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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 Bob Young 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: $69.50 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 Hifi equipment can be a big delusion One of our greatest pleasures in producing SILICON CHIP magazine is the great feedback we sometimes get from readers. It doesn’t happen all that often but when an enthusiastic reader does make contact, it can be a real buzz. On the other side of the coin, some electronics constructors are really keen to push circuits or designs that they have built and we are certainly not keen to disillusion. Sometimes though, it has to be done. We had one case recently where a keen reader was very im­ pressed with a class-A amplifier which he had built from an overseas publication. Being so sure of its worth, he wanted us to test and measure it. He was very persistent. Since he was an employee of one of our clients, I reluctantly agreed. As soon as I saw it, I knew that the performance results were certain to be bad. I didn’t know how bad but I knew he was going to be disappointed. Don’t get me wrong. He had done a very good job with its construction. It was housed in a 2-unit high rack case, used a big toroid power transformer and had lots of capacitance – cer­tainly enough to ensure 10 watts class A operation per channel. But when I asked him about the circuit and he sketched it out, I knew it was going to be even worse. In essence, it just used two power Mosfets, one set up as a constant current source for the other which operated as a simple common-source amplifier and with no feedback. And to make things even worse, the internal speaker wiring was run in Cat-5 cable (ie, single strand telephone wire). I told him this was bad practice but he still wanted meas­urements done. Ergo, I did them. And they weren’t pretty. Fre­quency response was OK, about 1.5dB down at 20kHz and signal-to-noise ratio was around -90dB – pretty good really. But distortion and power output? It was really only good for about 4 watts per channel into 8-ohm loads and at that level, the distortion was 10%, right across the band from 20Hz to 20kHz. This was from an amplifier which would have cost many hundreds of dollars to build! Even at low power, less than 200 milliwatts, distortion was 1% and it rose to 20% (yes, 20%) at just under 6 watts. Umm, what to tell him? Well, I could report that it was amazingly consist­ent in both channels. Worse still, the output impedance of the amplifier was really high, as you would expect with single strand telephone wire. How high would you expect? Try 3.5Ω. And even measured right at the amplifier modules terminals it was still 3.3Ω. The result is a gutless amplifier with boomy bass, due to the poor damping factor. But really, if you build such a simple amplifier with abso­lutely no feedback, the results are always going to be poor. And I can see some readers nodding to themselves but saying that their favourite amplifier is different – it sounds great. Well, dream on. You are bound to be deluded. As one of the staff mem­ bers here stated, “No parent has an ugly child!” The message is quite simple. If you build or buy an ampli­fier for which there are no published distortion figures, you will almost certainly get a load of rubbish. It might cost quite a lot of money but it will still be a load of rubbish. And what about the poor builder of this amplifier? How did he feel? I don’t know. At the time of writing this I still had not told him and I am not looking forward to it. Leo Simpson * Recommended and maximum price only. 2  Silicon Chip www.siliconchip.com.au