SECOND EDITION
•
Three Shillings & Sixpence Net •
Transformers and Chokes
for the
WILLIAMSON AMPLIFIER NATIONAL PHYSICAL LABORATORY REPORT
ON TESTS OF TRANSFORMER (Marked: Lab: Output, Series II)
Tested for: Vortexion Ltd., 257/263 The Broadway, London, S.W.19.
The series resistance and inductance of the primary The series resistance and inductance of the primary winding with the secondary winding open-circuited, winding with the secondary winding short-circuited, was measured at 50 cycles per second with 5 and 20 and of half the primary winding with the other half volts applied. The room temperature was 1 "oC. short-circuited were measured at 1,000 cycles per The results are given in Table 1. second. About 10 volts were applied. The results are given in Table n. TABLE I TABLE 11 Primary Resistance and Inductance at 50 c/s Resistance and Inductance at 1,000 c/s Resistance (0) Inductance (H) Connections Resistance (0) Inductance (mH) Voltage Pril�:;Y�ho�:��nd- 5 4700 95 495 5.3 20 7900 142 �!:rr;mar;hO�:�� 222 5.5
Date: 2nd June, 1951. Reference: E.475.100.
This Laboratory Model Transformer Series n easily permits 6 db more fe edback with complete stability than the maximum of our Series 1 which Mr. D. T. N. Williamson described.
PRI CES Series n Output Transformers as above £7. 7.0 Series I Output Transformers £6. 6. 0 Mains Transformers ... £4. 4.0 CHOKES 1012H, 150 mA ... £1. 11. 6 CHOKES 30/H, 20 mA £2. 0. 6
Please write for Catalogue WA/5 \r ()11 T E X I ()N LT'I) .
257·263, THE BROADWAY, WIMBLmON,LONDON,S.W.l9
Telephones: LIBerty 2814and 6242-3 Telegrams: "VORTEXION, WIMBLE, LONDON' 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
LONDON: ILIFFE & SONS, LT D. IT�S YOllH CHOICE If the components are below standard, there's not much justification for expecting first-class results. It has been our experience and that of many satisfied customers that in building the Williamson Amplifier it definitely pays to use the best components. All items supplied are new and guaranteed.
I SPECIAL I WILLIAMSON AMPLIFIER (Fig. I) TWO COMPLETE KITS OF ALL SPECIFIED PARTS EACH INCLUDING PARTRIDGE OUTPUT TRANSFORMER - DRILLED STEEL CHASSIS - MARCONIIOSRAMICOSSOR VALVES - G.E.C. AND T.C.C. CONDENSERS THROUG HOUT.
With ELLlSON MAINS TRANSFORMER AND CHOKES 19 Gns. With PARTRIDGE MAINS TRANSFORMER AND CHOKES 21 Gns. CARRIAGE IN U.K. 7s. 6d. PACKING 101- RETURNABLE.
MAIN AMPLIFIER (FIG. I) FADING CONTROL (FIG. 18) PARTRIDGE OUTPUT TRANSFORMERS: 1.7, 0.95 or 3.6 ohm RESISTORS-See Below. Secondaries: Wire ended £6}1}-; Tag Board £6}16}6; Potted CONDENSERS, T.C.C. AND DUBI LlER: £7}S}3. 1-4 ...... 17/2 ...... 2/3 mfd. 250v. .. . 1-.1 mfd. 350v. . . MAINS TRANSFORMERS: 1-2 mfd. 350v...... 8/6 . 74}- 56}3 PARTRIDGE ...... ELLlSON SMOOTHING CHOKES: TONE COMPENSATION AND FILTER UNIT (FIG. 19) 20}9 13H. 200 mA ...... 40/3 PARTRIDGE 25 H. 60 mA...... RESISTORS-See Below. 15}8 10 H. 150 mA 23/3 ELLlSON 30 H. 20 mA .. . . • ...... CONDENSERS, T.C.C. AND DUBILlER: VALVES: 3-50 mfd. 12 v...... ea. 2/9 1-.25 mfd. 500v. ... ea. 2/10 . . SO}2 MARCONI OSRAM KT66 MATCHED PAIRS ...... 1-.01 mfd. 350v. .. 1/8 1-IOOO pf. .. 1/6 14/4 . 21}6 MARCONI OSRAM L63 COSSOR53KU ...... 50/500 pf. Silver Mica±5% ea., 1/4-1/7; I ea., 1/6-2(3. RESISTORS. A full range is av ailable, including: 2-8 mfd. 450 v...... 3/6 I-ISO pf. Trimmer 1/6 2/- 3}- . 22K I watt Matched, Pr. 47K 2 watt Matched, Pr. 3-.05 mfd. 350v...... 2/- 2- 16 mfd. 500v...... 6/- 1}6 1500hm 3 watt ...... SWITCHES: CHASSIS : 17(6 . 3/8 I-I pole,S way,S bank.. I-S.P.D.T...... STEEL DRILLED FOR KITS 27/6. I-D.P.D.T...... 3/11 12/6...... STEEL 17in. x lOin. x 3in. UNDRILLED MAINS TRANSFORMER: 12}6. ALUMINIUM 17in. x lOin. x 2tin. UNDRILLED 325 v. 45 mA. etc.: PARTRIDGE 42/- ; EL L lSON 32/3. CONDENSERS T.C.C.: SMOOTHING CHOKE-PARTRIDGE 75H. 25 mA. 20/9. 7/6 2-8 mfd. 600v...... ea. 15/- 1-8+8 mfd. 450 v. ea. VALVES: 3-EF37A, ea., 25/1 ; 1-6X5, 15}1. 2-.05 mfd. 350v...... 2/- 1-200 pfd. 350 v...... 1/2 1-8 mfd. 500v...... 6/6 2-.25 mfd. 350 v. 2/8 RADIO FEEDER UNIT (FIG. 20) POTENTIOMETERS: 7/2. (a) VARIABLE TUNING MODEL 2- 100 ohms COLVERN ea. RESISTORS: See Below. PRE-AMPLIFIER (FIGS. 13 and 27) CONDENSERS: 2/3 1}4 RESISTORS-See Below. 3-.1 mfd. 350v ...... ea. 2- 100 pf...... ea. 1-2 Gang 500 pf...... 9/6 1-16 mfd. 450v. .. .. 5/- CONDENSERS, T.C.C. AND DUBILlER: . 1-.5 mfd. 350 v...... ea. 3/- 1-16 mfd. 450 v. . .. ea. 5/- COILS-ATKINS HIGH "Q "- I each AL2/R, AL2/W ea. 4/-. . 13/-; 12}9. I-lOO pfd. 350v...... 1/6 I-50 mfd. 12 v. . .. 2/9 DIALS-JB .. AI RPLANE" or .. SQUAREPLANE" . . . .. 2/6 . .... VALVES: I-EF39, 18/8; I-EF37A,25}1. 1-4000 pf. 10% ...... 1-.05 mfd. 500v...... 2}1 VALVE: EF37A...... 25/1 (b) PRE-SET TUNING MODELS I SWITCH 2 wafe rs, 9/6. PRE-AMP WITH HIGH PASS FILTER (FIGS. IS and 29) COILS-ATKINS HIGH " Q ", ea., 4}-. RESISTORS-See Below. TRIMMERS, ea., 1/6 to 2/3. CONDENSERS, T.C.C. AND DUBILlER: RESISTORS 3/- . 5/- 2-.5 mfd.350v •...... ea. 2- 16 mfd. 450v . ... . 1-4000 pf...... 2/6 1-.01 mfd.±2% . .. 6/3 COMPOSITION: * and t watt 20%, 6d. eac h...... 1/- 2-5000 pf. Matched 3/6 1-.5 mfd. 500v. . .. 3/10 ! and t watt 10%. 8d. each.; I watt each .. HIGH STABILITY * and t watt I % and 2%, 2/- each. 2-50 mfd. 12 v...... 2/9 1-.02 mfd. 350v. .. .. 1/9 . 2}10 * watt 5%. 1/- each. ; t watt 5%, 1/3 each. I-lOO pfd. 350v...... 1/6 1-.25 mfd. 500v...... 25}1 NOTE: PRICES AS RULING AT DATE OF PURCHASE. VALVES: 3-EF37A . . ..
Send for Detailed Price List, Post Free. Prompt Delivery. Terms C.W.O. or C.O.D. Postage Extra OVERSEAS EN QUIRIES IN VITED. IIOv. WINDINGS AND TROPICALISED COMPONENTS AVAILABLE. TELE-RADIO (1943) LIMITED 177a &. 189, EDGWARE ROAD, LONDON, W.2 Tel: PAD 6116 Shop Hours: Monday-Saturday 9 a.m.-6 p.m.
PAD 5606 Thursday 9 a.m. - I p.m.
2 e Williamson Amplifier
CONTENTS
Page Introduction 5
Basic Requirements: 7 Alternative Specification (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)
Design for a Radio Feeder Unit 30 (December 1949)
Replies to Queries Raised by Constructors 34 (January 1950)
Modification for high-impedance Pickups and Long-Playing Records 35 (May 1952)
3 The journal for all radio technicians
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ILIFFE & SONS LTD., DORSET HOUSE, STAMFORD ST., LONDON, S.E.l 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. is booklet includes all the arcticles 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 33⅓ 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 which produce the results. Editor, Wireless World.
5 � WODEN Potted components are in constant demand fo r the WILLlAMSON AMPLIFIER AND PRE-AMPLlFIER. note the advantages • Made to the author's exact specification. • Smart appearance and uniform layout. • Absolute Reliability. SEND FOR FURTHER PARTICULARS AND COMPLETE CATALOGUE. WODEN TRANSFORMER [0· LTD MOXLEl' nUllO BllSTON STJlFFS PHONE: BILS TO N 41959 J.T.L. 6 e Williamson Amplifier Basic Design Requirements: Alternative Specifications ECENT improvements in the operation of the loudspeaker. spectrum (but especially, at the field of commercial sound is in turn reconverts the elec- low-frequency end) be substan- recording have made prac- trical waveform into a corres- tially less than that at medium Rticable 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. e useful range of be a replica of the original. these frequencies before they reach shellac pressings has been ex- e performance of an amplifier the amplifier as otherwise severe tended from the limited 50-8,000 intended to reproduce a given intermodulation will occur. is c/s which, with certain notable waveform is usually stated in is especially noticeable during the exceptions, has been standard terms of its ability to reproduce reprodudction of an organ on from 1930 until the present, to a accurately the frequency com- incorrectly designed equipment range of some 20-15,000c/s. is 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 e 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 e 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. ( e term "non-linear changes in effective gain due to reproducing equipment with a distortion" includes the produc- current and voltage - 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.) is requires that of such components to a minimum. large-amplitude low-frequencysig 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. e transient Reproduction of sound by elec- quency spectrum of 10-20,000 c/s. causes the current in the output trical means involves the ampli- (b) Constant power handling stage to rise, and this is followed, fication ofan 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. e is 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 is requirement is concerned at a power level suitable for the response at either end of the with the attainment of good 7 e Williamson Amplifier Ê salient fea- The functions of negative feed- tures of ¥se back are:- methods are of (a) To improve the linearity interest. of the amplifier and output- Push-pull triode transformer. valves without (b) To improve the frequency the of response of the amplifier and negative feed output transformer. back form ¥ (c) To reduce the phase shift mainstay of pre- in the amplifier and output trans- sent-day high- former within the audible fre- (a) (b) equip- quency range. ment. A stage of (d) To improve the low-fre- Fig. I. Output/input characteristics (a) without this type has a quency characteristics of the out- feedback (b) with negative feedback number of dis- put transformer, particularly frequency and transient response advantages. With reasonable defects due to t˙ non-linear from the loudspeaker system by 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 (f) To reduce the of 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 of a loud is a gradual curve as in Fig. voltage changes, and of any speaker, the efficiency is rarely 1 (a). With this type of characteris- spurious defects. higher than 5-10 per cent, and the tic will be introduced at A stage of this type is capable air loading, which determines the all signal levels and intermodula- of fulfillîng ¥ highest 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 The intermodulation with such a characteristic is of the type shown in a high-grade loudspeaker is characteristic is very considerable in Fig. 1 (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 à “mushiness” which when it curves sharply with t˙ 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 ì considerable inter can bê reduced to a degree repre- This is proportional to the modulation will be introduced by sented by less than 0.1 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. Á proportional to the total resistance to give maximum output the frequency response of ¥ 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 will be about transformer secondary can be is short-circuited, hence 2, which is 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 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 of Ê output resistance, upon orchestral music in an average multiple valve capacitances and which the loudspeaker usually 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 a pplica tion of negative feed pedance/output resistance (some- of normal 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” out- 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 e Output Stage plifier from the output transform- to give a performance midway 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 negative feedback, or push-pull demands upon the output capabili- ciency and lower drive voltage tetrodes 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 c/s, fb may be taken as feedback used. The feedback (c) Intermodulation and har 3.3 c/s and ft as 60 kc/so 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 loadimpedance below limit may be reduced, as the feedback. Great care must be the safe limit, resulting in a very energy content of sound at thes� taken with the design and opera reactive load at low frequencies. frequencies is not usually high tion of the amplifier to achieve This may cause the valves to be The limiting factor will be the this, and troubles such as para5itic driven beyond cut-off since the necessity of achieving stability oscillation and instability are load ellipse will tend to become when feedback is applied across liable to be encountered. circular. the transformer, i.e., that the loop \Vhen 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 KT66 type valves to a operation and certainty of results. core exceeds the safe limit. I5-ohm loudspeaker load. It appears then that the design (e) Harmonic distortion intro Primary load impedance= IO,OOOn of an amplifier for sound repro duced by excessive resistance in duction to give the highest possible the primary winding. · IO,OOO Turns ra t10 = --- = 25. 8 :1 fidelity should centre round a The design of a practical trans J 15 push-pull triode output stage and former has to be a compromise Effective a.c. resistance of valves should incorporate negative feed between these conflicting require = 25000 back. ments. The most suitable types of valve At a low frequency fb' such that Low-frequency Response for this service are the PX25 the reactance of the output trans Parallel load and valve resist- and the KT66. Of these the KT66 former primary is equal to the 2500 X 10,000 ance = = 2000Q is to be preferred since it is a resistance formed by the load 12,500 more modern indirectly-heated resistance and valve a.c. resist fb = 3·3 C/S(Wb=2I) response type with a 6.3-volt heater, and ances in parallel, the output should be 3db down. will simplify the heater supply voltage will be 3db below that at Primary incremental inductance problem. Triode-connected it has medium frequencies. At a fre 2000 characteristics almost identical quency 3fb the response will be L = =95 H. with those of the PX25. well maintained, the transformer --21 Using a supply voltage of some reactance producing only 200phase High-frequency Response 440 volts a power output of 15 angle. Similarly at the high Sum of load and a.c. resistances watts per pair may be expected. frequency end of the spectrum the = IQ,OOO + 2500 response will be 3db down at a = 12500 0 The Output Transformer frequency ft such that the leakage The output transformer is prob reactance is equal to the sum of At ft = 60 kc/s (Wt = 376,000') ably the most critical component the load and valve a.c. resistances. response should be 3db down. in a high-fidelity amplifier. An Again at a frequency ft/3 the 12,500 Leakage reactance incorrectly designed component 376 is capable of producing distortion which is often mistakenly attribu =33 mHo ted to the electronic part of the A 20-watt transformer having 10 amplifier. Distortion producible primary and 8 secondary sections directly or indirectly by the and using one of the better grades output transformer may be listed of core material can be made to as follows :- comply with these requirements. (a) Frequency distortion due Winding data will be given in an to low winding inductance, high appendix (see page II). leakage reactance and resonance A Some confusion may arise when phenomena. specifying an output transformer (b) Distortion due to the phase as the apparent inductance of shift produced when negative ALTERNATING EXCITATION VOLTAGE 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-cored 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 driver stage to the previous arrangement, as in Fig 3 (b), provides a solution to most of the difficulties. Each stage then wor1{S 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 splittcr and push-pull driver stage may c ( ) be combined in a self-balancing " paraphase" circuit giving thc Fig. 3. Block diagrams of circuit arrangements discussed in the text. arrangement of Fig. 3 (c). The grid of one drive valve is fed component is a function of the will result, should a phase shift directly from the first stage, thc excitation, the variation being of 1800 occur at a frequency where other being fed from a resistancc of the form shown in Fig. 2. The the vector gain of the amplifier network between the anodes of exact shape of the curve is and feedback network is greater the driver valves as shown in dependent on the magnetization than unity. The introduction of Fig. 4. This arrangement forms characteristic for the core material. more than one transformer into a good alternative to the preceding The maximum induct:wce, the feedback path is likely to one where it is desirable to use the corresponding to point C occurs give rise to trouble from insta minimum number of valves. when the core material is nearing bility. As it is desirable to apply 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 +------1'--11- transformer the primary induct OUTPUT contain push-pull stages it is ance corresponding to the voltage � desirable that the input and output I-- 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. phase Shift The feedback is taken from the 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 ffedback 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 V2 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. ¶ first two stages are parameters. The cathode bias follows the basic arrangement of thus designed as a single entity. resistor of V1, to which feedback Fig. 3(b). ¶ 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 standardisation follower, its grid being some 100V 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 Á ãode of the first triode is also the phase-splitter is taken to the about 8,000 Û a.c. resistance arranged to be about 100V posi- push-pull driver stage. Provision Initial Stages.— In order to tive and is coupled to the phase- is made for varying the load re- Fig. 5. Circuit diagram of complete amplifier. Voltages underlined are peak signal voltages at 15W output. SEULAV TIUCRIC SEULAV R 1 1 MÛ ¼ watt ± 20 per cent R 15 , R20 1,000 Û ¼ watt ± 20 per cent C 8 8µF 550V, wkg. R 2 000,33 Û 1 ttaw ± 20 " R 16 , R18 001 Û 1 ttaw ± 20 " C 9 8µF 600V, wkg. R3 47,000 Û 1 watt± 20 " R 17 , R21 001 Û 2 ttaw -eriw CH 1 30 H at 20 mA (min.) R 4 Û074 ttaw¼ ± 10 wound variable. CH 2 10 H at 150 mA (min.) R 5 , R 6 , R 7 22,000 Û 1 watt± 10 " R 22 150 Û 3 ttaw ± 20 " T Power transformer. 0.47MÛ ¼ watt ± 20 R , R 100Û ½watt ± 20 R 8 , R 9 23 24 " Secondary 425-0-425 V. R10 093 Û ¼ ttaw ± 10 R 25 1,200 √ speech coil impedance, 150 mA (min.) 5 V. 3A ¼ watt ± 5 R 11 , R13 39,000 Û 2 watt ± 10 " " 6.3 V. 4A, c.t. R12 25,000 Û 1 ttaw -eriw C 1 ,C 2 , C5 8µF 450V, wkg. V1 to V4 L63 .elbairav dnuow .elbairav C 3 ,C 4 0.05µF 350V, wkg. V5 , V6 KT6 .6 R 14 , R19 0.1 MÛ ¼ watt ± 20 C 6 ,C 7 0.25µF 350V, wkg. V7 U52 The Williamson Amplifier 0 � "- -<>/ 5 ,," "/ � .. I S ., -- WITHOUT FEEDBACK 0 I�v I-- -- WITH FEEDBACK � / ... 1 1/ "- 5 I '0 / f---fv O'l J l./ .� �- .�,-.- ./ D V 05 10 IS INPUT VOLTS (R.M s.) 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 c!s with slight overload (2) At 300 eIs, output voltage 15% below maximum. (3) and (4) Conditions as in (I) and (2) respectively, but at 30 c/s. 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 1.7, 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 sole 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, 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 0f 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 reproducing organ music includ application of negative feedback, few degrees. ing pedal notes of the 20 c / sorder, 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 15-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 level 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. APPENDIX. Frequency Response.-The fre If desired, the power output of 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. . the output transformer had a transformer, power supply and t 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 R25 will require to tion. around this frequency. The char be modified. Amplifiers of this Turns ratio = 76 : I. acteristic within the audible range design with power outputs up to Primary inductance= IOO H (min.) from 10-20,000 c / s is linear with 70 watts have been produced. Leakage inductance= 30 mH (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: Ilin stack of Pattern No. 28A "Super Sileor" laminations (:\fagnetic and Electrical Alloys, Burnbank, Hamilton, Lanarks). The winding consists of two identical interleaved coils, each I�in wide, wound on I!in x I!in paxolin formers. On each former is wound: 5 primary sections each consisting '<:811111111 111111111 1111 of 5 layers (88 turns per layer) o 0 0 0 0 o- 0 0 0 of 30 s.w.g. enamelled copper wire q � � interleaved with 2 mil. paper, alter nating with 4 secondary sections, FREQUENCY IN CYCLES PER SECOND each consisting of 2 layers (29 turns per layer) of 19 s.w.g. enam. copper Fig. 7. Frequency response (without feedback) of 20 watt output trans wire, interleaved with 2 mil. paper. former described in appendix. Generator resistance 2,5000 load Each section is insulated from its resistance 1.70. Measured with 5V r.m.S. on primary. At higher neighbours by 3 layers of 5 mil. excitations the bass response improves progressively up to saturation. Empire tape. All connections are brought ont on one side of the wind ing, but the primary sections may be connected in series when winding, only two primary connections per coil being brought out. * Measured Performance. Primary inductance = 100 H. (measured at 50 cls with 5V r.m.s. on primary, equivalent to 2.5 mW) Leakage inductance = 22 mHo (measured at 1,000 c/s) Primary resistance = 250 ohms. (a) Input waveform, 300 c/s. (b) Output waveform with feedback and slight overload. (c) Output waveform with feedback but output voltage * Secondary connectionsfor dtfferent 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 R5, Rp Rll and R". plifier design suitable for high domestic sound-reproducing in A transitional phase-shift net quality reproduction of sound, stalla tion. work consisting of Rz• and ClO' 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, RI2, 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 p hone pickups and microphones of stages, the amplifier is largely this prevents a damaging voltage l ow 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 Fig. I. Circuit diagram of complete amplifier. Voltages underlined are peak signal voltages at IS watts output. RI IMO ! watt ± 20% R14, RID O.IMO ! watt ± 10% C6, C7 0.25J.LF 350V wkg. &2 33,0000 1 watt ± 20% RIo' R20 1,0000 ! watt ± 20% C9 8J.LF 600V wkg. R3 47,0000 1 watt ± 20% RI6, RIS1000 1 watt ± 20% CJO 200pF 350V wkg. R4 4700 ! watt ± 10% RI7, R2I1000 2 watt wirewound CHI 30H at 20mA variable R6, R7 1 watt ± 5°/ CH2 lOH at 150mA 22,0000 /0 R 1500 3 watt 20% (or matched) 22 ± T Power transformer R23, R24 1000 ! watt ± 20% R 22,0000 1 watt ± 20% Secondary 150 mA, 5V, aA, s R20 1,200 v'speech coil impedance 425·0-425V Ra, Rg 0.47MO ! watt 20% 6.3V 4A, centre· tapped ± t watt (see table) I 3900 10% Vj, V2 2xL63 or 6J5, 6SN7 or B65 R O ! watt ± R26 4,7000 ! watt ± 20% Ru,R S47,0000 V3, V4 do. do. I 2 watt ± 5% Cl' C2, Co, Cs 8J.LF 500V wkg. (or matched) C3, C4 0.05J.LF 350V wkg. Yo, V6 KT66 V7 Cossor 53KU, 5V4 14 The Williamson Amplifier 0 V- -- ..... I Z 0 3 I--.. 1/ 1:"- � o / " h- I / i --- AMPLlF IER WITHOUT FEEDBACK "- o . . I AMPLIFIER WITH FEEDBACK 3 / I ..... 2 ...... - -20 / - I -'- 0 1 V 0 0 /' ...-- - O� �:-f 0 8 FREQU[�iCY (cls) Fig. 2. Loop gain and phase-shift characteristics of the amplifier. available when the circuit was C'JTPUT originally published. A list of alternative valve types is also MAlt:£, shown. INPUT Amplitude and Phase I fre· quency Response. A curve show ing the transmission and loop gain of the amplifier at frequencies between 1 cl s and 1 Mc I s is shown in Fig. 2. Although only the sec tion between IQ cl s and 20,000 cl s is useful for sound reproduction, the curves outside this range are included as they may be of in Fig. 3. Suggested terest to those who may wish to layout of principal use the amplifier for other pur components of com poses. They may also serve to bined amplifier and . emphasize that, in a feedback power pack 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 standing 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 grol.: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 followi n g precautions end, is greatly to be preferred to signal balancing is as follows:- should be observed:- the use of the chassis as an earth I. The output transformer core return. Static Balancing. 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 12smA by means of R'l' 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 The Williamson Amplifier and the secondary impedance. being proportional to the square of the turns ratio, becomes 1.7 x 22 =6.8 0. Similarly if three sections are connected in series the impedance becomes I.7 x 32 = 15.3 0. Thus the available secondary impedances, keeping a 10,000 0 primary load impedance, are 1.7, 6.8, 15·3, 27, 42.5, 61, 83 and 109 O. The connections to obtain these values are shown in the table. Should it be necessary, in an CONNECTING LINK 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 Ra until the reading is zero, in high frequencies, and great varia impedances, namely I, 4, 9, 16, dicating balance. Rand 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,. signal levels, due to the use of must remain unaltered, as the Signal Balancing. core material with a low initial turns ratio is unchanged . The (a) Connect the low-im- permeability, or to careless values of R'5 are given in the pedance winding of a small out assembly of the core. The high table. put transformer in the lead to frequency characteristics are not Winding data for an output the centre tap of the output dependent on the core material to transformer to match loads in the transformer. Connect a detector a substantial degree . They are region of 3.5 n are given in the (headphones or a cathode-ray dependent only on the geometry of Appendix and the connections oscillograph if available) to the construction, and to some extent and other data are included in the other winding, earthing one upon the dielectric properties of lower section of the table. side for safety. the insulants used, and are there The two outer layers of the (b) Connect a resistive load fore reproducible with a high output transformer primary should in place of the loudspeaker. degree of accuracy. normally be connected together to (c) Apply a signal at a fre Comments are frequently ex form the centre tap, the inner sec quency of about 400 c / s to the pressed about the size of the out tions of the winding being taken amplifier input to give an out put transformer. It is true that to the valve anodes. This gives put voltage about half maxi it is considerably larger than the the mlIllmum external electric mum. transformers which are usually field. (d) Adjust R" for minimum fitted to I5-watt amplifiers . The Stability with Negative Feed output in the detector. 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 c / s 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 will 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 pri m a r y load impedance is gain-that is the transmission of cured only by altering the time 10,000 0 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 amount of feedback. At high fre is 10,000 0/762 or 1.7 0. When of the amplifier has reached 1800 • quencies the situation is more secondary sections are connected It is also possible for an amplifier complex, 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 occur 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 10-20 c / s, occur. It should be emphasized and, therefore, the time constant would require a transformer with that this will happen only very 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... 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 Primary Inductance time constants, such that its mini teristics which differ substan 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 groups of sections in series 1 2 3 4 5 6 7 8 --- � � ::--.- � Connections U- U- � � -11 -Jq � � � I Jf J� -Jl -l� _'-':� -� --- Correct secondary impedance (ohms) I 1.7 6.8 15.3 27 42.5 61 83 109 Original Output Minimum second- Transformer ary impedance I 1 4 9 16 25 36 49 permissible (ohms)1 64 10,000/1.7n Feedback resistor R25 (ohms) 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 i Alternative Coc",,,hooond,,,, Output impedance (ohms) 3.6 14.4 32.5 57.5 90 130 176 230 Transformer (See Appendix) Feedback resistor 10,000/3.60 2,200 4,700 6,800 9,000 11,500 13,500 16,000 18,000 R25 (ohms) . i Turns ratio ~52.5 26.25 17.5 13 10.5 8.75 7.5 6.5 I 11 I 17 The Williamson Amplifier below ID cl s. I t 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 Capacitive Loads which is most likely to show a 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-c /s rectifier heater wind phase-shift network consisting of little justification of this modifica ing of the mains transformer and R,. and Cl. in conjunction with tion on economic grounds, as the measuring the current in it. The the output resistance of V l' 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 mA gain at frequencies from 20 kc I s mon bias arrangement shows a a.c. range of a Model 7 Avometer, upwards without affecting the definite saving. should be 150 }lA 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 }lA. work consisting of a capacitor the effect of frightening those who If the output transformer is shunting R'5 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 1800 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'5 the oscil across R'5' 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 const'ructed 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'5 to reduce the loop gain . If such as the U5e of an output trans instruments are available, the former with completely different loop gain may be measured by high - frequency characteristics. disconnecting R'5 from the The writer has no information APPENDIX cathode of V I and reconnecting it about this. Output Transformer with 3.6-ohm via a 470 D± 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 : I!in. stack of 28A Super junction of R'5 and the 470 D 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/s and IO 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 I�in. wide on paxolin surement is being made. work is made sufficiently long to formers Itin. X Iiin. inside dimen The adjustment of the loop gain ensure that the low-frequency per sions. On each former is wound 5 primary sections, each con to its correct value at medium formance of the amplifier is un sisting of 440 turns (5 layers, 88 frequencies should render the 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- 18 The WilIiamson Amplifier 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. \V in Why the WILLIAMSON AMPLIFIER should eDlploy PARTRIDGE TransforDlers THE widest possible audio range-the lowest possible distor tion and an output of 20 watts . . . these critical demands of the designer of this now fa mous Amplifier implied the finest that technical skill and craftsmanship could provide fo r every component. Little wonder that from the inception of the Williamson Amplifier in 1947 Partridge have specialised in the transformers and chokes. The all important output transformer was the special care of Partridge and this "Williamson specification " component is now available fo r a varied range of impedance. CA model is also available fo r American 807 tubes, see the modified circuit in "Audio Engineering," November 1949.) All secondary windings are brought out as eight separate sections of equal impedance. Stock types comprise 0.95 ohm, 1.7 ohm, 3.6 ohm and 7.5 ohm sections ; this latter giving a 500 ohm secondary for American requirements. The Partridge " Williamson " Output Transformer is acknowledged the most efficient of its typ e. Available Technical data sheets with fullest details UNPOTTED (Style VDNj436B) (complete with alternative mountings) are available on request. Also available, complete or catalogue of the Partridge range including the POTTED (Style VDNj436B) mains components for this and other amplifiers. * IMMEDIATE DELIVERY can be made, PMliIPOGIJ and these components are ready for shipment to all parts of the world. TR ANSFORMERS LTD ROEBUCK ROAD : KINGSTON BY-PASS TOLWORTH : SURREY : Elmbridge 6737(8 19 The WiIliamson 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 M tended for sound repro to say that the matter is one in ameliorated . In addition, fixed 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 uniform 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 I frequency and sound and provides a common fects as are likely to occur in the phase / frequency response charae- basis for the design of individual units, which, when connected to gether, will provide a complete INPUT channel with a uniform gain / frequency characteristic. Considerations of an engineer 5 RISE l ing nature sometimes make it de r-----���__ JT REBLE sirable, and even essential, to FALL depart from this ideal of a uni form response in certain sections VALVE of equipment, and quite fre GRID quently the use of inferior equip MIN.TREBLE MAX ment or long and unsuitable trans mission lines leads to an undesir FALL able departure from uniformity . In cases like this, other "equal RISE izer " units have to be inserted in the channel to provide character istics which are the inverse of those of the offending section, so Fig. 5. Basicfrequency compensation circuit. Typical remedying the defect. values (for use after an EF37, triode-connected) are : When listening conditions de �O, 250kO, log ; Rfh IookO ; R42, 6.8kO ; R4,3, part from the ideal-and this, un IokO ; �4' IookO linear. C20, 150pF max. ; Cn, fortunately, happens frequently o.oIf'F, C22 0.o5f'F ; C2a, IooopF. since most rooms are unsuitable auditoria for the reproduction of orchestral music at realistic in tensities-it is sometimes bene � \ , 1 : I1 11 - ficial to modify the frequency re + 20 �� sponse characteristic of the equip �� I DECRE ment in an attempt to compensate - INCREASE C zz I � i I'-..A I for the more obvious defects in the .D + I I I � ID I V room acoustics. The word 52 RISE 5 RISE Z "'- V R40 MAX ...... 11 R44l MAX. "attempt " is used advisedly, '";c ,./ since only very complex equaliza '" R40 MIN...... R44 MIN. > _I 1 - - tion could ever hope to provide ;:: B/ ...... 1 � D - accurate compensation for room � �52FALL R40 MA� FAL� acoustics. This question of the -ID - 3 ...-- 5 frequency compensation which is I�CIR�A,S,E C21 R44 MAX.1 "- desirable when conditions depart C from the ideal is a very thorny 11 I11 �cRillE u" 0 0 0 0 o and subjective one. It provokes 0. much heated, dogmatic , and « /s) usually very unscientific discus fREQUENCY sion, and is beyond the scope of Fig. 6. Response curves of circuit of Fig. 5. 20 The WiIliamson Amplifier teristics, but the equipment to do this is complicated and expensive. When a considerable portion of the channel is outside the control of the listener, as is the case records or when reproducing CURVE LOOP GAIN A = 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 � 90° :rt; have occurred in that portion. the response rises to a fraction of teristic and the type of pickup its value below resonance and then used. 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 proportional to signal above this FREQUENCY 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 FREQUENCY shown the recording characteristic satisfactory . To attain the high Fig. 10. Final low-pass charac used by Decca. The KM,I, char attenuation rates necessary to se teristic resulting from addition of acteristic does not differ substan cure satisfactory results a norma} 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 disadvanta.ges 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 amplifiersto 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 satisfactory 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 IOo 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 lines, 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, IO and 13 kc /s 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, I I. 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. 'Thiessen, G. J. -, R·C Filter Circuits." compensation is required . The Noise Level.-The attainment Journal 0/ the Acoustical Society 0/ nature of this compensation will of a low noise level in high-quality America. VoL 16, No. 4, pp. 275-279 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 It 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 elec tro n partition between screen High - Pass Characteristic. applied to the valve by the and anode. In fac t, ho wever, Gramophone mo tors tend to pro po tential divider formed by R,. there are no high-gain trio des duce vibrations whic h can caus e and the impedance of CH' C1 5 commerc ially available with th e unpleasant rumbling noises in a and R33• At medi um frequencies requisite characteristics and elec wide-range system. Although the the reactance of C,. is small, and tro de structures for low-noise energy contained in the " rumble " that of C' 5 large compared with operation. A valve des igned fo r compo nents may be relatively the resistanc e of R33 and RH' and such conditions should have a lo w, the frequency is also very the gai n of the stage is determined rigidly brac ed elec tro de structure low, and consequently loud by the values of these resistors . to reduce microphony and a speaker cone mo vements of high As the frequency is lo wered the balanc ed " double helical" heater amp litude may be caus ed. If the impedance of the top limb in construc tio n to minimize the alter driving coil should move out of creases, giving a progressive re nating field surro unding the the regio n of uniform flux-dens ity, duc tion of feedback. This pro cathode. The Mullard EF37 has th e who le spectrum being repro duces a gain / frequency charac this construction and, connec ted duc ed will be disto rted in a par teristic whic h rises to a maximum, as a pento de, the no is e levels men tic ularly unpleasant manner. Dis determined by the circ uit con tio ned earlier are obtainable. Be tortio n in the outp ut transformer stants , and then dec reases due to fore commenc ing wo rk, the reader is also possible. the coupling components Cl &' Ra. who is no t familiar with the tech This situation can be imp roved and R3•• With increasing fre nique of high-gain amplifier con materially by the ins ertio n of a quency the imp edance of C'5 de struc tio n should consult an article high-pass fil ter with a cut-off fre 3 creas es , increasing the negative on this subject?' Co ns iderabl e quenc y of about 20 c / s and a feedback and produc ing a falling reduction of residual hum may fairly rap id attenuation belo w cut gain / freq uenc y charac teristic . usually be obtained by demagnet off. At thes e low frequencies , The capacitanc e between the iz ing the valve.' In order to such filters are convenientl y com inp ut transformer secondary wind obtain the best signal/ noise ratio, posed of resistance-c ap ac itance ing and earth may, if large, affect the princ ip le 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-compens ation pre end of the audible spectrum. This limiting fac to r in high-gain ampli amplifier. effect is negligible with a well fiers, is to put the whole of the Electrical Fading Control. des igned compo nent, but lo ng available signal into the valve When the pickup is placed on, or leads should be avoided. The grid, and to provide any fre remo ved from, the disc the gain transformer should be mo unted quency compensatio n whic h may mus t be reduc ed to avo id un on the pre-amplifier chassis, be nec essary after the signal has pleasant no ises. While this may which in turn may conveniently been amp lified. By this method be done by a mechanic al poten be fixed beneath the mo tor bo ard. valve noise is included in any tio meter the method is clumsy The overall characteristic with attenuating operations whic h may and does not facilitate rapid re an input fro m a perf ec t be perfo rmed and the overall sig cord chan ging. It has been found " velocity " pickup on a Decc a nal no is e ratio is imp ro ved . convenient to employ an el ec tric al di sc is shown in Fi g. 14. Low Distortion. - Numerous metho d in which the gain of one A mo re complex circ uit, whic h methods of providing a response of the stages is reduc ed to zero at gives nearly perfect compensatio n whic h varies with frequency are the flick of a switc h by a bias volt and a very rap id attenuation possible and, of course, each age applied and remo ved by (3 0 db/ octave) belo w 20C/S, is metho d has advantages and dis means of a network with a suit shown in Fig. 15. This pre advantages. Where the response able time constant. amplifier has a higher gain than has to be continuously variable the previous one, and is particu the method which gives greatest Pre-Amplifiers larly suitable for us e in equip simp lic ity of contro l usually Although all the refi nements ment where the pic kup is loc ated tri ump hs . Other things being outlined so far are desirable, in at some dis tance from the rest of equal, ho wever, methods whic h dividual requirements will vary the amplifier as the circ uit ter employ selec tive negative feed considerably and will determine bac k are to be preferred, as cir ho w much complication should be minates in a catho de follo wer. cuits of this nature generally have at temp ted. Two gramophone The construction of this circuit a high signal-handling capacity pre-amplifi er circ uits will there is no t recommended for those and non-linear disto rtio n is kept fore be described, which should witho ut access to facilities for to a mllllmum. In a pickup pre cover most requirements. chec king the response of the amp lifier this may be of import Fi g. 13 shows a simple circuit finished unit, as the perf ormanc e ance where pickups with widely which gives good compens ation may be seriously affec ted by an varying outp ut levels are to be for the Decc a recordi ng charac error in component values. used. teristic. The circ uit constants The frequency charac teristic of have been adjusted to give as this amplifier is produc ed by the I Baxandall, P. J., " Hum in High Gain Am plifiers." Wireless World, VD!. 53, No. high a degree of attenuatio n combinatio n of two curves shown 2, pp. 57-61 , February. 1947. . A. F., "Hum ReductIon," belo w 20 c / s as is consistent wi th at A and B in Fig. 16. These, • Dickerson, Electronics, VD!. 21, No. 12, p. 112, De- simplicity. This invo lves a slight when added, give the curve C. cember, 1948. ... ndence. Eledron!c Engzneenng, sacrifice of the response at 20 c / s. Curve A is produced by the cir 4 Correspo VD!. 20. No. 245, p. 235. July, 1948; No. The metho d of operatio n is as cuit assoc iated with V13, which is 248, p. 339, October, 1948; p. No. 250, follows : Negative feedback is similar in principle to that of 406, December, 1948. 24 The WiIliamson Amplifier Fig. 13· Simple List of Components for Fig. 13. gramophone pre- 350V Type Rating Tolerance amplifier designed for the Decca re- RZ7 Value to suit High·stability RSI transformer carbon cording character- istic. When playing R28 0. 1 MO do. �W E.M.I. records R29 0.68 MO do. !W C15 may be switched out R30 0.22 MO do. !W of circuit. Alter- R3l 47 kO do. !W natively, compensa- R10 CIl R32 4.7 kO do. tion can be effected R 29 C'6 R33 0.22 MO Composition 10% in the tone-control R34 22 kO do. 10% circuits. R35 2.2 MO do. C'4 All resistors may be iW rating tolerance 20% unless VI otherwise specified. ..., C ...,�z .... Rating '5 �VI (V d.c. 0% .... 2 Type R1S working) Tolerance .� 0.5 p.F Paper 250 . R1S S!< 011 :=� 012 50 p.F Electrolytic 12 5� 013 16 p.F Electrulytic 450 0 .... C14 4000 pF Silvered mica 350 10% CII 015 100 pF Silvered mica 250 10% R2I Rn R14 016 0.05 p.F Paper 500 Fig. 13. The attenuation at low frequencies is due to the combined -. effect of the intervalve couplings. ... ." ...... - ...... Curve B is produced by feedback z: /' C over V 14 through a parallel-T net .,. V work tuned to 20 c / s. � -I ;:: The overall frequency response c ....0 curve, taken under the same con � ditions as that of Fig. 14, is shown -2 0 0 g o in Fig. 17. q g. o Fading Control.-The circuits �N of Figs. 13 and 15 have no pro FREQUENCY (e/' ) 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 I Fig. 13 or VIS in Fig. 15, enables A very carefully designed and (Fig. 13) has a gain of 11 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 S5 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, 350 V at 20 mA. and the stage to about 150. 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, 11, 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 R57 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 p.V 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 I,ooo c/swhen 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 WiIliamson Amplifier 350V R" R'l ... R .. R .. R •• R" R7I R 78 Fig. 15. Pre-amplifier with high-pass filter. Component Values for Circuit of Fig. 15. Type Rating Tolerance Type Rating Tolerance 0.22 MO RS8 Value to suit High-stability R79 Composition 20% tmnsformer carbon Rso 10 kO do. lW 20% R 0. 1 MO do. W 20 * May require adjustment. o9 t % Reo 0.68 MO do. tW 20% All resistors may be tW rating, except where other- Ru 0.22 MO do. iW 20% wise stated. R62 4.7 kO do. 20% Hating R6s 0.22 MO Composition 10% (V d.c. RM 20 kO* do. Type working) Tolerance 22 kO High-stability W 20 C 0.5 f.LF Paper 250 20 R65 ! % so % carbon Cn 50 f.LF Electrolytic 12 R66 0.22 MO Composition 10% C52 16 f.LF Electrolytic 450 R67 0.20 MO* do. C53 0.02 f.LF Paper 350 10% R6s 4.7 MO do. 5% CM 4000 pF Silvered mica 350 10% R 1.0 MO do. W 20 C 100 pF Silvered mica 350 10 69 l % S5 % R70 0.22 MO do. tW 20% CS6 0.5 f.LF Paper 250 20% R 2.2 kO do. 20% C57 50 f.LF Electrolytic 12 n 0' R72 2.0 �1O Righ-stability carbon 1 /0 C5B 0.01 f.LF Silvered mica 350 1% or matched or matched 2.0 MO do. 1 C 0.25 f.LF Paper 500 20 R73 % 59 % or matched C60 5000 pF i:lilvered mica 3':;0 1 % 1 R74 1.0 MO do. % or matched or matched C61 5000 pF Silvered mica 350 1 % R75 10 MO Composition 5% or matched R76 47 kO do. 10% C62 7000 pF Silvered mica 3;-;0 10% 20 0.5 f.LF Paper R77 lkO do. % C63 500 20% lW 20 16 Electrolytic R78 47 kO do. % CG4 f.LF 450 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 ISin x 14in x The amplifier and loudspeaker taste . The suggestions which 20in 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 pick up 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 Williamson Amplifier the necessity of rising from one's / --, 350 V9 comfortable seat to attend to the \ r I controls or change a record. The main amplifier may be included IF;-'� C.7 Re4- Re3• ' --IL WILLIAMSON'S in the console, but this tends to �1-- 1 11 I Rei make it heavy and bulky, and Rez gives rise to problems of heat dis I I OFa I 5 O.P. sipation which are not easily �l, J }>'ON �� solved. I �: Acknowledgment.-The writer I I l' ! TRANSFORMER is greatly indebted to Ferranti, , I c •• Ltd., for permission to publish Fig. 18. Circuit of fading control. To Author's the results of work undertaken on their behalf, and wishes to Specification thank his colleagues for help List of Components for freely given. Fig. 18. Rating £4-13-6 RS1 0.22 MO �W c RS2 0.22 MO tW B RS3 47 kO :z: RS4 100 0 CHOKES FOR WILLlAMSON'S .., (Vd.c. SOH at 20 m/a. · . 22/- working) -. L-__L- ______250 350 MAINS TRANSFORMERS FREQUENCY 350 Fig. 16. Derivation of high-pass FS43. I n put 200/250v. characteristic. Output 42s/0/425v. at 200 m/a. 6.3v. 4 amps. CT. 6.3v. 4 amps. CT. sv. 3 amps. Fully Shrouded . ... 51/- W. I. Input 200/250v. Output 325/0/32sv. at 20 m/a. 6.3v. 0.6 amps. 6.3v. 1.5 amps. Chassis mounting ...... 23/- FREQUENCY (cls) Fig. 17. Response curve of circuit of Fig. 15· H. ASHWORTH OTHER "WIRELESS WORLD " REPRINTS Receiver Alignment Equipment : 1. Simple Cathode-Ray Oscilloscope (March 1950). 2. Design fo r a Wobbulator (October 1950). By M. G. Scroggie, B.se., M.LE.E...... 9d. net. By post lO!d. 676, GREAT HORTON Communications Receiving Equipment : 1. Ex-R.A.F. Communi cations Receiver (July 1946). 2. Band-Pass Converters (October 1950). 3. More about Band-Pass Converters (February 1951). 4. 21 mlcs Band Pass Converter (July 1952). . Is. net. By post Is. Hd. ROAD Radio Feeder Unit : High Quality Pre-tuned Receiver with Gramophone Pre-amplifier. By J. F. O. Vaughan (December 1951). 9d. net. By post lO!d. Television Oscilloscope : Simple Design with Five-Inch Cathode Ray Tube. By W. Tusting (June and July 1952). 91. net. By post lO!d. BRADFORD Midget Three-Valve A.C. Mains Receiver : Long and Medium Wave T.R.F. set. By S. W. Amos, B.Se. (HONS.) (February 1950). 6d. net. By pOSt nd. Sensitive T.R.F. Receiver : Embodying Automatic Gain Control. By S. VOR K S. W. Arnos, B.se. (HONS.) and G. G. Johnstone B.se. (HONS.) (October and November 1951)...... Is. net. By post Is. l!d. Obtainable direct from : 'Phone : BRADFORD 71916 ILIFFE & SONS LTD., DORSET HOUSE, STAMFORD ST., LONDON., S.E.1. 27 The Williamson Amplifier '50V Rs Rn o C C2 l9 0 INPUT FROM RSI C,s PICKUP OR R PRE -AMPLIFIER 3I C'4 R47 R48 Fig. 19· Complete tone compensation and filter unit. The input and output voltages underlined are peak values for full output from the main amplifier. List of Components for Fig. 19. Rating Tolerance Rating (V d.c. Ra6 0.25 MO log. l Type working) Tolerance R3; 47 kO W l Silvered mica 5 R38 47 kO W % do. 5% Ra9 3.3 kO do. 5 R40 0.25 MO log. % do. 5 RH 100 kO % do. 5 R42 6.8 kO % 10 kO do. 5% R43 do. 5 R44 0. 1 MO linear % Electrolytic 12 20 R4s 100 kO lW % Paper 500 R46 2.2 kO 0. 1 MO 10% Electrolytic 450 R47 { Silvered mica l R48 0.47 MO 10% % 10 do. 1% R49 0.47 MO % do. or 1 Rso 33 kO lW % do. matched 1 R51 100 kO lW % 3.3 do. 1 R52 kO % 1 MO do. 1% RS3 { do. 1 RS( 0.1 MO } High- or 1% % stability matched 1 do. Rss 0. 1 MO % carbon 1 Electrolytic 450 RS6 50 kO % 100 0 do. 500 RS7 All resistors may be !W rating, tolerance 20% unless otherwise specified. Choke. CHa 50H at 20 mA. Resistance about 1,500 O. Rating (V d.c. Mains Transformer. Type working) Tolerance Primary : 10-0-200-220-240 V, 50 c/s. JLF Electrolytic 12 Secondaries : 1. 325-0-325 V, 20 mA d.c. Cl; 50 C 8 JLF Electrolytic 450 2. 6.3 V, 0.6 A. 18 6.3 JLF Paper 500 20 3. V, 1.5 A. C19 0.25 % C20 150 pF max. Preset Switches. C21 0.01 JLF Paper 250 20% SI' Single pole double throw. C22 0.05 JLF do. 250 20% S . Double pole double throw. C23 1000 pF Silvered mica 20% t S . Single pole double throw. C24 50 JLF Electrolytic 12 a S ' 5 bank, 5 position selector switch. C2S 0.05 JLF Paper 500 20% 4 The WiIliamson Amplifier THE WILLlAMSON AMPLIFIER DESERVES JOINTS SOLDERED WITH OUTPUT RSS AHPl?FlER One imperfectly soldered joint may endanger the successful assembly of the Radio Feeder Un it (see page 30 10r general detail ) Williamson Amplifier. Solder with s Multicore and run no risks. Multicore contains 3 cores of extra-active, non ADDITIONAL COil DATA corrosive Ersin Flux ensuring speedy and reliable soldering without waste or The radio feeder unit described struction of coils for the reception trouble and guaranteeing that there are on succeeding pages was designed of the Droitwich transmitter on no lengths of solder without flux. Correct originally to provide high-quality 200 kc /s, and the author has sup reception from medium-wave sta plied the following additional data proportions of both flux and solder are tions and coil-winding data covered automatically applied. Fast-acting, fa st for those who get a higher signal a range of frequencies from 500 kc / s holding Multicore is used exclusively strength for the B.B.C. Light Pro to r.6Mc/s. by leading manufacturers of radio, T/V gramme from the long-wave trans Since then there have been many and electronic equipment. Make certain requests for guidance in the con- mitter. of a good job-with Multicore. COIL-WINDING DATA FOR THE LONG-WAVE RANGE THE SIZE 1 CARTON (shown above) has been designed sp ecifically fo r easy use. Simply pull out the length you Inductance Coellicient require. C16018 sp ecification (60/40 Transformer Winding No. of turns (,uH) of coupling alloy) is particularly recommended fo r (approx.) the Williamson Amplifier 180 750 Primary RADIO &; T/V SERVICE Aerial 0.3 ENGINEER'S lb. REEL. Secondary 330 2,000 1 For u;es which demand Primary 260 1,500 Coupling 0.6 fair quantities of solder. Contains approximately Secondary 330 2,000 167 feet of 18 S.W.G.5OJ50 alloy Ersin Multicore Solder. Coils are wound with 40-42 s.w.g., used, the minimum capacitance of the d.s.c. copper wire. ganged capacitor should be increased Ersin Multicore Solder can be obtained To give the correct coefficient of by the addition of a IOopF silvered fr om radio shops everywhere. Size I coupling the spacing between the mica capacitor across each secondary carton 5/- retail. I lb. reel 15/-. windings of the aerial transformer winding of the transformers, giving a should be increased to 0.2sin. The coverage of approximately 150-300 disposition of the coupling trans· kc/so MULTICORE SOLDERS LTD. former windings is unaltered. For fixed tuning, the capacitors MULTlCORE WORKS, MAYLANDS AVENUE, When continuous tuning is to be should be 300 pF. HEMEL HEMPSTEAD, HERTS (BOXMOOR 31136) 29 The Williamson Amplifier Design for a Radio Feeder Unit HE preceding articles in this transmitters, and which desires offered as an indication of the T series have described ampli only to receive transmissions from general lines on which to proceed, 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 na tive 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 snlts, 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 wonld be unnecessarily say 200 kc /so 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 +350V radio receiver. Posi- tions of selector Re6 switches for pre-set EF39 or tuning shown at X. KTW 61 T, C70 Res Re9 ::J C74 R92 C7S R94 OUTPUT C69 COMPONENT VALUES FOR CIRCUIT OF FIG. 2Cl Rating Type Rating (V d.c. RS5 0. 1 MO working) RS6 0.1 MO t w See text RS7 330 0 O.lfLF Paper 250 Rss 1.5 kO O.lfLF Paper 350 RS9 0.1 MO O. lfLF Paper 350 R90 10 IrO 2 W 16fLF Electrolytic 450 R91 47 IrO 100pF Silvered mica R92 4.7 IrO 100pF Silvered mica R93 22 IrO O. lfLF Paper 500 R,.4 2.2 MO All resistors may be ! W rating, tolerance 20 per cent unless otherwise specified. The Williamson Amplifier push-button or rotary switch . WINDING DATA FOR R.F. TRANSFORMERS The use of variable inductors in this arrangement provides a simple Coefficient method of achieving a uniform Transformer Winding No. of turns Inductance of coupling selectivity and sensitivity over the (ILH) (approx.) range, with the disadvantage that two coils or tuned circuits must be Primary 35 30 provided for each station to be Aerial 0.35 95 160 received. In the unlikely event of Secondary serious thermal drift, correction is Primary 60 80 easily applied by the use of nega Coupling 0.65 tive temperature coefficient capa Secondary 95 160 citors. 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 V,.' 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 Construction.-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 /so 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. 2I l! L against transmitter frequency. This curve has been cornputed for L an L/C ratio of unity (L in ILH, 1 V C in pF), which is nearly opti 30 0 � lL mum. The number of turns V necessary to produce the required "," � / !<.. inductance with the formers and "7 / � ,:$>'L 1 , dust-cores specified may then be � / ':'� ",,->.'<' ,:,'"r--- j--- obtained from Fig. 22. The �/ <:S / � � x",'v V /, !.'!-"- 40 0 I / <..<:S I � V :r:� r-� f- r- e- / // ::t u... �j- - )=- ;- c-!� r- � .,::; � j t- / V- w - -1-- - , u - I H��� r I w :z i '" u 300['0.,,' / // :z I t:; '" >-- I V [/ i :::> rt-H- I+ u 100 §! '" ./ Q. � I >- '" I u er'" 1"""'- I I 20 Cl J--" :z 0 :z 0 Z I :::> I � >-- Vl : - 1 I t-.... 100 r- o 500 1,000 1,500 1,600 70 80 90 100 110 120 130 140 FREQUENCY (kefs) 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 text. 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 ---.. � SLOTS CUT WITH tuning arrangements. "'- . .. /�. THIN SAW The Detector.-To give low distortion, the detector requires to work at a fairly high signal level-say 5V r.m.s. output. As the receiver is intended to feed the MATERIAL BAKELIZED FA BRIC ; 0'01 S .. OR PAXOLlN tone compensation unit, which requires an input of only 200 mV peak, the output is taken from a AER IAl 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.-(r) Set ganged capacitor at a position about five degrees from the mini PR I. ===:!!!!P===f'!!�= 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 WINDING. tance position and adjust dust CONNECTION) cores for maximum out NUMBER OF SECONDARY TURNS 'I, NUMBER OF SECONDARY TURNS 2{, put from the low-fre PRIMARY WINDING. (70ftFEEDER ) SECONDARY WINDING. SEE TEXT quency Third Pro- gramme. '{,o NUMBER OF SECONDARY TURNS r·�----- S'{ "------.4 (3) Repeat this pro SECONDARY WINDING. SEE TEXT 2 CONNECTOR BLOCK cess until both stations '\ are accurately tuned. Power Supplies. Fig. 23. Formers are standard 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. IN Fig. 24 view of top [0 of chassis. 4 Swi tched t::;:;:�::::::::::=��L SWITCH WAFERS model. to be supplied from the s'{;------� pre-amplifier power BLOCK t8 supply. The decoupling is not adequate to enable it to be fed from the main amplifier supply. Acknowledgment. - The writer ALUMINIUM OR COPPER SCREEN is indebted to Mr. A. T. Shepherd EXTENDING BELOW CHASSIS C of Ferranti, Ltd., for his assis n WITH AND ABOVE CHASSIS tance in the compilation of data G C OU3'{�'FOR GANGED CAPACITOR . I/ UT T h.,--l-.I'-...-=-....,...... , for these notes. CHASS IS 1'/;' DEEP cW Fig. 24. This diagram shows a plan view of top of chassis. 32 High quality output TRANSFORM ER It gives us great pleasure to announce our latest output we know, unique and permits of seven different transformer for the "Williamson " amplifier. The secondary impedances with virtually no change in result of some painstaking research, this instrument leakage reactance as against the normal four obtained represents quite a considerable advance on our 2B36B. with customary arrangements of eight sections. Typical technical details are as follows : In the 3C67A these impedances are 0.45n, 1.80, 40, 7£2, II 0, 160 and 300. One additional imped Primary ance-220-can be obtained if required with an D.e. Resistance 900 + 900. increase in leakage reactance of about 10 per cent. Incremental inductance taken at 5 v. 50 c,'s., 100 hys. minimum. Frequency Response Nominal impedance 10,0000 centre tapped. With no feedback and assuming a generator impedance Leakage reactance tested at I v. 800 cls. : of 2,5000 the frequency response, even at incremental Whole Primary to the secondary connected fo r 30[1 levels, is virtually flat (0.25 d.b.) from 12! cis. to and short-circuited, 16 m'Hys. Insertion loss 25 kc s. There is a smooth high lrequency "roll 0.5 d.b. (6t per cent.). off " with no objectionable resonances. Half primary to the 300 secondary, 8 miHys. Half primary to the other half short-circuited, Power handling capacity 17 m/Hys. At 15 watts at 12t c!s., the core material is being worked at 10,000 gauss and the distortion introduced Secondary by the iron at this fr equency and power level without Considerable thought has been given to the question any fe edback should not exceed 2.5 per cent. of secondary impedances, and we have decided that, At 25 � the transformer will handle 60 watts with for Loudspeaker matching, the most useful range is half this distortion, and in either case quite a modest given by a basic impedance of 0.450. amount of feedback will reduce the distortion to The arrangement of secondary sections is, so far as negligible proportions. ". . . As Mr. Williamson himself observes in one U outstanding results " of his Wireless Warld articles, the output trans former is probably the most critical component in a high-fidelity amplifier. Your transformer performs this exacting duty with full honours, �"� FOq",,,, and I shall always recommend its use to anyone undertaking construction of a "Williamson." (SAVAGE '-:�DEVIZES) With many thanks for a first-class job." E. Swindon. ('M 'Tt\) L. J., NURSTEED ROAD DEVIZES WILTS Telep hone Devizes 536 33 The Williamson Amplifier Replies to Queries Raised by Co nstructors HE series of articles recently published on the to adjust the anode currents to equality, but unless High-Quality Amplifier has aroused consider the transfo er has a split primary winding they T . � able interest and given rise to correspondence. are mconvement, 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 �ause oscillation which could give misleading read pondence, may be of assistance to readers who have mgs. similar difficulties. Construction.-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 mA The method of construction of the preamplifier to IIO mA by adjustment of R The use of these and tone-compensation units will usually be adapted 21 • valves with reduced rating entails a slight reduction to individual circumstances. One suggested method of the maximum output. The 807 may be used at ?f 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 gin x 3in x lin. The valve connections. valves and electrolytic capacitors are mounted in a Since the articles were written , a modification of group along the centre of this chassis, and the other the EF37 has appeared under the nUIilber 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 V'3' 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 . �he chassis at one end. With the sizes given, there IS ample room for a screened component of dimen Transformer.-When assembling the core Output sions up to 3in x 3in x 2in. The whole unit should of the transformer, care should be taken to ensure be fitted with screening covers, and mounted on the that the edges of the T and U laminations butt to underside of the motorboard as close as possible to gether. The magnetic properties of the core are de the pickup . pendent upon careful assembly and tight clamping. The tone compensation unit of Fig. 19 may be constructed on orthodox lines, the only essential being Static Balancing.-The method of balancing the 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 re sistances should 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 Wireless Warld.-Er>. 3rd Edition. Compiled by the staff of WIRELESS WORLD. Gives the RADIO VALVE main characteristics and base connections of over 2,000 types of British and American radio valves, and over 150 cathode-ray tubes. These are further classified into obsolete, replacement or current types as recom DATA mended by the makers. 80 pp. 3s. 6d. net. By post 3s. IOd. Characteristics of 2,000 Receiving Obtainable fro m all booksellers or direct from : Valves and C.R. Tubes ILIFFE & SONS LTD., DORSET HOUSE, STAMFORD ST., LONDON, S,R1. 34 The WilIiamson Amplifier Modifica tions fo r High-impedance Pickups and Long-playing Records HE 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 hig�-freque�cy T 1949, of gramophone pre-amplifier circuits for peaks which are greater than those expenenced 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 modlficatlOns energy level of music decreases with increase of fre necessary to provide alternative standard and long quency above about 500 c / s. 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 Cl5 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, C15 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 Cl. has been high orchestral levels, recorded waveforms with radii altered, so that the whole of the feedback netwcrk 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, Cl7' 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 fo r current Decca produce cymbals exceeds that normally required at long-playing records. medium frequencies. +20 This early experience has led to I the adoption of a characteristic which is a better compromise I V >- , between these conflicting factors to and gives much more satisfactory u i-- Cl o I - .... I I- results in practice. Fig. 26 shows > I 1--1- the provisional recording charac 8 <:> i teristic now in use by the Decca 15 -10 J--- I Record Company for L.P. � - -f-+-+1 a: records. The amount of treble -20 I boost is lower than the theoreti o o 0 0 o N o o o o o o cal optimum, but the use of even � o o this amount of compensation fREQUENCY (c/s ) N 35 * * * • * * * ADVANCED THEORY OF WAVEGUIDES By RADIO INTERFERENCE SUPPRESSION : AS L. Lewin. Sets out the various methods that have APPLIED TO RADIO AND TELEVISION RECEPTION. 2nd been found successful in treating the types of Edition. By G. L Stephens, A.M.l.E.E. Explains problems arising in waveguide work. the principles of suppression technique as applied to 30s. net. By post 30s. 7d. both radio and television reception and gives many examples of practical applications. RADIO DESIGNER'S HANDBOO K 4th Edition. Editor : F. Langford-Smith, B.SC., B.E., Senior 105. 6d. net. By post 105. lId. Member I.R.E. (U.S.A.), A.M.I.E. (AUST.). A com SHORT-WAV E RADIO AND THE prehensive reference book, the work of 10 authors IONOSPH ERE 2nd Edition. By T. W. Bennington. and 23 collaborating engineers. 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ILIFFE & SONS LTD., DORSET HOUSE, STAMFORD ST., LONDON, S.E. l 36 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. IS, 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 I: I 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 pick up directly in place of the transformer secondary. The limit 3<;0 V ing factor will be the capacitance between the leads and their screening, which will be shunted across R or R64, and which, if sufficiently l�ge, would upset the treble compensation. The value of this stray capacitance shoul� not be allowed to exceed 50 pF, and If C'5 or C55 is switched out, should be compensated by a capacit ance of one tenth of its value in parallel with R33 or R63, to give a linear frequency-response character istic at high frequencies. Resistors R27 and R.; s must be Left : Fig. 27. Simple two-position switch ing in Single-valve pre-amplifier fo r playing Decca 78-r.p.m. standard and 33t-r.p.m. L.P. records. Compensation fo r the E.M.I. 78-r.p.m. standard charac teristic should be applied separately by the treble tone control. Below : Fi�. 28. Alternative circuit (applic List of Components for Fig. 27 able to Figs. 27. 29 and 30) with three POSition switch giving compensation fo r Type Rating Tolerance Decca 33t. Decca 78 and E.M.I. 78-r.p.m. High-stability Value to suit recording characteristic!. transformer carbon R28 O.IMf.! do. CI60R Cn R29 0.68Mf.! do. Rao 0.22Mf.! do. ���----�-----+- R3l 47kf.! do. Ra2 4.7kf.! do. 10 % Raa 0.22Mf.! Composition O·OOlSJLF R34 22kf.! do. 10 % Ra5 2.2Mf.! do. All resistors may be lW rating, tolerance 20% unless otherwise specified. Rating (V d.c. Type working) Tolerance " C O.5ftF Paper 250 R ll OR CI2 50ftF Electrolytic 12 Rn CIa 16ftF do. 450 CI5 100pF Silvered mica 250 10 % CI6 0.05ftF Paper 500 C 10-50pF Silvered mica 250 n R'4 C78 2500pF do. 250 10 % OR C79 1500pF do. 250 10 % Ru. C80 300pF do. 250 10 % S6 Single-pole changeover switch 37 HANNEY OF BATH WILLlAMSON AMPLIFIER COMPONENTS 12 I % silver mica condensers for Small quantities of WOCEN mains trans PARTRIDGE type WWFBjOj l.7 fig. 19 ...... 15 0 formers, chokes and ouput transformers output transformers unpotted, 127 0 7 5% silver mica condensers for are avai lable from time to time, as de.. including packing ...... fig. 19 ...... 7 0 tailed in our current list supplement. Acas GP 20 pickups, standard or 25 Resistors and pots for William- LP ...... 71 33 9 .ion main amplifier...... T.C.C. .25 mfd. metal packs Heads only, standard or LP ...... 43 4 9 6 9 Resistors for fig. 13...... (coupling condensers for fig. I) B.S.R. type GU4 gram-motors ...... £9 19 6 23 Resistors for fig. 15...... 24 0 each 2 10 B.S.R. type MU 14 3 speed motors £7 3 4 22 Resistors and pots for fig. 19 .. . 37 0 T.C.C. .05 mfd. metal mites £3 14 4 6 Condensers for fig. 13...... 15 0 (coupling condensers for fig. I) B.S.R. type MU 10 2 speed motors 2 15 Condensers for fig. 15...... 47 0 each COLLARO type 3RC521 auto- 33 Condensers for fig. 19...... 54 6 KT 66 valves, matched pair, in- changers, 3 speed unmixed, with cluding tax ...... pair 50 2 2 Hi-Fi crystal heads...... £18 14 8 10 Resistors for fig. 20 ...... 6 8 EF37A valves (EF37 now obsolete) COLLARO type 3RC522 auto- 7 Condensers for fig. 20 (less C68 (including tax) ...... each 25 and Cn) ...... 12 9 I changers, as above but plays Mains transformer to specification 57 6 10 and 12 inch records MIXED £21 4 3 Matched pairof47 K 2watt resistors 4 0 10 henry 150 mA choke to speci- CHANCERY LP attachment, con- Matched pair of 22K It watt fication ...... " 20 0 resistors ...... 2 8 . . verts 78 r.p.m. motor to LP in a 30 henry 20 mA choke to speci- matter ofsec.onds ...... 72 6 5 Bank 6 position switch ...... 9 6 fication ...... 10 0 £10 6 S.P.D.T. genuine OAK switches .. . 4 4 GOOD MANS AXIOM 150 speakers Williamson output transformers £8 12 6 D.P.D.T. genuine OAK switches .. . 4 8 by ELSTONE (3.6 ohm secon- GOODMANS AUDIOM 60 speakers Reliance 100 ohm potentiometers 5 6 dary)...... 90 0 ROLA GI2 speakers ...... £8 5 0 The above is only 0 selection of our large stocks, send 6d. stomp now for our current list and supplement, and save yourself time and trouble. L. F. DANNEY 77 LOWER BRISTOL ROAD, BAT H Telephone : 381 1 ...... ,..,...... RADIO DESIGNER'S HANDBOOK A comprehensive reference, the work of 10 authors and 23 collaborating engmeers Previous editions of Radio Designer's Handbook especially interested in the design and appli have achieved exceptional success, many cation of radio receivers or audio amplifiers ; thousands of copies of this standard reference it contains an enormous amount of data work having been sold in all parts of the which has been made readily accessible by world. The book deals with general theory, means of a fully-detailed list of contents components, testing and design. The new and a comprehensive index. edition is more than fo ur times as large as the previous Yolume, and contains work The main subjects arp , valves and valve hitherto unpublished. It is the work of ten testing, general theory and components, audio authors and twenty-three collaborating en fr equencies, radio fr equencies, power supplies, gineers, under the editorship of F. Langford design of complete A-M and F-M receivers, Smith. The work is intended for those and reference data. Publication May, 1953. 42s. net. By post 43s. 6d. Obtainable from all booksellers or direct by post fr om the address below. ILIFFE & SONS LTD., DORSET HOUSE, STAMFORD ST., LONDON S.E.I 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 Overloading.-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. 0.1 MO, and its voltage gain at I,OOO C/s is g. 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, no pre-amplifier is necessary and any correc Below : Fig. 29. Revised three-stage pre-amplifier circuit with high-pass filter, to play Decca 33t- and 78-r.p.m. records. tion required may be achieved by means of simple +350V c" R" C" VI S C .. R61 � R" R" Cas � 331/'1 Css R" t; C" z R" o R" � .. � R' z 4 R C57 1I � ��-r--���--��--�--��--��--��----�----�------�--+-��S Component Values fo r Circuit of Fig. 29 Type Rating Tolerance Rating Rss Value to suit High-stability CV d.c. Transformer Carbon Type working) Tolerance RS9 O.IMn do. !W 20 % C50 O.5fLF Paper 250 20 % R60 0.68Mn do. !W 20 % C5l 50fLF Electrolytic 12 � R61 0.22Mn do. W 20 % C52 16fLF do. 450 R62 4.7kn do. 20 % C53 0.02fLF Paper 350 10 % 10 R63 0.22Mn Composition 10 % C55 100pF Silvered mica 350 % R64 20k n* do. C56 O.5fLF Paper 250 20 % � R65 22kn High-stability W 20 �� C57 50fLF Electrolytic 12 carbon Cos O.OlfLF Silvered mica 350 1 o·10 R66 0.22Mn Composition 10 % or matched R67 0.20Mn* do. C59 0.25fLF Paper 500 20 % 4.7Mn 5000pF Silvered 350 1% R6S do. S 0/.0 C60 R69 1.0Mn do. !W 20 % or matched R70 0.22Mn do. ftW 20 % C61 5000pF do. 350 do. % R71 2.2kn do. 20 % C62 7000pF do. 350 10 R72 2.0Mn High-stability 1% C63 O.5fLF Paper 500 20 �� carbon or matched C64 16fLF Electrolytic 450 R73 2.0Mn do. do. CSI 10-50pF Silvered mica 250 R74 1.0Mn do. do. CS2 O.lfLF Paper 500 R75 10Mn Composition 5% CS3 2500pF Silvered mica 250 10 % R76 47kn do. 10 % CS4 1500pF do. 250 10 % R77 lkn do. 20 % CS5 300pF do. 250 10 % R7S 47kn do. I W 20 % S7 Single-pole changeover switch. R79 0.22Mn do. 20 % Rso 10kn do. IW 20 % 20 % R95 2.2Mn do. * May require adjustment. All resistors may be !W rating, except where otherwise stated. 39 The Williamson Amplifier Fig. 30. Modified input cir RC networks, details of which have already been cuit fo r use without trans published.' fo rmer when one Checking the Pre·amplifiers.-When a pre-ampli side of the pickup fier has been constructed, it is advisable to measure must be eorthed. 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. I I To facilitate this measurement the 'n etworks of Figs. 3I and 32 have been devised. These circuits, when T fed with constant-voltage variable-frequency input, o·oo6J.LF PICKUP INPUT TAP AT RESISTANCE EQUAL TO IMPEDANCE OF PICK UP IF LOWER THAN 330D INPUT 3300 (CONSTANT VOLTAGE) OUTPUT TO PRE-AMPlIFIER O,oosJ.LF ( DISCONNECT FOR EM! CHARACTERISTIC) n Fig. 32. Simulator fo r Decca 33t r.p.m. L.P. characteristic. " 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 3"3kD p should be connected between an oscillator and the INPUT 33kfi pre-amplifier input. The output from the pre-ampli (CO�;TANT VOLTAGE ) fier for a constant voltage to the network should then OUTPUT TO follow the response curve already published for the PRE -AMPlIF IER appropriate circuit (Figs. I4 and I7, pp. 25 and 27) . Acknowledgment.-The writer is indebted to Decca for information about their recording characteristic . Fig. 31. Simulator fo r Decca and E.M.I. 78-r.p.m. recording I West and Kelly, "Pickup Input Circuits," Wireless World, characteristics. November, 1950, pp. 386-391. Printed in England by Cornwall Press Ltd., Paris Garden, LUlldun, S.E.I. L1604-IlKS1613 KS WILLlAMSON HIGH � FIDELlTY AMPLlFIER.S * The Goodsell version of the World famous Williamson Amplifier is to full specification, laboratory built and tested. * All components are adequately rated to ensure long life and trouble free operation. * Fully tropicalized version. Type GW I SIC is available for use overseas. * All models have separate power supply incorporated on the same chassis fo r use with multi-stage pre-amplifiers. tape pre-amplifiers and tuner units. A laboratory report of a Williamson Amplifier, our type GWI8/C, which was submitted to intermodu I.M. Distortion Power Output Frequency lation tests. The results using an Altec Lancing I Ratio I : 4 Intermodulation Distortion Meter and Boonton Beat Frequency Oscillator are given. 2% 15 watts 40 & 2,000 c!s. The report on the Goodsell pre-amplifier showed it 0 10 watts 40 & 2,000 cis. to be a comparable unit with similar figures. .3% I 0.9% I 15 watts 100 & 2,000 cis. Type GWI8 (standard model) Price £33 : 5 : 0 0.2% 10 watts 100 & 2,000 cis. Type GW 18/C (as illustrated) Price £36 : 0 : 0 I I PRE-AMPLlFIER TONE CONTROL UNITS Multi-stage, low noise tone control units with radio input and gramophone pre-amplifier. Eq ualization for micro groove and standard recording characteristics, S position steep cut low-pass filter and cathode follower out-put. Feedback over every stage ensures negligible distortion. Type PFA with equalization for British and American recording characteristics LP and 78. Price £21. : 10 : 0 Type FUTC with LP and 78 equalization and 15 mv sensitivity. Price £11.: 11. : 0 MANUFACT URED BY- GOODSELL LTD., 40, GARDNER STREET, BRIGHTO N, l Phone : Brighton 26735 Y ALYES for the L.63 WILLIAMSON AMPLI FIER KT.66 The fo llowing Osram valves are specified : L.63, *8.65, KT.66, U.52. U se of KT .66 is essential to obtain optimum performance of this equipment. The characteristics of KT .66 and 6L6G are not identical. Circuit information on a Quality Amplifier for DC! AC operation may be obtained from 8.65 the Osram Valves & Electromc Dept., Magnet House, Kingsway, W.C.2. * B.65 double triode as alternative for any two L.63 type. U.52 THE GENERAL ELECTRIC CO., LTD., MAGNET HOUSE, KINGSWAY, W.C.2