"MZ" wrote in message


Some amplifiers I've encountered have what amounts to a
gain adjustment that consists of nothing more than a
voltage divider at the input, oftentimes followed by an
op amp buffer, etc. Basically, the gain adjustment is
done prior to the input stage of the amplifier.

That seems to be pretty much the standard way to do things.

Some amps that utilize global NFB have a voltage gain
that's defined solely by the value of one resistor (per
channel). Why introduce the added components associated
with the input circuit I described if that resistor could
be replaced with a potentiometer? Is there something
that makes that resistor unsuitable for gain adjustment?

Yes, global NFB is not very suitable for the purpose of
controlling gain over a wide range.

For example, you can't reduce the gain of something to zero
with a typical NFB loop. The best you can do is reduce the
gain to unity. In a typical power amp that means that you
are limited to about 30 dB gain control range.

Furthermore, the world is full of perfectly servicable
amplifiers that are not stable over a wide range of NFB
settings.

Obtaining very low amounts of gain with good stability can
be hard to do. High gain is generally easier,but to have a
working gain control you need both.