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Andre Jute
 
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Default KISS 116 by Andre Jute

KISS 116 by Andre Jute
This text is copyright Andre Jute 1997, 2004 and may not be reproduced
except in the thread KISS xxx on rec.audio.tubes
THE VOLTAGES IN THIS AMP WILL KILL YOU. GET EXPERIENCED SUPERVISION IF
IT IS YOUR FIRST TUBE AMP.

Is distortion in SE amps euphonious?
André Jute

There are some who claim that distortion in single-ended amps don't
matter, and even some who claim that their distortion signature is
what creates the euphony of SE! The argument goes that what has gone
wrong with solid state amps is that the search for ever lower total
harmonic distortion is the wrong quest. To jump from there to SE
distortion being a good thing is a world record in logical long jumps.

In 1947 H. F. Olson did a series of amplifier distortion listening
tests, most accessibly summarized on p609 of the Radiotron Designer's
Handbook, 4th edition. The finding that attracts attention from the
proponents of the 'euphonious distortion' camp is that a number of
critical listeners to a class A SE amp with an HF bandwidth of 15kHz
judged perceptible total harmonic distortion to begin at 0.75%, with
tolerable distortion at 1.8% and distortion being noted as
objectionable at 2.5%.

At the time, this may have proven that below x% distortion did not
matter too much. But it seems to me that fifty years later further
consideration is required even if only because psychological research,
unlike the physical sciences, does not pretend to offer definitive
permanent answers.

First, toleration of THD decreases with increasing bandwidth. With the
complete human hearing range of 20Hz-20kHz now being the aspiration of
all serious SE amps, if the Olson tests were repeated with a modern SE
tube amp the results would, I believe, report lower tolerances for
that reason alone. This is merely the headline: the case is
complicated by the fact that with reduced bandwidth objectionable
distortion can be disproportionally higher than perceptible distortion
in relation to the tolerances at greater bandwidths.

Second, tolerance for distortion is related to experience; this was
well understood even in Olson's day. Those 'critical listeners' of
1947 were likely to have had less experience of very low THD
reproduction chains and recordings than a similar group sixty years
later. Clearly, today everyone interested has much greater exposure to
high quality reproduction and also to live concerts. It seems at least
possible that a similar group of 'critical listeners' selected today
to take part in an equivalent test would establish lower tolerances
for each parameter with a ceiling of 15kHz (and much lower tolerances
at 20kHz).

Thus far the case as it stands against the 'euphonious distortion'
argument. There are however also two serious arguments that tend to
support it:

Third, higher toleration for distortion on any parameter is reported
by Olson for SE amps than for PP.

Fourth, modern SE tube amps have THD which compared to modern SS amps
puts them in the neanderthal era of audio, even if vastly improved
when compared to the classic designs. Despite all this, SE amps
continue to be highly valued by significant numbers of 'critical
listeners'.

There is also a less incontrovertible point that I would like to add
from personal experience and talking to designers and DIYers: it is
possible by fitting the wrong combination of modern parts to refine
any amp, but particularly an SE amp, to a point where it sounds
sterile, at which point a common response from experienced listeners
is, 'It sounds just like a tranny amp.' (As I say, this is not an
uncontroversial point. There are many respected designers who believe
that any propertly designed amp should sound like any other properly
designed amp, regardless of its construction with transistors or
tubes. They proceed from the assumption that the purpose of an
amplifier is to reproduce the music as it was played: they believe
that the aim of hi-fi is ever higher fidelity in an ever closer
approach to the live performance. Others, having given up on that
dream, consider that the aim of hi-fi should be to tailor a
reproduction chain to their personal taste. Of course parties who
start from such radically opposed basic assumptions will not agree on
anything.)

Thus, if I am wrong on the second point, increased experience, and a
new set of Olson tests report approximately the same tolerances in a
by now much more experienced group of critical listeners, the
'euphonious distortion in SE amps' argument would receive a shot in
the arm at least to the extent that there would be corollary evidence
that distortion up to a certain level adds to the perceived sonic
advantage of SE over PP. (The modern critical listener will then be
seen to have traded off an irrelevant potential reduction in noise for
greater usable power. At that point we will then have the fascinating
new question to answer: what inspired that minority of well-regarded
critical listeners to reduce noise by choosing higher impedance output
transformers, thus opening the whole can of worms again!)

If my hypothesis is correct, and in a new study lower percentages are
reported at all Olson parameters, the euphonious distortion argument
will be exposed as a relative environmental factor - - with a further
lessening of its importance being foreseen over time.

Meanwhile, there is not enough evidence to take sides.

© 1997, 2004 André Jute
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