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Ultra-Linear Operation of the Williamson Amplifier

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Ultra-Linear Operation of the Williamson Amplifier Web: http://www.pearl-hifi.com 86008, 2106 33 Ave. SW, Calgary, AB; CAN T2T 1Z6 E-mail: [email protected] Ph: +.1.403.244.4434 Fx: +.1.403.245.4456 Inc. Perkins Electro-Acoustic Research Lab, Inc. ❦ Engineering and Intuition Serving the Soul of Music Please note that the links in the PEARL logotype above are “live” and can be used to direct your web browser to our site or to open an e-mail message window addressed to ourselves. To view our item listings on eBay, click here. To see the feedback we have left for our customers, click here. This document has been prepared as a public service . Any and all trademarks and logotypes used herein are the property of their owners. It is our intent to provide this document in accordance with the stipulations with respect to “fair use” as delineated in Copyrights - Chapter 1: Subject Matter and Scope of Copyright; Sec. 107. Limitations on exclusive rights: Fair Use. Public access to copy of this document is provided on the website of Cornell Law School at http://www4.law.cornell.edu/uscode/17/107.html and is here reproduced below: Sec. 107. - Limitations on exclusive rights: Fair Use Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, includ- ing such use by reproduction in copies or phono records or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for class- room use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include: 1 - the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes; 2 - the nature of the copyrighted work; 3 - the amount and substantiality of the portion used in relation to the copy- righted work as a whole; and 4 - the effect of the use upon the potential market for or value of the copy- righted work. The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors ♦ PDF Cover Page ♦ ♦ Verso Filler Page ♦ Ultra-Linear Operation of the Williamson A m p lifier DAVID HAFLER':: and HERBERT I. KEROES::: The Famous "Williamson" can be improved simply by replacing the output transformer and making a few minor changes in other components. The results are well worth the effort and expense. RO:\I Tllr. TIME an article I appeared descrihing the principle of Ultra­ F Linear operation of power output tuhes, considerable interest has been evidenced in rhe application of this new circuit improvement to the famous WiIliamson amplifier. The William­ son circuit ha$ hcen publicized in several arrangenlents including at least one commercial one, and the configur­ atiou is undoubtedly the most popular The �uthors' Ultr�· high-quality audio circuit ever devel­ Line" �mplifier com­ oped. For Illany people there is little bined with the power necessity to attempt to improve this supply on a single basic amplifier circuit. Its listening chassis. quality is excellent; it is easy to COil­ strtlct; and it provi(les top quality at a cost comparahle wirh units which cannot measure up to its capahili ties. The one category in which the \Vil­ liamson amplifier is significantly de­ .ficient is with regard to efficiency and power-output capabilities. Peak power output is less than 15 watts, and it takes a 450-voIt supply at approxi­ mately 130 ma to achieve this power screen grids of tetrodes are energized TO-300, which was designed for use output. It this limitation can he over­ from a tap on the primary of the out­ with tubes of the 6L6 type. Its 6600 come without deterioration of quality, put transiormer. This connection, on ohms primary impedance therefore, is a change in the original design is justi­ which patents are pending, modifies the also correct for 51:\81's and 807's in the fied. If simultaneonsly it is possible to operating characteristics of t.he tube. Ultra-Linear hook-up. In alhlition, KT- improve the amplifier hoth in measure­ Proper location of the tap results ill 66's can be Ilsed without deterioration able aspects and in listening quality, optimum input-output linearity simul­ of quality as the slight mismatch is in then a change is not only justifiable, it taneously with ellicient operation, a favorahle direction with respect to is mandatory. power capahilities approximatdy douhle distortion characteristics. Thereiore, It is difficnlt to impr"ove on some­ those of a triode connection, alld low­ this transiormer can be used with the thing which is really good. There .re impedance output such as is offered by tube types normally used in \\'illiam­ some audio enthusiasts who will scoff triodes. In short, it �>crmits better per­ son ampl ifiers w i thou t compromise of at lhe illea that the \Villiamson circuit. fonuance than either triode or tetrode characteristics. It is of interest to note can be improved. However, it has heen connection oi the tubes, and this is sub­ that the change in impedance to 6600 fi,'e years since l\[r. Williamson pub­ stantiated in comparative listening does not violate �[r. Williamson's de­ lished his circuit; and in the course of tests an(1 distortion measurements. sign considerations. The modi fied tube five years, there is little which can The unique merits of the Ultra-Linear characteristics of the Ultra-Linear con­ maintain supremacy without change or stage are particularly applicahle to the nection require this impedance if we renovation. \Vhen a basic circuit im­ Williamson circuit. The mating of the wish to preserve operating conditions provement-the Ultra-Linear output two seems to have been ine,·itable. The similar to those of the original amplifier. stage arrangell1ellt-<ame along, it was simple suhstitution of an 'output trans­ I n other words, the tuhes are still natural to see how it could fit in with former with primary taps for Ultra­ matched for minimum distortion rather the basic Williamsoll circuit. J .inear operation and a few minor than for maximum power output. The The Ultra-Linear output stage is 110t changes in circuitry, which will be transiormer, therefore, can he placed in a triode stage as is used with the 'ViI­ discussed he low, combined the basic the circuit directly and the screens of liamson circuit-nor is it a tetrode or circuits into an amplifier which prac­ the output tubes connected to t he ap­ pentode srage. It combines the ad­ tically cveryhody agrees is an improved propriate taps as shown in Fig. 1. This vantages of both triolle and tetrode hy version in all respects. Obviously, we eliminates the two lOO-ohm screen using an arrangement in which the must gain improvement if we substitute stopper resistors of the original circuit. a more linear outpnt tuhe and use a The plate and screen leads of the trans­ * Aero Products Co., 369 S/lIIrs LOIII!, Philadelphia 28, PI!ll110. transformer which exceeds the origin­ former are color coded to avoid phasing 1 Hailer and Keroes, "An ultra-linear ator's stipulations for performance. di fficttlt:ies. amplifier," AUDIO ENGINEERING, November The original Ultra-Linear circuit Several additional circuit changes 1951. utilizes a transformer, the Acrosound have hecn found heneficial for optimum 26 AUDIO ENGINEERING • JUNE, 1952 -'1----0,. \10.'1---.0 • ICI-----'O 4 c ",..+----:-911 =+---1- acllO To-� ---- '.TO�� __------ �--------t- -I�� __TQG 'AlII __ to-tO � 500 w" 11711 Fie. 1. Schematic of the Ultra-Lintar Williamson amplifier. The components in dotted circles life those which are changed from the origin,l circuit i" making the conversion of an ellisti�1 amplifiir to Ultra-Linear oper.ltion. performance. One of these is the all taps of a tapped - secondary winding. creasing one pair of coupling capacitors change in value of the feedback resistor The amplifier, as converted, now from .05 Ilf to .25 Ilf gives a five-to-one to 10,000 ohms in order to maintain 20 surpasses the original with respect to ratio of time constants for the two db of feedback. In the Ultra-Linear response, distortion, and transient char­ pairs of networks and increases the stage the gain of the stage is greater acteristics. In addition, it was con­ low-frequency stability margin at nom­ than for a triode stage. In addition, the sidered desirable to make certain other inal increase in cost. change in primary impedance changes slight changes which primarily increase The insertion of a lO,OOO-ohm para­ the proportion of voltage fed back. the stability under feedback conditions. sitic suppressor- in the input grid and Thus the feedback is increased unless The low-frequency time constants of a 100-1111£ capacitor across the feedback the feedback resistor is changed to the original circuit's interstage coupling resistor adds to the high-frequency compensate. The readjustment of this networks were the same for both such stability margin and eliminates a slight­ resistor to the desired value then per­ networks. This is not particularly de­ ringing in the vicinity of 200 kc. mits the added gain of the Ultra-Linear sirable in a feedback amplifier since a One last optional difference from Mr. output stage to increase the amplifier given frequency loss is accompanied by Williamson's original circuit lies in the sensitivity. It can now be driven with maximum phase shift. Separation of use of a bypass capacitor across the a little over 1 volt as compared to almost the time constants permits less phase cathodes of the output stage.
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