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SILICON
SILIC
CHIP
www.siliconchip.com.au
Publisher & Editor-in-Chief
Leo Simpson, B.Bus., FAICD
Production Manager
Greg Swain, B.Sc. (Hons.)
Technical Editor
John Clarke, B.E.(Elec.)
Technical Staff
Ross Tester
Jim Rowe, B.A., B.Sc
Nicholas Vinen
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Ross Tester
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Glyn Smith
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Rodney Champness, VK3UG
Kevin Poulter
Stan Swan
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2 Silicon Chip
Publisher’s Letter
The quest for ultra-low distortion
This month, we present the first article on the
Ultra-LD Mk.3 amplifier module. This is the second
time we have revisited this class-AB amplifier design
and it’s almost getting monotonous, as we announce
even lower distortion. Exceedingly low! In these
days of iPODs and MP3 players, why do we bother?
The reason is that those sound sources are so
mediocre. Why put up with mediocrity? The same
comment can be made of the vast majority of hometheatre systems and indeed, virtually any sound
source that most people listen to. Compared to where
we were with high-fidelity sound systems based on CD players 25 years ago,
we have gone backwards. Furthermore, it can safely be said that the vast majority of people under the age of 30 probably have never experienced a good
hifi sound system. If they have, it was probably years ago while they were still
living with their parents.
We at SILICON CHIP, on the other hand, believe that the best quality sound is
still worth striving for. We also have better tools and design methods to help
us get there than we did years ago. Our benchmark is the standard of performance available from the best CD players available today, such as the Marantz
CD player reviewed in last month’s issue. So in designing a no-holds-barred
amplifier, our task is to produce something that does not degrade the sound
quality of such a CD player in any way. Or for that matter, the sound quality
of a premium Blu-Ray DVD player.
Ideally, this means that its total harmonic distortion across the entire audible spectrum up to 20,000Hz must be less than .0015% and its signal-to-noise
ratio should be better than -110dB. If we get more distortion or more low-level
noise, we are degrading the signal quality. It turns out to be a very difficult
task to achieve such a standard. Indeed, home-theatre equipment, from even
the best brands, is hard-pressed to achieve .01% harmonic distortion – about
10 times worse. And the typical home-theatre system with class-D amplifiers
is a good deal worse again.
Our 20W Class-A Amplifier has been our best attempt to date and its distortion is far below that of the best CD and Blu-ray DVD players, for frequencies
below 10kHz at least. We have always aimed for the same standard with our
more powerful class-AB designs and that is a much more difficult task. But
with the Ultra-LD Mk3 amplifier module we have come tantalisingly close,
as readers who like to closely compare performance graphs will attest (the
relevant graphs for our 20W Class-A amplifier were featured in the May 2007
issue). The difference is that the Ultra-LD module is a great deal more powerful than the class-A design.
Significantly, the new module typically has half the harmonic distortion of
the module it supersedes. And while we are very pleased to be able to present
it, we have not left all those people who built the previous module in the
lurch. We intend to bring out a small adaptor PCB which will bring it up to
the same standard.
Is this the ultimate in amplifier design? In the true sense of the word, it is but
only for now. Who knows what new devices and new techniques may bring in
the future. In the meantime, our challenge is to produce a complete integrated
amplifier featuring the new module and with facilities for analog and digital
sound sources. If the development of the new Ultra-LD amplifier module is
anything to go by, that could take some time. Stay tuned.
Leo Simpson
siliconchip.com.au
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