This is only a preview of the January 2004 issue of Silicon Chip. You can view 33 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Studio 350 Power Amplifier Module":
Items relevant to "High-Efficiency Power Supply For 1W Star LEDs":
Items relevant to "Antenna & RF Preamp For Weather Satellites":
Items relevant to "Lapel Microphone Adaptor For PA Systems":
Articles in this series:
|
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
|