Amp/Preamp Asylum: RE: harmonic distortion does not get 'buried in the mix'. by morricab Looking for a new Amp or Preamp? If you're after tubes, post over here.

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Posted by morricab (S ) on July 5, 2012 at 01:13:20

In Reply to: RE: harmonic distortion does not get 'buried in the mix'. posted by villastrangiato on July 4, 2012 at 19:41:28:

Technically I don't think a manufacturer of tube amps can be a shill for tube amps.

There is plenty of literature out there dealing with this subject if you didn't have your head burried so far up...er, in the sand.

Read the writings of Norman Crowhurst in the late 1950s regarding negative feedback. Read the latest writings from Nelson Pass http://www.firstwatt.com/articles.html. Click the Audio distortion and Feedback article.

Read the master's thesis of Cheever: "A NEW METHODOLOGY FOR AUDIO FREQUENCY POWER AMPLIFIER TESTING BASED ON PSYCHOACOUSTIC DATA THAT BETTER CORRELATES WITH SOUND QUALITY" You can google it and read the .pdf of it.

Read the papers published by Earl Geddes:http://www.gedlee.com/distortion_perception.htm

Distortion perception and Distortion perception II

"Distortion by‐products that lie closer to the excitation are less likely
to be perceived (they are masked) than those that lie farther away
(masking is a localized effect – it mostly occurs in the vicinity of the
masker).
3. Distortion by‐products of any kind are likely to be more perceptible
at lower signal levels than at higher signal levels. (Less masking
occurs at lower signal levels)"

"The masking effect of the human ear will tend to make
higher order nonlinearities more audible than lower
order ones.
2. Nonlinear by‐products that increase with level can be
completely masked if the order of the nonlinearity is
low.
3. Nonlinearities that occur at low signal levels will be
more audible than those that occur at higher signal
levels."

These are some fundamental problems with high feedback (like your Crown) amplifiers. A non-feedback amplifier, if otherwise designed properly, will not have high order harmonics or if they are produced at higher volumes then become masked as the the higher levels increase the masking range. Clearly an amp with only low order distortion will often sound completely UNdistorted at all levels short of clipping because of the ear/brain masking.

Further:
"A sound systems quality is often judged (but not
exclusively) by its THD and/or IMD numbers.
�� In the context of the perception of distortion, it is not
unreasonable to question the validity of these numbers
for several reasons."

"They (THD and IMD) are purely mathematical relationships without
consideration for the characteristics of the receiver – the
human ear.
�� The recent application of psychoacoustics to problems in
audio data compression clearly indicates that masking
plays an almost dominate role in hearing acuity."

"THD and IMD have no correlation to the perception of
the distortion that they are intended to represent.
�� Correlation is possible with a metric that takes into
account the way the ear actually functions
�� One of the most important implications is that (non-linear)distortion
in loudspeakers could well be an insignificant factor"

This last point is interesting because generally loudspeakers only make low order harmonic and IM distortion. They DO NOT make the same kind of non-linear distortion that can be generated in amplifiers.

"Loudspeakers tend to have very low order nonlinearities
�� Higher orders would require very large forces in a
mechanical system
�� Loudspeaker distortion tends to increase with output,
but is generally low at lower levels
�� These factors imply that loudspeaker distortion may be
masked in the ear"

The Cheever thesis says much the same thing and he comes up with a new metric as well for distortion perception: Hint, your Crown and it's ilk don't fare to well...

So, even though it is clear you lack the perceptive qualities to actually hear what is wrong with most amplifiers at least you can follow some intellectual discussion on the matter as long as you have not blinded yourself to scientific investigation.

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