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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
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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.
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