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IMPROVING 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 The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors ♦ PDF Cover Page ♦ ♦ Verso Filler Page ♦ IMPROVING the WILLIAMSON AMPLIFIER By T ALBOT M. WRIGHT Changing a few resistors in the driver section results in reduced distortion in this hi-fi amplifier design. HE Williamson amplifier, whether it clium to high levels and instability. We'll A careful inspection of any U.S. tube be of the classic triode type or the discuss instability later-but first to the manual will disclose the fact that the T "Ultra-Linear" vel-sion, has to some distortion. 6SN7 or 6CG7, as used in the William­ extent fallen into disfavor with audio­ son amplifier circuit, is badly under­ IM Distortion philes since it is now possible to build an biased. amplit\er at lower cost and still equal or If you run an intermodulation distor­ Take the push-pull drivel' stage fO/' better the Williamson's performance. tion test on an avemge Williamson am­ instance. Usually the plate loads arc Many Williamson owners have dis­ plifier using the classic drive system you 47,000 ohms and the cathode resistor is carded their trusty old amplifiers in will find that the 1M exceeds 2'ir at only around 560 ohms. This produces a bias favor of 50 or 60 watts in the never­ slightly over half the rated power. By of around 51,2 volts with about 175 volts ending chase after unmeasurable dis­ the time you reach rated power you on the plates, assuming a supply of tortion. Many Williamsons have been find the distortion is well past the toler­ around 440 volts. The tube manuals. in­ converted to higher powel's by the use able level. The result of this testing dicate that the 6SN7 should be operated of newer tubes and heftier output trans­ usually leads to a substitution of all the with 250 volts on the plate and an 81J.a­ formers. In some cases the proud owner tubes in a wistful search for lower dis­ volt bias. has realized an audible increase in defi­ tortion. Sometimes this helps a little. All you have to do is change that nition and an accompanying drop in Basically {ne voltage amplifiers are cathode bias resistor from its present distortion. producing much greater amounts of dis­ value to 1000 ohms. A resistor of 1200 Frequently such an experimenter will tortion than they should. "But," you say ohms works well, too. With a lOOO-ohm add a high-power output stage to his to yourself, "I followed the design bias resistor, there .will be about 250 Williamson drive system-in most cases scrupulously; even the voltages are volts at the driver plates and the bias paying for 50 watts in order to get a what they should he." The point over­ will rise to approximately 9 volts. Not clean 20 watts. Even then he doesn't looked. however, is that the American only does this cut distortion but it always get the improvement he expects. 6SN7, 6CG7, or 12AU7 will not work means you can produce more drive volt­ The fault lies not in the power stage but well with the circuit values specified by age. Naturally, too, you can now put in the drive system. The two main faults Mr. Williamson in his original paper on more signal voltage into the 6SN7 driver of the Williamson are distortion at me- the subject. before it starts to draw grid current. The driver will stay class A, in fact long after the output stage has finally given Fig. 1. aeductlon In Intermodulatlon distortion of the amplifier shown in Fig. 2. up. I rhis change alone, of course. will not 14 eliminate the distortion fed into the out­ IJ put stage. The first two stages are still " 8£FO"£ underbiased and they hang together be­ It cause of the d.c. coupling between them. / Let's start with the first stage. The I 2' bias resistor usually found there is 470 I- 'I 11:10 ohms. This should be increased to twice o I­ its value-simply add another 470-ohm ell • / o / I resistor in series with the fil'st. This ' raises the bias to around 3 volts, but 2 I incl'easing this bias causes the plate � 7 voltage of this stage to go up, thereby I AFTER .1 • decreasing the grid-cathode bias aCI'oss 'I 'I the' phase-inverter-the next stage. i/ J There al'e now two things to be done: 4 lower the supply voltage to the first / / stage and increase the supply voltage 3 ./ to the phase-inverter. If each of these is t changed in the right proportion, both V stages will be biased correctly. ...... / , The average WiIliamson circuit uses a 33,OOO-ohm resistor from the 450-volt 4 I , 10 IZ 14 " I, to Zt Z4 ZI ze 30 3Z 34 3' 38 40 4Z 44 4, supply point to decouple the first stage; POWER-WATTS If0U4V. SINl-wAVE) this must be increased to about 47,000 104 ELECTRONICS WORLD ohlll�. This will cut the current through With a 450-volt supply you can get well the distortion should be well below 0.5";' . the stage somewhat, producing a cath­ over 100 volts to each gdd. With a 400- Performance of the amplifier of Fig, 2 nde uias of around 21, .. volts, and it will volt supply that drops to something near is illustrated in the curve of Fig. 1. For also lower the plate voltage. The de­ 350 volts due to lack of regulation in the all IM tests, the generator supplied fre­ coupling resistor to the phase-inverter power supply, you should still be able to quencies of 60 and 6000 cps at a ratio of is usually 22,000 ohms from the 450-volt get 80 volts of drive per grid. 4 to 1. Readers should not be too con­ point. This resistor should be l'ec1uced to There is one thing more which must cemed about the absolute values of 3900 ohms, pulling more current be done before you have the amplifier power indicated nor a comparison of the through the phase-invertel' and raising operating correctly. The output trans­ power in one amplifier we have worked the cathode uias, formers used in Williamson amplifi ers on with another. The important thing \\'hen these last two chan ges have will not usually hane-lIe more than about heing shown here is the definite im­ b(,l'1l made there should be about a 5 %­ 20 db of feedback, but increasing t.he I'e­ provement in performance that occurred to i-volt bias from grid to cathode of sistor to the cathode of the first stage with the changes incorporated. t Il!' phase-in\'el'ter. Of couI·se. if t he ini­ has already increased the feedhack. Amplifier No. 2 (see Fig. 3) uses four tial supply voltag\? is much below 450 Since the value of the cathode resistor 5881's in a push-pull parallt'1 triode out­ you will not get exact Iy the ri ght bias, has been doubled, it is necessary to put stage. In this one only the biasing but it will be close enough to allow the double the value of the feedback l'esistor modifications have been made, The /irst two stages to bias each othel' cor­ to put the feedback at its previous value.
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