by Greg Hanks time . . ”
Contents
1. Haven t you been there The lead vocalist asks if you can give him a more orange echo in his cans You kind of know what he means and turn for another 2 dB at 1 kHz only to discover that whatever you do the echo is somehow off The EMT 140 echo plate has appeared on more hits than you have years but yours doesn t seem to sound like them Maybe it s time for a tune up The EMT 140 Reverberation Plate is found in studios the world over Its popularity is evidenced in part by the introduction of the Studio Technologies Ecoplate and the Audicon Plate both of which work on the same basic principles These brands were introduced as soon as the various EMT patents expired There are a number of variations on the 140 140 Mono Tube 140ST Stereo Tube 140 Stereo Solid State Q Quad Return Solid State all with or without servo remote etc but for the time being the following considerations applies to them all Principle of Operation Let s look at what the EMT manual has to say sections below in quotes and try to see what it means The EMT 140 reverberation unit utilizes the physical properties of metal to achieve its effect It is a fact that a steel sheet which has been excited by an impulse setting up within it bending oscillations will deliver reflec tions which increase in density with time Reflections in a three dimensional room on the other hand become more dense as a function o2. The following is dedicated to the hardy soul who wishes to exercise great care is willing to dedicate 6 to 12 hours and wants to indulge in frustration I broke the drive coil during my first EMT plate tuning 1 Dis assemble the unit removing all side bolts and end bolts on the end panel without electronics Remove damper plate by removing the two top bars that retain the main ver tical support bars carefully lift ing out by the same vertical bars 2 Remove the driver magnet inspecting for any metal chips stuck inside the gap cleaning with double sided tape If neces sary cover with masking tape to prevent further contamina tion Remove magnet mounting plate 3 Inspect and repair damper plate If repairing an older unit check VERTICAL FRAME MEMBER VERTICAL PULL RESULTANT DIRECTION 4 A OF FORCE gt HORIZONTAL PULL Figure 1 DRIVE coll PLATE DRIVE DRIVE MAGNET MAGNET MOUNTING the integrity of the small tiles that make up the damper On newer models check thick damper for edge damage trim frayed edges and glue back any loose fibres Check flatness of damper assembly Remove the pick ups by unscrew ing the retaining screws on the damper side of the plate replacing the screw in the pick up since these are impossible to replace and leaving the pick up in the shield can Remove the drive coil by unscrew ing as above and tape in place on the phenolic strip O U R Now you can choo
3. echo results There are a number of things that plate then with an impulse type input THE CONNECTION FOR USED AUDIO EQUIPMENT CONSOLES TAPE MACHINES OUTBOARD EQUIPMENT MICROPHONES BUY e SELL e TRADE Summit Audio P O Box 1678 Los Gatos Ca 95031 408 395 2448 R e p 50 June 1982 Front view of drive coil assembly with the magnet removed affect the smooth reflection packing density of the plate and these are as follows A The plate density uniformity If a plate has rust or oil on it then in these affected areas the damping is greater and this unequal damping can produce echo flutter B Tension differences in the plate suspension cause area tension differen tials which in turn will cause low frequency time intervals resulting in roll around or low frequency echo flutter C Centering of the plate in the mounting which given the mounting method can cause uneven tension dis tribution throughout the plate with equal torque on each mounting bolt D To a small extent magnetization of the place around the driver resulting in even order harmonic distortion and a lowering of overall driver efficiency E The uniformity of damping across the plate which on short delay settings can affect how much one area of the plate is damped without equally affecting the other parts of the plate in the same manner The result will
4. PANDABLE SYSTEM 200 Fitter Display _ Input Out 3 Resolution _ Fitters _ Tecand Options 2 or Resolution Pink Noise Generator 15 V Microphone Power e Oscilloscope and Plotter Drive e Optional Function Generator e Interchangeable Filters Peak and Average Processing Precision 16 Measurements 45 Dynamic Range e 16 x 31 High Intensity 0 5 LED Display e 6 Non volatile Nemories All Functions Microprocessor Controlled Plug in Options Available June 1982 R e p 53 For additional information circle 35 For additional information circle 34
5. able Rare TUBE microphones mastering equipment video and support gear the threads of the eyelets to 11 12 13 14 Suspension springs and clips at corner of plate assure that the thread drag does not skew your readings from actual tension torque relation ships After initially tensioning the plate to a given torque figure check the centering of the plate within the frame and make adjust ments as necessary More on this later Remount the pick ups taking care not to damage the connecting wires eyeball the perpendicularity and adjust the shield can to be centered around the pickup Remount the drive coil again tak ing care not to damage or distend the connecting wires If any stretching of the coiled leads takes place it can be rectified by rewinding the lead around a 0 Phillips screwdriver shaft The voice coil should be made perpen dicular to the plate using a mach inist s square measuring any two points that are displaced by 90 degrees Center the drive coil axially on the magnetic gap of the magnet This step requires some measurements and calculations because you must measure the distance from the plate to the center of the coil then measure the distance from the plate to the center of the gap and make up the difference with the spacer washers removed dur ing disassembly of the driver plate The spacer count necessary for centering is almost always less than were removed I
6. be a minute degree of unevenness in damp ing across the plate resulting in coloration of the reverb by small degrees of low frequency echo flutter Plate Tensioning The plate is non reverberantly sus pended in a rigid tubular frame Sus pension and decoupling are provided by springs and clips mounted perpendicu larly to the plate edge at all four corners These clips and springs are under ten sion and because of their location pro vide tension to the plate as a vectorial product In other words the tension is equal to the algebraic sum of the indi vidual tensions and centered along a line that is tangential to their applica tion Figure 1 If these tensions are all equal then the tension at all points on the plate will be uniform falling to a minimum at the plate center This implies circumferential dispersion of the wave emanating from the driven point which in turn means that the major natural resonance is the product of the direct dimensions of the plate and not an integer thereof thereby reducing the equidistant or time repetitive reflec tions or echo flutter The problem with this mounting technique is that the springs are tensi oned beyond their elastic limit and that the tension provided by the stretched spring is not necessarily stable To my knowledge all EMT plates with the exception of s n 100 located at Nola Recordings New York City and proba bly dating from the early Fifties are eq
7. d will be in the echo Insert all echo delay devices in the send bus so as to use the high frequency attenuation of the chamber to mask the additional noise of the delay Kk OK The prime objective of this article is to impart some of my experiences in obtaining a reverberation device that has a smooth decay characteristic free of echo flutter and how to use this device with a minimum addition of noise These objectives may be reached through attention to mechanical detail in set up and judiciously avoiding the accentuation of reverb noise in application Addendum In reference to the chosen torque fig ures for tightening the plate it is best to make this choice for the particular plate and spring material in question In order to arrive at a torque figure tension a single spring clip combination until you see the elastic limit has been reached This is the point at which the torque does not go up as you continue tightening but in fact may even decrease Such a tension is a good place to put your torque with double clips it usually works out to be somewhere between 13 and 19 inch pounds N Bibliography Service Manual for EMT 140 and 140 ST Gotham Audio Instruction Manual EMT 140 ST s n 1658 U S Patent 2 923 369 Kuhl U S Patent 2 986 228 Rettinger et al U S Patent 1 843 553 Gladstone The Engineer s Companion by Mott Souders Wiley amp Sons omea MONITOR maray THE EX
8. e to sell used equipment for less money You see we don t have money tied up in inventories warehouses and showrooms So unlike dealers we don t have to inflate our equipment prices with a lot of overhead We simply don t have that overhead That s why YOU SAVE MONEY with Sye Mitchell Sound LARGER SELECTION We curently offer over two million dollar sworth of used recording gear including 43 con soles and 45 multitrack tape machines We re not limited to just a few brands we have consoles tape machines microphones and outboard gear from every major manufacturer And we re constantly searching for more BETTER SERVICE Buying from us is easy We amange for shipping and R e p 52 June 1982 Sound insurance and handle other details for you And if you re not sure which gear is right for you call us for help We ll gladly make recommendations tailored to yourneedsand budget More services We put together complete studioequipment packages from 8 track to 24 track We do custom installations and wiring We can even help with your studio design FREE EQUIPMENT LIST Our regu lary published used equipment list will be sent free upon request Callor write today SYE MITCHELL SOUND COMPANY INC 22301 Cass Avenue Woodland Hills California 91364 213 348 4977 All equipment available for inspection e Financing available e We ship anywhere in the world e No order too small Also avail
9. ent The steel plate s losses are additively formed by the non frequency dependent and frequen cy dependent parts which are caused by the heat conductivity losses of the bending modes This roughly translates to The losses of the plate can be attributed to friction within the plate s molecular structure such losses are termed damping For high frequencies the non frequency dependent terms are predominant and for mid and low frequencies the damping is frequency dependent Damping through heat conduc tivity is through practically the entire audible range directly proportional to the frequency and inversely proportional to the plate thickness These losses occur through bending friction the higher the frequency the more damping and the thicker the plate the less damping The plate used must not only be completely undamped but must also be extremely flat The low frequency response of the reverberation unit is a function of the decay time The term Echo Plate is really a misnomer since echo is a distinct repetition of the original direct sound with given time delay s Reverberation on the other hand is a series of rapid repetitions of the original sound which if of sufficient density of repetition form a smoothly decaying sound devoid of distinct echo Reverberation density how many reflections occur per unit time is determined by a number of factors the
10. f the square of the time The human ear is unable to recognize the difference between R e p 48 June 1982 by Greg Hanks these two operating modes The main component of the reverberation plate is a steel plate which is suspended in a tubular frame Parallel to this plate another panel made of porous material is suspended in such a way as to permit it to be swung towards or away from the steel plate with an extreme distance ratio of about 30 1 The choice of plate material requires great care and takes into consideration its internal damping characteristics and the resulting reverberation time Through the use of appropriate steel and critically chosen dimensions it is possible to produce a plate which possesses an adequate number of self resonances The length and frequency response of the decay time produce the reverberation effect These quotes have been taken out of sequence and are incomplete but they basically sum up the heart of the EMT In order to obtain optimum results from an EMT 140 echo plate it must be set up properly To adjust one of these devices it is necessary to have an understanding of what mechanisms are at work and how to optimize them individually To this end this article will examine the basic operation of an echo plate outline the various adjustments and explain the subtleties of the mechanical adjustments and their interactions Borrowing from the owner s manual a mom
11. first of which is decay time The greater the density of reflections the less echo slap is perceived and the better the reverb sounds Where there are density discontinuities in time these frequencies continued overleaf amp 2 Ss A S wt z 5 aj A 3 yn amp wn 3 as S Q S 3 _ v pa jo Q A uw uw amp 5 gt 3 WY For additional information circle 31 wa Top View of drive coil assembly showing mounting for drive magnet and packing washers exhibit decay time anomolies which create what is known as echo flutter To minimize this echo flutter effect the signals obtained from the contact mic rophones are entirely incoherent in terms of the physical mounting rela tionships of the driver and pick up assemblies The mountings are not spaced an equal integer of the plate s horizontal or vertical dimensions as given by the manufacturer s mounting method If the damping is equal across the surface of the plate for short decay times and if the suspension is such that the tension of the plate is equalized from the four corners to the center of the Drive magnet removed from mounting plate the reflections perceived are statistically distributed In other words the reflec tions will be random in relation to one another rather than being equidistant in time resulting in echo Thus the reflections smoothly increase with time and a good sounding
12. have a bag of extras that you would not believe Center the magnet assembly with the Lucite ring that is supplied with the unit for this purpose as per the instruction manual avoid ing the step that says to make small adjustments by bending it non perpendicular Re assemble the damper plate assembly making sure that the top and bottom of the damper are the same distance from the plate at all four corners throughout the adjustment range re adjust if necessary Make sure that the damper does not touch the plate at the shortest decay setting Re assemble the cabinet and res tore the plate to its proper home Carefully 16 Adjusting the electronics is best done by following the procedure outlined in the manual then rol ling off all the bottom end and setting the drive level to maximum Studio Applications Receive levels Adjust so that the stereo returns are the same level and the chamber noise is approximately equal to the noise floor Drive level Adjust to desired subjective effect Equalization 1 To attenuate the highs EQ the returns 2 To boost the highs EQ the send 3 To attenuate the lows EQ the send if rolling waves of echo are the prob lem or EQ the returns if chamber or outside noises are the difficulty 4 To boost lows EQ the sends whe never possible clipping of the chamber being the limiting factor Keep in mind that the longer the decay setting the more apparent bottom en
13. o install the clips on the springs but only to hold the plate in place with the springs alone When installing the clips on the springs I use a pair of long nose pliers and a pair of 4 inch Channel lock ignition pliers and I install the clips so that I am always applying pres sure away from the edge of the plate The procedure outlined in the EMT owner s manual works well when double clipping but stresses the springs unduly when only one is used per position 9 With all corners clipped the next step is to center the plate within the frame with a slight offset to allow for tensioning This means positioning the plate slightly lower than center and a shade Pickup coils attached to front side of plate closer to the electronics end 10 The next step is to tension the plate I tension the top two and end two eyelets only on the assumption that if the plate is centered within the frame assem bly the tension applied to the top bolts and end bolts will be by nature of the symmetry of the plate mounting the same on the bottom and the other end To ten sion the plate I use a torque wrench supplied by Snap On Tools which has a bidirectional range of 30 inch pounds When using this method be sure to apply some light machine oil to Used recording equipment is a better And the best buys are at Sye LOWER PRICES We re not a dealer we re a broker Without all the high costs of running a dealership we re abl
14. se from a whole family of dbx compressor limiters to match PL AN sion curve for inaudible transition into compression RMS detec 210 TO tion for the most natural sound Patented VCAs for precise low distortion control of audio levels That s the plan Now join the family 6 MOUNTING MAGNET Figure 2A Figure 2 the plate and remove all but the top two and bottom end two Inspect the plate for rust and dents remove and repair as necessary on a large flat surface like a floor Slightly enlarge all of the eyelet holes so as to accommodate two springs in the hole Note break the sharp edge of the hole on the non countersunk side as this operates as a clip shear under high tensions Re install the plate into the frame I do this by re clipping the top two the top end two the bottom two and then the bottom ends When initially hanging the plate in Slacken the eyelet bolts around See your dbx Pro dealer or write for information Manufacturer s suggested retail price Over Compressor Limiters dbx Incorporated Professional Products Division 71 Chapel St Newton Mass 02195 U S A Tel 617 964 3210 Telex 92 2522 Distributed in Canada by BSR Canada Ltd Ontario dbx June 1982 0 R e p 51 information circle 32 For For additional information circle 33 ib Damper plate and operating mechanism place it is not necessary t
15. uipped with two holes per spring posi tion Utilizing both of these holes I dou ble clip each corner so that each spring can operate in its stable elastic region This maintains the same tensions as a stretched spring but ensures long term stability The drive signal is electromagnetically coupled to the plate by means of a rig idly mounted voice coil The voice coil is positioned in a magnetic gap with the magnet attached to a mounting plate which is supported by the rigid tubular frame by means of two vertical pieces of angle iron In order to most efficiently couple the drive signal to the plate this coil should be positioned A Perpendicular to the plate so as to impart maximum energy to the plate B Radially centered in the gap C Axially centered in the gap I have found the axial centering to be the most common misalignment with an EMT plate Figure 2A and 2B Axial miscentering of the coil in the gap is probably done at the factory to allow for more freedom of movement of the plate without bottoming and damaging the voice coil in transit The pick up assembly is a piezo electric accelerometer whose output is nearly inversely proportional to fre quency The most important considera tion when dealing with this apparatus is that they are positioned perpendicu lar to the plate and that the leads have sufficient clearance to prevent any additional damping to the plate Tuning and Mechanical Refurbishment
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