The Williamson Amplifier A Collection of Articles, reprinted from “ Wireless World,” on “Design for a High-quality Amplifier” By D. T. N. WILLIAMSON (formerly of the M.O. Valve Company, now with Ferranti Research Laboratories) Published for Wireless World LONDON : ILIFFE & SONS, LTD. Digitized march 2011 by Thomas Guenzel for www.radiomuseum.org The Williamson 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 (October and November 1949) Design for a Radio Feeder Unit 30 (December 1949) Replies to Queries Raised by Constructors 34 (January 1950) Modifications for High-impedance Pickups and Long-playing Records 35 (May 1952) Digitized march 2011 by Thomas Guenzel for www.radiomuseum.org The Williamson Amplifier Introduction Introduced by Wireless World in 1947 as merely one of a series of amplifier designs, the “ Williamson ” has for several years been widely accepted as the standard of design and performance wherever amplifiers and sound reproduction are discussed. Descriptions of it have been published in all the principal countries of the world, and so there are reasonable grounds for assuming that its widespread reputation is based solely on its qualities. This booklet includes all the articles written by D. T. N. Williamson on the amplifier. Both the 1947 and 1949 versions are reprinted, as the alternative output transformer ratios cover a wide range of require- ments. Modifications and additions include pre-amplifier circuits and an r.f. unit, with recently published information on adaptation to high- impedance pickups and correction for 33 1 r.p.m. records. 3 We would stress the importance, if the full potentialities of the amplifier are to be realized, of following the author's recommendations in detail. Even in the U.S.A., where several modified versions have been described, many users adhere to the designer’s exact specification with the original valve types. It is not the circuit alone, but the properties of the valves and such components as the output transformer together with the welding of theory and practice into a rational layout which produce the results. Editor, Wireless World. Digitized march 2011 by Thomas Guenzel for www.radiomuseum.org The Williamson Amplifier Basic Design Requirements: Alternative Specifications ECENT improvements in the operation of the loudspeaker. spectrum (but especially, at the field of commercial sound This in turn reconverts the elec- low-frequency end) be substan- R recording have made prac- trical waveform into a corres- tially less than that at medium ticable the reproduction of a ponding sound pressure waveform, frequencies, filters must be wider range of frequencies than which in an ideal system would arranged to reduce the level of hitherto. The useful range of be a replica of the original. these frequencies before they reach shellac pressings has been ex- The performance of an amplifier the amplifier as otherwise severe tended from the limited 50-8,000 intended to reproduce a given intermodulation will occur. This 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 20-15,000c/s. This ponents of a mythical Fourier where pedal notes of the order of increase in the frequency range analysis of the waveform. While 16-20 c/s cause bad distortion, has been accompanied by an this method is convenient and even though they may be in- overall reduction in distortion and indeed corresponds to the manner audible in the sound output. the absence of peaks, and by the in which the mechanism of the (3) Negligible phase shift with- recording of a larger volume range, ear analyses sound pressure wave- in the audible range. Although which combine to make possible a forms into component frequencies the phase relationship between standard of reproduction not pre- and thereby transmits intelligence the component frequencies of a viously attainable from disc re- to the brain, the fact that the complex steady state sound does cordings. Further improvements, function of the system is to repro- not appear to affect the audible notably the substitution of low- duce a waveform and not a band quality of the sound, the same is noise plastic material for the of frequencies should not be not true of sounds of a transient present shellac composition, are neglected. Sounds of a transient nature, the quality of which may likely to provide still further nature having identical frequency be profoundly altered by disturb- enhanced performance. contents may vet be very different ance of the phase relationship The resumption of the television in character, the discrepancy being between component frequencies. service with its first-class sound in the phase relationship of the (4) Good transient response. In quality, and the possible extension component frequencies. addition to low phase and fre- of u.h.f. high-quality trans- The requirements of such an quency distortion, other factors missions, increase the available amplifier may be listed as :— which are essential for the accu- sources of high-quality sound. (1) Negligible non-linear dis- rate reproduction of transient Full utilization of these record- tortion up to the maximum, rated wave-forms are the elimination of ings and transmissions demands output. (The term “ non-linear changes in effective gain due to reproducing equipment with a distortion ” includes the produc- current and voltage cut-off in any standard of performance higher tion of undesired harmonic fre- stages, the utmost care in the than that which has served in the quencies and the intermodulation design of iron-cored components, past. Extension of the frequency of component frequencies of the and the reduction of the number range, involving the presence of sound wave.) This requires that of such components to a minimum. large-amplitude low-frequency sig- the dynamic output/input char- Changes in effective gain during nals, gives greater likelihood of acteristic be linear within close “ low frequency ” transients occur intermodulation distortion in the limits up to maximum output at all in amplifiers with output stages of reproducing system, whilst the frequencies within the audible the self biased Class AB type, enhanced treble response makes range. causing serious distortion which this type of distortion more (2) (a) Linear frequency re- is not revealed by steady-state readily detectable and undesirable. sponse within the audible fre- measurements. The transient Reproduction of sound by elec- quency spectrum of 10-20,000 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 electrical waveform capacity for negligible non-linear at a rate determined by the time which should be an exact counter- distortion at any frequency within constant of the biasing network, part of the air pressure waveform the audible frequency spectrum. by a rise in bias voltage which which constitutes the sound. The This requirement is less strin- alters the effective gain of the purpose of the amplifier is to gent at the high-frequency end of amplifier produce an exact replica of the the spectrum, but should the (5) Low output resistance. electrical input voltage waveform maximum power output/frequency This requirement is concerned at a power level suitable for the response at either end of the with the attainment of good Digitized march 2011 by Thomas Guenzel for www.radiomuseum.org 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-pull triode transformer. valves without (b) To improve the frequency the refinement of response of the amplifier and negative feed- output transformer. back form the (c) 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. Fig. 1. Output/input characteristics (a) without ment. A stage of (d) To improve the low-fre- feedback (b) with negative feedback. this type has a quency characteristics of the out- number of dis- put transformer, particularly frequency and transient response advantages. With reasonable defects due to the non-linear from the loudspeaker system by efficiency in the power stage relation between flux and magne- ensuring that it has adequate such an arrangement cannot be tizing force. electrical damping. The cone made to introduce non-linearity to (e) To reduce the output movement of a moving-coil loud- an extent less than that represen- resistance of the amplifier. speaker is restricted by air loading, ted by about 2-3 per cent (f) To reduce the effect of suspension stiffness and resistance, harmonic distortion. The output/ random changes of the parameters and electro-magnetic damping. In input characteristic of such a stage of the amplifier and supply voltage the case of a baffle-loaded loud- is a gradual curve as in Fig. changes, and of any spurious speaker, the efficiency is rarely 1 (a). With this type of characteris- defects. higher than 5-10 per cent, and the tic distortion will be introduced at A stage of this type is capable of air loading, which determines the all signal levels and intermodula- fulfilling the highest fidelity radiation, is not high. In order to tion of the component signal requirements in a sound repro- avoid a high bass-resonance frequencies will occur at all levels. ducing system. The output/input frequency, the suspension stiffness The intermodulation with such a characteristic is of the type shown in a high-grade loudspeaker is characteristic is very considerable in Fig.