Is there a way to make preamp gain dependent on supply voltage?

[PietS]

On the surface, it seems like a simple question but I can't seem to find a schematic that allows variable (clean) gain due to supply voltage variation. What I would like to do is use something like a potmeter to drop the V+ on a transistor or op-amp and in that way control the gain. The circuits I've seen and tested only reduce headroom when you reduce V+ but the gain stays the same.

[antonis]

A (very brute) solution may be a dual gang pot..

[MP_Quokka]

What is the application? Is there a reason why the supply voltage needs to change?

I think changing the supply voltage on a transistor stage would change the bias and cause distortion at some part of the adjustment range

... What I would like to do is use something like a potmeter to .......control the gain.

Using a pot in the feedback of an op amp in addition to or instead of Rf would allow adjustable gain without appreciable distortion, but not sure if this is what you're after?

[PRR]

Yeah, why?

The potentiometer can directly pot-down the signal. If you don't like signal on the pot, use an LM3080/etc and feed the Iabc pin from a pot-variable voltage. The THAT Corp VCAs are another possibility.

A too-simple BJT amplifier _will_ change gain with supply voltage (actually current), but you have no design control of impedances or gain-range. It tends to be both hissy and distorty.

[iainpunk]

you can use Delta-Modulation (turning the audio in to super sonic PWM) and then the amplitude of the input signal is referenced to a triangle oscillator's amplitude and basically turns the audio in a data stream of percentages of this triangle's amplitude, these percentages are turned back in to voltages corresponding to a percentage of the supply voltage.

its how i plan to build a feed forward compressor.

cheers

[PietS]

Yeah, why?

The potentiometer can directly pot-down the signal. If you don't like signal on the pot, use an LM3080/etc and feed the Iabc pin from a pot-variable voltage. The THAT Corp VCAs are another possibility.

A too-simple BJT amplifier _will_ change gain with supply voltage (actually current), but you have no design control of impedances or gain-range. It tends to be both hissy and distorty.

Why? I was just brainstorming on a simple way to control the gains of multiple parallel signal paths. I thought there might just be a simple way using supply voltage.
Thanks for pointing out the LM3080. Its schematic might just do the trick.

[merlinb]

Not forgetting the venerable LM13700, which could also accomplish this

[PRR]

> gains of multiple parallel signal paths.

That's a reason worth mentioning.

Why an amplifier? How about an attenuator? FET shunting a series resistor. Craig Anderton(or John Simonton?) used a CMOS hex inverter as a 10-cent hex voltage controlled attenuator. LDRs were widely used until ROHS.

Most simple plans have to knock signal down below 100mV. Distortion in BJT rises past 20mV and the push-pull 3080/3700 goes only somewhat past that. JETs and LDRs and vari-Mu tubes have ranges which take volts but usually have an important transition where >100mV is sour.

The dBx/THAT VCAs are the epitome of high level low-THD wide-range gain control, but cost more than a buck.

[Rob Strand]

On the surface, it seems like a simple question but I can't seem to find a schematic that allows variable (clean) gain due to supply voltage variation. What I would like to do is use something like a potmeter to drop the V+ on a transistor or op-amp and in that way control the gain. The circuits I've seen and tested only reduce headroom when you reduce V+ but the gain stays the same

There's something missing here because you can simply use a volume pot after the clean stage.   What's the intention behind complicating things by controlling gain via the supply voltage?