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PUBLISHER'S LETTER Publisher & Editor-in-Chief Leo Simpson, B.Bus., FAICD Production Manager Greg Swain, B.Sc.(Hons.) Technical Staff John Clarke, B.E.(Elec.) Robert Flynn Rick Walters Reader Services Ann Jenkinson Advertising Manager Brendon Sheridan Phone (03) 9720 9198 Mobile 0416 009 217 Regular Contributors Brendan Akhurst Rodney Champness Garry Cratt, VK2YBX Julian Edgar, Dip.T.(Sec.), B.Ed Mike Sheriff, B.Sc, VK2YFK Ross Tester Philip Watson, MIREE, VK2ZPW Bob Young SILICON CHIP is published 12 times a year by Silicon Chip Publications Pty Ltd. A.C.N. 003 205 490. All material copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Printing: Macquarie Print, Dubbo, NSW. Distribution: Network Distribution Company. Subscription rates: $59 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. ISSN 1030-2662 and maximum * Recommended price only. 2  Silicon Chip Measuring low level audio signals This month we’re very pleased to feature an ultra-low dis­tortion amplifier module running in class-A. The module is the direct result of readers’ letters asking for a state-of-the-art design over a number of years. Quite frankly, we haven’t been keen on the concept but having decided to do it, we were a little stunned at how good it has turned out to be. Sure, it has the usual drawback of class-A operation in that it is very ineffi­cient, using a lot of power for not very much power output – only 15W into 8Ω. But the distortion is a great deal lower than any amplifier we have produced in the past. And because the distortion is so low, it has caused us real problems in trying to determine just how low it is. Our audio distortion test set, made by Audio Precision, is perhaps the best commercially available equipment in the world but even it is not good enough to fully test this new amplifier module. And this caused us to use a new distortion monitoring technique, which was suggested by Doug Self in a recent issue of the English magazine “Electronics World”. Briefly, it makes use of the signal averaging facility in a digital sampling scope, to remove random noise from very low level repetitive signals. This is an interesting turn of events and shows how a digital oscilloscope, normally not regarded as ideal for observ­ing low-level analog signals, actually can be used with much greater effect than a conventional analog scope. By way of expla­nation, even though digital oscilloscopes are becoming widely used in laboratories around the world, designers still tend to turn to their trusty analog scopes when they want to look at low-level analog signals. The same situation occurs in our own lab. We use the digital scope all the time and often feature its recorded waveforms in our articles. But there are times when only the analog scopes will do. The reason is not hard to find. Digital scopes inevitably show even the cleanest of waveforms as having little wiggles all over them. But analog scopes show clean sinewaves when the wave­form is clean. Is that not true? Well, it all comes down to perceived reality and what we’re used to seeing on scope screens. Because analog scopes display repetitive waveforms as many hundreds or even thousands of sweeps of the beam across the screen, they filter out low level noise. Digital scopes don’t; their sampling method inevitably catches the noise and the glitches and so we see all the garbage on the waveform. We’re not used to this and it clashes with our normal perception of reali­ty. Most designers don’t like it. However, when analog scopes are called upon to display noisy waveforms they have a problem because the noise obliterates the repetitive signal buried under it. But it turns out that we can use the averaging mode of a digital scope to effectively remove the random noise from a low-level signal and allow the repetitive component to be clearly displayed. So the often-des­pised digital scope turns out to have unseen advantages. The problem with this averaging technique is that it again challenges our concepts of how signals should appear on a screen. Is this “filtered” version reality? The truth is that every method of measurement only gives us a partial view of what’s really happening in a circuit or piece of equipment. We become used to this “partial view” and accept it as the whole truth but again, it isn’t, is it? There will always be another way of measuring a circuit or producing a better result. There will always be something new; some new circuit technique, product or measurement method. That’s the challenge of electronics and it is very satisfying. Leo Simpson