Dumb Question About Opamps

[Paul Marossy]

I was messing around with a simple two stage opamp distortion design the other day and thought I'd put the gain control in the second feedback loop and have a fixed resistor in the first feedback loop. Problem was that gain control in the second feedback loop did absolutely nothing to change the gain (nothing perceptible anyways). But in the first stage, a variable resistor in the feedback loop makes a mojor difference. (and I did confirm that I didn't mess up the circuiting, so I can rule that out)

So my question is, why is this? Is it because the first stage is pummelling it so much that it's just basically useless as a gain control? I know it depends on the circuit, but I can't reveal the details of it, so generalities would be fine. Just trying to understand this...

[R.G.]

...why is this? Is it because the first stage is pummelling it so much that it's just basically useless as a gain control?

Almost certainly.

[daverdave]

I reckon if the gain of the first stage is so high that the signal is literally crammed in to the power supply rails then having the second stage variable isn't gonna make a great amount of difference to the sound, as it's already clipped into a sorta squarewave, that could be a plausable reason, depending on your closed loop gains, you could try making the first stage unity, and see how varying the second stage gains sounds from there, then just increase the gain of the first in increments. Hope that helps a bit.
Dave

[Paul Marossy]

...why is this? Is it because the first stage is pummelling it so much that it's just basically useless as a gain control?

Almost certainly.

That's what I figured. I guess I was just expecting something different to happen! 

I reckon if the gain of the first stage is so high that the signal is literally crammed in to the power supply rails then having the second stage variable isn't gonna make a great amount of difference to the sound, as it's already clipped into a sorta squarewave, that could be a plausable reason, depending on your closed loop gains, you could try making the first stage unity, and see how varying the second stage gains sounds from there, then just increase the gain of the first in increments. Hope that helps a bit.

Maybe I'll try this for kicks in the near future.