I have some PC speakers (Altec-Lansing?) that I figured out that the volume control is a gain control, because when I turn the volume all the way down with headphones plugged into the aux jack, i hear noise/hissing.
I doubt the control changes the gain of the amplifier -
it attenuates the signal. The argument is whether
the attenuator is better at the input or the output
side of the fixed gain amplifier.
GoingFastTurningLeft wrote: IMO this is a bad design, since you will have a varying level of "music" and a constant level of hiss.
Assuming that the hiss is being generated in the
amplifier and not part of the input signal or a ground
loop or power supply noise, or detected RF noise from
a nearby digital computer box.
A negative feedback amplifier (such as an op amp)
will have no control of the input referred internal noise,
which is amplified by the open-loop gain. Luckily, very
low noise op amps are readily and cheaply available.
GoingFastTurningLeft wrote:I think it is much better to have a constant gain that ramps the audio up where you need it and overpowers the intrinsic hiss then drops it down with an audio taper pot.
If the attenuator is on the input side of the amplifier
it protects the amplifier from distortion caused by
An attenuator on the output has two problems, it
increases the 'sending' impedance of the amplifier,
and is non-constant across the range of the control.
Secondly, it forms a low pass filter with the capacitance
of the output cable and whatever is being driven.
A lot of audable noise in amplifiers is caused by a
lack of bandwidth limiting. The circuit I presented
earlier has a 20dB gain, rolling off at 50kHz. It
would be quite easy to roll that off earlier to allow
an upper -6dB point of, say, 20kHz. Thus reducing
any HF noise that can't be heard but may cause
distortion in the audio passband.
One final point, the link to a simple bipolar transistor
circuit has a couple of limitations. The gain is set
by the hFE of the transistor, not the circuit components
(I doubt two of these will work exactly the same).
Also, the output side volume controls are in parallel,
turning down one will cut off the other.