reversed somewhat c
42. urrent budget dac s are quite a bit better than their forerunners a decade ago yet the best chips are still old multibit or differential output hybrid designs The generation before the latest designs is a bit more heterogeneous We find many dac s with differential output but various different schemes of on chip filtering and buffering and the occasional dac with single ended output and really good on chip analogue silicon In the latter case CS4328 certain OEM Products the same as above holds true For all other differential output dac s the circuit shown further down should could be used and should work under any conditions offering good filtering and ensuring an easy load for the dac chip keeping distortion low Finally the original first generation timeslicing dac s have no analogue stages some of these are early Philips chip s as used in the QED Digit external dac also the NPC SM5872 Marantz CD 53 57 63 67 among others and NPC SM5864 Arcam Alpha lower models and surprisingly many OEM chips even in current use All these will require some buffering and due to the lack of on chip filtering a direct connection without any additional low pass filtering even to high impedance inputs is not advisable Again the circuit shown below will work just fine amy 4 7k ECC8S8 3 ECC8S 6 out from DAC 4 7mH k Ok 25 DC VE ANA ANA ore 7 wea O 1uFL i Toko ii wy core 47 22k S 620 B4 PS PS appx SV ECC88 KP
43. ve the function must be provided where needed in the analogue stage In this case R2 and C2 provide it together with the switch The switch is ideally a high quality relay mercury wetted reed relays are best and needs to be controlled from the Players dac s internal logic Nowadays pre emphasised CD s are gone and should you own one or two they are easily replaced wth often better sounding modern copies plus most dac chips digital filters and receivers de emphasise signals in the digital domain The analogue de emphasis circuit is no longer recommended The two parallel output capacitors are used to achieve a suitable load compatibility The Capacitors used are very high quality industrial Foil amp Film types from Arcotronics or LCR and sold by respectively RS Components or Farnell in multiples of 5 and for a sane price Their performance is broadly in line with Hoveland Musicaps The resulting 0 94uF output coupling capacitance will allow loads of 20kOhm or higher to be driven without audible bass roll off In the 21 century high quality capacitors have become more easily available and a little less extremely boutique priced While the Arcotronics KP2 72 and KP2 73 series remain excellent choices on a budget my current recommendation would be Mundorf M Cap Zn or Audyn KP SN and ONLY the tinfoil types their other capacitors are not so good to these ears or other types of tinfoil amp polypropylene types Tinfoil amp PTFE capacitors ar
44. ways there and inside the Black Box you think of as dac This trend has somewhat been reversed Most time slicing and hybrid dacs still integrate analogue stages on board hovever the latest top of the range offerings from Analog Devices Burr Brom and ESS have gone back to direct current outputs giving back a lot of control about the performance of the final product to the designer Well done But back to basics the big advantage of the timeslicing dac is that it is an almost pure digital circuit All and everything can be done in the digital domain so much so the output from earlier time slicing dac s is a pure pulse width pulse density modulated square wave And unlike precision analogue chip s even very complex digital chip s can be made dirt cheap Even the integrated analogue circuits are usually of the 5 pence the piece if you buy 1000 type making not much of an impact on production cost And let s not forget that many multibit dac s require four to six separate supply voltages The time slicing dac usually requires only one and that of course costs a lot less again And the production cost is exactly the reason for the introduction of time slicing dac s Any claims for superior performance or any better technology and such can be safely relegated into the realm of fairytales spun by marketing futzies Neither sonically or measurable are there ANY advantages for the current timeslicing dac s over classic multibit
45. with large quantities of ultrasonic noise This is actually much more problematic than the already problematic situation with Multibit dac s As a result many designers screwed up their analogue stages for the early time slicing dac s up so badly that most dac manufacturers started integrating various parts of the analogue stage onto the chip to avoid this notable exception Nippon Precision Circuits NPC Later Phillips bit stream chip s like TDA1547 are famous for having been crippled sonically by some Idiot who integrated an analogue stage based on the NE5532 Op Amp onto the silicon correction it was not as good as the 5532 actually Another dac vendor who really made a sonic mess is Cirrus Logic under the Crystal Brand After their superb CS4303 which offered pure digital outputs and which almost no one got to sound good as the analogue stages could not cope with all that RF Noise and the CS4328 which implemented an excellent internal analogue stage most of their other dac s CS4327 4329 4390 and so on have incorporated switched capacitor low pass filters sonic poison and lousy CMOS Op Amp output buffers While these measures make the Crystal chip so easy to implement that even the most incompetent and ham fisted designer cannot screw up the sound of the chip too much they seriously massacre the inherent sound of the device This makes all other things being equal and a competently designed analogue stage assumed
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