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MZ
 
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Default gain adjustment in an amplifier utilizing NFB

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.

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? Somehow more prone to noise?
Thanks.

PS - I'm not looking to modify or construct an amp in this fashion; I'm
curious more than anything else.


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Karl Uppiano
 
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"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.

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? Somehow more prone to noise?
Thanks.

PS - I'm not looking to modify or construct an amp in this fashion; I'm
curious more than anything else.


Sometimes gain is adjusted by changing the feedback, but amplifiers are
often optimized for a specific gain/feedback, and changing the feedback
makes the overall performance sub-optimal. Furthermore, it is usually
advisable to keep feedback wiring as short and straight as possible, which
is made more difficult with a variable resistor in the circuit. A simple
adjustable voltage divider (potentiometer) that is responsible for just one
purpose (variable gain control) is usually more reliable.


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Arny Krueger
 
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"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.


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