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SESCOM, INC, ORDER TODAY! ORDERS (800) 634-3457 2100 WARD DRIVE FAX ORDERS (800) 551-2749 PREPAID ORDERS ARE OFFICE (702) 565-3400. FAX (702) 565-4828 HENDERSON,NV SHIPPED SURFACE AT NO CHARGE TECH LINE (702) 565-3993 M-Th 8 am to 4 pm (PST) II[ 89015·4249 USA (U.S. 48 STATES,MEXICOAND CANADA) SESCOM, INC. is not responsible"" inodvertent typographicalemn Pricesal>liipocficetions are subject to change wiIhouI nolice The Williamson Amplifier

A Collection of Articles, reprinted from" Wireless World," on "Design for a High-quality Amplifier"

By D. T. N. WILLIAMSON

(formerly of the M.O. Valve Company, now with Ferranti Research Laboratories)

Published for Winless I'Qrld • LONDON: ILIFFE & SONS, LTD. THIRD EDITION THIRD PRINTING 1994 Copyright ©1990 by Audio Amateur Publications, Inc.

Distribution Agents: Old Colony Sound Lab Post Office Box 243 Peterborough, New Hampshire 03458-0243 USA

Library of Congress Card Catalog Number: 90-083998 ISBN: 0-9624191-8-4

No patent liability is assumed with respect to the use of the information contained herein. While every precau­ tioh has been taken in the preparation of this document, the author assumes no responsibility for errors or omissions. Neither is any liability assumed for damages resulting from the use ofthe Information contained herein. The Williamson Amplifier

CONTENTS

Page Introduction 6

Basic Requirements: 7 Alternative Specifications (April 1947) Details of Chosen Circuit and Its Performance 11 (May 1947) NEW VERSION Design Data: 14 Modifications: Further Notes (August 1949) Design of Tone Controls and Auxiliary Gramophone Circuits 20 (October and November 1949) Designfor a Radio Feeder Unit 30 (December 1949) Replies to Queries Raised by Constructors 33 (January 1950) Modificationsfor High-impedance Pickups and Long-playing Records 34 (May 1952)

5 Introduction

Introduced by Wireless World in 1947 as merely one of a series of amplifier designs, the "Williamson" has for several years been widely accepted as the standard of design and performance wherever amplifiers and sound reproduction are discussed. Descriptions of it have been published in all the principal countries of the world, and so there are reasonable grounds for assuming that its widespread reputation is based solely on its qualities. This booklet includes all the articles written by D. T. N. Williamson on the amplifier. Both the 1947 and 1949 versions are reprinted, as the alternative output transformer ratios cover a wide range of require­ ments. Modifications and additions include pre-amplifier circuits and an r.f. unit, with recently published information on adaptation to high­ impedance pickups and correction for 33t r.p.m. records. We would stress the importance, if the full potentialities of the amplifier are to be realized, of following the author's recommendations in detail. Even in the U.S.A., where several modified versions have been described, many users adhere to the designer's exact specification with the original valve types. It is not the circuit alone, but the properties of the valves and such components as the output transformer, together with the welding of theory and practice into a rational layout, which produce the results. Editor, Wireless World.

6 The Williamson Amplifier

Basic Design Requirements: A lternative Specifications

E CE NT improvements in the operation of the loudspeaker. spectrum (but especially, at the field of commercial sound This in turn reconverts the elec­ low-frequency end) be substan­ R recording have made prac­ trical waveform into a corres­ tially less than that at medium ticable the reproduction of a ponding sound pressure waveform, frequencies, filters must be wider range of frequencies than which in an ideal system would arranged to reduce the level of hitherto. The useful range of be a replica of the original. these frequencies before they reach shellac pressings has been ex­ The performance of an amplifier the amplifier as otherwise severe tended from the limited 50-8,000 intended to reproduce a given intermodulation will occur. This cis which, with certain notable waveform is usually stated in is especially noticeable during the exceptions, has been standard terms of its ability to reproduce reproduction of an organ on from 1930 until the present, to a accurately the frequency com­ incorrectly designed equipment range of some 20-15,000 cis. This ponents of a mythical Fourier where pedal notes of the order of increase in the frequency range analysis of the waveform. While 16-20 c / s cause bad distortion, has been accompanied by an this method is convenient and even though they may be in­ overall reduction in distortion and indeed corresponds to the manner audible in the sound output. the absence of peaks, and by the in which the mechanism of the (3) Negligible phase shift with­ recording of a larger volume range, ear analyses sound pressure wave­ in the audible range. Although which combine to make possible a forms into component frequencies the phase relationship between standard of reproduction not pre­ and thereby transmits intelligence the component frequencies of a viously attainable from disc re­ to the brain, the fact that the complex steady-state sound does cordings. Further improvements, function of the system is to repro­ not appear to affect the audible notably the substitution of low­ duce a waveform and not a band quality of the sound, the same is noise plastic material for the of frequencies should not be not true of sounds of a transient present shellac composition, are neglected. Sounds of a transient nature, the quality of which may likely to provide still further nature having identical frequency be profoundly altered by disturb­ enhanced performance. contents may yet be very different ance of the phase relationship The resumption of the television in character, the discrepancy being between component frequencies. service with its first-class sound in the phase relationship of the (4) Good transient response. In quality. and the possible extension component frequencies. addition to low phase and fre­ of u.h.f. high-quality trans­ The requirements of such an quency distortion, other factors missions, increase the available amplifier may be listed as :­ which are essential for the accu­ sources of high-quality sound. (I) Negligible non-linear dis­ rate reproduction of transient Full utilization of these record­ tortion up to the maximum rated wave-forms are the elimination of ings and transmissions demands output. (The term .. non-linear changes in effective gain due to reproducing equipment with a distortion" includes the produc­ current and voltage cut-off in any standard of performance higher tion of undesired harmonic fre­ stages, the utmost care in the than that which has served in the quencies and the intermodulation design of iron-cored components, past. Extension of the frequency of component frequencies of the and the reduction of the number range, involving the presence of sound wave.) This requires that of such components to a minimum. large-amplitude low-frequency sig­ the dynamic output/input char­ Changes in effective gain during nals, gives greater likelihood of acteristic be linear within close " low-frequency" transients occur intermodulation distortion in the limits up to maximum output at all in amplifiers with output stages reproducing system, whilst the frequencies within the audible of the self-biased Class AB type, enhanced treble response makes range. causing serious distortion which this type of distortion more (2) (a) Linear frequency re­ is not revealed by steady-state readily detectable and undesirable. sponse within the audible fre­ measurements. The transient Reproduction of sound by elec­ quency spectrum of 10-20,000 cis. causes the current in the output trical means involves the ampli­ (b) Constant power handling stage to rise, and this is followed, fication of an electrical waveform capacity for negligible non-linear at a rate determined by the time which should be an exact counter­ distortion at any frequency within constant of the biasing network, part of the air pressure waveform the audible frequency spectrum. by a rise in bias voltage which which constitutes the sound. The This requirement is less strin­ alters the effective gain of the purpose of the amplifier is to gent at the high-frequency end of amplifier. produce an exact replica of the the spectrum, but should the (5) Low output resistance. electrical input voltage waveform maximum power output/frequency This requirement is concerned at a power level suitable for the response at either end of the with the attainment of good

7 The Williamson Amplifier

The salient fea­ The functions of negative feed­ tures of these back are:­ methods are of (a) To improve the linearity interest. of the amplifier, and output Push-pulltriode transformer. INPUT valves without (b) To improve the freqnency the refinement of response of the amplifier and negative feed­ output transformer. back form the (e) To reduce the phase shift mainstay of pre­ in the amplifier and output trans­ sent-day high­ former within the audible fre­ (~) (b) fidelity equip­ quency range. ment. A stage of (d) To improve the low-fre­ Fig. I. Output/input characteristics (a) without feedback (b) with negative feedback. this type has a quency characteristics of the out­ number of dis­ put transformer, particularly frequency and transient response advantages. With reasonable defects due to the non-linear from the loudspeaker system by efficiency in the power stage relation between flux and magne­ ensuring that it has adequate such an arrangement cannot be tizing force. electrical damping. The cone made to introduce non-linearity to (e) To reduce the output movement of a moving-coil loud­ an extent less than that represen­ resistance of the amplifier. speaker is restricted by air loading, ted by about 2-3 per cent (fl To reduce the effect of suspension stiffness and resistance, harmonic distortion. The output/ random changes of the para­ and electro-magnetic damping. In input characteristic of such a stage meters of the amplifier and supply the case of a baffle-loaded loud­ is a gradual curve as in Fig. voltage changes, and of any speaker, the efficiency is rarely I (a). With this type of characteris­ spurious defects. higher than 5-10 per cent, and the tic distortion will be introduced at A stage of this type is capable air loading, which determines the all signal levels and intermodula­ of fulfilling the highest fidelity radiation, is not high. In order tion of the component signal requirements in a sound repro­ to avoid a high bass-resonance frequencies will occur at all levels. ducing system. The output/input frequency, the suspension stiffness The intermodulation with such a characteristic is of the type shown in a high-grade loudspeaker is characteristic is very considerable in Fig. I (b), and is virtually kept low, and obviously the power and is responsible for the harsh­ straight up to maximum output, loss in such a suspension cannot ness and "mnshiness" which when it curves sharply with the be large. Electro-magnetic damp­ characterizes amplifiers of this onset of grid current in the out­ ing is therefore important in type. In addition, further non­ put stage. Non-linear distortion controlling the motion of the cone. linearity and considerable inter­ can be reduced to a degree repre­ This effect is proportional to the modulation will be introduced by sented by less than o. I per cent current which can be generated the output transformer core. harmonic distortion, with no in the coil circuit, and is therefore If the load impedance is chosen audible intermodulation. The proportional to the total resistance to give maximum output the frequency response of the whole of the circuit. Maximum damp­ load impedance/output resistance amplifier from input to output ing will be achieved when the coil ratio of the amplifier will be about transformer secondary can be is effectively short-circuited, hence 2, which is insufficient for good made linear, and the power the output resistance of the loudspeaker damping. handling capacity constant over amplifier should be much lower It is difficult to produce an a range considerably wider than than the coil impedance. adequate frequency response char­ that required for sound reproduc­ (6) Adequate power reserve. acteristic in a multi-stage ampli­ tion. The realistic reproduction of fier of this type as the effect of The output resistance, upon orchestral music in an average multiple valve capacitances and which the loudspeaker usual!" room requires peak power capa­ the output transformer primary depends for most of the damping bilities of the order of 15-20 and leakage inductances becomes required, can be reduced to a watts when the electro-acoustic serious at the ends of the a.f, small fraction of the speech coil transducer is a baffle-loaded spectrum. impedance. A ratio of load im­ moving-coil loudspeaker system The application of negative feed­ pedance/output resistance (some­ of normal efficiency. The use back to push-pull triodes results times known as "damping fac­ of horn-loaded loudspeakers may in the more or less complete sol­ tor ") of 20-30 is easily obtained, reduce the power requirement to ution of the disadvantages out­ " Kinkless " or "beam" ou t­ the region of 10 watts. lined above. Feedback should put tetrodes used with negative be applied over the whole am­ feedback can, with care, be made The Output Stage plifier, from the output transform­ to give a performance midwav An output of the order of 15-20 er secondary to the initial stage as between that of triodes with and watts may be obtained in one of this method corrects distortion without feedback. The advantages three ways, namely, push-pull introduced by the output trans­ to be gained from the use of triodes, push-pull triodes with former and makes no additional tetrodes are increased power effi­ negative feedback, or push-pull demands upon the output capabili­ ciency and lower drive voltage tet ro.les with negative feedback. ties of any stage of the amplifier. requirements. 8 The Williamson Amplifier

It must be emphasized that the the form of parasitic oscillation response will be well maintained. characteristics of the stage are due to phase shift produced in the If then the required frequency dependent solely upon the char­ high frequency region by a high range in the amplifier is from acter and amount of the negative leakage reactance. 10-20,000 cis, fb may be taken as feedback used. The feedback (c) Intermodulation and har­ 3.3 cis and ft as 60 kc/s. A trans­ must remain effective at all monic distortion in the output former which is only 3db down at frequencies within the a.f. stage caused by overloading at low frequencies as widely spaced as spectrum under all operating con­ frequencies when the primary these would be difficult to design ditions, if the quality is not to inductance is insufficient. This is for some conditions of operation, degenerate to the level usually primarily due to a reduction in and where this is so the upper associated with tetrodes without the effective load impedance below limit may be reduced, as the feedback. Great care must be the safe limit, resulting in a very energy content of sound at these taken with the design and opera­ reactive load at low frequencies. frequencies is not usually hig-h. tion of the amplifier to achieve This may cause the valves to be The limiting factor will be the this, and troubles such as parasitic driven beyond cut-off since the necessity of achieving stability oscillation and instabilitv are load ellipse will tend to become when feedback is applied across liable to be encountered. ­ circular. the transformer, i.e., that the loop When equipment has to be (d) Harmonic and intermodula­ gain should be less than unity at operated from low-voltage power tion distortion produced by the frequencies where the phase shift supplies a tetrode stage with non-linear relation between flux reaches 180°. negative feedback is the only and magnetizing force in the core To illustrate the procedure, choice, but where power supplies material. This distortion is always consider the specification of an are not restricted, triodes are present but will be greatly aggra­ output transformer coupling two preferable because of ease of vated if the flux density in the push-pull I{T66 type valves to a operation and certainty of results. core exceeds the safe limit. Is-ohm loudspeaker load. It appears then that the design (e) Harmonic distortion intro­ Primary load impedance= ro.ooot) of an amplifier for sound repro­ duced by excessive resistance in duction to give the highest possible the primary winding. . 8 The design of a practical trans­ Turns ratio = JIO'OOO .._- = 25. :1 fidelity should centre round a 15 former has to be a compromise push-pull triode output stage and Effective a.c. resistance of valves should incorporate negative feed­ between these conflicting require­ = 2500 f! back. ments. . The most suitable types of valve At a low frequency fb' such that Low-frequency Response for this service are the PX;Zs the reactance of the output trans­ Parallel load and valve resist­ former primary is equal to the and the KT66. Of these the Kt66 ance = 25°o._~~,000 = 2000!.l is to be preferred since it is a resistance formed by the load 12,500 resistance and valve a.c, resist­ more modern indirectly-heated fb = 3·3 c!S(w response type with a 6.3-volt heater, and ances in parallel, the output b=2I) voltage will be jdb below that at should be jdb down. will simplify the heater supply Primary incremental inductance problem. Triode-connected it has medium frequencies. At a fre­ quency 3fb the response will be 2000 characteristics almost identical L =-- = 95 H. with those of the PX2S. well maintained, the transformer 21 0phase Using a supply voltage of some reactance producing only 20 High-frequency Response 440 volts a power output of IS angle. Similarly at the high Sum of load and a.c. resistances watts per pair may be expected. frequency end of the spectrum the response will be jdb down at a = 10,000 + 2S00 = 12SOO f! The Output Transformer frequency ft such that the leakage 6,000) The output transformer is prob­ reactance is equal to the sum of At ft = 60 kc/s (Wt = 37 ably the most critical component the load and valve a.c. resistances. response should be jdb down. Again at a frequency ftl3 the 12,5 00 in a high-fidelity amplifier. An Leakage reactance incorrectly designed component 376 is capable of producing distortion = 33 m H. which is often mistakenly attribu­ ted to the electronic part of the A zo-watt transformer having 10 amplifier. Distortion producible primary and 8 secondary sections w directly or indirectly by the u and using one of the better grades z output transformer may be listed ....< of core material can be made to u :::> comply with these requirements. as follows :­ c (a) Frequency distortion d lie :!: Winding data will be giv!"n in an to low winding inductance, high appendix (see page II). leakage reactance and resonance 'A Some confusion mav arise when phenomena. specifying an output- transformer (b) as the apparent inductance of Distortion due to the phase ALTERNATING EXCITATION VOLTAGE shift produced when negative the windings will vary greatly feedback is applied across the Fig. 2. Variation of iron-cored with the method of measurement. transformer. This usually takes inductance with a.c, excitation, The inductance of an iron-coreI 9 The Williamson Amplifier

to a low value as it contains the minimum number of stages. The arrangement, however, has a number of disadvantages which render it unsuitable. The input voltage required by the phase splitter is rather more than can be obtained from the first stage (a) for a reasonable distortion with the available h.t, voltage, and in addition the phase splitter is operating at an unduly high level. The gain of the circuit is low even if a pentode is used in the first stage, and where a low-impedance loudspeaker system is used, in­ sufficient feedback voltage will be available. The addition of a push-pull (b) driver stage to the previous arrangement, as in Fig. 3 (b), provides a solution to most of the difficulties. Each stage then works well within its capabilities. The increased phase shift due to the extra stage has not been found unduly troublesome provided that suitable precautions are taken. The functions of phase splitter (c) and push-pull driver stage may be combined in a self-balancing Fig. 3. Block diagrams of circuit arrangements discussed in the text. " paraphase" circuit giving the arrangement of Fig. 3 (c). The component is a function of the will result, should a phase shift grid of one drive valve is fed excitation, the variation being of 1800 occur at a frequency where directly from the first stage, the of the form shown in Fig. 2. The the vector gain of the amplifier other being fed from a resistance exact shape of the curve is and feedback network is greater network between the anodes of dependent on the magnetization than unity. The introduction of the driver valves as shown in characteristicfor the core rnaterial. more than one transformer into Fig. 4. This arrangement forms The maximum inductance, the feedback path is likely to a good alternative to the preceding corresponding to point C occurs give rise to trouble from insta­ one where it is desirable to use the when the core material is nearing bility. As it is desirable to apply minimum number of valves. saturation and is commonly 4-6 feedback over the output trans­ times the "low excitation" or former the rest of the amplifier " incremental" value at A, which should be R-C coupled. corresponds to operation near the origin of the magnetization curve. Alternative Circuits In a correctly designed output Although the amplifier may transformer the primary induct­ contain push-pull stages it is ance corresponding to the voltage desirable that the input and output swing at maximum output at should be "single ended" and 50 c / s will lie in the region of B have a common earth terminal. in Fig. 2. Three circuit arrangements suggest In specifying the component, themselves. the important value is the incre­ The block diagram of Fig. 3 (a) mental inductance corresponding shows the simplest circuit arrange­ to point A, since this value deter­ ment. The output valves are mines the frequency response at preceded by a phase splitter low outputs. which is driven by the first stage. The feedback is taken from the Phase Shift output transformer secondary to The reduction of phase shift in the cathode of the first stage. Fig. 4. .. Paraphase" circuit amplifiers which are to operate This arrangement is advantageous combining the functions of with negative feedback is of in that the phase shift in the phase splitter and push-pull prime importance, as instability amplifier can easily be reduced driver stages. 10 The Williamson Amplifier

Details of Chosen Circuit and Its Performance

HE considerations under­ keep the phase shift in the ampli­ splitter grid. Due to the cathode­ T lying the design of a high­ fier at low frequencies as small as follower action of V. the operating quality amplifier were dis­ possible the first stage has been conditions are not critical and no cussed in the first part of this directly coupled to the phase trouble is likely to be encountered article. A circuit of the complete splitter, eliminating one R-C from normal changes in valve amplifier is shown in Fig. 5. This coupling. The first two stages are parameters. The cathode bias follows the basic arrangement of thus designed as a single entity. resistor of V l' to which feedback Fig. 3(b). The design of the indi­ The phase-splitter section, which is applied from the output trans­ vidual stages will not be treated consists of a triode with equal former secondary, is kept as small in detail. but a review of the loads in anode and cathode cir­ as practicable to avoid gain reduc­ salient features may be of value. cuits, operates partly as a cathode tion in the first stage, due to series As a measure of standardization follower, its grid being some 100 V feedback. all valves except those of the out­ positive with respect to chassis. Driver Stage.-The output from put stage are type L63. triodes of The anode of the first triode is also the phase-splitter is taken to the about 8.000 ohms a.c. resistance. arranged to be about 100 V posi­ push-puU driver stage. Provision Initial Stages.-In order to tive and is coupled to the phase- is made for varying the load re-

CH I 4S0V CHz 4'4mA !s'ZSmA l'0mA IZSmA R6

R7 C z C~ l-

Rs liT

..!:ll , It JJ ... ir :: R, a,

R4 Calijil ~Rzs

Fig. 5. Circuit diagram of complete amplifier. Voltages underlined are peak signal voltages at IS watts output.

CIRCUIT VALUES

R 1 1 MO 1 watt ± 20 per cent R 16,R20 1,000 0 1watt ± 20 per cent Ca 8 /LF 550 V, wkg. ; A 8 I£F 600 V, wkg. R 2 33,000 0 1 watt ± 20 R 18, R 1S 100 0 1 watt ± 20 Cg .. .. 30 H at 20 mA (min.) R8 47,000 0 1 watt ± 20 .. R)7,R2 1 100 0 2 watt wire- CHI , R~ 470 0 1 watt ± 10 wound variable. CH2 10 H at 150 mA (min.) .. Power transformer. R 6, R 8, R7 22,000 01 watt ± 10 R 22 160 0 3 'Watt ± 20 T .. .. Secondary 425-0-425 V. N Rs, R g 0.47 MO 1 watt ± 20 .. R 23, R24. 100 O! watt ± 20 .. RIO 390 0 1 watt ± 10 R 1,2oov'speech coil impedance, 150 mA (min.) 5 V. 3A,6.3 .. 26 V. 4A, c.t. R n , R 18 39,000 0 2 watt ± 10 .. 1 watt. R 25,000 0 1 watt wire- VI to V.. L63 1- YI' 12 C1, C2, C6 8 /LF 450 V, wkg. wound variable. c; C4. 0.05 /LF 350 V, wkg. V6, V8 KT66 R V U52. U,R19 0.1 MO 1 watt ± 20 .. C8, C7 0.25/LF 350 V, wkg. 7 11 The Williamson Amplifier

,~," 0 ':t

-, ~I/ 5 co "/ ....o ,.,...... " z / ..,. S-•... ~ ~ ~ ,.,1/ -- WITHOUT FEEOBACK 0 l-f­ '"~ .,IJ -- WITH FEEDBACK ~ ~ ",17 ~ g IJ 5 .. I '0 I- If'.!! "'-?I' z o / J....; (O'S t= f­ ~ 1/ _.-1­ Y Ci 0 0'5 1'0 ..1'5 2,3 INPUT VOLTS (R.M 5.) Fig. 6. Input-output characteristic and harmonic distortion curves, with and without feedback. (Right)­ Oscillograms of input-output characteristic ; left-hand column, without feedback; right-hand column, with feedback. (I) At 300 cIs with slight overload (2) At 300 cis, output voltage 15% below maximum. (3) and (4) Conditions as in (I) and (2) respectively, but at 30 cis.

sistors of this stage which, in con­ impedance that by series-parallel specified no trouble should be ex­ junction with a common unby­ arrangement a number of suitable perienced from instability due to passed cathode bias resistor, load impedances may be provided the effects of unintentional posi­ allows a considerable range of utilizing all the sections of the tive feedback. Should instability adjustment to be made in the transformer. A suitable value of arise it will probably appear as drive voltages to the output valves impedance is 1.7 ohms per sec­ oscillation at a supersonic fre­ to compensate for any inequality tion, giving alternatives of I.7, quency. This may be transient, in gain. 6.8, 15.3, 27 ohms, etc. occurring only at some part of the Output Stage.-The balance of Winding data for a suitable cycle when the amplifier is oper­ quiescent anode current in the transformer are given in the ated near maximum output. Its output stage is a matter of some Appendix. cause may be bad layout or an importance, as it affects the per­ output transformer with a higher formance of the output trans­ Negative Feedback Network.­ leakage reactance than specified, former to a marked degree. In The design of this amplifier is such or it may be due to resonance in this amplifier, provision is made, that no difficulty should be experi­ the output transformer. by means of a network in the enced in the application of nega­ A remedy, which should only cathode circuits of the KT66 tive feedback up to a maximum be used as a temporary measure, valves, for altering the grid bias of some 30 db. Provided that the is to reduce the high-frequency of each valve, giving complete threshold of instability is not response of one of the amplifier control of the static conditions of reached, the benefits of negative stages, so reducing the loop gain the stage. A feature of this feedback increase as the amount at the frequency of oscillation to arrangement is that the valves of feedback is increased, at the a value below unity. This may operate with a common unby­ sale expense of loss of gain, but conveniently be done by connect­ passed cathode bias resistor, there will be little if any audible ing a small capacitor (say 200 pF) which assists in preserving the improvement to be gained with in :series with a 5,000 n resistor balance of the stage under this amplifier by increasing the from the anode of V J to chassis. dynamic conditions. amount of feedback beyond 20 db. Output Transformer. - The The feedback network is a Performance turns ratio of the output trans­ purely resistive potential divider, Linearity.-The linearity of the former will be determined by the the bottom limb of which is the amplifier is well illustrated by the impedance of the loudspeaker cathode bias resistor of the first series of oscillograms. These show load. It is convenient to make stage. that, up to maximum output, the each secondary section of such an With component values as linearity is of a high order, and

12 The Williamson Amplifier

that the overload characteristic is istic indicates that little phase supported the measured perform­ of the desirable type shown in shift is present. Phase shift is only ance. No distortion can be de­ Fig. I(b) in the previous issue. apparent at the extremes of the tected, even when the amplifier is . The improvement due to the a.f. spectrum and never exceeds a reprod ucing organ music includ­ application of negative feedback, few degrees. ing pedal notes of the 20 c / s order, especially at low frequencies, is Output Resistance.-The out­ which reach the threshold of clearly demonstrated by the put resistance of the amplifier is maximum output. Transients are oscillograms. 0.5 ohms measured at the ry-ohm reproduced with extreme fidelity; Equipment for measuring inter­ output terminals. tests using a direct microphone modulation products was not Noise Level.-In the amplifier circuit with noises such as jingling available, but measurement of the tested, the measured noise level keys reveal extraordinary realism. total harmonic distortion was was 85 db below maximum output. The amplifier can be described made with an input frequency of The noise in this amplifier was, as virtually perfect for sound­ 400 c / s. The result is shown in however, almost entirely 50 c / s reproducing channels of the high­ Fig. 6, from which it will be seen hum, caused by coupling between est fidelity. It provides an ideal that the harmonic distortion at the mains and output trans­ amplifier for sound-recording pur­ maximum rated output (15 watts) formers. By more careful ar­ poses, where " distortionless " is less than o. I per cent. Inter­ rangement of these components it amplification and low noise le-ve-l modulation, with this degree of appeared that the noise level are of prime importance. linearity, is not present to an could be reduced to better than audible degree. 100 db below maximum output. Frequency Response.-The fre­ If desired, the power output of APPENDIX. quency response of the amplifier the amplifier may be increased Output Transformer. is greatly dependent upon the beyond 15 watts by the use of characteristics of the output trans­ several pairs of output valves in Specification. former. In the amplifier tested, parallel push-pull. The output Primary load impedance = 10,000 ohms c.t, the output transformer had a transformer, power supply and Secondary load impedance resonance at about 60 kc / s which bias arrangements, and the feed­ = 1.7 ohms per sec­ caused a sharp dip of 2.6 db back resistor R 2 5 will require to tion. around this frequency. The char­ be modified. Amplifiers of this Turns ratio = 76 : t , acteristic within the audible range design with power outputs up to Primary inductance e- 100 H (min.) from 1o-20,oooc/s is linear with­ 70 watts have been produced. Leakage inductance= 30 mB (max in 0.2 db. Listening tests carried out in Phase Shift.-The excellence of conjunction with a wide-range Windin~ Data. the frequency response character­ loudspeaker system have fully Core: din stack of Pattern No. 2SA "Super Sileor" lamiuations (Magnetic and Electrical Alloys, Burnbank, Hamilton, Lanarks.) The winding consists of two + I ....., identical interleaved coils, each l~ill wide, wound on [lin x I tin paxolin db 01.-­ formers. On each former is wound: I 5 primary sections each consisting of 5 laye-rs (SS turns per layer) o of 30 s.w.g. enamelled copper wire 8 8. 8. interleaved with 2 mil. pappr, alter­ o ~ nating with 4 secondary sections. FREQUENCY IN CYCLES PER SECOND each consisting of 2 layers (2<) turns per layer) of 19 s.w.g. enam. [OpPN wire, interleaved with 2 mil. paper. Fig. 7. Frequency response (without feedback) of 20 watt output trans­ Each section is insulated from its former described in appendix. Generator resistance 2,500n load neighbours by 3 layers of 5 mil. resistance I.7n. Measured with 5V r.m.s, on primary. At higher Empire tape. All connections a re excitations the bass response improves progressively up to saturation. brought out on one side of the wind­ ing, but the primary sections may be connected in series when winding. ·6nly two primary connections per coil being brought out."

Measured Performance. Primary inductance = 100 H. (measured at 50 cis with SV r.rn.s on primary, equivalent to 2.5 mW) Leakage inductance = 22 mH. (measured at 1,000 cis) Primary resistance = 250 ohms.

(a) Input waveform, 300 cis. (b) Output waveform with feedback and slight overload. (c) Output waveform with feedback but output voltage • Secondary connections for dIfferent 15% below maximum. ratios are given in the Table on p.17. 13 The Williamson Amplifier

The New Version Design Data: Modifications: Further Notes

INCE the publication in the amplifier, and in subsequent adjustment. Accordingly, revised April and May, 1947, issues articles to present the design of values and tolerances are shown S of Wireless World of an am­ auxiliary equipment to form a for resistors R" R., Ru and RU' plifier design suitable for high­ domestic sound-reproducing in­ A transitional phase-shift net­ quality reproduction of sound, stallation. work consisting of R.o and ell' correspondence has revealed that Circuit Diagram. The list of which was previously recom­ a more complete explanation of component values are printed mended as a temporary measure, some of the features of the design, again. These differ in minor detail has been added as a permanent with the addition of some informa­ from the originals. In the circuit feature to increase the margin of tion about construction, would be previously printed a potentio­ stability at high frequencies. This of interest. The correspondence meter, R u ' was provided in the will be discussed later when the also shows that considerable de­ penultimate stage to enable the stability of the amplifier is con­ mand exists for a pre-amplifier signal to be balanced. Due to the sidered. unit to enable the amplifier to be use of common unbypassed Finally, an indirectly - heated used in conjunction with gramo­ cathode resistors for the push-pull rectifier has been substituted as phone pickups and microphones of stages, the amplifier is largely this prevents a damaging voltage low output. In the present article self-balancing to signal, and it is surge when the amplifier is it is proposed to deal with the permissible to dispense with this switched on. No suitable type was

CHI 450V CHZ 4'4mA 5'25mA 10mA IZ5mA R Rz o

3SV

RII

RI3 ... -O:'Pv+" ~ R 105 ;!! 1

RS

R4 Cs RZ5

Fig. I. Circuit diagram of complete amplifier. Voltages underlined are peak signal voltages at 15 watts output.

0.25~ R 1 IMO 1 wa.tt ± 20% R14 , R 1, O.lMO 1 watt ± 10% Cs, C7 350V wkg. RI 33.0000 1 watt ± 20% R16• RIO 1.0000 1 watt ± 20% C, 8~ 600V wkg. R R a 47.0000 1 wa.tt ± 20% 1S, RIa 1000 1 watt ± 20% C10 200pF 350V wkg. R , R 2 R 4 4700 1 watt ± 10% 17 l 1looO watt wirewound CHI 30H at 20mA variable R • R 22,0000 1 watt 5% CRt lOR at l50mA 6 7 ± 1500 (or matched) R" 3 watt ± 20% T Power transformer R 28 • R24 1000 1 watt ± 20% R s 22.0000 1 watt ± 20% Secondary 425·0·425V 150 mA, 5V. 3A. R 26 1,200 V speech coil impedance HR, R, 0.47MO 1 watt ± 20% 6.3V 4A, centre-tapped 1 watt (see table) RIO 3900 1 watt ± 10% VI' V 2xL63 or 6J5. 6SN7 or B65 R2S 4,7000 1 watt ± 20% 2 It • R ll 1I47,oooO 2 watt ± 5% C , C • C , c, 81'F 500V wkg. Va, V4 do. do. (or matched) 1 2 6 Ca. C4 0.051'F 350V wkg. V6, Vs KT66 V7 Oossor 53KU, 5V4 14 The Williamson Amplifier

0 1---­ --t-... ~~

':0' I 0 ;S 1/ z: ec s o / ~ k ~ / / '. , II --- AMPLIFIER WITHOUT FEEDBACK ~ 13300 :n~ 011 / AMPLIFIER WITH FEEDBACK ... ~ 12 00 f~ ~~ -~ - OO~­ -21o / I e,..­ 1­ 1/ ~'= ~'0 0 -'iJi /' i~ I(Xl [-~­ -­ ~e200 :~ ~~ 300 -'iJi - 8 g § o 8 FREQUENCY (cis) i Fig. 2. Loop gain and phase-shift characteristics of the amplifier

OUTPUT available when the circuit was PLUG originally published. A list of alternative valve types is also c. ~ F411~NA~~~ shown. Iri*"~~~ Amplitude and Phase / fre­ quency Response. A curve show­ ing the transmission and loop gain of the amplifier at frequencies 10. between 1 c / sand 1 Mc / s is shown in Fig. 2. Although only the sec­ tion between 10 c / sand 20,000 c / s is useful for sound reproduction, the curves outside this range are included as they may be of in­ Fig. 3. Suggested layout of principal terest to those who may wish to components of com- INPUT use the amplifier for other pur­ t PLUG bined amplifier and poses. They may also serve to power pack. I. 14' .I emphasize that, in a feedback amplifier, the response must be carefully controlled at frequencies shows, the amplifier has consider­ the amplifier is put into service very remote from the useful range able gain at low radio frequencies, there are a few adjustments which if stability is to be achieved. and care is necessary to avoid require to be made. These con­ General Constructional Data. oscillation. cern the balancing of the sta nling The layout of the amplifier is not 3. Signal wires, especially grid currents in the output stage, and critical, provided that a few leads, should be kept as short as (with the original circuit) balanc­ simple precautions are observed. possible, and the stopper resistors ing of the signal currents in the Many different arrangements have associated with the output stage push-pull stages. been used satisfactorily to suit must be mounted on the valve­ Accurate balance of the stand­ differing circumstances. An ex­ holder tags, and not on grot:.p ing currents in the output stage cellent plan is to construct the panels. is essential, as the low-frequency power supply and the amplifier on 4. A bus-bar earth return characteristics of the output trans­ separate chassis, as this gives formed by a piece of 12 or 14 former deteriorate rapidly with greater flexibility in accommo­ s.w.g. tinned copper wire, con­ d.c. magnetization. The proce­ dating the equipment in a cabinet. nected to the chassis at the input dure to be adopted for static and The follow in g precautions end, is greatly to be preferred to signal balancing is as follows:­ the use of the chassis as an earth should be observed:­ Static Balancing. 1. The output transformer core return. should be positioned at right 5. Electrolytic and pap e r (a) Connect a suitable milli­ angles to the cores of the mains capacitors should be kept away ammeter in the lead to the transformer and the main smooth­ from sources of heat, such as the centre tap of the output trans­ ing choke. output and rectifier valves. former primary. 2. The output transformer and Figs. 3 and 4 show the positions (b) Set the total current to loudspeaker leads should be kept of the major components in two 125 rnA by means of R ... at a reasonable distance from the alternative layouts which have (c) Connect a moving-coil input leads, which should be been used successfully. voltmeter (0-10 V approx.) screened. As the response curve Initial Adjustments. Before across the whole of the output 15 The Williamson Amplifier

and the secondary impedance, I' 11" • • 9"------0 being proportional to the square l of the turns ratio, becomes 1.7 x 2'=6.811. Similarly if three sections are connected in series the impedance becomes 1.7 x 3'= r, 15.311. Thus the available secondary impedances, keeping a e" 10,00011 primary load impedance, FUSES are 1.7, 6.8, 15·3,/ 27, 42.5, 61, 83 and 10911. The connections to

MAINS obtain these values are shown in l~_ INPUT the table. Should it be necessary, in an emergency, to match loads of other impedances to the ampli­ Fig. 4. Layout when using separate power pack. fier, it is permissible to reduce the primary load impedance to 6,000 n transformer primary and adjust transmission of the component at giving another series of secondary impedances, namely I, 4, 9, 16, R 17 until the reading is zero, in­ high frequencies, and great varia­ (heating balance. Ran d 0 m tions are possible. 25, 36, 49 and 64 n. Under these fluctuations of this instrument In the output transformer speci­ conditions the power output will may be noticed. These are due fied, the only parameter which is be increased slightly and the dis­ to mains and valve fluctuations likely to vary appreciably is the tortion will be doubled. The and should be disregarded. inductance of the primary at low value of the feedback resistor R" must remain unaltered, as the Signal Balancing. signal levels, due to the use of core material with a low initial turns ratio is unchanged. The (a) Connect the low-im­ values of R" are given in the pedance winding of a small out­ permeability, or to careless assembly of the core. The high­ table. put transformer in the lead to Winding data for an output the centre tap of the output frequency characteristics are not dependent on the core material to transformer to match loads in the transformer. Connect a detector region of 3.5 n are given in the (headphones or a cathode-ray a substantial degree. They are dependent only on the geometry of Appendix and the connections oscillograph if available) to the and other data are included in the other winding, earthing one construction, and to some extent upon the dielectric properties of lower section of the table. side for safety. The two outer layers of the (b) Connect a resistive load the insulants used, and are there­ fore reproducible with a high output transformer primary should in place of the loudspeaker. normally be connected together to (c) Apply a signal at a fre­ degree of accuracy. Comments are frequently ex­ form the centre tap, the inner sec­ quency of about 400c/s to the tions of the winding being taken amplifier input to give an out­ pressed about the size of the out­ put transformer. It is true that to the valve anodes. This gives put voltage about half maxi­ the minimum external electric mum. it is considerably larger than the transformers which are usually field. (d) Adjust R for minimum 12 fitted to Is-watt amplifiers. The output in the detector. Stability with Negative Feed­ fact that the peak flux density of back.-Much has been written The Output Transformer. As 7,250 gauss for maximum output about the stability of amplifiers stated previously, the output at 20 cis lies on the upper safe under conditions of negative feed­ transformer is the most critical limit for low distortion is sufficient back, and the criteria for stability component in the amplifier and comment on current practice. are now widely appreciated. The satisfactory performance wiII not Some confusion arose regarding article by "Cathode Ray" in the be obtained with a component the method of connection of the May, 1949, issue, states the differing substantially from the transformer secondary windings to matter simply and with character­ specification. The effect of de­ match loads of various im­ istic clarity. creasing the primary inductance pedances, whilst utilizing all the Continuous oscillation will occur will be to produce instability at secondary sections. The correct in a feedback amplifier if the loop low frequencies, which can be p rim a r y load impedance is gain-that is the transmission of cured only by altering the time 10,00011 and as the turns ratio in the amplifier and the feedback constants of the other coupling the original design is 76: I the im­ network-is greater than unity at circuits, or by decreasing the pedance of each secondary section any point where the phase shift 0 amount of feedback. At high fre­ is 10,00011/762 or 1.711. When of the amplifier has reached 180 • que-ncies the situation is more secondary sections are connected It is also possible for an amplifier conplex, as there are more in parallel. the turns ratio, and to be unstable in the absence of variables. The leakage induc­ hence the impedance ratio, re­ continuous oscillation if these con­ tance, the self-capacitance of the mains unchanged. If now two ditions should occur in a transient windings, the capacitance between secondary sections, or sets of manner at a critical signal level. windings and the distribution of paralleled sections, are connected This latter condition is particu­ these parameters determine the in series the turns ratio is halved, larly likely to or-cur in badly de­ 16 The Williamson Amplifier

signed amplifiers with iron-cored transformer distortion at fre­ margin of stability, oscillation will components, where the inductance quencies of the order of IO-20C/S, occur. It should be emphasized and, therefore, the time constant would require a transformer with that this will happen only velY controlling the phase and ampli­ a very large initial primary induc­ rarely, and when it does the tude characteristics of one or more tance. This would necessarily be remedy is obviously to reduce the stages may increase by as much as expensive, and a compromise loop gain to its correct value. a factor of five between zero and must be drawn between the three To assist the unfortunate few maximum signal levels. If this factors. Because of this, the who experience instability, the variable time constant is shorter margin of stability must be kept following procedure is recom­ than those of the fixed coupling to the lowest practicable value. mended. If oscillation should circuits, an increase in its value When the amplifier is repro­ occur at a low frequency (about due to a high signal level may be duced, the "spread" in tolerance 2 c / s) the first step should be to sufficient to render the system un­ of components will normally be disconnect the feedback resistor stable. In order to avoid this such that changes in character­ R'5' If the oscillation continues condition the fixed time constants istics due to departure from the the decoupling circuits should be must be made much longer than nominal value of one component checked and any faulty compon­ that of the variable stage. This will be balanced by opposite ents replaced. The amplifier condition would lead to undesir­ changes produced by departure in should also be examined to ensure ably large interstage couplings if another component, and the that it is operating correctly good low-frequency response were amplifier as a whole is likely to balanced in push-pull, and not in required. Alternatively, the var­ have characteristics close to the an unbalanced manner due to the iable time constant must be average. Individual amplifiers failure of some component. chosen in relation to the fixed may. however, have charac­ time constants, such that its mini­ teristics which differ substan­ Primary Inductance mum value is sufficiently longer tially from the average, due Assuming that the amplifier is, than the fixed values to produce to an upward or downward or has been rendered, stable with stability. An increase in its value trend in the changes produced by the feedback disconnected, the then serves only to increase the component deviations. If the next step should be to check the stability margin. This method is trend is in a direction such that phase and amplitude character­ used in the amplifier under dis­ the loop gain is reduced, no in­ istics at low frequencies. It is not cussion. stability will result, the only effect practicable to make direct mea­ To ensure a wide margin of being a slight degrading of the surements of these characteristics stability, whilst at the same time performance. If, on the other without very special equipment, preserving the high loop gain hand, the loop gain is increased as inspection of Fig. 2 will show necessary to reduce the effect of by an amount greater than the that the interesting region lies

OUTPUT TRANSFORMERS. TABLE OF CONNECTIONS. No. of secondary groupsofseetlons in series 1 2 3 4 5 6 7 8 --. ---­ ::--...­ ~ ~ ~ ~ Connections N U­ ~ ~ ~ -J~ ~ ~ Jr J~ --11 _i1 --l~ -~ --. -"

Correct secondary I impedance (ohms) 1.7 6.8 15.3 27 42.5 61 S3 IOU Original Output Minimum second. Transformer ary impedance permissible (ohms) 1 4 9 16 23 I :16 49 64

10,000/1.70 Feedback resistor R 26 (ohms) 1,500 3,300 4,700 6,800 8,200 10,000 11,000 12,000 Turns ratio 76 38 25.4 19 15.2 12.6 10.8 9.5 ------Alternati ve Correct secondary Output impedance (ohms) 3.6 14.4 32.5 57.5 90 130 176 230 I Transformer (See Appendix) Feedback resistor 10,000/3.60 R 25 (ohms) 2,200 4,700 6,800 9,000 11,500 ia.ooo 16,000 18,000 I I Turns ratio 52.5 26.25 17.5 13 10.5 8.75 7.5 6.5 I I I I I I I

17 The Williamson Amplifier

below 10 C / s. It is therefore quencies of individual amplifiers oscillation. If, on the other hand. necessary to arrive at the desired will deviate appreciably from nor­ it is made sufficiently short to result by indirect means, namely mal unless the layout is very poor avoid this, the ability of the by measurement of the component or the transformer is not to speci­ amplifier to handle low fre­ parameters which determine the fication. quencies will be impaired. The characteristics. The parameter use of separate bias impedances which is most likely to show a Capacitive Loads destroys the self-balancing pro­ large deviation from specification The amplifier is absolutely perties of the amplifier, and if two is the initial primary inductance stable at high frequencies with a dissimilar valves are used in the of the output transformer, since resistive or inductive load, but it output stage "motor boating" is the quality of the core material is is possible for oscillation to occur likely, due to the presence of sig­ not easy to control accurately, and when the load impedance is capa­ nal in the h.t, line. The perform­ careless assembly of the core may citive at very high frequencies, ance of the output transformer cause considerable variations in its for example, when a long cable is may be seriously affected by the permeability. used to connect the amplifier and out-of-balance current caused by The initial primary inductance loudspeaker. To avoid this pos­ valves whose anode currents lie should be checked by connecting sibility, and to give an increased within the manufacturer's toler­ the primary winding across the margin of stability, a transitional ance limits. Finally, there can be 5-V, 50-cis rectifier heater wind­ phase-shift network consisting of little justification of this modifica­ ing of the mains transformer and R.o and C,o in conjunction with tion on economic grounds. as the measuring the current in it. The the output resistance of V" has costs are roughly similar. Indeed, secondary windings should be on been included in the circuit. This if the question of replacement due open circuit. The current, which has the effect of reducing the loop to failure is considered, the com­ can just be read on the 10 rnA gain at frequencies from 20 kc / s mon bias arrangement shows a a.c. range of a Model 7 Avometer, upwards without affecting the definite saving. should be 150flA or lower. The phase shift in the critical region. It is to be hoped that these re­ component should be rejected if The use of a phase advance net­ marks on stability will not have the current exceeds 200 flA. work consisting of a capacitor the effect of frightening those who If the output transformer is shunting R,. has been advocated already possess amplifiers of this satisfactory the values of the other as a means of stabilizing this type or are contemplating acquir­ components should be checked, amplifier. The effect of such a ing them. Their purpose is to particular attention being paid to network is to increase the loop help the occasional" outer limit .. the coupling components. Should gain at high frequencies, at the case where instability is experi­ the time constants of the coup­ same time reducing the amount of enced, but if they serve to impress lings, that is their RC product. be phase lag. It is sometimes pos­ upon the reader that negative feed­ higher than the nominal values by sible by this means to steer the back amplifiers are designed as more than 20 per cent, the resis­ phase curve away from the 180· an integral unit, and that any tors should be adjusted to give point as the loop gain is passing modifications, however insignifi­ the correct value. through unity, thus increasing the cant they may appear, may seri­ The trouble will probably have margin of stability. ously affect the performance or revealed itself by this time. but, The connection of a capacitor stability, a useful purpose will if upon reconnecting R,. the oscil­ across R••, however, will not have been accomplished. Such lation is still present, it is very stabilize this amplifier if it has modifications should be attempted likely to be due to the use of been constructed to specification, only by those who are confident valves with mutual conductances although it may produce improve­ that they know what they are do­ higher than average, and it is ment if oscillation is due to some ing, and who have access to mea­ legitimate to increase the value of large departure from specification, suring equipment to verify results. R,. to reduce the loop gain. If such as the use of an output trans­ instruments are available, the former with completely different loop gain may be measured by high - frequency characteristics. APPENDIX disconnecting R,. from the The writer has no information cathode of V 1 and reconnecting it about this. Output Transformer with 3.6-ohm via a 470 n ±10 per cent resistor to The use of separate RC bias Secondaries chassis. The voltage gain, mea­ impedances for the output valves Winding Data sured from the input grid to the has also been suggested. This Core: Ifin. stack of 28A Super junction of R,. and the 470 n re­ procedure is not endorsed by the Silcor laminations. (Magnetic and sistor, should be 10 at frequencies writer, as there are numerous dis­ Electrical Alloys, Burnbank, Ham­ between 30 c / sand 10 kc / s. advantages in its use and no re­ ilton, Lanarks.], The winding con­ Care must be taken not to over­ deeming features whatsoever. If sists of two identical interleaved load the amplifier when this mea­ the time constant of the bias net­ coils each Ilin. wide on paxolin surement is being made. work is made sufficiently long to formers Itin. X Ifin. inside dimen­ The adjustment of the loop gain ensure that the low-frequency per­ sions. On each former is wound to its correct value at medium formance of the amplifier is un­ 5 primary sections, each con­ frequencies should render the sisting of 440 turns (5 layers, 88 impaired, the phase shift of the turns per layer) of 30 s.w.g. amplifier stable at high fre­ bias network will have its maxi­ enamelled copper wire interleaved quencies. It is unlikely that the mum at or near the lower critical with 2 mil. paper, alternating with phase characteristic at high fre- frequency and may provoke 4 secondary sections, each con- Continued on page 33 18 THE WILLIAMSON AMPLIFIER THE AUDIO GLOSSARY D.T.N. Williamson BKAA6 J. Gordon Holt BKAA7 In Arril1947. Willi.~. The author takc."i the reader deep yet at times humorous fashion. Three edition': Softcover into his design corwidcranons. offcrin~ practical advice (S); Hardcover with Dust [acker (H); and Limited, Auto­ on how [0 build the units pim; concise instructions on graphed Hardcover with Gold-Embossed l\inding

LOUDSPEAKERS: THE WHY AND HOW OF GOOD REPRODUCnON G.A. Brilllls BKAA5 This easy-to-read classic, Ian revised in I949, introduces the reader to concepts such as impedance. phons and decibels, frequency response, response curves, volume and watts, resonance and vibration, cabinets and baffles, horns, room acoustics, transients, crossovers, negative feedback. Doppler and phase effects, and much more. A provocative survey of the right questions abour sound reproduction. • INUSA, PLEASE ADD $3SHIPPING FOR FIRST AUDIO ANTHOLOGIES, VOLS. 1-5: Year 1949, 1990 Pages 88 BOOK. INCANADA, $5. OVERSEAS SURFACE, 0-9624-1913-3 Softcover 5V1 x8V1 US $6.95 WHEN AUDIO WAS YOUNG $10. OVERSEAS AIR, $20. ALL CUSTOMERS: C.G. McProud, editor BKAA3 ADD US$.75FOR EACH ADDITIONAL BOOK. These are the outstanding articles from Audio Engint'~ing KILLER CAR STEREO ON A BUDGET: 1947~1959. • ASK TODAY FOR YOUR FREE CATALOG OF magazine during Amps. preamps, tuners, AN EASY CURE FOR speakers, and more. Vol. 1covers 1947~50and 38 articles. HO-HUM AUTO SOUND KITS, COMPONENTS, CABLE, SOFTWARE AND TEST CDs! 2, 1950-1952.45.3, 1952-1955,43.4, 1955·1957.34.5, Daniel L. Ferguson BKAA4 1958-1959.48. Providing quick, tested methods for upgrading your car's • MCMSA WELCOME! CHECKSIMONEY ORDERS Year 1950, 1987 Pages 124 sound, this manual provides .'10 excellent. easy-ro-under­ INUSFUNDS DRAWN ON USBANK 0-a338.Q195-3 Vol. 1Softcover 8V1 x 11 US$16.95 nand, hands-on treatment of the cost- effective design and Installation of high-quahrv vehicle sound systems. Year 1952. 1989 Pages 124 Included are (lenin.':' Started: Sedan, Hatchback. and OLD COLONY SOUND LAB O-a338.Q197-X Vol. 2 Softcover 8V1 x 11 US$16.95 Pickup Truck Systems; Speaker Cabinet Design: Filter Year 1955. 1990 Pages 124 Crossovers: and Installarion. PO BOX 243, DEPT. W94 0-9624-1911-7 Vol. 3 Softcover8V1 x 11 US$16.95 PETERBOROUGH, NH 03458 USA Year 1989 Pages 118 Year 1957, 1991 Pages 144 0-9624-1910-9 Spiralbound 5V1 x 8V1 US $9.95 24-HOUR LINES: 0-9624-1919-2 Vol. 4 Softcover 8V1 x 11 US$16.95 TEL (603) 924-6371 Year 1959, 1993 Pages 144 FAX (603) 924-9467 1-8825-8OO1-X Vol. 5 Softcover 8V1 x 11 US$16.95

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19 The Williamson Amplifier

Design of Tone Controls and A uxiliary Gramophone Circuits

OST power amplifiers in­ the present article. It must suffice source, and are capable of being tended for sound repro­ to say that the matter is one in ameliorated. In addition, fixed M duction are designed to which the individual must exer­ compensation must be provided have a uniform response to fre­ cise his own judgment and act for deviations from a uniIorm re­ quencies within the audible range, accordingly. sponse which are deliberately in­ and It is the aim of designers of In order that he may have troduced in gramophone records. pickups, microphones and loud­ scope to do this, a pre-amplifier The degree of complication speakers to give similar character­ designed to be used in conjunction which is worthwhile in such a istics to their products. This re­ with gramophone recordings and unit must be considered. In presents an attempt to fulfil one radio transmissions should there­ theory, it is possible to compen­ of the conditions for the creation fore be capable of providing vari­ sate precisely for deficiencies in of a perfect replica of the original able compensation for such de­ the amplitude / frequency and sound and provides a common fects as are likely to occur in the phase / frequency response charac­ basis for the design of individual units, which, when connected to­ or gether, will provide a complete INPUT channel with a uniform gain / t 10 frequency characteristic. R., Considerations of an engineer­ RISE 51 FALL ing nature sometimes make it de­ TREBLE ~ ~ sirable, and even essential, to FALL R. RISE I l depart from this ideal of a uni­ ell form response in certain sections B~SS VALVE R.. R•• GRID of equipment, and quite fre­ c- MIN MAX I MIN. MAX quently the use of inferior equip­ 151 BASS TREBLE ment or long and unsuitable trans­ I :~ell mission lines leads to an undesir­ FALL ..! able departure from uniformity. - :~ell In cases like this, other "equal­ RISE izer" units have to be inserted in R.l the channel to provide character­ istics which are the inverse of those of the offending section, so remedying the defect. Fig. 5. Basic frequency compensation circuit. Typical values (for use after an EF37, triode-connected) are : When listening conditions de­ R , 250k£l, log; ~1> look£l ; R4 2 , s.sxn . R , part from the ideal-and this, un­ 4 0 4 3 lok£l; R4 4 , look£l linear. C20 1 150pF max.; C2 11 fortunately, happens frequently o.olJ

20 The Williamson Amplifier

teristics, but the equipment to do z this is complicated and expensive. :;;c w When a considerable portion of '"> the channel is outside the control of the listener, as is the case ~ when reproducing records or CURVE A lOOP GAIN = 0 broadcast transmissions, he has B • • - 20 no means, apart from the sensi­ C • • ~ 100 tivity and training of his ears, of ....~ '" determining the defects which :5 90° have occurred in that portion. ~ 01 ~ ~ 3 90° ~ ~,.;::-:::::::::!!!!'---- Since it is impossible to determine .... I '" = the nature and amount of phase distortion by listening to a trans­ fREQUENCY mission, and since it is not usual for much attention to be paid to Fig. 8. Characteristics of circuit this form of distortion at the re­ of Fig. 7. cording or transmitting end, there would seem to be little justifica­ The attenuation introduced by tion for the inclusion of phase the network when controls are at correcting networks in domestic the level position is 24 db, and the equipment. In the case of a network must, of course, be sound reproducing system which introduced into the system at a is, completely under the control of signal level such that the valve the user, particularly if stereo­ feeding is not overloaded. phonic, phase distortion should Low-Pass Filter.-The majority not be allowed to occur if the Fig. 7. Basic filter circuit. of medium-wave broadcast trans­ finest possible quality is to be ob­ missions, when reproduced with tained. This is especially true at stray alternating magnetic fields, wide-range equipment, exhibit a low frequencies, where consider­ especially if they are air-cored. most objectionable form of non­ able time delays are involved. Metal- or dust-cored toroids are linear distortion. This takes the Low phase distortion is best less troublesome in this respect, form of a rattle or buzz often achieved by designing a system but are expensive and not readily accompanying transient sounds with a bandwidth considerably obtainable. such as pianoforte music. This greater than the audible range, Frequency Compensation.-Fig. type of distortion is commonly but where this is not possible com­ 5 shows a" simple compensation caused by minor discontinuities in pensation may be provided. circuit which will accomplish bass the transfer characteristic and is Consideration of the causes of and treble accentuation and frequently associated with Class frequency distortion leads to the attenuation without the use of in­ " B " amplifiers. conclusion that it is normal for the ductors. The controls consist of Recording and processing de­ levels at the ends of the spectrum two potentiometers, each asso­ fects, record wear and imperfect to be accentuated or attenuated ciated with a changeover switch. tracing by the pickup produce a progressively with respect to the Consider the low frequency con­ similar type of distortion from level at middle frequencies and a trols R•• and S2' When R .. is gramophone records. form of compensation to correct fully anticlockwise (minimum re­ The most offensive frequency this fulfils most requirements. It sistance) the response to fre­ components of the rattle or is not possible to lay down hard quencies below I,OOOC/s is uni­ buzz are generally present at and fast rules about the amount form. If the switch S2 is set to the extreme upper end of the of compensation necessary, but " rise," as R •• is rotated clock­ audible spectrum, and spread rates of attenuation or accentua­ wise, the amplitude/frequency downwards as the severity of the tion greater than 6 db / octave are characteristic will rise at low fre­ effect increases. Fortunately, the not usually required. quencies to the maximum shown concentration of this type of dis­ As it is often desirable to change at A in Fig. 6. If S2 is set to tortion into the extreme upper end the amount of compensation dur­ " fall" and R,. rotated clockwise of the spectrum makes it possible ing a programme without calling from the minimum position, pro­ to effect considerable improve­ attention to the fact, methods gressive low-frequency attenua­ ment by removing or reducing the which give continuous control tion will be introduced, up to the energy in the signal at these fre­ over the response are to be pre­ maximum shown at B. In a simi­ quencies. A low-pass filter with ferred to switched systems, unless lar manner, by the use of R" and a cut-off frequency variable be­ the latter are graded in very fine S3 the high-frequency response is tween the limits of 5 and 13 kc / s steps. continuously variable from a level and a fairly high rate of attenua­ The use of inductors to provide response to the extremes shown at tion above the cut-off frequency gain/frequency compensation is C and D with the values given. is a great asset in securing the best to be deprecated as, apart from The curves may be shifted bodily possible aural result from indiffer­ possible troubles due to resonance along the horizontal axis by ent transmissions or recordings. effects and non-linearity, they are modifying the capacitance values Although it is practicable to very liable to pick up hum from as shown by the arrows in Fig. 6. provide a filter with a continu­ 21 The Williamson Amplifier

the response rises to a fraction of teristic and the type of pickup its value below resonance and then used. z falls off due to the attenua­ For reasons now too well known '" tion produced by the capacitor C. to require repetition, lateral disc ...'" The addition of a further R-C recordings are usually cut with a '> ;:: attenuating network external to groove amplitude which is propor­ ~ the circuit will produce a fre­ tional to signal below some arbi­ '" quency response characteristic as trarily selected frequency in the 300-400 c / s region and with a lateral groove velocity which is FREQUENCY proportional to signal above this z :;( frequency. To improve signal/ Fig. 9. Modification of basic ...'" noise ratio it is now common prac­ filter characteristic produced by '> tice to increase the level recorded additional phase shift. ;:: ...'" at high frequencies. This is par­ '" ticularly effective, since the noise ously variable cut-off frequency, energy per cycle increases with the expense and complication are frequency due to the structure of not normally justified and a the record material. In Fig. 12 is switched selection of frequencies is FREOUENCY shown the recording characteristic satisfactory. To attain the high Fig. 10. Final low-pass charac­ used by Decca. The E.M.I. char­ attenuation rates necessary to se­ teristic resulting from addition of acteristic does not differ substan­ cure satisfactory results a normal external R-C attenuator. tially at low frequencies but the resonant-section type of filter rise above 3,000 c / s is absent. It could be used, but this carries shown in Fig. 10. The similarity is proposed to use the Decca char­ with it the disadvantages asso­ of this curve to the response of a acteristic as a basis for design. ciated with the use of inductors. resonant element L-C filter will When playing E.M.I. recordings, An alternative type of filter readily be appreciated. There is one fixed capacitor in the pre­ using only resistive and capacitive a practical limit to the rate of amplifiers to be described later may elements based on the parallel-T attenuation which can be achieved be switched out of circuit, giving a network' is capable of giving very with a single stage, since the level response. Alternatively the sa tisfactory results. Briefly, the attenuation rate and the level to gramophone pre-amplifier may be principle of this filter is as fol­ which the response rises above the left unchanged and correction pro­ lows. In Fig. 7 is shown an ampli­ frequency of maximum attenua­ vided by means of the variable fier feeding a parallel-T null net­ tion are interrelated. Thus a high treble control in the tone compen­ work, the output from the net­ rate of attenuation is achieved sation unit. This, when C•• is set work being fed back to the input with simplicity only at the expense to 100 pF and R.. (Fig. 5) ad­ of the amplifier. Such a system of a low ratio of response below vanced by one quarter of maxi­ has amplitude and phase charac­ cut-off to peak response above mum rotation, gives almost per­ teristics of the general shape cut-off. However, a rate of fect correction. shown in Fig. 8. By altering the attenuation of 40 db/ octave can The majority of pickups, with loop gain of the amplifier, it is be obtained from one stage with the exception of piezoelectric possible to produce a resonance a minimum attenuation above types, give an electrical output characteristic of any desired de­ cut-off of nearly 30 db, which is which is proportional to the lateral gree of sharpness. quite satisfactory. By cascading velocity of the stylus, The out­ If now a lagging phase shift is a number of these filter stages any put of such a pickup when play­ introduced into the amplifier, for desired attenuation characteristics ing a Decca recording will be of example, by connecting the capa­ may be achieved, and high-pass the form shown in Fig. 12, with citor C from grid to earth, it will filters may be similarly formed by ordinates of voltage instead of be seen that the total phase shift the addition of leading phase shift velocity. A pre-amplifier suitable due to network and amplifier just to the amplifier. for such a pickup should have a below resonance will be greater A filter designed on these line-s, frequency characteristic which is than 90° and the feedback volt­ with five switched positions giv­ the inverse of this. age will have a positive compon­ ing nominal cut-off frequencies of Some desirable properties of a ent, whilst above resonance a 5 7,10 and r j kc rs and a pickup pre-amplifier are:­ greater negative component will " linear" position is incorporated I. Low noise level. exist. The effect of this is to un­ in the final circuit. The perform­ 2. Low distortion at signal balance the amplitude character­ ance is shown in Fig. II. levels likely to be encountered istic as shown in Fig. 9. A rise Gramophone Pre-amplifier.­ with pickups in common use. in response occurs just before the The arrangements just described 3. Sharp attenuation below resonance frequency due to the are generally all that is necessary 20 c / s to suppress turntable positive component of feedback, to compensate for defects in radio rumble, etc. and above the resonant frequency transmissions. For record repro­ 4. Provision for varying the duction, however, additional fixed gain electrically. I Thiessen, G. J. .. R·C Filter Circuits." compensation is required. The Noise Level.-The attainment jou",al the Acoustical Sodety 0' Am erice, 0' Vol. 16, No, 4, pp. 275-279 nature of this compensation will of a low noise level in high-quality April. 1945 depend on the recording charac­ sound systems is of such vital im­ 22 The Williamson Amplifier

portance that a few remarks of sponse flat to 20,000 c (s operat­ several megohms--since the volt­ a general nature will not be out ing at a realistic volume level pro­ age output from the transducer of place at this juncture. duces, in the absence of a signal, will increase simultaneously, re­ lt is an unfortunate fact that noise which is just audible as a ducing the gain required from the improvements in microphones and very gentle rustle and is com­ electronic equipment and the pickups in the direction of wider pletely inoffensive. amount of noise contributed by it. frequency range and absence of Most modern microphones and lt is not practicable, however, other forms of distortion are pickups are electromagnetic. to increase the secondary imped­ almost invariably achieved at the although there is a tendency for ance much beyond O.I MO if a expense of the electrical output. microphone design to gravitate flat frequency response is required This does not necessarily mean towards carrier-operated capacitor from the transformer over the that the efficiency of the trans­ types. These have problems of audible range. ducer is reduced by the other im­ their own and will not be treated The noise generated by thermal provements, but merely that it re­ here. Electromagnetic micro­ agitation in a o. I MO resistor at moves less energy from the phones and pickups are manufac­ room temperature is about 6 p.V acoustical field or from the record tured with impedances ranging for a bandwidth of 20,OOOC(s. To groove which actuates it, causing from a few milliohms to several this must be added the noise pro­ less disturbance of this field, or thousand ohms, but are normally duced in the first valve of the less wear of the record groove. used in conjunction with a trans-­ amplifier. By careful design and There is, however, a limit to former which raises the impedance construction, and by the use of a this tendency set by the noise to a suitably high value to match suitable valve, the noise from all generated by thermal agitation in the input impedance of a valve. causes, including mains hum, can the transducer and its auxiliaries For obvious reasons it is desir­ be reduced to a value equivalent and by the noise produced in the able to make this secondary im­ to about 3 p.V at the grid, but first valve of the amplifier. lt is pedance as large as possible-say under normal conditions a figure desirable in a wide-range, high­ quality sound system to attempt to maintain a peak signal ( noise II ratio of at least 70 db. This 0v­ I"'f figure represents the best that can l 4 f--­ be achieved with a direct cellulose } disc recording when everything is :! -I 0 FILTER POSITION 1 Z .. just right," and it is to be ex­ z: \ pected that the standards of com­ .. '" mercial disc recordings will ap­ ~ -20 proach this level when improved i= techniques are combined with new ...~.. '" .....r--. disc materials. A well-designed -} J magnetic tape recorder will give 7N rl' a signal ( noise ratio of 70-80 db, VV~ and the increasing use of this type , -4vQ ~ = = of equipment will doubtless give -s ~ o o impetus to the research necessary ~ ~ for the achievement of similar FREQUENCY (C!s) standards in other forms of re­ Fig. I I. Measured overall response of low-pass filter, in conjunction cording. With a signal ( noise with pre-amplifier circuit (Fig IS, page 26). ratio of 70 db, a sound reproduc­ ing system with a frequency re­ of 5 p.V is fairly representative. The total noise may be taken as +1 C the square root of the sum of the I...... squares of these values, or about .-.. 10­ 8 p.V. To obtain a signal ( noise .I> 0 ~ ratio of 70 db, then, the peak sIg­ ...>­ 1/­ nal must be 70 db above this level, say 25 mV Lm.S. The pre-ampli­ ~ -I 0 fier should have sufficient gain to V S I/ enable the main amplifier to be Q fully loaded by a signal at this '"0 / :.1 -2 level. '" J/ The choice of a valve type for the first stage must be made care­ , - }vo o 00 fully. In theory, for equal gain -:5 ­ 00 8. ~. ~- the noise level in a triode stage is lower than that produced by a FREQUENCY (c/s) pentode, since the pentode has an Fig. 12. Decca recording characteristic. additional noise component due to 23 The Williamson Amplifier

electron partition between screen High. Pass Characteristic.­ applied to the valve by the and anode. In fact, however, Gramophone motors tend to pro­ potential divider formed by l{..

there are no high-gain triodes duce vibrations which can cause and the impedance of C... C I • commercially available with the unpleasant rumbling noises in a and R.,. At medium frequencies

requisite characteristics and elec­ wide-range system. Although the the reactance of C14 is small, and trode structures for low-noise energy contained in the " rumble" that of C15 large compared with operation. A valve designed for components may be relatively the resistance of R •• and R .., and such conditions should have a low, the frequency is also very the gain of the stage is determined rigidly braced electrode structure low, and consequently loud­ by the values of these resistors. to reduce microphony and a speaker cone movements of high As the frequency is lowered the balanced " double helical" heater amplitude may be caused. If the impedance of the top limb in­ construction to minimize the alter­ driving coil should move out of creases, giving a progressive re­ nating field surrounding the the region of uniform flux-density, duction of feedback. This pro­ cathode. The Mullard EF37 has the whole spectrum being repro­ duces a gain / frequency charac­ this construction and, connected duced will be distorted in a par­ teristic which rises to a maximum, as a pentode, the noise levels men­ ticularly unpleasant manner. Dis­ determined by the circuit con­ tioned earlier are obtainable. Be­ tortion in the output transformer stants, and then decreases due to fore commencing work, the reader is also possible. the coupling components Cl8' R as who is not familiar with the tech­ This situation can be improved and R. o• With increasing fre­ • nique of high-gain amplifier con­ materially by the insertion of a quency the impedance of C I de­ struction should consult an article high-pass filter with a cut-off fre­ creases, increasing the negative on this subject.2,' Considerable quency of about 20 c / s and a feedback and producing a falling reduction of residual hum may fairly rapid attenuation below cut­ gain / frequency characteristic. usually be obtained by demagnet­ off. At these low frequencies, The capacitance between the izing the valve." In order to such filters are conveniently com­ input transformer secondary wind­ obtain the best signal/noise ratio, posed of resistance-capacitance ing and earth may, if large, affect the principle which should be fol­ networks and may be incorpor­ the response at the extreme upper lowed, when valve noise is the ated in the bass-compensation pre­ end of the audible spectrum. This limiting factor in high-gain ampli­ amplifier. effect is negligible with a well­ fiers, is to put the whole of the Electrical Fading Control.­ designed component, but long available signal into the valve When the pickup is placed on, or leads should be avoided. The grid, and to provide any fre­ removed from, the disc the gain transformer should be mounted quency compensation which may must be reduced to avoid un­ on the pre-amplifier chassis, be necessary after the signal has pleasant noises. While this may which in turn may conveniently been amplified. By this method be done by a mechanical poten­ be fixed beneath the motor board. valve noise is included in any tiometer the method is clumsy The overall characteristic with attenuating operations which may and does not facilitate rapid re­ an input from ape r f e c t be performed and the overall sig­ cord changing. It has been found " velocity" pickup on a Decca nal noise ratio is improved. convenient to employ an electrical disc is shown in Fig. 14. Low Distortion. - Numerous method in which the gain of one methods of providing a response of the stages is reduced to zero at A more complex circuit, which which varies with frequency are the flick of a switch by a bias volt­ gives nearly perfect compensation possible and, of course, each age applied and removed by and a very rapid attenuation method has advantages and dis­ means of a network with a suit­ (30 db/octave) below 20C/S, is advantages. Where the response able time constant. shown in Fig. IS. This pre­ has to be continuously variable amplifier has a higher gain than the method which gives greatest Pre-Amplifiers the previous one, and is particu­ simplicity of control usually Although all the refinements larly suitable for use in equip­ triumphs. Other things being outlined so far are desirable, in­ ment where the pickup is located equal, however, methods which dividual requirements will vary at some distance from the rest of employ selective negative feed­ considerably and will determine the amplifier as the circuit ter­ back are to be preferred, as cir­ how much complication should be minates in a cathode follower. cuits of this nature generally have attempted. Two gramophone The construction of this circuit a high signal-handling capacity pre-amplifier circuits will there­ is not recommended for those and non-linear distortion is kept fore be described, which should without access to facilities for to a minimum. In a pickup pre­ cover most requirements. checking the response of the amplifier this may be of import­ Fig. 13 shows a simple circuit finished unit, as the performance ance where pickups with widely which gives good compensation may be seriously affected by an varying output levels are to be for the Decca recording charac­ error in component values. used. teristic. The circuit constants The frequency characteristic of • Baxandall, P. J .. "Hum in High Gain Am­ have been adjusted to give as this amplifier is produced by the plifiers." Wireless World, Vol. 53, No. high a degree of attenuation combination of two curves shown 2, pp. 57-61, February. 1947. • Dickerson, A. F., "Hum Reduction." below 20 c / s as is consistent with at A and B in Fig. 16. These, Electronics. Vol. 21, No. 12, p, 112, De· simplicity. This involves a slight when added, give the curve C. cernber, 1948. • Correspondence. Electronic Engineering, sacrifice of the response at 20 C / s. Curve A is produced by the cir­ Vol. 20. No. 245. p. 235. July, 1948; No. The method of operation is as cuit associated with V'3' which is 248, p. 339, October, 1948; No. 250, p. 406. December, 1948. follows: Negative feedback is similar in principle to that of 24 The Williamson Amplifier

Fig. 13· Simple gramophone pre­ amplifier designed I 3S0V for the Decca re- cording character­ R" istic. When playing E.MJ. records Cl 6 may be switched out of circuit. Alter­ i natively, compensa­ ~R,o fC15 t ion can be effected fl 29 Cl6 in the tone-control circuits. , I' I .... - , CI4 ...oil C" i!i...... OZ'" R .... !:! ·.IR ~ " ...... 55 ~~ ....z "cs~ 0 , u R2'~ :;: r* ~R'21 I ~R'4

Fig. 13. The attenuation at low frequencies is due to the combined ...... effect of the intervalve couplings. .0 ~ v- Curve B is produced by feedback Z /' over V.. through a parallel-T net­ ~ V work tuned to 20 c / s. ~ -I The overall frequency response 5 curve, taken under the same con­ ~

0_ ~ ditions as that of Fig. 14. is shown -z - s -sq g g in Fig. 17. o· 0 Fading ControI.-The circuits N of Figs. 13 and 15 have no pro­ FREQUENCY (c/s} vision for electrical fading. Fig. 18 Fig. 14. Response curve of circuit of Fig. 13 with ideal "velocity" shows a network which. when pickup. connected to the cathode of V. in Fig. 13 or VI' in Fig. 15. enables A very carefully designed and (Fig. 13) has a gain of II at 1,000 the gain to be reduced to zero in necessarily expensive decoupling c / s. Thus. when this unit is used, about a second when the switch system is required if a high-gain full output may be obtained with S5 is closed. On opening S., the pre-amplifier is to operate satis­ a pickup which produces 18 mV gain is restored to its normal value factorily from the amplifier power peak. Should it be required to in a similar period. supply. The cost of such de­ use the system with an insensitive Complete Variable Compensa­ coupling is higher than that of a microphone, disconnection of C.. tion Unit.-It is now necessary to separate power supply unit pro­ in Fig. 13 will raise the gain of connect together the circuits just ducing, say. 350Vat 20mA. and the stage to about ISO, with a described to form a flexible tone therefore the use of a unit of this sensibly linear frequency re­ compensation unit. This must be type is strongly recommended. sponse. Full output will then be done in such a manner that each Performance.-Frequency Re­ obtained with an input of 1.3 mV works well within its signal­ sponse.-Reference to Figs. 6. II, peak. The more complex pickup handling capacity and does not 14 and 17 will enable the fre­ pre-amplifier (Fig. 15) has a gain influence the others adversely. quency response of any combina­ of approximately 250. Fig. 19 on pages 28 and 29 shows tion of units and control settings Noise Level.-With careful the final arrangement. to be determined. The effect of construction and by adjustment Power Supplies.-The High intermediate control settings may of R" to give minimum hum. the Quality Amplifier has a frequency be arrived at by interpolation. noise level may be reduced to an response which is useful down to Gain.-The figures underlined equivalent input signal of 3-5}loV 2 c / s. This necessitates a few in Fig. 19 are the peak signal at the pickup pre-amplifier grid, precautions when auxiliaries are voltages necessary to give maxi­ excluding the noise due to the connected to the input. At these mum output at r.ooo c rs when the pickup transformer and very low frequencies, the balance pre-amplifier is used in conjunc­ auxiliaries. of the push-pull stages may not tion with the High Quality Distortion.-The total har­ be good, and there may be con­ Amplifier. monic distortion produced by the siderable signal in the supply line. The simple pickup pre-amplifier units when used up to the signal 25 The Williamson Amplifier

R.. ROD ",Visov

Rll ROD R" :~C5l R.. R,O Rl] : ..COZ Rn C'" I I ~:' ~ .. I .. .. l II=C.. ~.- C~ COl r """V" (50: ~ r """VI. r """VI> ...... O' rt~_.. h:-~_.. [1:=:1 R R,. !:C" ...­ EF37 C".--­ ,....L i~ .~ l~ S~J R ~ \a) --..­ ~ PIC ~ ,0, n III R" R" "'1ii!i.... :I R .... ." Rot n '--.­ ~ oz : ~C'" ::C" ....0 R,. R.. R" .... S ...... ~~ R•• ~ R'lI ~~ ClI " Ru C,,: ~ R'I 0", S

Fig. 15. Pre-amplifier with high-pass filter.

nr

levels indicated is considerably other the turntable. This pre­ a multicore-screened cable, which less than o. I per cent. vents mechanical and acoustical connects the console with the Form of the Equipment.-The feedback. amplifier and loudspeaker unit, outward form which a complete The control unit may be a con­ and carries the mains and aerial domestic sound equipment takes sole of armchair height (overall connections. is very much a matter of personal dimensions about 18in x I4in x The amplifier and loudspeaker taste. The suggestions which zoin high) easily movable on unit may be a triangular corner follow have been found in prac­ castors. This may contain the cabinet, with the amplifier built tice to provide ease of operation pickup and turntable, the pre­ into the lower portion, and the combined with absence of trouble­ amplifier unit and, if desired, a loudspeaker occupying the upper some feedback effects. radio receiver, complete with its section, arranged at a convenient The equipment is best con­ power supply. The output from level for listening. structed in two units, one con­ the pre-amplifier may be con­ This arrangement gives great taining the loudspeaker and the nected via a cathode follower to ease of manipulation, avoiding 26 The Williamaon Amplifier

+, the necessity of rising from one's }SOV ,"-- "\ comfortable seat to attend to the I I controls or change a record. The l~-"I main amplifier may be included ~,~.., Cl 7 R.. RI) in the console, but this tends to II r~ R make it heavy and bulky, and •.-1 R IZ gives rise to problems of heat dis­ I I 1t-1~ I l sipation which are not easily ~I., c. ...c c solved. Aclcnowledgment.-The writer Tl' I IS IIl is greatly indebted to Ferranti, & I Ltd., for permission to publish FREQUENCY the results of work undertaken Fig. 18. Circuit of fading control. on their behalf, and wishes to Fig. 16. Derivation of high-pass thank his colleagues for help characteristic. freely given.

~

~ ~ ~ -I ~ c I '"~ I ;:: ~ ~... -2 I '" I II -~ - o g ~. o- §. ~ :;: FREQUENCY (c/') Fig. 17. Response curve of circuit of Fig. IS,

A Treasure Trove of Authoritative Power Amp and Pre-amp Designs AN APPROACH TO AUDIO FREQUENCY AMPLIFIER DESIGN First published in 1957, this volume is the most comprehensive tubed power amp design guide available. Produced from the research laboratories of General Electric Company Limited, U.K., it covers designs from five watts to eleven hundred watts. The"authors include W. I. Heath, D. M. Leakey, and G. R. Woodville. GEC and its SUbsidiary, M-O Valve Co., Ltd. produced some of the world's finest audio tubes, including the highly regarded KT66 and KT88. The book's nine chapters and four appendices cover Class AB and B power amps with their associated power supplies as well as two pre-amplifier designs. One of the appendices includes tube base diagrams, US and European equivalents for GEC types, as well as ratings. The book is a thorough and complete introduction to high quality amplifier design. $19 95, 126pp, + 10p, 6" X 9", softbound BKAA31 ONLY • • AVAILABLE SEPTEMBER 15,1994 ------­ ~YES! PLEASESENDME __EKAA31@$19.95 NAME TODAY'S DATE FOR SIH I INCLUDE; D $3 USA STREET $5 OTHER SURFACE $10 OTHER AIR o 0 CITY STATE ZIP

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27 The Williamson Amplifier

lSOV .... Rn ~Rso CIO C20 tFr~ INPUT FROM PICKUP OR ~ RISE fR4S *Css R41 S TREBLE RSlf PRE - AMPLIFIER ~!S Sl>FAll --­ R47 R48 CS4

C 23 Rn R<2

R CI7 Rso C2 Cn S2

Fig. 19. Complete tone compensation and filter unit. The input and output voltages underlined are peak values for full output from the main amplifier.

28 The Williamson Amplifier

NEW BOOI(S! CHS Edwin H. Armstrong: Operating Features of the Audion First published in 1917, the essay first ap­ peared in the Journal ofthe New York Acnd emy of Sciences. Reprinted as a 32-pagc, illustrated pamphlet, it is reproduced In fac­ simile from the original. Major Armstrong was the inventor of the superheterodyne CIr­ cuit, FM broadcasting, and dozens of other 1'9V (48 major engineering designs. This booklet con­ tains the very first au thoritative report on the triode vacuum tube's capabilities. OUTPUT R BKAA26 $6.95 551 AMPI?FIER Mullard Circuits for Audio Amplifiers First published in 1959 by Mullard, one of RS4 (49 Britain's premier "valve" manufacturers. this ~ID book is a complete guide to building eleven power and control amps for a sound system · with vacuum tubes. A four-chapter tutorial on sound quality is classic guidance on RS6 achieving the best sound with tubes. A do-it­ yourself classic. with sheet metal diagrams. parts lists. and pcrfonnance data. BKAA27 $16.95 ... MC, Visa, Check, Money Order ... Shipping: $3USA, $5OTHER SURFACE, $10 OTHER AIR OLD COLONY SOUND LAB PO BOX 243, Dept W94 Peterborough, NH 03458 USA ' 24-Hour Lines: Radio Feeder Unit (see page 30jor general de/ails) : ' .. (603) 924-6371, (603) 924-6526 0 .' FAX(603) 924-9467 ADDITIONAL COIL DATA

The radio feeder unit described struction of coils for the reception on succeeding pages was designed of the Droitwich transmitter on THE AUDIO DESIGNER'S originally to provide high-quality 200kc/s. and the author has sup­ TUBE REGISTER reception from medium-wave sta­ plied the following additional data uons and coil-winding data covered for those who get a higher signal a ran ge of frequencies from 500 kc I s volume 1 to 1.6 Me I s. strength for the B.B.C. Light Pro­ Since then there have been many gramme from the long-wave trans­ ALL THE CURVES AND DATA requests for guidance in the con- mitter. YOUVEBEENLOO~NGFOR AND MORE! COIL-WINDING DATA FOR THE LONG-WAVE RANGE FOR TUBE TYPES 6C41 EC90, 6C10, 6CG7 6DJ8/69221 ECC88, 6EU7, 6K11 Inductance CoelJicient 12AT7 / ECC81, 12AU7AI ECC82, Transformer Winding No. of turns (I'H) of coupling 12AX7A/ECC83, 12AY7, 12AZ7A, (approx.) 12BH7A, 12DW7, AND 5751 S/H" Primary 180 750 ONLY $17.95 plus Aerial 0.3 "$3USA, $5Other Surface, $10 Other Air Secondary 330 2,000 OVER 140 PAGES INCLUDING: Primary 2GO 1,500 o 11 CURVES FOR EACH TYPE Coupling i o.n 07 DATA TABLES FOR EACH TYPE Secondary 330 2,000 o ELECTRICAL & PHYSICAL SPECS I o ALL NEW DATA, NOT REPRINTS ... J Compiled by Tom Mitchell, author of Coils are wound with 40-42 s.w.g., used, the minimum capacitance of the "How to Service Your Own Tube Amp" d.s.c. copper wire. ganged capacitor should be increased To give the correct coefficient of by the addition of a IOopF silvered­ AVAILABLE FROM coupling the spacing between the mica capacitor across each secondary OLD COLONY SOUND LAB windings of the aerial transformer winding of the transformers, giving a PO BOX 243, Dept W94 should be increased to o.z yin. The co verage of approximately 150-300 Peterborough, NH 03458-0243, USA disposition of the coupling trans kc/s. TEL (603) 924-6371 I (603) 924-6526 former windings is unaltered. For fixed tuning, the capacitors FAX (603) 924-9467 \\'hen continuous tuning is to be should be 300 pF. 29 The Williamson Amplifier

Design for a Radio Feeder Unit

H E preceding articles in this transmitters, and which desires offered as an indication of the series have described ampli­ only to receive transmissions from general lines on which to proceed, T fier, tone compensation and these by the simplest possible and is capable of being adapted to gramophone pre-amplifier units means. individual requirements and con­ which are capable of driving a In order that the units described ditions. loudspeaker from the output of a in the series should form a com­ The basic circuit, shown in pickup or a radio receiver. The plete domestic sound installation, Fig. 20, consists of an r.f. ampli­ design of a radio receiver which it is proposed to outline the design fier, transformer-coupled to a would be suitable for use under of a small two-stage receiver suit­ negative-feedback detector. Cir­ the varied reception conditions able for the reception of medium­ cuit values for a number of alter­ which exist in the populous parts wave transmissions within the native tuning arrangements are of the country, and which at the primary service area. The type of given. Possibly the simplest same time could be constructed receiver to be described gives satis­ scheme, from the point of view of simply and with certainty of re­ factory results where the spacing construction, is to use a twin­ sults, would be a difficult under­ between the carrier frequencies of ganged capacitor to cover the taking. In addition, such a the principal transmitters is high, range, although by this method it receiver would be unnecessarily say 200 kc / s. It is not suitable is not easy to secure a uniformly complex for the needs of that for use in districts where closely­ good performance at each end of section of the community which spaced powerful transmissions the medium-wave band. Alter­ lives within the primary service exist, or where interference is natively the receiver may be pre­ area of high-powered twin-wave severe. The receiver circuit is tuned, stations being selected by a

Fig. 20. Circuit dia­ gram of local station R.. R,o +350V radio receiver. Posi- R tions of selector 16 switches for pre-set tuning shown at X. (n

(76 R9I Rn

(70

Res Ru ::J (74 R,z (n:;: R94 OUTPUT

30 The Williamson Amplifier

push-button or rotary switch. WUfDlNG DATA FOR R.F. TRANSFORMERS The use of variable inductors in this arrangement provides a simple Coemclent method of achieving a uniform Transformer Winding No. of turns Inductance of coupling selectivity and sensitivity over the (1'8) (approx.) range, with the disadvantage that two coils or tuned circuits must be Primary 35 30 provided for each station to be Aerial I 0.35 received. In the unlikely event of Secondary 95 I 160 serious thermal drift, correction is Primary 60 80 easily applied by the use of nega­ Coupling 0.65 tive temperature coefficient capa­ Secondary \10 160 I citors. I R.F. Transformers. - Winding data are given to enable r.f, trans­ formers to be wound simply on dimensions of the coil formers and of instability is the presence of standard formers without the use windings are shown in Fig. 23. undue stray capacitance between of a wave-winding machine. The When the capacitance is being the anode and control grid of VII' correct number of turns are pile­ chosen, allowance should be made The valve types used have an wound in a random manner be­ for strays. which will probably be anode-grid capacitance of less than tween thin Paxolin or cardboard about 25 pF. The values used 0.003 pF, and a layout should be cheeks, which serve to guide and should therefore be less than those chosen which does not materially support the edges of the winding. indicated by this amount. In prac­ increase this figure. The design. This gives an approximation to tice the nearest standard value based on this value. has a factor the performance of a wave-wound should be chosen and allowance of safety of about 4. Although coil. made in the value of inductance. the valve is metallized. a screening The table gives winding data for Movement of the core will enable can may be necessary to reduce transformers to be used with a a variation of approximately ±18 leakage to the valve base. All twin-ganged capacitor with a per cent to be made in the induct­ components in the grid circuit capacitance swing of 485 pF with ance. should be kept above the chassis, trimmers. covering a frequency Constroction.-In order to pre­ and all components in the anode range of approximately 550-1.550 serve stability, precautions must circuit below the chassis. Where kc/s. be observed when constructing the components in the anode circuit. When separately-switched tuned receiver. The most likely cause or in the following grid circuit transformers are to be used. the values of secondary inductance 400 and tuning capacitance may be read from the curve of Fig. 21 1/ J against transmitter frequency. 17 This curve has been co-nputed for / an L/C ratio of unity (L in "H, V C in pF). which is nearly opti­ ~ mum. The number of turns lOOI / necessary to produce the required ~.::;-"":Z / 17 inductance with the formers and ~'7 ~"'l 1/ <57 .,.~ I dust-cores specified may then be ~ /; '5:' f-- obtained from Fig. 22. The / ~'9' ~" ­ -3 ~ ... V '<~" ~ 20 ) &~'" 40 0 ~ ...... / ./ ::r:: ...... !i ::l..~ 7 -r :» 1/ ~ /v z uw 30 Dr\. ~ l/ ... z: ...... V l/ :::>o u­ 10 ) z: ... - Q. /' >- ... 1...... ­ ~ ~ 20 0 oz: z:_ o z: ...... u :::> .... '" r- I'­ I~O 1 1,000 1,500 1,600 70 80 90 100 110 120 IlO 140 FREQUENCY (kc/.) NUMBER OF TURNS

Fig. 21. Curve relating tuned circuit parameters Fig, 22. Curve relating inductance and num­ and resonance frequency. ber of turns for windings discussed in ten. 31 The Williamson Amplifier

COIL FORMERS must be brought above the chassis. as is the case when tuning is by means of a ganged capacitor, they must be screened carefully from the aerial circuits. Figs. 24 and 25 show suggested layouts for con­ " tinuously variable and switched 0'12S' -+- r0'3"r <, I '--...... SLOTS CUT WITH tuning arrangements. rf' THIN SAW The Detector.-To give low distortion, the detector requires to work at a fairly high signal L level-say 5V r.m.s. output. As the receiver is intended to feed the MATERIAL; O'OIS" BAKELIZED FABRIC OR PAXOLIN tone compensation unit, which requires an input of only 200 mV peak. the output is taken from a AERIAL TRANSFORMER COUPLING TRANSFORMER tapping on the detector load resis­ tance. This greatly reduces the a.c. loading on the detector and enables it to handle high modula­ tion levels without distortion. SEC. Alignment Procedure.i--fr) Set ganged capacitor at a position about five degrees from the mini­ PRI. 'ffl lffl mum capacitance end. and adjust trimmers for maximum output from the high-frequency Third Programme. (2) Set capacitor about twenty DATA FOR FIXED TUNING degrees from maximum capaci­ PRIMARY WINDING. (DIRECT AERIAL PRIMARY WINOING. tance position and adjust dust­ CONNECT ION) cores for maximum out­ '" NUMBER OF SECONDARY TURNS ~, NUMBER OF SECONDARY TURNS put from the low-Ire­ PRIMARY WINDING. (lon FEEDER) SECONDARY WINDING. SEE TEXT quency Third Pro­ '/'0 NUMBER OF SECONDARY TURNS gramme. • 5,/;1 •I (3) Repeat this pro­ SECONDARY WINDING. SEE TEXT cess until both stations r are accurately tuned. Fig. 23. Formers are standard Power Supplies.­ moulded type, fitted with 8-mm The receiver is intended threaded iron-dust cores. All coils are wound with Litz wire consisting of 7-9 strands of 45-48 s.w.g. SCREEN AS Fig. 25. Plan enamelled copper wire. ffP IN Fig. Z4 view of top 4" • of chassis. '---"'" . l\" ...~'.....~'~.. ~...... ?;•. lUI ~ ,...... :._110 0 0 0 .•.•. SWITCH Swi tched WAFERS model.

5'/; " to be supplied from the .. . , .... J pre-amplifier power .!. 1 1 ... supply. The decoupling l is not adequate to R. enable it to be fed from the main o amplifier supply. Acknowledgment. - The writer is indebted to Mr. A. T. Shepherd of Ferranti. Ltd.• for his assis G tance in the compilation of data for these notes. rrr=o~ CHASS'S 1'/," DEEP

Fig. 24. This diagram shows a plan view of top of chassis.

32 The Williamson Amplifier

Replies to Queries Raised by Constructors

H E series of articles recently published on the to adjust the anode currents to equality, but unless High-Quality Amplifier has aroused consider­ the transformer has a split primary winding they T able interest and given rise to ccrrespondence. are inconvenient, and great care should be taken to It is hoped that these notes, which deal with ensure that the insertion of instruments does not matters of general interest arising from the corres­ cause oscillation which could give misleading read­ pondence, may be of assistance to readers who have ings. similar difficulties. Constroction.-There is little to add to the con­ Valves.-There is no exact equivalent for the structional data on the main amplifier given in the Osram type KT66, and its use is recommended where August, 1949, issue, except perhaps to explain that possible. When the equipment is to be used over­ the purpose of the sub-chassis screen, shown in Fig 3 seas, the KT66 may be difficult to obtain, and 6L6 (see page 15). is to prevent feedback from the anode glass and metal types may be regarded as direct connections of the output valves to the input of the replacements, with the proviso that the total anode amplifier. It should extend downwards to the full and screen dissipation should be reduced from 25 W depth of the chassis. to 21.5 W by reducing the total current from 125 rnA The method of construction of the preamplifier to IIO mA by adjustment of R'I' The use of these and tone-compensation units will usually be adapted valves with reduced rating entails a slight reduction to individual circumstances. One suggested method of the maximum output. The 807 may be used at of construction for the preamplifier circuit of Fig. 15 the full rating of 25 W, with modifications to the is to use a shallow chassis about 9in x 3in x lin. The valve connections. valves and electrolytic capacitors are mounted in a Since the articles were written, a modification ot group along the centre of this chassis, and the other the EF37 has appeared under the number EF37A. components mounted vertically above the chassis on This has improved heater construction giving greater tag strips arranged on each side of the central group. freedom from hum, and its use may be advantageous The connections to the valveholders are taken for V. and VIJ' through slots cut in the top of the chassis. The No other changes in valve types can be recom­ input transformer should be mounted on the top of mended, as their use would involve radical redesign. the chassis at one end. With the sizes given, there Output Transfonner.-When assembling the core is ample room for a screened component of dimen­ of the transformer, care should be taken to ensure sions up to 3in x 3in x 2in. The whole unit should that the edges of the T and U laminations butt to­ be fitted with screening covers, and mounted on the gether. The magnetic properties of the core are de­ underside of the motorboard as close as possible to pendent upon careful assembly and tight clamping. the pickup. The tone compensation unit of Fig 19 may be Static Balancing.-The method of balancing the constructed on orthodox lines, the only essential being standing currents in the output valves, which was to provide sufficient frontal area to accommodate suggested in the article in the August, 1949, issue, is seven controls. Grid leads should be kept short to dependent for its success on close matching of the avoid hum pick-up. The blank valveholder terminals d.c. resistances of the halves of the output trans­ (pin 6) should not be used as anchors for the leads former primary. Nominally the sections are identi­ to the top-cap grids. The power supply components cal. and when carefully machine-wound from the can, with advantage, be assembled on a separate same reel of wire, the resistances shculd not differ chassis. materially. It is possible, however, due to varia­ Conclusion.-The circuits published in the series tions in wire diameter and insulation thickness, for have been evolved over a considerable period of time the resistances to differ by up to 5 per cent and even, and are capable of giving a very high standard of in extreme cases. 10 per cent. Should this occur, a performance. Requests have been received for data compensating resistor should be added in series with on modifications, but as it is rarely possible to the low-resistance side in order to equalize the resist­ determine the full effect of these without carrying ances, and the meter connected across the equalized out tests, in general. no such data can be supplied sections. by the writer.* Other more direct methods may. of course, be used • Or. for that matter, by Wi,eless Wa,/d,-Eo,

from page 18 sisting of 84 turns (2 layers, 42 neighbours by 3 layers of 5 mil. two primary connections only per turns per layer) of 22 s.w.g. Empire tape. All connections are bobbin being brought out. Windings enamelled copper wire interleaved brought out on one side of the wind­ to be assembled on core with one with 2 mil. paper. ing, but the primary sections may be bobbin reversed. and with insulating Each section is insulated from its connected in series when winding, checks and a centre spacer. 33 The Williamson Amplifier

Modifications for High-impedance Pickups 'and Long-playing Records

H E introduction of long-playing records in Great means that the first stages of the pre-amplifier must Britain, after the publication in November, be capable of handling occasional high-frequency T 1949, of gramophone pre-amplifier circuits for peaks which are greater than those experienced with the "High Quality Amplifier" which were suitable standard records, unless the pick-up is a constant only for the 78-r.p.m. standards, has made it neces­ amplitude one, or its output at high frequencies is sary to revise these designs. attenuated before reaching the pre-amplifier. The principle of recording with a rising frequency The original designs of pre-amplifier employed characteristic at high frequencies and reproducing negative-feedback methods of compensation, and with a correspondingly falling characteristic, in order hence are particularly suitable for a wide range of to effect a reduction in the level of surface-noise from inputs. However, pickups are available with such a the material, is a well-established and useful one. In wide variety of output levels that no single circuit the case of long-playing records it results, in conjunc­ will cope adequately with them, and external attenu­ tion with the use of a homogeneous plastic for the ators may have to be used. record material, in an almost silent background. Modifications.-Dealing first with the single-valve There are, however, dangers attendant upon its pre-amplifier (original circuit, Fig. 13, p. 25), the use. The scheme is based on the hypothesis that the revised circuit of Fig. 27 shows the modifications energy level of music decreases with increase of fre­ necessary to provide alternative standard and long­ quency above about 500 cis. Thus it should be pos­ playing characteristics. To simplify the switching, sible steadily to increase the gain of the recording by using a single-pole changeover switch, the capa­ channel above this frequency. This appears particu­ citor CIS is left permanently in circuit, giving a larly attractive at first sight, since with the normally Decca 78-r.p.m. characteristic in the " 78" position. used constant-velocity characteristic the recorded am­ Alternatively, CIS may be removed to give the E.M.I. plitude for a constant recording level is inversely characteristic. In either case, correction for the proportional to frequency and is therefore very small other 78-r.p.m. characteristic may be made by means at high frequencies. of the treble control on the tone compensation unit. Initially, a rising frequency response characteristic The advantage of this simplified switching is that producing practically constant amplitude at constant it becomes practicable to gang the switch to the level was used, the energy level distribution being motor speed-change control to give automatic com­ relied upon to restrict the amplitude at high frequen­ pensation. If this arrangement is not desired, a two­ cies. The effect of this was, in practice, to cancel the pole multi-position switch may be used, to give three improvement in tracing, which the small-groove or more combinations, as in Fig. 28. system offered, by producing, at high frequencies and It should be noted that the position of CI6 has been high orchestral levels, recorded waveforms with radii altered, so that the whole of the feedback network is of curvature too small to be traced accurately. The at earth potential. This avoids switching transients resulting distortion manifested itself as a tearing which would otherwise occur, due to charging and sound superimposed on the full orchestra. discharging of capacitors as the switch is operated. There is additional evidence to suggest that the A small capacitor, CIT' has been connected across original hypothesis required revision, since it is the input transformer secondary. This is to prevent demonstrable that it breaks down when such per­ any tendency to instability or peaking at high fre- cussion instruments as cymbals and castanets are con­ sidered, particularly when the frequency range is wide. Indeed, the peak power level required to re­ Fig. 26. Recording characteristic used for current Decca produce cymbals exceeds that normally required at long-playing records. medium frequencies. This early experience has led to +20 ...... the adoption of a characteristic D -e ,.,./ which is a better compromise '- +10I >­ between these conflicting factors to u and gives much more satisfactory g :.--...­ ~ I results in practice. Fig. 26 shows '> ...... the provisional recarding charac­ 8 - o V­ - teristic now in use by the Decca § -10I Record Company for L.P. a: records. The amount of treble -­ -20 boost is lower than the theoreti­ o o o o o N !2 g o g­ cal optimum, but the use of even ~ o this amount of compensation FREQUENC Y (c/s) N 34 The Williamson Amplifier

quencies, caused by the presence in the feedback loop a switch inaccessible, consideration should be given of the stray secondary reactances of the transformer. to the use of a relay in place of the selector switch, The necessity for this capacitor and its minimum rather than the use of extension leads. This has the value will vary with the individual transformers. Its additional advantage that it could easily be operated value should be kept as small as possible, consistent from the speed-change lever by means of a micro­ with stability. switch or from the additional switched pin which is Modifications to the three-stage high-pass pream­ a feature of some pickups with interchangeable heads. plifier (original circuit Fig. 15. p. 26) are on the same Pickups without Transformers.-A number of pick­ lines, and Fig. 29 shows the revised circuit. ups are available which do not normally require a With these pre-amplifier circuits, the wiring to the transformer. It is possible to use the majority of selector switch must be kept short, and the switch these with the pre-amplifier circuits. by interposing a should, if possible. be mounted on the pre-amplifier. suitable 1: 1 transformer. In other cases, when the Should the position of the pre-amplifier render such connecting leads are short, it may be practicable to connect the pickup directly in place of the transformer secondary. The limit­ }50V ing factor will be the capacitance between the leads and their screening, R)I which will be shunted across R,. or R ••, and which, if sufficiently large. would upset the treble compensation, The value of this stray capacitance should not be allowed to exceed 50 pF, CI6 and if CIS or Css is switched out, RUf should be compensated by a capacit­ ance of one tenth of its value in V6 .f-P T parallel with R" or R.,. to give a n C I C n linear frequency-response character­ 78 ,. 56 e istic at high frequencies. DECCA "'cz '" ... Resistors R and Roo must be 0",... 21 }'V1 o~ ... ­ R)) R,) ...!C o..z"'''' Left: Fig. 27. Simple two-position switch­ "'0...... ing in single-valve pre-amplifier for cor: °E playing Decca 78-r.p.m. standard and o CII ... R _... R 33t-r.p.m. L.P. records. Compensation 'nZ1 rt n R)4 for the E.M.I. 78-r.p.m. standard charac­ teristic shouldbe appliedseparately by the treble tone control.

Below: Fig. 28. Alternativecircuit (applic­ able to Figs. 27. 29 and 30) with three­ position switch giving compensation for Decca 33t. Decca 78 and E.M.I. 78-r.p.m. recording characteristiC!.

CI10RC , --II Il, , ,

t-SMn O,ooISJ.IoF

Rn OR Rn

R,. OR ~

35 The Williamson Amplifier

retained to provide a conducting path to the valve of Fig. 14, p. 25. This circuit is suitable for most grid when the pickup heads are being interchanged. moving-iron variable-reluctance pickups, and can be There may be cases, where one side of the input used with piezoelectric pickups which have been must be earthed, in which it is impracticable to utilize loaded to give an output proportional to recorded the pre-amplifiers in this way. In this event the velocity. circuit may be modified as shown in Fig. 30. This Danger of Overioading.-The input to the pre­ circuit applies to both pre-amplifiers. In it, the trans­ amplifiers should be restricted to 200 mV in the case former had been replaced by a resistive network R"" of the single-stage circuits and 50 mV for the three­ R.", mixing the input and feedback voltages. stage circuit, and if necessary a potential divider The input resistace of this circuit is approximately should be used. O.I Mfl, and its voltage gain at I,OOOC/s is 9. The Piezoelectric Pickups.-Lightweight piezoelectric frequency-response curve is almost identical with that pickups have recently become popular, particularly for L.P. recordings. Since these give a relatively high output, 110 pre-amplifier is necessary and any correc­ Below: Fig. 29. Revised three-sta,e pre-amplifier circuit with high-pass filter, to play Decca 33,.- and 78-r.p.m. records. tion required may be achieved by means of simple

R" R..

+55l v R" Roo I ~ Cll R 70 R ~C" R,o R.. R" n :!' c.~~ c.;./ ...... ~ l~ c.. ~ ..... v,' ':t'v,. [G~ C"! C..l Coo COl .:"_~ CII ...... , .:"_- R.. i'. --­ :.." '8 5, DECCA g~_ --­ :~ --- \-~ Hl' Hl' R,. C" --- EF37 I~ ~~l PIC @ ll'/'1 : @ .. w I. ~. R91 R"...... '" ~II \~ R" C" R" ... CIS R" ~ ~ R.. z - C" o R.. * R" ;: R,. - ~ ~C R RII R" R" 61 C" :$: CIOclf ~

36 The Williamion Amplifier

Fie. 30. Modified' cir­ RC networks, details of which have already been ns­ published.' one Checking the Pre-amplifiers.-When a pre-ampli­ ~jd~'-~f ih~-'PiCkup must be earthed. fier has been constructed. it is advisable to measure its response curve over the audible frequency range and beyond, in order to ensure that nothing is amiss. This is particularly so in the case of the three-stage pre-amplifier. Rg6 To facilitate this measurement the networks of Figs. lookfi 31 and 32 ha.ve been devised. These circuits, when rr fed with constant-voltage variable-frequency input. o·006,...F R97 100kfi PICKUP INPUT

TAP AT RESISTANCE EQUAL TO IMPEDANCE OF PICKUP IF LOWER THAN ssen INPUT 1100 (CONSTAN! VOLTAGE)

OUTPUT TO PRE-AMPLIFIER

0·005,...F (DISCONNECT FOR EM.I. CHARACTERISTIC) Fig. 32. Simulator(or Decca 33! r.p.m. L.P. characteristic. f~~ produce outputs which are, respectively, replicas of TAP AT RESISTANCE EQUAL TO the standard and L.P. characteristics. IMPEDANCE OF PICKUP IF To test a pre-amplifier. the appropriate network LOWER THAN l'lkD should be connected between an oscillator and the INPUT Bkn pre-amplifier input. The output from the pre-ampli­ (CONSTANI VOLTAGE) fier for a constant voltage to the network should then OUTPUT TO follow the response curve already published for the PRE -AMPLIFIER appropriate circuit (Figs. 14 and 17. pp. 25 and 2]). Acknowledgment.-The writer is indebted to Decca for information about their recording characteristic. Fig. 31. Simulator for Decca and E.M.I. 78-r.p.m. recording , West and Kelly, "Pickup Input Circuits," Wireless World. characteristics. November, 1950, pp. 386-391.

37

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111 A Woodwinds IndustrialCourt, Cary, NC 27511 Announcing... The~~ThbeLibrary ere's your chance to own the most comprehensive library ofaudi.o tube infonna­ tion ever published - complete and up-to-date! Published in five volumes, this library will H give you all the lnfonnation you've ever wanted, and could never find. What tubes were in what equipment? WhIch oftoday's tubes arethebestsubstitute? How doI tellthediJferenc8 between goodtubes and bad? Wklt is thehistory ofa certain tubeand wklt is itsbestapplication? What arethetubes usedin classic circuits? These, and many more questions will be answered in our five-volumes, which will be published in the fol­ lowing manner: Vol. 1 • Tube Compliment and Substitution Guide Section one gives OEM compliments for nearly every piece ofaudio tube gear sold in the past 45-years, with tube upgrade recommendation. Section twogives a com­ plete tube substitution guide from the late 19406 upto the present day - the first of .1 ns kind printed in several decades. 1. 1 J Approximately 350-pages. 8-112"x11", 3-holed drilled. Due November, 1994. I~ w .Iii C5 ... oM·II :Ii! ...II! .. w ~ '29.95 }.i ~.g'" ma o!i :g _w ii u. .!! .... 9. .11 Vol. 2 - Classic Circuits and Their Tubes .10 .DoD",,,, .!l.••• Ii 15 This volume will give specification guides and specs of all the classic tube circuits, u~ zw li .... 110 ... :::c and the tubes they use. Perfect for the hobbiest. With lots of graphs, charts and ... • circuit drawings -i j i ;: Approximately 300-pages. 8-112"x11 ", 3-holed drilled. Due late February, 1995. '29.95 Vol. 3 - New-Old-Stock Spotter's Guide This volume will be the definitive guide to differences between true N-O-S and current production tubes, Includes over 150-pho­ tosto help you separate the good from the bad, and is sure to stir up a lotof controversy. Approximately 300-pages. 8-112"x11", 3-holed drilled. Due outlate June. 1995. '29.95 Vol. 4 - Historical Perspectives This volume will cover allvacuum tubes used inaudio applications, and who did what, when, how and where. Approximately 350-pages. 8-112"x11", 3-holed drilled. Due late October, 1995. '29.95 Updates and Addendums Because we are constantly finding and adding new information that belongs in one of the previous fourvolumes, we will come out with addendums for each of the fourvolumes. The entire fourvolumes, plus addendum, will be 8-112"x11" in size, and be 3-hole drilled. We will offer avery nice, gold-embossed 3-hole binder to hold all ofthe volumes and addendums in one great package. Due outwith the shipment of volume fourin late October, 1995 with prices to be announced. OtIBPublications from CoolSounds Press... How to Order... •TheVacuum Tube Logic Book by David Manley. Now In Ifs 3rd prlnUng. this "audio classic" Is essenUai reading Mail or Fax us at: for audlopbfles Interested In tube technology, or Just Cool Sounds Press plainaudlo•.•$19.95 . Michael Green's Tuning Revolution bythe founderof P.O. Box 110695 RoomTunes. Tells you how to get the mostout of your Campbell, C4 95011-0695 system and tune "all" of your components.•.Fall, 1995 . Tube Talk. Aquarterly newsletter for tube fanatics. No 408/377-9050 (phone). 4081377-1282 (fax). Cartoons, no reviews· Just thousands of people sharing Visa!Master Cards ~tOO.. Add $4 for S&H on books in U.S. their knowledge and loveof audio tubes...$19.95/year. All prices quoted are in U.S. funds. CaB or Fax for exact quote.

Over 2,000 Tube Types for All Applications RECEIVING • SPECIAL PURPOSE • POWER • INDUSTRIAL • ANTIDUE * Highest Quality from the World's Most Popular Manufacturers * TYPE EACH TYPE EACH TYPE EACH TYPE EACH 3A3C 4.50 6EJ7 3.00 12AZ7A 3.25 811A 11.70 5AR4 7.25 6GH8A 2.50 12BA6 3.00 812A 21.95 5R4GY 8.40 6GJ7 3.54 12BE6 3.00 813 23.00 5U4G 3.90 6HA5 3.25 12BH7A 9.80 2050/2050A 12.00 5Y3GT 3.85 IJ5 4.90 15CW5 3.25 5749/6BA6W 2.40 BAJ8 3.75 IJ7 5.60 17JZ8 8.50 5814A 3.50 8AU 2.38 IJZ8 11.70 30AE3 4.10 5881 6.00 BAQ5 4.85 IK7 6.50 33GY7A 9.50 5965 4.00 6AU6 1.90 6SA7 5.00 35W4 3.25 6146A/B 14.25 IAX50T 4.50 18G7 3.25 38HE7 12.80 6350 4.20 BBAI 2.25 18J7 2.80 50C5 3.50 6463 4.00 IBEI 2.75 6SK7 2.80 6267 4.00 IBH6 3.85 6SN7GTB 2.50 1973 20.00 I VOLUME lILa 1.80 6807 3.60 7025A 4.50 DISCOUNTS 6CA4 4.00 6U8A 3.25 7189A 4.40 ICA7 5.50 6X4 4.48 7581A 15.00 6CG3 6.00 6X5GT 3.20 KT88 11.00 Solid State leX8 4.00 6X8 3.50 2D211EN91 4.50 Replacements 6CW5 2.35 12ATI 3.60 85A2/0G3 4.25 as low as $4.00 6DL5 3.25 12AUI 2.80 108Cl/0B2 2.70 • IOOIB 5.75 12AU7 2.90 150C4/0A2 2.00 Write or calf for IDR7 4.00 12AV6 2.50 572B 73.25 complete tube IDXI 3.75 12AV7 2.80 805 30.75 range. price list 6EA8 3.00 12AX7A 3.25 807 6.80 and accessories 6EH7 3.15 12AY7 5.50 810 80.00 catalog.

Prices are subject to change ~ ~ ~ International International Components Corporation 105 Maxess Rd..Melville. NY 11747- NY State 516-293·1500-FAX 516-293-4983• Call Toll-Free 800·645·9154. 1 TESLA,CZECHOSLOVAI!A In 1024 !'" 8.20 7,60 SIND, CHINA ,."v 8,80 8.30 1-9 10-24 25+ 300B $59.00 $49.00 $45.00 807 6.90 5.90 4.90 845 22.50 19.50 17.50 2A3 12.90 11.90 10.90 211 19.50 16.50 14.90 SOLID STATE RECTIFIER 6L6GC "Coke" 5.40 4.90 4.40 BUILT INTO TUBE SOCKET 1-9 10, 6L6STR 6.90 6.40 5.90 Direct Plug-in Replacement for a1l5Y?'. .11'u", """--" Philips -~ 6CX8 5.15 7868GE 18.90 6189W 4.50 6C10 9.90 81lA 11.90 $2.00 extra per tube. IlZAU1W Induslrl,1) 6DR7 5.15 B13 25.00 626UEF88 4.20 6EAB 5.50 6550GE 24.70 6EU7 -Tf}Q­ 872A 12.50 IZI Visa/MasterCard. 6973 14.90 12AU6 4.90 6FHB 15.50 12AV7 3.90 r. 6AL5 2.40 6GW8 6.90 6AM8 5.90 12AY7 8.50 615 5.50 12AZ7 4.90 ffii5A 4.85 617 6.50 6AS7 4.95 6IE616LQ6-~ 12BH7A 10.50 12BY7 7.90 6AQ8 5.85 6K6 4.75 6Aii6----~ 12DW7 14.90 6Kll 6.70 ~ 12FQB 9.00 __..1JL 6SC7 7.90 EF50 4.90

N 5 "/ 5 ENS (fY (0 R P 0 RAT I~ --() 111 Fifth Avenue, New York City, NY 10001 • (2121 529-04~6 • 1-800-611-5477 • Fax (2121 529-0486 -" ~l~lJ1~lJl~Jl~lJl~J~~[w~JT~lJl

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Disrupting the usual flow of events can cause Svetlana is a manufacturer of heads to turn, and that's exactly what R&G Power Grid and Audio Tubes. ~.Y,~otl!!!:!!!= International, Inc. is doing. Through our joint • partnership with Svetlana, we engineer and Audio Glassic Tubes are manufacture tubes for all kinds of applications graded, matched and tested. and carry over 2,000 different types of tubes. Whether you need one tube or a whole • Pride Tubes are RF tested for inventory, your head will spin at the service pl)ET U • • , amateur radio. you will receive from R&G International, Inc:.--i4; Call today for more information (}IV and pricing from any of the ~~ following divisions of R&G. ~ ~ R&G INTERNATIONAL, INC. 8200 South Memorial Parkway • Huntsville, Alabama 35802 800-456-5642 • outside the US: 205-880-3660 • Fax: 205-880-8077