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Building the Williamson Amplifier

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Building 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 The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors ♦ PDF Cover Page ♦ ♦ Verso Filler Page ♦ Building the "WILLIAMSON" AMPLIFIER By HERBERT I. KEROES Acro Products Co. Fiq.1. Two views 01 the audio ampli­ fier. The power supply is built on the same chassis as the amplifier. Hi"h 'lualitrl performance is obtaine.l cathode. The two stages are self bal­ ancing and bias themselves to a low distortion operating point. froll' this AII,erican ve"sion of a B,·iti.�h .lesi"n. The heart of the amplifier· is the output transformer. This device must provide response that extends well be­ · · HE Williamson amplifier circuit* effort was made to choose substitute yond the limits of the audio band in T originated in England by D. T. N. parts that would permit the same high order to limit phase shift to the requi­ Williamson has attracted world­ degree of performance attributed to site degree in the feedback circuit. It wide attention from high fidelity en­ its prototype. is the degree of success with which thusiasts because of the almost perfect The circuit diagram of the amplifier this is achieved that makes for fidel­ quality of the reproduced output. is shown in Fig. 2. The circuit con­ ity in musical transient reproduction. There are many features in the am­ tains four resistance-coupled stages The original specifications of the out­ plifier that make it an attractive con­ and is operated with 20 decibels of put transformer call for response struction project for the builder. The voltage feedback taken from the sec­ which is down not more than 3 db. at ' circuit is simple, free from critical ad­ ondary of the output transformer and 3 c.p.s. and at 60 kc. An American justments, and may be built economi­ carried around the complete amplifier. counterpart, the Acrosound TO-290, cally of top quality parts at a cost of Medium mu triode tubes are used faithfully copies the performance of less than $50. Within its power rating throughout and are biased to operate the original and is used in the circuit of 10 watts at less than 1% intermod­ with minimum distortion. A note­ here described. ulation, the amplifier proves to be worthy feature of the amplifier lies The type 7N7 tube has been selected ideal for home and small auditorium in the selection of the type of output for use in the voltage amplifier stages installa tions. tube. This is a power tetrode which because of its shielded construction, The performance of the amplifier, is connected in the circuit to function low internal capacity, and symmetri­ based on listening tests, can best be as a medium mu power triode. The cal base layout that permits direct described as containing the elusive driving voltage required is much point-to-point wIrmg. The output "presence effect," a quality inherent smaller than that taken by the more tubes are the type 807, the character­ in low distortion equipment with the conventional 2A3 or 6B4 type of out­ istics of which are similar to the fiat frequency response and low phase put tube, and the driver operates with KT-66 British type used in the orig­ shift that enables speech and musical considerably lower distortion. inal. The plate resistance, however, transients to be correctly reproduced. The first two stages of the amplifier is about 20% greater for the 807, and The bass response is solid and free are somewhat unusual. The first stage, this requires a corresponding increase from harmonic distortion. Highs are which is a voltage amplifier, is di­ in the plate-to-plate match of the out­ clean and crisp with none of the shrill­ rectly coupled to the second, a catho­ put transformer in order to obtain the ness so often experienced with other dyne inverter. This method of cou­ same low figures of distortion in the amplifiers. pling is made possible by the high output. The plate-to-plate impedance Since a number of the components operating potential on the inverter of the transformer is, therefore, 12,000 specified in the original amplifier are ohms instead of the 10,000 ohms speci­ • Wireless Worl(j: April. May 1947. August of English manufacture, considerable 1949. fied for the original. Another excel- RADIO & TELEVISION NEWS lent output tube which may be used is the Western Electric type 350A. This tube has greater power capabili­ ties than the 807 and will provide up to 25% more output. One deviation has been permitted in the circuit and is based on the operat­ ing characteristics of the 807. An elec­ trolytic condenser has been added across the cathode biasing resistor ar­ rangement of the output stage and has been found to be of material ad­ vantage in further reducing distortion at high output levels. £onstrnction The amplifier is constructed on a single 10 x 14 x 3 inch chassis. Consid­ erable care has been taken in the lay­ out to permit direct point-to-point wiring in all signal circuits and to avoid extraneous couplings which may introduce instability into the feed­ back loop. In the construction of the amplifier a ground bus is not used since no improvement will be effected through its use provided several pre­ cautions are observed. Ground re­ R1- 1 megohm ..,01. control Cn, C.-10/10 fold., 450 �. elec. cond. turns should be made closely adjacent R.---470 ohm, If, w. res. C" C,-.25 !Lld., 600 �. cond. to the stage affected. This may be Raj Rs-22,OOO ohm, Y2 w. res. (matched pair C.-500 !Lld., 50 �. elec. cond. ±1%) C,-4 /lId., 600 �. cond. easily done by using sockets with R4-47,OOO ohm, 1 'W. res. C", Cll-20 !Lld., 450 Y. elec. cond. ground lugs projecting from the R.-33,000 ohm, 'Iz w. res. T,-Oulpul Irans. 12.000 ohm plale.lo-plale, ohm, 1/2 W. res. 10 . 8, 16 ohms (Acrosound TO·290) mounting ring. It is also desirable, R'"j-22tOOO 4 Rs, R,-470,000 ohm, % w. res. T2-Power Irans. 425·0·425 �. @ 130 ma.; 6.3 but not essential, to solder one mount­ R1o-560 ohm, 112 'W. res. �. @ 3.5 amp.; 6.3 �. @ 3.5 amp.; 5 �. @ Ru, R2lo-4700 ohm, 'Iz w. res. 3 amp. (Acrosound TP·310, Slancor P.4004, ing tab of each electrolytic condenser or equiy.) R12J R13-47,000 ohm, 2 w. res. (matched pair to the mounting plate and one point CH,-4 hy., 150 ma. filler choke ±1 %) 1/2 CH,,--4 hy., 50 ma. filler choke on the plate to the chassis. The fila­ RH, Rn-lOaD ohm, w. res. ],-Inpul jack ments are wired by running a sepa­ R15, R;xJ-IOO,OOO ohm, V2 w. res. FI, F�Fused line plug R16• Rl�h RZ'2J R�3-100 ohm, V2 w. res. rate two-conductor pair to each stage. P,-Oclal plug R17-lOO ohm, 2 w. wirewound pot. V" V2-7N7 tube When wired in this manner only one RIB-300 ohm, 10 w. res. V •. V,-807 tube pair of leads carrying the stage cur­ C" C" C,-.05 Itld., 600 �. cond. V5-5V4 lube rent enters into proximity with the stage, and the hum field is reduced. Fig. 2. Complete circuit diagram 01 the American version 01 the "Williamson" amplifier.
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