[Arny Krueger]

"Phil" wrote in message


Nevertheless, it is a given, in my mind, that a very high
open loop gain, with its need to constantly "correct"
every input signal by 99% (in the case of 40 dB
feedback), *regardless* of the inherent linearity of the
amp's devices and circuit, MUST cause problems for
signals 60 dB to 80 dB below the main signal, and perhaps
also phase shift the high frequency components, as
Aczel's summary of Otala's paper states, thereby robbing
the circuit of much of its "life" and "air," the
criticisms one normally hears about high feedback amps,
and also solid state amps, in which the solid state
capacitances and high thermal variations also interfere
with low level signals.

Could you be more presumptious or wrong, Phil?

This will not show up as TIM or
SID, unless the amp has been very poorly designed, and
I'm still not sure how one would measure it.

There's really no way to measure your imagination, Phil.


My best
guess has been to use a 20 Hz signal and a much smaller
(-60 to -80 dB) 10 KHz signal, filter out the 20 Hz
signal from the output with a notch filter plus high pass
filter, and either look directly at the 10 KHz signal for
signs of distress, or filter it out with another notch
filter, and see if phase shifting causes "sidebands" to
appear and disappear when the 20 Hz signal is put in and
out of the test.

You're looking for the unholy Grail, Phil.

People don't use notch filters that much any more. They just apply the test
signal and analyze the amp's output with a very good spectrum analyzer.


Assuming that normal feedback causes problems

That takes a lot of ignorance or paranoia.

-- and as
Patrick says, with low feedback and tubes it isn't too
bad, but SS amps have more problems and generally need
more feedback

SS amps don't have more problems, if well-designed.

-- it would be nice if we could figure out
a way to tremendously reduce the need for feedback to
"correct" every normal signal by 99% even when there is
no device distortion, meaning allow the feedback to
"focus" *only* on actual device and load non-linearities.

The error here is that there are any unavoidable problems with the
application of lots of feedback.


Here is where Black's "feedforward" circuit may allow for
a real advance in SS amps, especially if tubes, with
their (generally) superior ability to handle a mix of
high and low level signals without messing up the low
level information, are used to provide the error signal.

In fact tubes have no such advantages.

Properly applied, Black's feedforward scheme (but not the
feedforward designs by many others!) does exactly this,
it allows feedback to appear and affect the signal *only*
when actual deviations caused by device or load
non-linearities appear.

This is nuts.

It may even be possible to
correct the effects of a typical transistor's parallel
capacitances -- which, being made of silicon, are of
*very* poor quality -- and thermal variations!

Making really good power amps with silicon transistors is an old art that is
quite well perfected at this time.

Normally,
this would be a "why waste the time, just use tubes,"
situation, but good output transformers are heavy, big,
and expensive, and if the amps used in compact disk
players, as well as TV's, could be considerably improved,
that would be nice! And of course, inexpensive amps that
sound very good are always in demand.

Thats why so many of them are made and sold - there's lots of demand for
them.