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WILLIAMSON AMPLIFIER NATIONAL PHYSICAL LABORATORY REPORT
ON TESTS OF TRANSFORMER (Marked: Lab: O tp t, Series Il) u u Tested for: Vortexion Ltd., 257/263 The Broadway, London, S.W.19.
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The series resistance and inductance of the primary The series resistance and inductance ofthe primary ,I winding with the secondary winding opcn-circuitcd, winding with the secondary winding short-circuited, was measured at 50 cycles per second with and 20 and of half the primary winding with the oilier volts applied. The room temperature was5 short-circuited were measured at 1,000 cycles halfper The results are given in Table 1. l<'°C. second. About 10 volts were applied. The results are given in Table IL TABLE I TABLE 11 Primary Resistance and Inductance at 50 c/s Resistance and Inductance at 1,000 c/s Voltage Resistance (0.) Inductance (H) Connections Resistance (0.) Inductance (lnH) 1'Ii':�;r�h,,!:�' This Laboratory Model Transformer Series easily permits 11 6 db more feedback with complete stability than the maximum of our Series 1 which Mr. D. T. N. Williamson described. PRICES Series Output Transformers as above ... Il £7. 7.0 Series Output Transformers I £6. 6.0 Mains Transformers ... £4. 4.0 CHOKES 1O.12H, 150 mA ... ll. £1. 6 CHOKES 30/H, 2e mA £2. 0. 6 Please write for Catalogue WAj5 VORTEXION IJ T I) . 257·263, THEBROADWIIY,WIMBLEDON, LONDON, S.W.l9 Tllepbones: LIBut, 2814 and 6242-3 TdelJ'aml: "VORTEXION. WIlIIIBLE. LONDON' POTTED COMPOUND FILLED TRANSFORMERS & CHOKES WODEN Potted components are in constant demand for the WILLlAMSON AMPLIFIER AND PRE-AMPLlFIER_ note the advantoges Made to the author's exact specification. • Smart appearance and uniform layout . • Absolute Reliability. • SEND FOR FURTHER PARTICULARS AND COMPLETE CATALOGUE. WODEN TRANSFORMER [0· LID I\IIOXLEf ROIlO BIlSTO� STJ\JJS PHONE: BllS TO N 41959 J.T.l. 6 The journal for all radio technicians WIRELESS WORLD is Britain's leading technical journal devoted to radio, television anj electronics - its pages providing a complete and accurate survey of the newest British techniques in design and manufacture. Articles of a high standard cover every phase of radio and allied technical practice, well as the wider aspects of international 3S radio. Theoretical articles by recognized experts deal with new developments, while design data and circuits for every application arc published regularly. Monthly, 25. Annual Subscription, £1 75. Order a regular copy DJ WIRELESS WORLD from your newsagent-to-day. The journal for all advanced workers WIRELESS ENGINEER - produced for research engineers, designers and students in radio, television and electronics - is the international source of information for advanced workers; the journal publishes only original work. It IS served by an Editorial Advisory Board romposed of representatives from the National Physical Laboratory and the British Post Office, and publishes Abstracts and References compiled by the Radio Research Organiza tion of the Department of Scientific and l:1dustrial Research. Monthly, 3s. 6d. Annual Subscription, £2 4�. 6d. Your newsagent ,oi/l be glad to supply regular a copy of WIRELESS ENGINEER. See him to-day! ILlFFE & SONS LTD., DORSET HOUSE, STAMFORD ST., LONDON, S.E.l The Williamson Amplifier A Collection of Articles, reprinted frolll "Wireless World," 011 "Design for a High-quality Amplifier" By D_ T_ N. WILLIAMSON (formerly of the M.O. Val"c: Company, now with Ferranti Research Laboratories) Published for • LONDON: ILIFFE & SONS, LTD. The W iIIiamson Amplifier CONTENTS Page Introduction 5 Basic Requirements: 7 Alternative Specifications (April 1947) Details of Chosen Circuit and Its Performance 11 (May 1947) NEW VERSION Design Data: 14 Modifications: Further Notes (August 1949) Design of Tone Controls and Auxiliary Gramophone Circuits 20 (Octoberand November1949) Design for a Radio Feeder Unit 30 (December 1949) Replies to Queries Raised by Constructors 34 (January 1950) Modificationsfor High-impedance Pickups and Long-playing Records 35 (May 1952) 3 Introduction Introduced by Wireless World in '947 as merely onc 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 arc discussed. Descriptions of it have been published in all the principal countries of the world, and so there are reasonable grounds for assuming that its widespread reputation is based solely on its qualities. This hooklet includes all the articles written by D. T. N. Williamson on the amplifier. Both the 1947 and 1949 versions are reprinted. as the alternative output transformer ratios cover a wide range of require ments. Modifications and additions include pee-amplifier circuits and an d. unit. with recently published information 011 adaptation bigb to impedance pickups and correction for 331 r.p.m. records. We would stress the importance, if the full potentialities of Hle amplifier are to be realized, of following the author's recommendations in detail. Even the .S.A., where several modified versions have in U been descri d, many users adhere to the designer's exact specification be with the original valve types. It is not the circuit alone, but the properties of the valves and such components the output tnlnsfonner, as together with the welding of theory and practice into a rational layout, which produce the results. Editor, Wireless World. , .' The Williamson Amplifier Basic Design Requirements: Alternative Specifications improvements in the operation of the loudspeaker. spectrum (but especially, at the ECENTfield of commercial sound This in turn reconverts the elec· low-frequency end) be substan recording ha made prac trical waveform into a corres tially less than that at medium vc 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. The useful range of be a replica of the original. these frequcncies be/ore they reach shellac pressings has been ex The performance of an amplifier the amplifier as otherwise severe tended from the limited 50-8,000 intended to reproduce a given intermodulation will occur. This 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 reproduction of an organ on from 1930 until the present, to a accurately the frequency com incorrectly designed equipment range of some 20-15,000 c/s. This ponents of a mythical Foutier where pelial notes of the order of increase in the frequency range analysis of the waveform. While c / s cause bad distortion, 16-20 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, car analyses sound pressure wave in the audible range. Although which combine to make possible a forms into component frequencies the phase relationship bctweell standard of reproduction not pre and thcreby transmits intelligence the componcnt frcquencies of a viously attainable from disc re to the brain, the (act that the complex steady-state sound ooes cordings. Further improvements, function of the s),stem is to repro not appear affect h t e audible to 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 transien t 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 he very different ance oC the phase rel ationsh ip The resumption of the tclcvision 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 JX>S.<;ible extension component frequencies. addition to low phase and fre· of u.h.£. high-quality trans The requirements of such an quency distortion, other factors missions, increase the available amplifier may be listed as :- which arc essential for the accu sources of high-quality sound. (1) Negligible non-linear dig· rate reproduction of transient Full utilization of these record· tortion up to the maximum rated wave-forms arc the elimination of inss and transmissions demands output. (The term " non-linear changes in effective gain due to reproducing equipment with a distortion" includes the produc current and voltage cut-off in any standard of performance higher tion of undesired harmonic fre· stages, the utmost care in the than that whieh has served in the quencies and the intermodulation design of iron-cored components. past. Extension of the frequency of component frcquencies of the and the re duction of the number range, involving the prcs<:nce of sound wavc.) This requir� that of such components to a minimum. large-amplitude low-frequency sig the dynamic output/input char Changes in effective gain during nals, gives greater likelihood of acteristic be line.."l.r within close " low· fr equency " transients occur intermodulation distortion in the limits up to maximum output at all in amp lifiers with output stages reproducing system, whilst the frequencies within the audible of the self·biased CIa.'iS AB ty pe, enhanced treble response makes range. causing serious distortion which this type o( di�1.ortion more (2) (a) Linear frequency re· is not revealed by steady-state readily detectable and undesirable. sponse within the audible fre· measurements. The transient Reproduction of sound by elec. quency spectrum of 10-20,000 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 of an electric al waveform capacity for negligible non-linear at a rate detennined 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 bi voltage which as which constitutes the sound. The This requirement is less strin alters the effective gain of the purpose of the amplifler is to gent at the high-frequcncy end of am plifier. produce an exact replica of the the spectrum, but should the (5) Low output resistance. electrical input voltage waveform maximum power output/frequency This requirement is concerned at a power level suitable for the response at cither end of the with the attainmcnt of good 7 The Williamson Amplifier The salient fea The functions of negative feed tures of these back are:- " , methods are of (a) To improve the linearity interest. of the amplifier, and output Push-pulltriodc transformer. valves without To improve the frequency INPUT IMPUT (b) �V the refinement of response of the amplifier and negative feed output transformer. back form the (cl To reduce the phase shift / mainstay of pre- in the amplifier and output trans sent-day high- former within the audible fre fidelity equip- quency range. (a) (b) ment. A stage of (d) To improve the low-fre Fig. I. Output/input characteristics (a) without this type has a f�dback (b) with negative feedback. quency characteristics of the out number of dis put transformer, particularly frequency and transient response advantages. With reasonable defects due to the non-linear . from the loudspeaker system by efficiency III 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 per cent (/) To reduce the effect of 2-3 suspension stiffness and resistance, harmonic distortion. The output/ random changes of the para and electro-magnetic damping. In input characteristic of such a stage meters of the amplifier and supply the case of a baffle-loaded loud is a gradual curve as in Fig. voltage changes, and of any tiipeaker, the efficiency is rarely 1 (a). With thistype of characteris spurious defects. higher than 5-10 per cent, and the tic distortion will be introduced at A stage of this type is capable air loading, which determines the all signal levels and intermodula of fulfilling the highest fidelity radiation, is not high. In order tion of the component signal requirements in a sound repro to avoid a high bass-resonance frequencies will occur at all levels. ducing system. The output/input frequency, the suspension stiffness The intennodulation 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, 10tifI in such a suspension cannot ness and "mushiness" which when it curves sharply with the belarge. Electra-magnetic damp characterizes amplifiers of this onset of grid current in the out ing is therefore important in type. In addition, further non put stage. Non-linear distortion controlling the motion of the cone. linearity and considerable inter can be reduced to a degree repre This effect is proportional to the mcxiulation will be introduced by sented by less than o. I per cent current which can be generated the output transformer core. harmonic distortion, with no in the coil circuit, and is therefore If the load impedance is chosen audible intermodulation. The proportional to the total resistance to give maximum output the frequency respon� of the whole of the circuit. Maximum damp load impedance/output resistance amplifier from input to output ing will achieved when the coil ratio of the amplifier will be about transformer secondary can be be is effectively short-circuited, hence which is insufficient for good made linear, and the power 2, the output resistance of the loudspeaker damping. handling capacity constant over amplifier should be much lower is difficult to produce an a range considerably wider than It than the coil impedance. adequate frequency response char that required for sound reproduc (6) Adequate JX1wer reserve. acteristic in a multi-stage ampli tion. The realistic reproduction of fier of this type as the effect of The output resistance, upon orchestral music in an average multiple valve capacitances and which the loudspeaker 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 application of negative feed pedance/output resistance (some of normal efficiency. The use back to push-pull triodes results times known as "damping fac of horn-loaded loudspeakers may in the more or less complete sol tor") of 20-30 is easily obtained. reduce the power requirement to ution of the disadvantages out " Kinkless" or "beam" 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 The Output Stage plifier, from the output transform to give a perfonnance midway An output of the order of er secondary to the initial stage as between that of triodes with and 15-20 watts may be obtained in onc of this method corrects distortion without feedback. The advantages three ways, namely, push-pull introduced by the output tran!'> to be gained from the use of triodes, push-pull triodes with fonner and makes no additional tetrodes are increased power effi negative feedback, or push-pull demands upon the output capabili ciency and lower dri ve voltage tetrodC5 with negative feedback. ties of any stage of the amplifier. requirements. S 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 frcquency region by a high range in the amplifier is from acter and amount of the negative leakage reactance. 10-20,000 eIs, fb may be taken as feedback used. The feedback (cl Intermodulation and har 3.3 cls and It as 60 ke/s. A tram;· 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 effectiveload impedance below limit may be reduced, Uti 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 parru,itic driven beyond cut-off since the necessity of achieving stability oscillation and instability arc load ellipse will tend to become when feedback is applied across liable to be encountered. circular. the transformer, i.e., that the loop When equipment has to be (d) Harmonic and intermodula gain should be less than unity at operated from low-voltage power tion distortion produced by the frequencies where the phase shift supplies a tetrode stage with non-linear relation between flux reaches 8 0 1 0 . negative feedback is the only and magnetizing force in the core To illustrate the procedure, choice, hut where power supplies material. TillS distortion is always consider the specification of an are not restricted, triooes arc present but will be greatly aggra output transformcr coupling two preferable because of case 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. 15-ohm loudspeaker load_ It appears then that the design (el Harmonic distortion intro Primary load impcdance= IO,OOOD: of an amplifier for sound reprO duced by excessive resistance in duction to give the highest possible the primary winding. " IO,OOO T urns rat ___ = 25.8 :1 fidelity should centre round a The design of a practical trans· 10= former has to be a compromise '5 push-pull triode output stage and Effective a.c. resistance of valves should incorporate negative feed betwccn these conflicting require J 25000 back. ments. = Low-frequency Response The most suitable types of valve At a low frequency Ib' such that Parallel load and valve resist- for this service are the PX25 the reactance of the output trans 2500 10,000 and the KT66. Of these the KT66 former primary is equal to the = ance X = 2oo0D: 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 Ib = c/S(wb�21) response ances in parallcl, the output 3·3 type with a 6.3-volt heater, and should be 3db down. will simplify the heater supply voltage will be 3db below that at Primary incremetllal inductance problem. Triode-connected it has medium frequencies. At a fre 2000 characteristics almost identical quency 3/b the response will be L � -- =95 H, with those of the PX25. well maintained, the transformer " Using a supply voltage of some reactance producing only 200phase fligh-frequency Response 440 volts a power output of 15 angle. Similarly at the high Sum of load and a.c. resistance:; watts per pair may be expected. freq llency end of the spectrum the = 10,000 + 2500 response will be 3db down at a = 125°0 n The Output Transformer frequency It such that the leakage The output transformer is prob At I, 60 kc/s (Wj = 376,000.) reactance is equal to the sum of = response should be 3db down. ably the most critical component the load and valve a.c. resistances. in a high-fidelity amplifier. An Again at a frequency the 12,500 It/3 = incorrectly designed component Leakage reactance � is capable of producing distortion = mHo which is oftcn mistakenly attribu 33 ted to the electronic part of the c A 20-watt transformer having 10 amplifier. Distortion prodllciblc primary and 8 secondary sections directly or indirectly by the B and using onc of the bettec 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 11). leakage reactance and resonance Some confusion may arise when A phenomena. specifying an output transformer Distortion due to the phase as the appa.rent inductance of (b) ALTERNATlNC shift produced when negative 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 inductan«; with a,c. excitation. The inductance of an iroll-cored 9 The Williamson Amplifier to a low value as it contains the minimum numhcr 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 frolll the first stage ( , ) for a reasonable distortion with the available h.t. voltage, and in addition the phase spl itter is operating at an unduly high level. The gain of the circuit is low even if a pentode is used in t.he first stage, and where it 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 thc difficulties. Eaeh stage then works well within its capabilities. The increa&.d phase shift due: to the OtJTPUT extra stage has not been found STAi;E unduly troublesome provided that suitable precautions are taken. The functions of phase splitter and push-pull driver stage may (c) be combined in a self-balancing " paraplJase" circuit giving tl:c Block diagrams of c rcu t arrangements discussed in the text. Fig. 3. i i arrangement of Fig. 3 (c). The grid of one drive valve is fed component is a. function of the will reau It, should a phase shift directly from the first stage, the excitation, the being of occur at a frequency where varia.tion other being fed from a resistance of the form shown in Fig. The t.he1800 vector gain of the amplifier network between the anodes of exact shape of the curve2. is and feedback network is greater dependent on the magnetization than unity. The introduction of the driver valves as shown in Fig. This arrangement forms characteristic for the core material. more than one t.ransfonner into 4. a good alternative to the preceding The maximum in(lucbnce, the feedback path is likely to onc where it is desirable to use the corresponding to point o:.;curs give risc to troublc from insta C minimum number of valves, when the core material is nearing bility. As it i� desirable to apply saturation and is commonly feedbaek over the output tran!l times the "low excitation" 4-6 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 ]n a correc1.ly designed oUlput Although the amplifier may I---�HI- transformer the primary induct contain push-pull stages it is OUTPUT ance corresponding to the vol tage desirable that t.he input and output �WV--r'ww+--l'1- swing at maximum output at should be "single ended" and cls will lie in the region of B c 50 have a common earth terminal. in Fig. 2. Three circuit arrangementssuggest III spccifying the component, themselves. the important value is the incre The block diagram of Fig. 3 (a) mental illducta!ICe corresponding R shows the simplest circuit arrange to point A, since this value deter· ment. The output valves are mines the frequency response preceded by a phase splitter low outputs. at which is driven by the first stage. The feedback is taken from the phase Shift output transformer secondary to The reduction of phase shift in the cathode of the first stage. Fig. 4. " Paraphase" circuit amplifiers which arc to operate This arrangement is advantageous combining the functions of with negative ffcdback is of in that the phase shift in the phase splitter and push-pull prime importance. as instahility amplifier can easily be reduced driver stages. >0 The Williamson Amplifier Details of Chosen Circuit and Its Performance consider a ions under keep the phase shift in the ampli split te grid. Due to the cathode t r HElying the design of a high fier at low frequencies as small as follower action of V, the operating quality amplifier were dis possible the first stage has been conditions arc not critical and no Tcussed in the first part of th is directly coupled to the phase trouble is likely to be encountered article. A circuit of the complete splitter, elimin ating one R·C from normal changes in valve amplifier is shown Fig. 5. This coupling . The first two stages are parameters. The calhode bias in follows the basic arrangement of thus designed as a single entity. t o which feedback resistor of V" Fig. 3(b). The de ign of the indi The phase-splitfer section, which is applied from the outpllt trans s vidual stages will not be treated consists of a triorle with equal forIller secondary, is .kept a.s small in detail, but a review of the loads in anode and cathode cir us practicable to avoid gain reduc s Circuit diagram of amplifier. Voltages umrerlin!d are peak signal vcltages at IS watts output. Fis.5. complete CIRCUIT VALUES R) MO t watt 20 per cent 1.000 n iwatt ± 20 por ('cnt c, 8 !.F 550 V, wkg. I aa,ooo n watt± ± 20 " Joo n I watt ± 20 C, V, wkg. R I Sill' 600 U"2 47,000 n I watt 20 100 n 2 watt wirc- CH, at 20 lOA (min.) ± :lO H 470nlwatt:l:10 .. wound v-ariable. CH, 100 mA (min.) H,,,, IQ II at n5• RG• R7 22,000 n 1 watt ± 10 .. 1[;0 n 3 WlItt ± 20 " " Power tran�formcr. 0.47 MO: i Wi\tt± 20 100 n i watt ± 20 .. Seeondarv V. " The Witliamson Amplifier . , " 0 -• , • , - •- WITHOUT fE(DUC� � :; , WITH FEEDSAC� - >• ,• • > . , ... . • , �• " ,. " IMPUT VOlTS (�. .. 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% low maximum, (3) and (4) Conditions as in (1) and be (2) respectively, but at 30 e/s. sistors of this stage which, in COll 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 voltagcs 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 gi ven in the ated near maximum output. Its output stage is a matter of some Appendix. c.."tuSC may bad layout or an importance, as it alTects the per output transformerbc 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 U1e 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 ca pacitor (s.."\y 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 20db. I Output Transfonner. - The The feedback network is a Performance turns ratio of the output trans purely resistive potential divider, Linearity.-Thc linearity of the former will be determined by the the bottom limb of which is the ampli.fier is well illustrated by the impedance of the loudspeaker cathode bias resistor f)f 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 <2 The Williamson Amplifier that the overload characteristic is i8 tic indicates that little phase supported th� ml::'<1sured pt'rform 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.£. spectrum and never exceeds a reproducing organ music includ application of negative feedback, few degrees. ing pedal notes of the 20 (,,/S order. especially at low frequencies. is Output Resistance.-The out which reach the threshold of clearly demonstrated by the put resistance of the amplifier is maximum output. Transient8 are oscillograms. 0.5 ohms measured at the Is-ohm reproduced with extreme fi delity; Equipment for measuring inter output terminals. tests using a dircct microphone modulation products was not Noise Level.-In the amplifier circuit with noises such as jing ing l available, but measurement of the tested, the measured noise level keys reveal e traordina y realism. x r harmonic distortion was was 85 db below maximum output. The amplifier can be de!;Cribed total made with an input frequency of The noise in this amplifier was, as virtually perfect for sound 400c I s. The result is shown in however, almost entirely soc/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 " distortion less .. is less than 0.1 per cent. Inter rangement of these components it amplification and low lIoise level modulation. with this degree of appeared that the noise level art: of prime importance. linearity. is not present to an could be reduced to better than audible degree. lOodb below maximum output. Frequency Response -Thc fre If desired, the power output of . APPENDIX. quency response of the amplifier the amplifier may be increased is greatly dependent upon the beyond IS watts by the use of Output Transrormer. characteristics of the output trans several pairs of output valves in c i n. Specifi at o fonner. In the amplifier tested, parallel push-pull. The output Primary load impedance = 10,000 ohms c.t. the output transformer had a transformer, power supply and Secondary load impedance resonance at about 60 kc/s which bias arrangements, and the feed = 1.7 ohms per sec- caused a sharp dip of 2.6db back resistor .. will require to R tion. around this frequency. The char he modified. Amplifiers of this Turns ratio = 76 : I. acteristic within the audible range design with power outputs up to Prim ry inductance= 100 (min.) from 10-20,ooo c/s is linear with 70 watts have been produced. Leakagea inductance= 30 mHH (max.) in o.2 db. Listening tests carried out in Phase Shift_-The excellence of conjunction with a wide-range Winding. the frequency response charaeter- loudspeaker system havc fully Data. COre: �lin 5tack of Pattern No. �8A .. Super Silcor " laminations (.\'lagnetic and EIt:ctrical Alloys, Burnbank, Jlamilton, Lanarks). The winding consists of two + I identical interlcaved coih. each Ilin " wide. wound Oil liin x in pax01in db l formcf';. On each formeIr wound: i!; I 5 primary sections each c()n!;i�tin� of 5 layer.; (8B turns per layer) 0 0 0 of 30 s.w.g. enamelled copper wire 8 interleaved with 2 Illil . paper, alter nating with secondary sections. HR 4 fREQUENCY IN CYClES SECOND each cOllsi�ting of 2 layers (29 turns per layer) of 19 s.w.g. enum. copper Fig. 7. Frequency response (without feedback) of 20 watt output trans wire, interleaved with mil. p;ll�r. former described in appendix. Generator resistance 2,5000 load :l Each section is insulated Jrom it� resistance 1.7n. Measured with SV r.m.S. on primary. At higher llI�ighbour5 by layers of mil. excitations the bass response improves progressively up to saturation. Empire tape. ;.I All connections5 are brought out on one side of the wind ing. but the primary may section!; lJe connected in series when winding. only two prilllury connec;tioll!; per coil being brought out.'" Measured Performance. Primary inductance = 100 H. (m as red ers with V e u at 50 .'i- r.m.s. primary, equivalent to mW) on �.5 Leakage inductance = mHo (measured at 1,000 cts)2� Primary resistance = �50 ohms. (a) Input wav form, c/s. (b) Output wavefonn with feedback and overload.e (c) Output300 waveform with feedback but output voltage .. Secondary cQ1mectio"s for different slight 15% below maximum. ratios tue given in the Table OH p.17. '3 The WiIliamson Amplifier The Ne w Ve rsion Design Data : Modifications: Further No tes 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 form a for resistors and to Rs' RI' RIL R " plifier design suitable for high domestic sound-reproducing in A transiti onal phase-Shift netIJ quality reproduction of sound. stallation. work consisting of •• and Cu R correspondence bas revealed that Circuit Diagram. The list of which was previolls y recom l a morc 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 origi nals. In tile circuit feature to increase the margin of tion about construction, would be previollsly printed a potentio stability at high frequencies This . of interest. The correspondence meter, was provided in the will be discuss ed later when the RIl• also shows that considerable de penultimate stage to enable the stability of the ampli fier is con mand cxlsts for a pre-amplifier signal to be balanced Due to the sidered. . unit to enable the amplifier to be use of common unbypassed Finally, an indirectly - heated used in conjunction with gramo cathode resistors for the push-pull rectifier has been substituted as phone pickups and microphones 01 stages, the amplifier is largely this p!"cvenls a damaging voltag£' low output. In the present article se balancing to signal, and it is surge when the amplifier is lf- it is proposed to deal witb the permissible to dispense with this switched all. No suitable type was '.0 Fig. 1_ Circuit diacram of complete amplifier. Voltages underlin� are peak s:gnal o t es at watts output v l ag IS . R, IMO J watt ± 20% H.u, RII,O.BHl J watt ± 10% Cs, C1 0.2SI'F 350V wkg. 33,OOOn I watt ± 20% Rn, 1.0000 i watt ± 20,% C, 81'F 600V wkg. , R2(J " watt ± Rl8 1000 1 watt. ± 20% 200pr' a.=;O V wkg. . 47,OOOn 1 20% RI?'I{.I&, R2IlOOO 2 watt. wirewound c" R," 4-iOn watt ± 10% CH, 30H at 20mA t variable R" R7 22,0000 1 watt ± 5% CIll! lOll at 150mA R" 1500 3 watt ± 20% (or ma.te I H od) Power transformer .. R�3. Ru lOOn t watt ± 20% T 22,0000 wlllt ± 20% Secondary 4.25_0_42.'lY 150 Ill."-. aY. 3A, 1,200 R. I V lpee<:b coil impedanOt 6.3V 4.-\., oollt.re-tappod •• R. 0.47Mn ;I watt ± 20,% R26 R -i wa.tt (see tabla) R" 3!lOO i watt 10% VI> V2 2xL63 or 6J5, GSN7 or BM ± R" 4,700n t watt ± 20% Ru,Ru47,OOOn 2 watt ± 5% do. do. Cl' C6• Cs 500V V3, V. wkg. (ur matchro) C3• O2, 0.05/,FS/,l<' 350V wkg. '-a, Ve KT66 "7 Coasor 53 , 5V4 C. KU '4 The Williamson Amplifier - >0 1--- III t- " Wll 0 I / , " .= / !��t:�:H �:����(ri���(K tI - , - , / , " 0 - I - I 00 00p- - I H - § § Fig. 2. Loop gain and phas shift characteristics of the amplifi r. e- e available when the circuit was originally published. A list of alternative valve types is also shown. Amplitude and Phase I fre- quency Response. A curve show ing the transmission and loop gain of the amplifier at frequencies between I cl s and I Mc I s is shown in Fig. 2. Although only the sec· tion between lO e/s and 20,ooo c/5 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 power pack. emphasize that. in a feedback a.mpl:ifier, the response must be carefully controlled at frequencies shows, the amplifier has consider the amplifier is pUl into service very remote from the uselul ra.nge able gain at low radio frequencies, there arc a few adjustments which if stability is to be achieved. and care is necessary to avoid require to be made. These COIl General Construction al Data. oscillation. cern the balancing of the standing The layout of the amplifier is not Signal wires, especially grid currents in the output stage, and 3. 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 S<1.tisfactorily 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 cha.racteristics of the output trans separate chassis, as this gives formed by a piece of 12 or former deteriorate rapid.ly with 14 greater flexibility in accommo s.w.g. tinned copper wire, con d_e. magnetizatioll . The proc� dating the equipment in a cabinet. nected to the chassis at the input dure to be adopted for static and The f 0 11 0 wing precautions end. is greatly to be preferred to signal balancing is as follows : - should be observed :- the use of the chassis as an earth 1. The output transformer core return. Slatic BalaJ1( illg. should be positioned at right Electrolytic and p a.per Connect a suitable milli 5. (a) 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 tram, ing choke. output and rectifier valves. former primary. 2. The output transformer and Figs. 3 and show the positions (b) Set the total current to 4 loudspeaker leads should be kept of the major components in two mA by means of H. , . 125 at a reaso able distance from the alternative layouts which have (c) Connect a moving-coil n input leads, which should be been used successfully. voltmeter (O-IO p ro . V a p x ) screened. As the response curve Initial Adjustments. Before across the whole of the output '5 The Williamson Amplifier and the secondary impedance. being proportional to the square of the turns ratio, becomes I.7 l!( 2'=6.80. Similarly three if sections are connected in series the v, impedance becomes -7 x 3� "'" I OUTrtJl 15.30. Thus the available lIlANSfORM£1I secondary impedances. keeping a v, 10,000 !l primary load impedance, "'�IMS are 1.7, 6.8, 15·3, 27, 42.5, 61, 83 and I()()!l. The connections to ''"'""'' obtain these values are shown in the table. I �='='::::='='=---,�======::'..J Should it be nectlssary, in an (.ONN[tTlN� lI�K emergency, to match loads of ® 0 other impedances to the ampli Layout when using power pack. Fig. 4. separate fier, it is permissible to reduce the primary load impedance to 6,000 n transformer primary and adjust transmission of the c.omponent a.t giving another series of secondary R" until the reading is zero, in high frequencies, and great varia impedances, namely I, 16, 4. g, dicating balance. Random tions arc possible. 25, 36, 49 and Under these 64 n. fluctuations of this instrument In the output transformer �peci conditions the power output wj]] 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 a.nd should be disregarded. inductance of the primary at low value of the feedback resistor Ru signal levels. due to the use of must remain unaltered, as the Signal Baltmc;'lg. core material with a low initial turns ratio is unchanged. The the low-im- (a) Connect permeability. or to careless values of R.� 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 arc region of 3.5!1 are givcn in the (beadphonl::s 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 4oo e/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 tha.t to the valve anodes. This gives p ut voltage about hall maxi it is considerably larger than the tile mmunum external electric mum. transformers which are usually field. (d) Adjust RI. for minimum fitted to 15-watt amplifiers. The Stab li y with e at e Feed output in the detector. fact that the peak flux density of i t N g iv back.-Much has been written The Output Transfonner. As 7,250 gauss for maximum output about the stability of amplifiers stated previously, the output at 20C/S lies on the upper safe under conditions of negative feed transformer is tht: most critical limit for low is sufficient distortion back, and the criteria for stability component in the a.mplifu::r and comment on current practice. arc now widely appreciated. The satisfactory performance will not Some confusion arose regarding article by .. Cathode Ray " in the be with a. component the method of connection of the obtained May, 1949, i sue, states the differing substantially from the transformer secondary windings to s matter simply and with character specification. The effect of de match loads of various im istic clarity. creasing the primary inductance whilst utilizing all the pedances. Continuous oscillation will occur will be to produce instability at correct secondary sectioDs. The in a feedback amplifier if the loop load impedance low frequencies. which can be pri m a r y is gain-that is the transmission of cured only by altering the time 10,0000 and the turns ratio in as the amplifier and the feedback h other constants or t e coupling the original design is 76: I the im network-is greater than unity at circuits. or by dt:Creasing the pedance of each secondary section any point where the phase shift is lo,ooo nl 76� or 1.70. When amount of feedback. At high fre of the amplifier has reached 180°. quencies the situation is more secondary sections arc 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-capacita.nce of the mains unchanged. If now two ditions should occur in a transient windings, the ca.pacitance between secondary sections, or sets oC 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. Jarly likely to occur in badly de- ,6 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 IG-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 chamcteristics of one OT more tance. This would necessarily be remedy is obviously reduce the stages may increase by as much as expensive, and a compromise loop gain to its corrcctto 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 the lowest practicable value. mended. oscillation should to If 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 2C/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 avoid this such that changes in character R.J• If the oscillation continues to 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 he examined to ensure ably large interstage couplings that it is operating correctly if another component, and the good low-frequency response were amplifier as a whole is likely to balanced in push-pull, and not in required. Alternatively, the var have characteristics close to the an unbalanced manner due to the iable time constant must be average. Individual amplifiers failure of some component. chosen in relation to the fixed may. however, have charac time constants, such that its mini teristics wbich differ substan Primary Inductance mum value is sufficiently longer tially from the average, due Assuming that the amplifier is, than the fixed values to produce an upward or downward or has been rendered, with stability. An increase in its value to stable trend in the changes produced by the feedba.ck disconnected, the then serves only to increase the component deviations. the next step should be to check the stability margin. This method is trend is in a direction Ifsuch 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 CUSSlOn. stability will result, the only efiect practicable make direct mea To ensure a wide margin of being a slight degrading of the surements ofto 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 TRA.NSFORMERS. TA.BLE OF CONNECTIONS. No. of seeondary groups of sections in series I 2 • • • • 7 8 --. ::-,, - :--:- Connections � � � � � � Jr J� U- "� ::::: I .J1 J� � -� - --. -11 -1� Correct impeda.m.'eseoondary (ohms) 6.S 15.3 27 42.5 61 109 gin l 1 L7 83 OriOutputa Mi nimum second. Transformer impedance "Yperm issible (ohms I 9 25 3. 49 , I. 64 10,000/1.70 R2r.Foodback (ohms)resistor 3,300 4,700 6,800 8,200 10,000 11,000 12,000 1,500 Turns ratio 76 38 25.4 19 15.2 12.6 10.8 '.5 Altcroative Correct secondary utput impedance (ohms) 3.6 l4A 32.5 57.5 90 130 17. 230 TrnmformerO (800 Appendix) TCsistor 10,000/3.60 RuFoodba.ck (ohms) 2,200 4,700 6,800 9,000 11,I'iOO 13,500 16,000 18,000 Turnsratio 52.5 26.25 17.5 13 10.5 8.15 7.5 �, I I " The Williamson Amplifier below 10 c/s. Tt 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 re::>ult by indirect means, namely mal unless the layout is very poor avoid this, the ability of the by measurement of the component or the transformer is not to speci amplifier to handle low fre parameters which determine the fication. quencies will be impaired. The characteristics. The parameter use of separate bias impedances which is most likely to show a. Capacitive Loads destroys the self-balancing pro large deviation from specification The amplifier is absolutely perties of the amplifier, and if two is the initial primary inductance stable at high frequencies with a dissimilar valves are used in the of the output transformer, since resistive or inductive load, but it output stage " motor boating" is the quality of the COre material is is possible for oscillation to occur likely, due to the presence of sig not easy to control accurately, and when the load impedance is capa nal in the h.t. line. The perlorm careless assembly of the core may citive at very high frequencies, ance of the output transfonncr cause considerable varia.tions 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 C,o in conjunction with tion on economic grounds, as the measuring the current in it. The the output resistance of V" has costs are roughly similar. Indeed, secondary willdings 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 20kc/s mon bias arrangement shows a 8..c. range of a Model 7 Avometer, upwards without affecting the definite saving. should be Ijop.A 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 p.A. work consisting of a capacitor the effect of frightening those who If the output transformer is shunting R.� has been advocated already possess amplifiers of this satisfactory the values of the other as a means of stabilizing this type or are contemplating acquir components should be checked, amplifier. The effect of such a ing them. Their purpose is to particular attention being paid to network is to increase the loop help the occasional " outer limit " the coupling components. Should gain at high frequencies, at the case where instability is experi the time constants of the coup same time reducing the amount of enced, but if they serve to impress lings, that is their RC product, be phase lag. It is sometimes pos upon the reader that negative feed higher than the nominal values by sible by this means to steer the back amplifiCrs are designed as more than 20 per cent, the resis phase curve away from the 180Q 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 performa.nce or revealed itself by this time, but, The connection of a capacitor stability, a useful purpose will if upon reconnectillg R'5 the oscil across Ra' however, will not have heen accomplished. Such lation is still present, it is very stabilize this amplifier if it bas modifications should be attempted likely to be due to the use of been consfructed to specification, only by those who are confident valves with mutual conductances although it may produce improvc that they know what they are do higher than average, and it is menl if oscillation is due some ing, and who have access to mea· la legitimate to increase the value of brge departure from specifIcation, suring equipment to verify results. R,s to reduce the loop gain. If such as the u�e of an output trans instruments are available, the former with completely different loop gain may be measured by high - frequency characteristics. disconnecting RH from the The writer has no information APPENDIX cathode of V and reconnecting it about this. Output Trun.;Jonner with 3.6-ohm I via a 470!l± 10 per cent resistor to The use of separate RC bias Secondaries chassis. The voltage gain, mea. impedances for the output valves Winding Dala sured from the input grid to the has also been suggested. This Core : l1in. stack of Super junction of RH and the 4700 re procedure is not endorsed by the 28A Si1cor laminations. (Ma ne ic and sistor, should be 10 at frequencies writer, as there are numerous <1is Electrical Alloys, Burnbauk,g tHam between 30c/s and IOkc/s. advant..--tges 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 eaeh I �in. wide on paxolin surement is being made. formers liin. Ijin. inside dimen work is made sufficiently long to X The adjustment of the loop gain ensure that the low-frequency per sions. On each ormer wo n primary ficctions,f eachis con_u d to its correct value at medium formance of the amplifier is un 5 sisting of 440 turns (5 layers, frequencies should render the impaired, the phase shift of the 88 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- frequcncy and may provoke 4 secondary sections, each con· ,8 The WilIiamson Amplifier sisting of 84 turns (2 layers. 42 neighbours by 3 la ers of roil. two primary connections ollly prr y .5 turns per layer) of 22 s.w.g. Empire tape. All connections are holobin being brought out. \Villdings enamelled copper wire interleaved brought out on onc side of the wind· to he assembled on core with the roil. paper. ing. but the primary sections may bobhin reversed. and with iusulating wi th :1 be Each section is insulated from its connected in series when winding. cheeks and centre spacer. 'V hy the lVILLlA1USON A1UPLlFIER should eDlploy PARTRIDGE TraosforDlers THE widest possible audio range-the lowest possible distor tion and an output of watts . . . these critical demands of the designer of 20 this now fa mous Amplifier implied the finest that technical skill and craftsmanship could provide for every component. Little wonder that fr om 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 H Williamson specification " component is now available for a varied range of impedance. (A model is also available for 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 fo r American requirements. The Partridge H Williamson " Output Transformer is acknowledged the most efficient 0/ its typ e. Available Te chnical data sheets wuh fullest details UN'DTTED (complete with alternatIVe mounrmgs) are (Style VDN/436B) available on Also available, complete rt4uesl. or catalogue of the Partridge range including the mains components for this and orher amplifiers. 'DTTED (Style VDN/436B) * IMMEDIATE DELIVERY can be made. and these components are ready for shipment PARfiPOGE to all parts of the world. TR ANSFORMERS LTD ROEBUCK ROAD : KINGSTON BY·PASS TOLWORTH : SURREY : Elmbridge 6737f8 19 The Williamson Amplifier Design of Tone Controls and uxilia ry Gramophone Circuits A power amplifiers in the present article. It must suffice source, and are capable of being OSTtended for sound repro to say that the matter is one in ameliorated. In addition , fixed duction are designed which the individual must enr· compensation must be provided to Mhave a uniform response to fre cise his own judgment and (or deviations from a uniform re act quencies within the audible range, accordingly. sponse which are deliberately in and it is the aim of designers of In order that he may have troduced in gramophone records. pickups, microphones and loud scope to do this, a pre-amplifier The degree of complication speakers to give similar character designed to be used in conjunction which is worthwhile in such a istics to their products. This re with gramophone recordings and unit must be considered. In presents an attempt to fulfil one radio transmissions should there theory, it is possible to compen of the conditions for the creation fore be capable of providing vari sate precisely for deficiencies in of a perfect replica of the original able compensation for such de the amplitude/frequency and sound and provides a common fects as are likely to occur in the phase / frequency response cbanc- basis for the design of individual units, which, when connected to .., gether. will provide a complete IHPUT channel with a uniform gain / C" frequency characteristic. 0 . Considerations of an engineer . III SE SJ,...- ing nature sometimes make it de fAU TRE6l[ ::-- sirable, and even essential, to �I�E II fAll depart from this ideal of a uni 00> C" form response in certain soctions I VALVE e�ss . G�ID of equipment, and quite fre 0·1 ' . ' MIN . ' quently the use of inferior equip "" " :51 " ment or long and unsuitable trans BASS I Cn TREBLE mission lines leads te an undesir FlIt I able departure from uniformity. (n cases li ke this, other "equal AIS( Cn 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. Basic frequency compensation circuit. Typical remedying the defect. values (for use after an EF37. triode-connected) are ! When listening conditions de R.cn. 3sokO, log ; �h lookO ; Ru. 6.8kO ; R.u, part from the ideal-and this, un lokO ; Rc •• IookO hnear. C20• ISopF mu. ; Cn. fortunately, happens frequently o.oIl'F. Cu O'OSl'F ; CUt looopF. since most rooms are unsuitable auditoria for the reproduction of orchestral music at realistic in tensities-it is sometimes bene 11111 - ficial to modify the frequency re sponse characteristic of the equip ment in an attempt to compensate for the more obvious defects in the I- " 11 , � �"" room acoustics. The word I (!i I "1'-, 1 "attempt " is used advisedly. ' ,i. V I, since only very complex equaliza I .; I tion could ever hope to provide � r- accurate compensation for room :.1,,[ , acoustics. This question of the f-- ,� frequency compensation which is I '''[ (n desirable when conditions depart '�. ",' from the ideal is a very thorny and subjective one. It provokes II much heated, dogmatic, and (c/.) usually very unscientific discus FR£QUlIICY � sion , and is beyond the scope of Fig. 6. Response curves of circuit of Fig. S. '0 The Williamson 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 ca..<;e when reproducing records or .. broadcast transmissions, he has CURVE A lOOP r.AIN 0 no means, apart from the sensi B .. 20 tivity and training of his cars, of ( ...100 determining the defects which have occurred in that portion. Since it is impossible to determine the nature and amount of phase " 90° distortion by listening to a trans .... � mission, and since it is not usual HEOUENCY for much attention to be paid to Fig. 8. Characteristics of circuit this form of distortion at the re - of Fig. 7. cording or transmitting end, there I w l would seem to be little justifica .... The attenuation introduced by tion for the inclusion of phase / '1-- the Iletwork when controls are at correcting networks in domestic -�- the level position is 24 db, and the equipment. In the case of a I� network must, of COllrse, be sound reproducing system which introduced into the system at a is completely under the control of signal level sllch that the valve the user, particularly if ::.iereo feeding is not overloaded. phonic, phase distortion should c'!:Vl 1 Low-Pass Filter,-The majority not be allowed to occur if the Fig. 7. Basic filter circuit. of medium-wave broadcast trans finest possible quality is to be ob missions, when reproduced with tained. This is especially true at stray alternating magnetic fields, wide-range equipment. exhibit a low frequencies, where consider especially if they are air-cored , most objectionable fonn of non able time delays are involved. Metal- or dust-cored toroids are linear distortion. This takes the Low phase distortion is best less troublesome in this respoct, form of a mttle or buzz often achieved by designing a system but are expensive and not readily accompanying transient sounds with a bandwidth considerably obtainable. such as pianoforte music . This greater than the audible range, Frequency Compensation.-Fig. type of distortion is commonly but where this is not possible com_ 5 shows a simple compensation caused by minor discontinuities in pensation may be provided. circuit which will accomplish bass the transfer characteristic and is Consideration of the causes of and treble accentuation and frequently associated with Class frequency distortion leads to the attenuati on without the use of in " B" amplifiers. conclusion that it is normal for the ductors. The controls consist of Recording and processing de levels at the ends of the spt."Ctrum two potentiometers, each asso fects, record wear and imperfect to be accentuated or attenuated ciated with a changeover switch. tracing by the pickup produce a progressively with respect to the Consider the low frequency con similar type of distortion from level at middle frequencies and a trols R •• and 52' When RH is gramophone records. form of compensation to correct fully anticlockwise (minimum re The most offensive frequency this fulfils most requirements. It sistance) the response to fre components of the rattJe or is not possible to lay down hard quencies below I,OOOC / S is uni buzz are generally present at and fast rules about the amount form. If the switch S. is set to the extreme upper end of the of compensation necessary, hut " rise," as R •• is rotated clock audible spectrum, and spread rates of attenuation or accentua wise, the amplitude/frequency downwards as the severity of the tion greater than 6db/octave are characteristic will rise at low fre effect increases. Fortunately, the not usually required. quencies to the maximum shown concentration of this type of dis As it is often desirable to change at A in Fig. If 53 is set to tortion into the extreme upper end 6. the amount of compensation dur " fall .. and Rn rotated clockwise of the spcctrum makes it possible ing a programme without calling from the minimum position, pro to effect considerable improve attention to the fact, methods gressive low-frequency attenua IDent by removing or reducing the which give continuous control ti on will be introduced, up to the energy in the signal at these fre over the response are to be pre maximum shown at B. In a simi quencies. A low-pass filter with ferred to switched systems, unless lar manner, by the use of R .. and a cut-oil frequency variable be the latter are graded in very fine S. the high-frequency response is tween the limits of 5 and r3kc/s steps. continuously variable from a level and a fairly high rate of attenua The use of inductors to provide response to the extremes shown at tion above the cut-off frequency gain/frequency compensation is C and D with the values given. is a great asset in securing the best to be deprecated as, apart from The curves may be shifted bodily possible aural result from indiffer_ possible troubles due to resonance along the horizontal axis by ent transmissions or recordings. eflects and non-linearity, they are modifying the capacitance values Although it is practicable to very liable to pick up hum from as shDwn by the arrows in Fig. 6, provi de a. filter with a continu- .. The Williamson Amplifier 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 recordings are usually cut with a R-C attenuating network external to groove amplitude which i!lpropor+ th e circuit will produce a Ire tional to signal below some arbi quency response characteristic as trarily selected frequency in the 300-4oo C/s region and with a lateral groove velocity which is propo ional to signal above this rt FREQUENCY frequency. To improve signal / Fig. 9. Modification of basic noise ratio it is now common prac_ filter characteristic roduced by ttee to increase the level recorded p additional phase shift. at high frequ encies. 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 tbe structure of not normally justified and a the record material . In Fig. 12 is switched selection of frequencies is shown the recording characteristic FREQUENCY satisfactory . To attain the high Fig. 10. Final low-pass charac used by Decca. The E.M.I. char attenuation rates necessary to se teristic resulting from addition of acteristic does not differ substan cure satisfactory results a norma} external tially at low frequencies but the resonant R-C attenuator. -section type of filter rise above 3,OOO C/S is absent. It could be used, but this carries shown in Fig. 10. The similarity is proposed to use the Decca cbar+ with it the disadvantages asso of this curve to the response of a acteristic as a basis for design. ciated with the use of inductors. resonant element L-C lter will When playing recordings, fi KM.1. An alternative type of filter readily be appreciated. There is onc fixed capacitor in the pre using only resistive and capac tive a practical limit to th e rate of amplifiers to be described later may i elements based on the parallel-T atlenuation which can be achieved be swi ched out of circuit, giving a t 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 prc-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 parall el T null net tion are interrelated . Thus a high treble control in the tone compen - work. the output from the net rate of attenuation is achieved sation unit. This. when C�. is set work being fed back to the input with simplicity only at the expense to 100 pF and .. (Fig. 5) ad R 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 ickups. with p loop gain of the ampli er, it is be obtained from one stage with the excep ion of piezoelectric fi t 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 propo ional to the lateral rt 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. with 12, 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-ampli er suitable fi 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, 10 and kc/s and a pickup pre-amplifier are : - 13 greater negative component will " linear " position is inco orated Low noise level. rp I. exist. The effect of this is to un in the nal circuit. The perform Low distortion at signal fi 2. balance the amplitude character ance is shown in Fig. 11. levels likely to be encountered istic as shown in Fig. 9. A rise Gramophone Pee.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 varyi ng the duction. however. additional fixed gain electrically. compensation is required The Noise Level.-The attainment 'Thie.sen. G. J. . R·e filter Circuits."' . nature of this compensation will of a low noise level in high-quality Jo.m.�l 0/ Vo1.the 16. Aoou.lioal pp.Sotiety 275-279 of Am",;o". No. 4. depend on the recording charae- sound systems is of such vital im- April. 19-15 22 The Williamso Amplifier n partance that a few remarks of sponse fiat to 20,OOO 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 oC 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 a.nd absence of Most modern microphones and lt is not practicable, however, other fonns of distortion are pickups are electromagnetic, to increase the secondary imped almost invariably achieved at the although there is a tcndency for ance much beyond 0.1 MO if a expense of the electrical output. microphone design to gravitate flat frequency response is required This does not necessarily mean towards carrier-operated capacitor from the transformer over the that the efficiency of the trans· types. These have problems of audible range. ducer is reduced by the other im their own and will not be treated The noise generated by thermal provements, but merely that it re here. Electromagnetic micro agitation in a 0.1 MO resistor at moves less energy from the phones and pickups are manufac room temperature is about 6/AV acoustical field or from the record tured with impedances ranging for a bandwidth of 20.000 C/S. To groove which actuates it, causing from a few milliohms to several this must be added the noise pro less disturbance of this field, or thousand ohms, but are normally duced in the first valve of the less wear of the record groove. used in conjunction with trans- amplifier. By careful design and a There is, however, a. limit to former which raises the impedance construction, and by the use of a this tendency set by the noise to a suitably high value to match suitable valve, the noise from all generated by thermal agitation in the input impedance of a valve, causes, including mains hum, can the transducer and its auxiliaries For obvious reasons it is desir be reduced to a value equivalent and by the noise produced in the able to make this secondary im to about 3P.V at the grid, but first valve of the amplifier. It is pedance as large as possible-say under normal conditions a figure desirable in a wide-range, high quality sound system to attempt to maintain a peak signalJ noise ratio of at least 70 db. This ,. figure represents the best that can ---- be achieved with direct cellulose Cl disc recording when everything is " just ri ght," and it is to be ex pected that the standards of com mercial disc recordings will ap proach this level whel; improved techniques are combined with new disc materials. A well-designed magnetic tape recorder will give a signal /noise ratio of 70-80 db, and the increasing use of this type of equipment will doubtless give impetus to the research necessary for the achievement of similar (els) FREOUENCY standards in other forms of re Fig. I I. Measured overall response of low_pass filter, in conjunction cording. With a signal /noise with pre_amplifier circuit (Fig, IS, page 26). ratio of 70 db, a sound reproduc ing system with a frequency re- of 5/AV is fairly representative. The total noise may be taken as the square root of the sum of the + squares of these values. or about BI'V. To obtain a si gnal/noise 1 ratio of 70 db, then, the peak sig nal must be 70db above this level. /' I say 25 mV Lm.s. The pre-ampli fier should have sufficient gain to o enable the main amplifier to be fully loaded by a signal at this V leveL 1/ The choice of a valve type for I the first stage must be made care fully. In theory, for equal gain . the noise level in a tri ade stage is lower than that produced by a fREQUENcY (c�) pentode, since the pentode has an Fig. 12. Decca record ng characteristic. additional noise component due to i '3 The Williamson Amplifier electron parlItLOu between screen High _ Pass Characteristic. applied to the valve by the and anode. In fact, however, Gramophone motors tend to pro potential divider formed by Ru there are no h1gh-gai.n triodes which can cause duce vibrations and the impedance of CH ' en commercially available with the unpleasant rumbling noises in a and nU' At medium frequencies requisite characteristics and elec wide-range system. Although the the reactance of CL� is small, and trode structures for Jaw-noise energy contained in the " rumble " that of Cu large compared with operation. A valve designed for components may be relatively the resistance of Ru and Ru' and such conditions should have a low, the frequency is also very the gain of the stage is determined rigidly braced electrode structure low, and consequently loud by the values of these resistors. to reduce microphony and a speaker cone movements of high As the frequency lowered the is balanced " double helical " heater amplitude may be caused. If the impedance of the top limb in construction to minimize the alter_ driving coil should move out of creases, giving a progressive rc nating field surrounding the the region of uniform flux-density, duction of feedback. This pro cathode. The Mullard EF37 has the whole spectrum being repro duces a gain/frequency charac this construction and, connected duced will be distorted in a par teristic which rises to a maximum, as a pentode, the noise levels men ti cularly unpleasant manner. Dis determined by the circuit con tioned earlier are obtainable. Be tortion in the output transformer stants, and then decreases due to fore commencing work, the reader is also possible. the coupling components C LI ' Rn who is not familiar with the te<:h This situation can be improved and Ru' With increasing fre nique of high-gain amplifier con materially by the insertion of a quency the impedance of Cu de struction should consult an article high-pass filter with a cut·off fre creases, increasing the negative on this subje<:t.2. I Considerable quency of about 'lo c/s and a feedback and producing a falling reduction of residual hum may fairly rapid attenuation below cut gain/frequency characteristic. usually be obtained by demagnet off. At these low frequencies. The capacitance between the izing the valve.' In order such filters are conveniently com to input transformer s(:condary wind obtain the best signal /noise ratio, posed of resistance-capacitance ing and earth may. large, affect if the principle which should be fol networks and may be incorpor the response at the extreme upper lowed, when valve noise is the ated in the bass-compensation pre end of the audible spectrum. This limiting factor in high-gain ampli amplifier. effect is negligible with a well fiers, is put the whole of the Electrical Fading Control. to designed component, but long available signal into the valve When the pickup is placed on, or leads should be avoided. The grid, and to provide any fre removed from. the disc the gain transformer should be mounted quency compensation which may must be reduced to avoid un the pre-amplifler chassis, be ne<:essary after the signal has on pleasant noises. While this may which in turn may conveniently been amplified. By this method be done by a mechanical poten be fixed beneath the motor board. valve noise is included in any tiometer the method is clumsy The overall cbaracteristic with attenuating operations which may and does not facilitate rapid re an input from a perfect be performed and the overall sig cord changing. It has been found .. velocity " pickup on a. Dccca nal noise ratio is improved. convenient to employ an electrical disc is shown in Fig. 14. Low Distortion. - Numerous method in which the gain of one A more complex circuit, which methods of providing response of the stages is reduced to zero at a. gives nearly perfect compensation which varies with frequency are the flick of a switch by a bias volt· and a very rapid attenuation possible and, of course, each age applied and removed by (30db/ octave) below 'loc/s, is method has advantages and dis means of a network with a suit shown in Fig. 15. This pre advantages. Where the response able ti me constant. amplifier has a higher gain than has to be continuously variable the previous onc. and is particu the method which gives greatest Pra-Amplifiers larly suitable for use in equip simplicity of control usually Although all the refinements ment where the pickup is located triumphs. Other things being outlined so far are desirable. in at some distance from the rest of equal. however. methods which dividual requirements will vary the amplifier as the circuit ter employ selective negative feed considerably and will detennine minates in a cathode follower. back are to be preferred. as cir how much complication should be cuits of this nature generally have attempted. Two gramophone The construction oL this circuit a high signal-handling capacity pre-amplifier circuits will there is not recommended for those and non-linear distortion is kept fore be described. which should without access to facilities for to a mlnLmum. In a pickup pre COver most requirements. checking the response of the amplifier this may be of import Fig. 13 shows a simple circuit finished unit, as the performance ance where pickups with widely which gives good compensation may be seriously affected by an varying output levels are to be for the Decca recording charac error in component values. used. teristic. The circuit constants The frequency characteristic of . have been adjusted to give as this amplifier is produced by the • Buandall. P. J . . . Hum in lIi.h Gain Am· plilien." No. high a degree of attenuation combination of two curves shown 2. pp. 57·61,lI'iu/eJJ Februa �II'Drld.. 1947. VDI. 53. . Dickenon A. F., Hum Reduction. . below 'l0 / s as is consistent with at A and B in Fig. 16. These, • , C EI'clro�iu. Vol. 21, No. 12, 112. De simplicity. This involves a slight when added, give the curve C. cember. lM8. p. • Correspondence_ sacrifice of the response at 20 e/s. Curve A is produced by the cir 20, No. Eledro�icp. 235. July. Efttiut";ftl.1948: The method of operation is as cuit associated with V,.. which is 248,Vo1. I'. 339. 245, 1948; No. 250.No. p. .06. Dccembtr,October, 1948. follows : Negative feedback is similar in principle to that of '4 The Williamson Amplifier Fig_ Simple List or Components for Fig. 13. gramophone pre- Typo Rating Tolerance amplifier designed "'V Rn Value to suit High·stability for the Decca re- cording transformer carbon I character- R". istic. When playing It" 0.1 ]'0[0 do. !W E.M.I. records CH Ru 0.68 MO do. IW R" 0.22 MO do. j may be switched out W of circuit. Alter_ 't" 47 kO do. IW 4.7 kO do. natively, compensa"l R. Rn n' 0.22 MO Compoaition 10% tion can effected R . " n" be c �:. RM 22 kO do. 10% in the tone-control f Ru 2.2 MO do. circuits. c.. : All resistors may rating tolerance 20% iW nle • otherwise specified.00 u ss w w" z· Ra.ting � �:;; (V d.e. � OZ Type working) c Tolerance I ,!.- "£ Paper 2f)ij o�" h R 'z Electrolytic 12 ", "w L-I"--r- r - 5� ElectrvJylic 450 0 " Silvered mica 350 10% �" Silvered mica. 250 10% RR : : .". C R" : " Paper ����( , ; R 500 :R,. Fig. 13. The attenuation at low frequencies is due to the combined effect of the intervalve couplings. Curve B is produced by feedback mu: over V 14 through a parallel-T net t- work tuned to 20 c I s. f-- The overall frequency response curve. taken under the same con f-- ditions as that of Fig. 14. is shown !;----; in Fig. 17. Fading Con rol.�The circuits t of (t/l) Figs. 13 and 15 have no pro FRf:QUEHC.Y vision for electrical fading. Fig. 18 Fig. 14. Response curve of circ t of Fig. 13 ith ideal "velocity " shows a network which. when pickup.ui w connected the cathode of V. in to Fig. 13 or V 11 in Fig. 15. enables A very carefully designed and (Fig. 13) has a gain of H 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 55 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 dcscribed to fonn 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.�F,.equency Re obtained with an input of 1.3mV works well within its signal sponse.�Reference Figs. 6, H, peak. The more complex pickup to handling capacity and does not I4 and 17 will enable the fre pre-amplifier (Fig. IS) 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 Letlel.�With careful the final arrangement. to be detennined. The effect of construction and by adjllstment Power Supplies.�The High intermediate control settings may of R51 to give minimum hum. the Quality Amplifier has a frequency be arrived at by interpolation. noise level may be reduccd to an response which is useful down to Gain.�The figures underlined equivalent input signal of 3-5 IAV 2 e/s. This necessitates a few in Fig. 19 are the peak signal at the pickup pre-amplifie r 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 '5 The Williamson Amplifier Fig. IS. Pre-amplifier with high-pass filter. Component Values ror Circuit of Fig. 15. Rating Tolornnce Ty pe Hating Tolerance R" Value to suit High'fYre.�tability R,. 0.22 MO Composition 20% trllns ormer cnrbon Hao IO kO do. IlY 20% f R" 0.1 '1\10 do. �W 20% • lIIay require adjustment. Reo 0.68 MO do. IlY 20% All resistors may be !W rating, except where other. R" 0.22 1\10 do. IlY 20% wise stated. R" 4.7 kQ do. 20"/0 Hating 0.22 1\[0 om o ition 10% (V d.G. C p s R.Rn 20 kO· do. Type working) Tolerance R6/j 22 kO High·stability !lY 20% 0.5 1'F Puper 250 20 0 % carbon C"60 50 1'1<' E:1ectro1ytic 12 0.22 l\lO Composition 10% C" 16 ,..F Electrolytic 450 H"R.. 0.20 MD· do. C� 0.02 1'P Paper 10% :I;}() R. 4.7 do. 5% C" 4000 pF Sih'ered mica. 3W 10% . i\Hl R. . 1.0 1\1 Q do. lIV 20% Coo 100 pF Sih-ered mica 350 10% R,o 0.22 MO do. �W 20% C. 0.5 /LF Paper 250 20% R" 2.2 kQ do. 20% C" /,F Electrolytic 12 "' f,Q R" 2.0 :\1 fl: Jligh.stability carlx)JJ I C. 0.01 I'F mica 350 1% " . Silt'ered or matched or matched 2.0 MD: do. 1% C. . 0.25 1'.1<' Papcr 500 20% Ro3 or matched Coo pF t::iitvercd mica 1% 1 f>OOO :IJO It,, 1.0 MD: do. % or matched C 5000 pF Sih-ered mica. 3;:;0 OT mutch(ld O' 1% R" 10 1\J Composition 5% or matched 0: HoG 47 kO: do. 10% C" 7000 pF Silvered mica 3.'>0 10% Rn I kQ do. 20% C� 0.5 /LF Papcr 500 20% 4i kn do. IlY 20% 16 1'1' EleetrviyLic 450 R" CM levels indicated is considerably other the turntable, This pre a multicore-screened cable. which less than 0.1 per cent. vents mechanical and acoustical connects the console with the Form of the Equipmcnt.-The feedback. amplifier and loudspeaker unit, outward form which a complete The control unit fiay be a con and carries the mains and aerial domestic sound equipment takes sole of armchair height (overall connections. is very much a matter of personal dimensions about 18in x l4in 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 pickup and turntable, the pre into the lower portion, and the combined with absence of trouble amplifier uDit 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 ea.se of manipulation, avoiding 26 The WilIiamson Amplifier the necessity of rising from one's comfortable seat to attend to the " --, \ 350V controls or change a record. The , I main amplifier may be included ' .....- ,1 1- .... C" R R WILLlAMSON'S in the console, but this tends to ,�:v .. n 1I make it heavy and bulky, and R" R" I OFFI gives rise to problems of heat dis I,- I � I l ON O.p. sipation which afe not easily "-I.,, " r.� c. .:h, solved. AcknowIedgment.-The writer I , I I TRANSFORMER is greatly indebted to Ferranti, 1 I Ltd., for pennission to publish Fig. 18. Cr'"irc t of fading control.C'T the results of work undertaken ui To Author's on their behalI, and to thank his colleagues wishc!lfor help Specification freely gIven. List ot Components for Fig. 18. fl.ating £4-13-6 0.22 Mn IW Ru 0.22 Mn IW RS2 kn Ra;slOOn 47 RBI CHOKES FOR WILLlAMSON'S .� All resistors may iW rating. 00 AMPLIFIER � ol--�/- - "" tolerance 20,% uules8 otherwise " · . � specified. 20 m/a. w JOH at 18/6 10H 150 · ...... Rating at m/a. 35/6 (Vd.c. 50H 20 m/a. · . working) at 22/- --'-L--__ 250 350 J fREQUENCY MAINS TRANSFORMERS � B;/I', :150 Fig. 16. Derivation of high_pass FS4J. Input 200/250v. (;baracteristi(;. Output 425/0/425v. at 200 m/a. 6.3v. 4 amps. 0 CT. 6.3v. 4 amps. CT. � • 5v. 3 amps. '"• � Fully Shrouded 51/ - 10 .." w > W.I. Inpuc 200/250v. • � w Output 325/0/32Sv. at -lO - // 20 m/a. 6.3v. 0.6 amps. 6.3v. 1.5 amps. Chassis -300 8 mounting . . . . . - . .. 23/- (�Is) fREOUENCY Fig. 17. Response curve of cirwit of Fig. 15· OTHER "WIRELESS WORLD " REPRINTS Receiver Alignment Equipment : I. Simple Cathode-Ray o�c;nos�..,pc H. ASHWORTH (March 950). e ign for � Wobbu1atQr October 1950). M. ScrQggie} !I.se., M.l.f.f.1 2. D. . •.• ... (...... 9d.By ""'. ByG. POSI 10! RH R" c" INPIIT fROM PICI(UP OR S[ " R" Cu PilE-AMPLIfiER � R" S,.� R FALL Cu R" R . . c" R C" "l R� et:: , v" JR"�S t�V' I" =� 11.1.010 -1'�r;;;"'LJ I1IN. 11.1. I I1IN. .. , . 'NPIIT BASS I I� � �ll , CU � RISE� Cu R" C" R" Fig. 19· Complete tone compensation and filter unit. The input and output underlined are peak values voItages for full output from the main amplifier. List of Components for Fig. 19. Tolerance Rating R" 0.25 L�ati!lg �I n log. (V d.c. R .. 47 kO Typ<> working) Tolerance IW 1<" 47 kO Silvered mica 5% 3.31.0 IW do. 5 R. . % R. . 0.25 MO log. do. 5 100 % R" kn do. 5% Ru 6.8 kO do. 5% R 10 n kO do. 5% R 0.1 .. MO linear do. 5% R" kO Electrolytic 20 100 IW 12 % R. . 2.2 kO Paper r.oo R" 0.1 MO 10% Electrolytic '50 OA7 Mn 10% Silvered mica 1% R. . 0,47 MO 10% do. 1 R. . % 33 kn do. or R., IW 1% kO do. matched 1 R. . 3.3100 IW % kO do. 1% R�R62 11'.10 do. 11% Rw 0.1 1\10 High. " % do. R" 1% 0.1 MO stability matched 1% do. 50 kn } caroon R 1% Electrolytic M lOO n '''' R" do. All resistors may be 1 rating, tolerance 20%C unless r.oo \V otherwi${: sptlcified. Choke. eHa SOH 20 mA. Resistam:e about 1,500 O. Rating a.t (V d.c. Mains Transformer. Type working) Tolerance Primary : 10·0-200-220·240 V, 50 cls. 50 Electrolytic 12 Secondaries : 1. 325-0·325 V, 20 mA d.e. C" /AF 8pF Electrolytic 450 2. 6.3 V, 0.6 A. C .. 0.25 �· Pa.per 500 20 3. 6.3 V, 1.5 C" % A. 150 pF ma.x. Proset Cro 0" 0.01 f'F Paper 250 20% Switche�. 0.05 f'F do. 250 20 % S,. Singlc pole double throw. Ctt 1000 pF Silvorod mica 20 S. C� % U{lublo pole double throw. 50 pF Electrolytic 12 C 8,. Single pole double throw. C"u 0.05 500 20 S,. 5 5 f!F Paper % bank, po�ition selector switch. ,8 Tht WiIliamson Amplifier THE WILLlAMSON AMPLIFIER DESERVES JOINTS SOLDERED WITH One impe r c ly may r e t Jomt endanger th� successful assembly of the Willillm n Amplificr. Solder with (.!u puge 30 jor 9trlerlll detail.!) so Radio Feeder Un it Multicore and run no risks. Multicorc contains 3 of non cores extra-.lctive, ADDITIONAL COIL DATA corrosive Ersin Flux ensuring speedy and rdiable soldering without waste The radio feeder unit described struction of coils for the reception or trouhl and gua dn eein that there are on succeeding pages was desigoed of the Droitwich transmitter on e r t g originally to provide high-quality no lengths without flux. Correct :wo kc/s, and the author has !Sup of solder reception from medium-wave !ita of both and solder are plied the following additional data proportions flux lions and coil-winding data covered automatically applied. Fast-acting, fast for those who get a higher signal a range of frequencies from 500 kc/s holding Muhicore is used exclusively strength for the RRC. Light Pro to r.6Mc/s. hy 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 Mult;core. COIL-WINDING DATA FOR THE LONG-WAVE RANGE (shoWI! abol·e) has beell'"I SllE(Iesiglled 1 CARTON specifically for easy Ilse. Simply pull out the length you Inductance Coefficient require. CI6018 specijirotiQn (60/40 Transformer No. of turns of coupl� Wlndlng (.... H) al/oy) is particularly recommended for (apprOL) the Wi/liomSOI! Amplifier 180 700 RADIO TIV SERVICE Primary 4 Aerial 0.3 330 2,000 ENGINEER'S REEL Secondary 1 1tl. Primary 260 For UIoCI whio;h dcmal>d 1,500 Coupling 0.6 raor q ...anll!� of solder. Secondary 330 2,000 Contain. approximately ------167 feel of 18 S. W.G.5O/SO Idlo), El'$in Mulliooft Coils are wound with 40-42 s.w.g., used, the minimum capacitance of the SOlder. d.s.c. copper wire. ganged capacitor should increased US;" MlIlticore Solder can he obloilled To give the correct coefficient of be by tbe addition of a JoopF silvered_ fr om radio shops el·ery.,.,·here. Size I coupling the spacing between the capacitor across each secondary mica ('orlon 5/- relail. I lb. reel 15/-. windings of the aerial transformer winding of the transformers, giving a should increased to 0.25ill. The coverage of approximately 150-300 be disposition of the coupling trans· kc/so MULTICORE SOLDERS LTD. former windings is unaltered. For fixed tuning, the capacitors MULTICQRE WORKS, MAYUNDS AVENUE, When continuous tuning is to be should be 300 pF. HEMEL HEMPSTEAD, HERTS (IOXMOQRMM) 29 The Willialllson Amplifier Design for a Radio Feeder Unit preceding articles in this transmitters, and which desires offered as an indication of the HEseries have described ampli only to receive transmissions from general lines all which to proceed, fier, tone compensation and these by the simplest possible and is capable of being adapted to Tgramophone pre-amplifier units means. individual requirements and con which are capable of driving a In order that the units described ditions. loudspeaker Lrom the output of a in the series should form a com The basic circuit, shown in pick up or a radio receiver. The plete domestic installation. Fig. consists of an r.f. ampli sound 20, design of a radio receiver which it is proposed to outline the design fier. transformer-coupled to a would be suitable lor use under of a small two-stage receiver suit negati ve-feedback detector. Cir the varied receptio n conditions able for the reception of mediurn cuit values for a number alter of which exist in the populous parts wave transmissions within the native bIlling 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 be described givessatis scheme. from the point of view of to simply and with certainty of re factory results where the spacing construction, is to use a twin sults, would be a difficult under between the carrier frequencies of gallged ca acitor to cover the p taking. In addi tion, such a the principal transmitters is high, range, although by this method it receiver would be unnecessarily say kc/s. Tt is not suitable is not easy to secure a uniformly 200 corn plex for the needs of tha t for use in districts where dosely good performance at each cnd of section of the community which spaced powerful transmissions the medium-wave band. Alter lives within the prim ry service e ist where interference is natively the receiver may be pre a x , or area of high-powered twin-wave severe. The receiver circuit is tuned, stations being selected by a Fig. 20. Circuit dia- gram of ocal station I R " + 150V 1 .. ' rad io rece iver. Posi_ I tions of selector " I switches for pre-set ' EF:Woc ni own at u I Cn t ng sh X. � KTW61 ..., - . - . -" I -" I r7 R" R" C" T I I T, I\s;� .. ,� I� c- C,.� I ; R" I R .. Coo I '---- CH R" Cn R" OUTPUT "--r-� , t I Coo � RIl (Ill i COMPONENT VALUES FOR CIRCUIT OF FIG_ 20 atin ating (V d.c. R g Type R working) R" 0.[ 'l\H1 jW See text R" 0.1 Ma O.lJ.lF Paper 25O R 330 a ., 1.5 kil O.II'F Pa.per 350 R M MO O.IJ.lF Paper 35. R 0.1 .. kO 2TV lI.il'lo' Electrolytic 450 R" IQ 47 kQ OOpF Silvered mica n Jl] 4.7 kO lOOpFI Sil�-ered R" mica R" 22 kfl O.I,IJJ � Paper 500 2.� MO .. ",. e i or ! W rating, tolerance All r s st s may be 20 per cent unless specified. otllcrwisc The Williamson Amplifier push-button or rotary switch. WINDING DATA FOR R. F. TRANSFORMERS Tbe use of variable indudors in this arrangement provides a simple method of achieving a uniform Transformer WindIng No. of turns Inductance ofCoefficient coupling selectivity and sensitivity over the (f.lH) (approx.) range, with the disa dvantage that two coils or tuned circuits must be Prima.ry 35 30 provided for each st..1.tion be Aeria.l 0.35 to conda 95 lOO received. In the unlikely event of Se ry serious thermal drift, correction is easily applied by the use of nega Primary 60 80 Coupling tive temperature coefficient capa 0.65 citors. Secondary 95 100 ... R. F. Transformers. - Winding data are given to enable Lf. trans formers to be wound simply on dimensions of the coil formers and of instability is the presence of standard formers without the llse windings are shown in Fig. 23. undue st.ray capacitance between of a wave-winding machine. Tbe \Vhen 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 Ior 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 llie 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 Although 4. 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 leakage to the valve base All ±I8 . 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, t.rimmers, covering a Irequency Construction.-In order to pre and all component.<; 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 t.he anode circuit, When separately-switched tuned receiver. The most likely cause or in the following grid circuit transformers are to be llsed, the values of secondary inductance 400 and tuning capacitance may be read from the curve of Fig. 21 against transmitter frequency. This curve has been cOTJJ.puted for / an L/e ratio of unity (L in ,..H, C in pF), which is nearly opti mum. The number of turns 0 .. 1/ necessary to produce the required �, U inductance with the formers and 1/ dust-cores specified may then be <.,..I::i ,,-'" ," I/� f- obtained from Fig. 22. The �M.,.� :$� ;0" f- ,�/'.0 1+ 0 .' 40' - '" 1/ V- l.-' 1/ V- I/ , , 10 0 t- , 200 Ii I 100 0 ' " '" ", '.000 70 " " ' 12' 140 (k�IS) fREQUENCY NuMBER Of TURN'> Fig. 21. Curve relating tuned circuit parameters Fig. 22. Curve relating inductance and num. and resonance frequency. bee of turns fOf windings discussed in teat. 3' The Willi_mlon Amplifier Call FOFlHE.RS must brought ablJve the chassis. be is the case when tuning is by as means of a. ganged capacitor. they must be screened carefully from the aerial circuits. Figs. 24 and ,-:=::�-I" 25 show suggested layouts for con tinuously variable and switched �SLOTS CUT WITH I,1° 0'3" tuning arrangements. THIN SAW The Detector.-To give low ! i distortion, the detector I'equires , ,__ . <, _ . L .. _ L . ,, work at a fairly high signal ---, to I_L level-say 5V r.m.S. output. As " the receiver is intended to feed the MATERIAL; O'OI� ORBAKElIZfO PAXOllN fABRIC tone compensation unit, which requires an input of only 200 mV peak, the output is taken from a TRANSFORMER COUPLING TRANSfORMER tapping on the detector load resis AERIAL tance. This greatly reduces the a.c. loading on the detector and enables it to handle high modula tion levels without distortion. SEC. SEC. Alignment Procedure.-(l) Set ganged capacitor at a position PR I. PRI. about five degrees from the mini mum capacitance cnd, and adjust trimmers for maximum output from the high-frequency Third Programme. (2) Set capacitor about twenty FOR FIXED TUNING OATA degrees from maximum capaci PRIMARY WINDIN(;. (OIR£O URI.U PRIMARY WINDING. tance position and adjust dust CONNECTION) cores for maximum out NUMBER Of SECONDARY TURNS lh NUMBER or SECONDARY 'UA�S 'fI put from the low-fre PRIMARY WINDING. (lOn fEEDER) srCONDARY WINDING. TEXT quency Third Pro- SEE 'hv Nur'I8ER OF SECONOARY TURNS gramme. ' . ----- (3) Repeat this pro SECONDARY WIMOIN(;. SE[ TEXT cess until both stations e--l'- --- II, "'"" "" IIl0C� � are accurately tuned. Fig. 23. Formers are standard Power Supplies. moulded type, fitted with a-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. Fig. 25. Plan enamelled copper wire. view of top of chassis. Swi tched model. to Sll pplied from the bc s'f;------>j pre-amplifier power supply. The decoupling is not dequate a to enable it to be fed from the main amplifier supply. Acknowledgment. - The writer llUMltllUM OR COPPH is indebted to Mr. A. T. Shepherd EXTENOING BELOW CH,t.,SSIS5tR£{H Cn of Fcrranti, Ltd" for his assis . AND llf: ABO�E CHASSIS 'oIITH 4 >, fOil GANGrD CAPACIT()� tance in the compilation of ' data ��:::::��q._C_'_'_O_-I::ll::ji�3l:lG l/CUTDUT for these notes . ""'" ,'I; "" .. � . 0 C O . ��g. 24· This di�ram ,...ows a p, an V1ew 0f L.l��:::::::�===n�----j�J top of chassis_ 3' the 3C67A High quality output TRANSFORMER It gives us great pleasure to announce OUT latest OU1 u, we know, unique and permits of seven different transformer for the "Williamson" amplifier. •F he secondary impedances with virtually no change in result of some painstaking research, this instrument leakage reactane<: as against thc normal four obtained represents quite a considerable advance on our 2836R. with CUSlOmary arrangements of eight sections. Typical technical details are as follows : the 3C67A these impedances are 0.450, 1.8n, 10 40, 70, 110, 16n and 300. Onc additional impcd Pritnary ance-220----can be obtained if required with an Resistance 900 ·f 900. increase in leakage reactance of about 10 per cent. D.e.Incremental inductance taken at ') v. 50 1'.'5" 100 hys. minimum. Frequency Response Nominal impedance 10,0000 centre tapped. With no feedhack and assuming a generator impedance Leakage reaClance tested at v. 800 ds. : of 2,5000 the frequency response, even at incremental Whole Pdmarr to the secondaryI connected for levels, is virtually fiat (0.25 d.b.) from 12� c.'s. to JOn and short-cm�uited, 16 m 'Hys. Insertion loss 25 kc s. There is a smooth high Irequency "roll 0.5 d.h. (6i per cent.). off" with no objectionable resonances. Half primary to the 300 secondary, 8 m'liys. !-Ialf primary to the other half short-circuited, Power handling capacity 17 m,lHys. At 15 watts at 121 e{s., the core material is being worked at 10,000 gauss aod the distortion intmduced by the iron at this frcquency and power level Secondary Considerable thought has been given to the question an)' feedback should not exceed 2.5 per cent. w;lho1/t of secondary impedances, and we havc decidcd 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 givcn by a basic imrcdancc of 0.450. amount of feedback will reduce the distortion to The arrangcmcnt 0 secondary sections is, so far as negligible. proportions. .. outstanding results " " ...As Mr. Williamson himself observes in one U of his artieles, the output trans Wireless World former is probably the most critical component in a high-fidelity amplifier. Your transformer performs this exacting duty with full honours, and I shall always recommend its use to anyone undertaking construction of a " Williamson." With many thanks for a first-class job." E. L. J., Swindon. NURSTEED ROAD DEVIZES WILTS Telep hone Devizes 536 33 The WilIiamson Amplifier Replies to Queries by Raised Co nstructors series of articles recently published on the to adjust the anode currents to equality, but unless High-Quality Amplifier has aroused consider the transf0n ter has a split primary winding they HE . : able interest and given rise to correspondence. are Inconvement, and great care should be taken to TIt is hoped that these notes, whieh deal with ensure that the insertion of instruments does not matters of general interest ansing from the corres �ause oscillation which could give misleading read pondence, may he of assistance to readers who have mgs. similar difficulties. Construction.-There is little to add to the con Valvcs.-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 IS), 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 f om 25 W depth of the chassis. r to by reducing the total current from 125 The method of construction of the preamplifier 21.5 \V mA to llamA by adjustment of R". The use of these and tone-compensation units will usuaUy be adapted Onc suggested method valves with reduced rating entails Cl. slight reduction to individual circumstances. IS of the maximum output. The 807 may be used at ?f construction for the preamplifier circuit of Fig. the full rating of W, with modifications to the IS to use a shallow chassis about 9in x 3in x lino The 25 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 nUIi1bcr 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 use may be advantageous The connections to the valveholde!'s are taken its for VI and V,., 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 Output Transfonner.-Vvhen assembling the core sions up to 3in x3in x 'Zin. 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 laminations butt to U 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 Static Balancing.-The method of balancing the constructed on orthodox lines, the only essential being standing currents in the output valves, which was to provide sufficient frontal area to accommodate suggested in the article in the August, 1949, issue, is seven controls. Grid leads should be kept short to dependent for its success on close matching of the avoid hum pick-up. The blank valveholder tenninals 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 I"esistances should not differ chassis, materially. It is possible, howeve1, due to varia Conc1usion.-The circuits published in the series tions in wire diameter and insulation thickness, for have been evolved over a. considerable period of ti me the resistances to differ by up to S per cent and even, and are capable of giving a. very high standard of in extreme cases, per cent, Should this occur, a performance . Requests have been received for data 10 compensating resistor should be added series with on modifications, but as it is rarely possible to in 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 ean be supplied sections. by the writer,- Other more direct methods may, of course, be used • Or. fn' that matl�r. by Wirel.n lVaT/d.-Er>. 3rd Edition. Compiled by the staff of WIRELESS WORLD. ives the RADIO VALVE main characteristics and base connections of over types Gof British and American radio valves, and over cathode-ray2,000 tubes. These arc DATA further classified into obsolete, replacementISO or current types as recom mended by the makers. 80 pp. 3s. 6d. net. By post 3s. IOd. Ch aracteristics 2,000 Receiving Obtaillable from all booksellers or direct from : of Valves and C.R. Tubes ILIFFE & SONS LTD., DORSET HOUSE, STAMFORD ST., LONDON, S.E.t, 34 The WilliamsoQ Amplifier Modifications fo r High-impedance Pickup s and Long -playing Records introduction of long-playing records in Great means that the first stages of the pre-amplifier must cy HEBrit ain, after the public..1.tion in November, be capable of handling occasional hig�-frequel� 1949. of gramophone pre-amplifier circuits for peaks which arc greater than those expenenced With standard records. unless the pick-up is Tthe " High Quality Amplifier " which were suitable a constant only for the 78-r.p.m. standards. has made it nece s amplitude one, or its output at high frequencies is attenuate before reach ng the pre·amp ifier. sary to revi5e these designs. d i l The principle of recording with a rising frequency The original designs of pre-amplifler 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 wide range of a to effect a reduction in the level of surface-noise from inputs. However, pick ups are available wilh s�ch a . . the material, is a well-established and useful one. In wide variety of output levels that no Single ClrcUlt 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 atars may have to he used. record material, in an almost silen t background. Modifications.-Dealing first with the single-valve There arc. 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. shows the modifications 27 energy level of music decreases with increase of fre necessary to provide alternative standard and long quency above about c J s. Thus it should be pos playing characteristics. To si plify the swit hing, 500 m c sible steadily to increase the gain of the recording by using a single-pole challgeover switch, the capa channel ab ove this frequency. This appears particu citor C.1 is left permanently in circuit, giving a larly attractive at first sight. since with the normally Decca. 78_Lp.m. characteri stic in the " 78 " position. used constant-velocity characteristic the recorded am Alternatively, Cl_, may be removed to give the KM.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-Lp.m. characteristic may be made by means at high frequencies. o[ the treble control on the tone compensation unit. Initially. a rising frequency response characteri stic 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 spccd·change control to give automatic com relied upon to restrict the amplitude at high frequen. pensation. this arrangement is not desired, a two H 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 Cl6 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 sollnd su erimposed on the full orchestra. p discharging of capacitors as the switch is operated . There is additional evidence to suggest that the A small capacitor, C,;, has been connected across original hypothesis required revision, since it is the input tra.nsformer secondary. This is to prevent demon strable that it brea s down when such per k any tendency to instability or peaking at high fre- cussion instruments as cymbals and castanets are COII sidered, particularly when the frequency range is wid e. Indeed, the peak power level required to re Fig. 26. Recording characteristic used for current Decca produce cymbals exceeds that normally require d at long-playing records. medium frequencies. This early experience has led to "'20 the adoption of a characteristic -� which is a better compromise + 0 - v I between these conflicting factors t: and gives much more satisfactory g results in practice. Fig. 26 shows 0 � the provisional recording charac- I :;l , --- teristic now in use by the Decca -I - Record Company for L.P. � 0 - � I I records. The amount of treble -20 0 0 0 0 boost is lower than the theoreti N g- cal optimum, but the use of even § o N this amount of compensation (cfo) rREQUENCY 35 * * * * * * ADVANCED THEORY OF WAVEGUIDES By RADIO INTERFERENCE SUPPRESSION : AS L. Lewin. Sets out the various methods that have AI'PLIED TO RADIO AND TELEVISION RECEPTION. 2nd been found successful in treating the types of Edition. By G. L Stephens, A.M.I.E.E. Explains problems arising in waveguide work. the principles of suppression technique as applied to 30s. nee. By post 30s. 7d. both radio and television reception and gives many examples of practical applications. RADIO DESIGNER'S HANDBOOK 4th Edition. lOs. 6d. lItt. By post IOs. lid. Editor : F. Langford� Smith, Senior B.se., B.E., Member I.R.E. CU.S.A.), A.M.I.E. (AUST.). A com SHORT-WAVE RADIO AND THE prehensive reference book, the wurk of 10 authors IONOSPHERE 2nd ByT. W. Bennington. and 23 collaborating engineers. Intended especially Shows how existing ionosphericEditi on. data can be applied for those interested in the design and application of to everyday problems ofshon-wave transmission and radio receivers or audio amplifiers; contains a vast reception. IOs. 6d. net. By post 105. l Od. amount of dara in a readily accessible form. preparatioll. 42s. lIft. By post 435. 6d. SOUND RECORDING AND REPRODUCTION 11/ By the Staff of the Engineering Training Dept.. FOUNDATIONS OF WIRELESS 5th Edition. B.B C. Covers the theory and practice of disc, By M. G. Scraggie, M.r.E.E. Covers the whole R.se., magnetic and film r(':cording, with special refercnce basic theory of radio, the reader requiring no to B.B.C. equipment. 30s. net. By post 30s. Bd. previous technical knowledge. 12s. 6d. lIet. By post 13s. TELEVISION ENGINEERING : PRINCIPLES AND GUIDE TO BROADCASTING STATIONS 6th PRACTICE. Volume 1 : Fundamentals. Camera Tubes. Edition. Compiled by " Wireless World." Tabu Television Optics, Electron Optics. A B.B.C. En lates, both geographically and in order of frequency, gineering Training Manual, by S. W. Amos, B.SC., all European long-wave and medium-wave stations A.M.I.E.E., and D. C. Birkinshaw, M.B.E., M.A., M.I.F..E., and 1,400 short-wave transmitt<:rs throughout the in eo!1aboration with 1. L. Bliss, A.M.I.E.E. The first world. 2s. /la. By post 25. 2d. volume of a comprehensive work on the funda mentals of television theory and practice. HIGH-QUALITY AUDIO AMPLIFIERS In preparation. Popular circuits reprinted from «Wireless World." Includes "Wireless World A.C./D.C. Quality TELEVISION EXPLAINED 4th Edition. By Amplifier," « High-Quality Amplifier Design " and w. E. Mi!1er. M.A. (CANTAB), M.BRIT.I.R.E. A non « Economical 50-watt Amplifier." mathematical, step-by-step description of television 25. 6d. lIet. By post reception circuits, aerials and aerial systems, receiver 15. &1. installation and operation. Ss. net. By post 5s. 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ILIFFE & SONS LTD., DORSET HOUSE, STAMFORD ST., LONDON, S.E.I 36 The Witliamson Amp lifier quencies, caused by the presence in the feedback loop a switch inaccessibLe, consideration should be gi\'cn of the stray secondary reactances of the to the of a relay in place of the selector switch. transformer. usc 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 sbo uld be kept ali lilllall as possible, consistent from the speed-change lever by means of a mino· with stability. switch or from the additional switched pin which is Modifications to the three-stage high-pass prealll a feature of some pickups with interchangeable heads. plifier (original circuit Fig. p. 26) are on the samc Pickups without Transformers_-A number of pkk· 15, lines, and Fig. shows the revised circuit. tipS are available which do not normally require a 29 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 prc-amplifier circuits. by interp:Hing a should. if possible. be mounted on the pre-amplifier. suitable 1: 1 transformer. In other cases. when thc Should the position of the pre-amplifier render suc:h connecting leads an: short. it may be practicable to connect the pickup directly in place of the transformer seeondary. The limit· ing factor will be the capacitance )I'V between the leads and their screening. R" which will be shunted across R or Ru' and which, if sufficiently l�ge. would upset the treble compensatiotl. The value of this stray capac itance shoul not be allowed. to exceed pF. R" � 50 and If e'l or CU IS switched out. R,,: FC�.,.. should be compensatod by a capacit ance of one tenth of its value in r �arallel with Rn or R", to give a C" hnear frequency-response character Cn - t"'- �, istic at high frequencies. . " . ' Resistors R" and R�. must he o.- - o·- R" " - " j C R o· l - [ I 'm:oo: � " � ,':" �5, . Left : Fie. �7. Simple two.position twitch R -' . " o· �i I- o Ine In smtle-volve pre-omplifier (or L..- r [C" -� playin, Decca l8-r.p.m. standord and " Ht-r.p.m. records. Comf't;nsatian ,C L.P. Rn ; R,. (or !he E..M.I. 7B-r.p_m. stondard choroc C, R" terlsric should be opplied seporotely by the � treble tone control. Below : FI.e. 28. Alternoti'le circuit (applic List of Components for Fig. 27 able to F'es. 27, 29 ona 30) with three poSition switch eMng compensation (or Type Rating Tolerance Decco 33!. Decca 7B ond 78-r.p.m. R" Value to suit High-stability E..M./. transrormer carbon recording characteristi". W R" O.IMn do. I C"OR Cu R" 0.68MCl do. jW R" 0.22Mn do. IW -f R" 47kn do. IW R32 4.7kn do. l·sMn : Q-004.uF R" 0.22Mn Composition 10% IS/--I-F R�4 22kO do 10% '·00 DE':�/_ R" 2.2Mn . do. All jW rating, tolerance 20% :n ' ) resistors may � unless 14 [.H.J. OIherwise specified. Rating 78?, CV d_c. Type working) Tolerance C" 0.5f'F Paper 250 C" 5Of'F Electrolytic 12 " C" 16f'F do. 450 IOOpF Rn C" 1000F Silvered mica 250 10% "1 1 R" C" O.05f'F Paper 500 100PF C" 1�5OpF Silvered mica 250 Coo 2500pF do. 250 10% " C" 1500pF do. 250 10% R,. Coo 300pF do. 250 10% R,. S, Single-pole changeover switch '7 HANNEY OF BATH WllLlAMSON AMPLIFIER COMPONENTS 12 1% .ilye. mira condensers for Small quantiti". of WODEN main. tran.· PARTRIDGE type WWFB/O/1.7 fir. fo.mer., cholc , and ouput tran,form ro output tran.formers unpotted. 7 5% 1'.1,ilver mica condensers for 15 • are available ..from time to time, ..e includi", packin, our current list 19 ...... tailed in lupplement.as d Acos GP pick"p', ... ndard or 117 0 fi , . 7 • 25 Resistors lnd pOts for William . LP 20 ...... on main amplif,er " , • T.C.C. .25 mfd. metal packs Hnds only, sc�ndard Or LP ...... 9 Rui.torl lor fig. I] " . (co�plin, conden.ers for n,. I) " B.S.R. type GU-4 ,ram_motors .. 23 R".istors for fir. 15 ______. .. • • ndt £9 19 • " . '" B.S.R. type MUI4 ] speed motors -4 22 Resi.tor. and potslor fig. 19.. mId. metalmitu £1 J T.C.C.(cou plOn,.05 cond"Mer. for I) B.S.R. type MUIO 2 .peed moto . 4 6 Condens"., for "I. 13 .. " . I� 15 • n,.each COLLARO type ]RC521 . U 15 Condensers for 15...... " f" . . valv�., motchcd p�ir, in- , chan,er., 3 opeed unmixed,luto_ with 33 Condenser. for fi,. 19 .. . KTclud,n, 66 ...... p" r 2 Hi_Fi . l he.d,.... 10 Resistors for fi,. 20 ... ..• tax cry • EF37A valve. (EF37 now ob.olete) " , ... £18 1� 8 7 Conden.er< for fir. 20 (le COLLARO type 3RC511 auto- (includins ...... each '" chan,erl. as .bove but play • nden) .. C68 " . • Ma,ns Telep hone : 38 11 RADIO DESIGNER'S HANDBOOK A compreh�nsivc:: rdc::rc::ncc::, the work of 10 amhors and 23 colbborating cnglllcers 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 enormous amount of data an 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 four times as large as the previous ,·olume, and contains work The main subjects valves and valve ar'"' hitherto unpublished. It is the work of ten testing, general theory and COmponents, audio authors and twenty-three collaborating en fre quencies, radio fr equencies, power supplies, gmeers, 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 from the address below. JLlFFE SONS LTD., DORSET HOUSE, STAJ\·fFORD ST., LONDON S.E.l & The Williamson Amplifier retained to pro vide a conducting path to the valve of Fig. 14, p. Thi circuit is suitable for most 25. 5 grid when the pickup heads are being interchanged . moving-iron va.riable-reluctance pick ups. and can be There may be cases, where one side of the input used with piezoelectric pickups which havc 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 tbis event the velocity. circuit may be modified as shown in Fig. 30. This Danger of Overloading.-The input to the prc ci«:uit applies to both prc-amplifiers. In it. the trans amplifiers 5hould restricted to :.IoomV in the case be formcr had been replaced by a resistive network R"", of the single-stage circuits and 50 mV for the three Ra,' 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 MD, alld its voltage ga in at I.OOO C S is The / 9. Piczoelec::tric Pickups.-Lightweight piezoeleclric frequency-response curve is almost identical with that pickups have recently be come popular, particularly for L.P. recordings. Since these give a relatively high output, no pre-am plifier is necessary and any correc Below ; fig. Revised three-stage pre-ampli(ier circuit with 29. tion required may be achieved by means of simple high-pass (ilter, play Decca 33t- and lB-r.p.m. records. fO , .. '" "'" " " .. c .. ' ' ' c" ' .. '.. �:. �:. C. :, . . �!! I 1 .�,; Coo c ..' r .. , . , , .. '" " ; ; r t'-,'� , • c ..' ' s ' c" [(."' � "-'" O(ctIAI�:' '.. f r!& . : " : '.. : ,.. c .. ' � ' " �'" rc " ' ®' L�:' ''''' c" ' .. ' .. ' i! .. ' ' : '" . - c" ' . ic .�· f' Component Va)ues for Circuit of Fig. 29 Type Rating Tolerance Rating R" Value to suit High-stability (V d.c. Transfonner ""'oon Type working) Tolerance R.. O.IMO do. !W 20% C," O.5p.F Paper 250 200;., 12 R" O.68MO do. jW 20 % C" 50p.F ectrolytic El 16 F 450 Rn O.22MO do. !W 20% Cb2 p. do. 1O'}(, R" 4.7kO do. 20% C�3 O.02p.F Paper 350 10% R" O.22MO Composition 10% CM, l pF Silvered mica 350 Coo OO Pape 250 2 % R. . 20kO* do. O. 5" F r % 0 R" 22kO High-stability IW 20 C" 50pF ElccU"olytic 12 carbon Cos O.01,..F Silvered mica 350 1% R.. O.22MO: Composition 10% oc matched R" O.20M!l* do. Coo O.2Sp.F Paper 500 20 % R" 4.7M!l do. 5 "" COO 5000pF Silvered 350 1 % ' oc at ed R. . 1.0MO do. jW 20 % m ch R" O.22Mn do. ,W 20% C" SOOOpF do. 350 do. R" 2.2kO do, 20% C62 7000pF do. 350 10 �� R" 2.oMO High-stability 1% C" O.5p.F Papcr 500 16,..F 20 �� carbon or matched CH Electrolytic 450 Rn 2.0Mn do. do. C" 10-SOpF Silvered mica 250 R,. l.OMD do. C" O.lp.F Paper 500 do. R" lOMQ Composition 5% C" 2500pF Silvered mica 250 10 % R" 47kO do. 10% C" 1500pF do. 250 10% 10% R" lkn do. 20 % Coo 300pF do. 250 R" 47kfl. do. IW 20% S, Single-pole changeover SWilCh. R" 0.22Mn do. 20% R" lOkO do, IW 20 % R .. 2.2MQ do. 20 % • May require adjusuncnt. All resistors may be lW rating, except where otherwise stated. 39 The WiJliamson Amplifier ,g. M0 difi�d input cir- RC networks, details of which have alre3.dy been F" JO cuit for use without tran, . published. ' former when °"' Checking the Pre·amplifiers.-When a pre·a.mpli· side of the pick "P [J er has been constructed, it is advisable to measure must be earthed. response curve over the audible frequency range its and beyond, in ordcr to ensure that nothing is amiss. I This is particularly so in the case of the three·stage pre-amplifier. To facilitate this measurement the networks of Figs. .. loo' kfi and have been devised. These circuits, when --- I I JI 32 -.....(t- fed with constant-voltage variable·frequency input, r .� o"OO6fLf R" IOOkn � O·03,uF "" CKUr.uT I 101t0 1 TAP AT RESISTANCE EOUAL TO mn IMPEDANCE Of PICKUP IF I I WER THAN llOn INPUT ""n (CONSU NT VOLTAGE) ; t/ OUTPUT TO PRE-AMPLlFIER (DISCONNECT FOA EM.l CHARACTEFIISTIC) C·OOSfLF Fig. 32. Simulator for Decco 33t r.p.m. characterisric. L.P. produce outputs which are, respectively, replicas of I TAP AT RESISTANCE EQUAL the standard and L.P. characteristics. IMPEOAtlC< OF PICKUP IFTO To test a. pre-a.mplifier, the appropriate network THAN 1·110.0 should be connected between an oscillator and the INPUT /0'<101 pre-amplifler input. The output from the pre-ampli (COIo "iTANT VOLTA(",[) fier for a. constant voltage to the network should then ::;--OUTPUT TO follow the response curve already published for the PRE -AMPLIfiER appropriate circuit (Figs. and pp. and :; q). 14 n, 25 Acknowledgment.-The writer is indebted to Decca for information a.bout their recording characteristic. Fig. 3/. Simulator for Deaa and E.M.I. 78-r.p.m. recording Kelly, "PicJmp Input ...... characteristics. , Wen and1950. pp. 386-391. Ciceuiu," WiT l War/d, November. 1'",lIed in Englalld by ('"uTI""all I'TC$$ Lld., Pa,i. Ga"fell. Lum/VII, 5.f..1. Ll00t-nR!,;l�l� KS --..... GOODSELL � WILLIAMSON HIGH - FI DELITY AMPLIFIERS * The Goodsell version of the World famous WHliamson Amplifier is to full specification, I laboratory bullt and tested. * All components are adequately rated to ensure long life and trouble free operation. t * Fully troplcalrzed version. Type GWr8/C is avai lable for use overseas. * All models have separate power supply incorporated on the same chassis for use with : multi-stage pre-ampllfiers, tape pre-ampllfiers and tuner units. � I � i I � : A laboratory report of i WHUamson Amplifier, our type GWr9/C, which w;as submitted to Intermod". I.H. Distortion Power Output Frequeney I lation tesu. The results using an Altee lanCing I Ratio I:" � Intermodulation DiUortion Meter and Boonto" Seat Oscill ...tor ...re given. Frequency O.l�2% 15 watts 10 & 2, 000 c/ •. The report on the Goodsell pre· ...mplifier showed it 10 wattS 10 & 2,000 eis, to be ... compar.able unit With s(mUar figures. 0.900 15 Wiltts .00 & 2,000 els. Type GWIB (standard model) Pr(ce Ul ; 5: 0 0.2% 10 watts 100 & 2, 000 e's. ! Type GWrB;C (as IUu.tr... ted) Price £J6 : 0 : 0 I I PRE-AMPLIFIER TONE CONTROL UNITS : Multi.$tage, low noise tone eontrol units with radio input and gramophone pre·ampHfiBr. Equalization for micro groove and stand... rd rec ording eharacterinics, 5 �sition .teep cut low.pu. filter and cathode follower out.put. feedback over every suge ensures negti,ible distortion. Type PFA with equalization for British and American recording characteristics and 5 mv sen,ltivlty on LP and 7B. Price m: 10 : 0 ! Type FUTC with LP ... nd 78 equalization ... nd 15 mv sensitivity. Price ll2 : 12: 0 I MANUFACT URED BY_ : j GOODSELL LTD., 40, GARDNER STRE ET, BRIGHTON, 1 I f Phone: Brighton 167J 5 • i I ...... ------...... ------_ . _ ------___. .- ----,� � - ..... VALVES fo r the L.63 WILLIAMSON KT.66 AMP IFIER The following Osram valves are specified : L.63. *8.65. KT.66. U.52. Use of KT.66 is essential to obtain optimum perfonnance of this equipment. The characteristics of KT.66 and 6L6G are not identica1. Circuit information on a Quality Amplifier for DC/AC operation may be obtained from 8.65 the Osram Valves & Electronic Dept., Magnet House, Kingsway, W.C.2. * B.G:j dOl/ble triade as altemarive for any two LJjJ type. U.52 THE GENERAL ELECTRIC CO .• LTD., MAGNET HOUSE, K!NG5WAY, W.C.2