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PUBLISHER’S LETTER www.siliconchip.com.au Publisher & Editor-in-Chief Leo Simpson, B.Bus., FAICD Production Manager Greg Swain, B.Sc.(Hons.) Technical Staff John Clarke, B.E.(Elec.) Peter Smith Ross Tester Jim Rowe, B.A., B.Sc, VK2ZLO Rick Walters Reader Services Ann Jenkinson Advertising Enquiries Leo Simpson Phone (02) 9979 5644 Fax (02) 9979 6503 Regular Contributors Brendan Akhurst Rodney Champness, VK3UG Julian Edgar, Dip.T.(Sec.), B.Ed Mike Sheriff, B.Sc, VK2YFK Philip Watson, MIREE, VK2ZPW Stan Swan SILICON CHIP is published 12 times a year by Silicon Chip Publications Pty Ltd. ACN 003 205 490. ABN 49 003 205 490 All material copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Printing: Hannanprint, Noble Park, Victoria. Distribution: Network Distribution Company. Subscription rates: $76.00 per year in Australia. For overseas rates, see the subscription page in this issue. Editorial & advertising offices: Unit 8, 101 Darley St, Mona Vale, NSW 2103. Postal address: PO Box 139, Collaroy Beach, NSW 2097. Phone (02) 9979 5644. Fax (02) 9979 6503. E-mail: silchip<at>siliconchip.com.au ISSN 1030-2662 * Recommended and maximum price only. 2  Silicon Chip Valve preamplifier a big hit We were really surprised at the overwhelming response to the valve preamplifier featured in the November 2003 issue. It has generated more correspondence in just a month or so than any other project that we can remember. Not all of the response has been favourable, of course. Some people have said “Hiss, Boo” for featuring a circuit using ancient technology. That is partly the response we did expect and it is generally in line with our overall attitude to valves. Generally though, the response was very favourable and not just because older readers regarded it as a trip down memory lane. Quite the contrary actually, because people realised that we had attempted to present a very realistic and detailed description of the technology and its capabilities. We did this because we had not seen a magazine article anywhere which explained the graphical method of gain calculations. However, some of the responses were quite negative because we had used negative feedback to improve the performance and thereby negate the distortion characteristic of valves. Shock, horror! The circuit would now not be a musical or as “warm sounding” as “true” valve circuits really are. My response to that is “what a load of garbage!” In hindsight, we should have published the distortion curves for the first circuit we produced, which did not have any feedback. Its distortion rose to over 6%. Sure most of that would be low-order harmonics but anybody who thinks that level of distortion is OK or even desirable clearly doesn’t understand sound reproduction. Why? Because any circuit producing high harmonic distortion ALWAYS produces high intermodulation distortion as well. And while low-order harmonic distortion might be regarded as innocuous or even preferable, intermodulation (production of sum and difference frequencies between two or more input frequencies) is always unpleasant. In fact, intermodulation over a couple of percent is just horrible. It is also clear that some musicians think that valve amplifiers have benign “soft overload” or “soft” clipping, as opposed to the “undesirable” hard clipping typical of solid-state amplifiers with lots of negative feedback. Well, that ain’t the case either, as the scope photos on page 6 of this issue clearly demonstrate. Most push-pull valve amplifiers do use modest feedback but once they go into clipping, the weaknesses in the output transformer generally conspire to produce truly horrible distortion as you drive them seriously into overload. We took these measurements a year or so back on a commercial valve guitar amplifier. It was quite instructive for me, as I had forgotten just how bad valve amplifiers could sound! In fact, with a nominal power output of about 50 watts, its performance could be summarised in one word: gutless. Will we publish another valve audio circuit? Possibly. A new valve power amplifier? Maybe. But if we do, you can be sure that we will pull every trick in the book to make sure that it is as “state of the art” as possible. It would be very quiet, have very low distortion and probably be very expensive. And if we couldn’t make it very quiet and with low distortion, we would not publish it. Leo Simpson www.siliconchip.com.au