Can We Use Inverting Opamp for Volume Control?

[Agung Kurniawan]

Hi guys
I have been playing with distortion for 3 years since my first time get my own electric guitar. A week after get my electric guitar, I start to DIY.

I playing with Boss MT-2 for the first time using a distortion. Making some fuzz for the first time with a lot of fail. Make (of course)MXR Dist+ with succesful but using 47k for the high-pass filter resistor instead of 4,7k cause that time I cant read resistor color band value.

Almost all of dist pedal have loud volume, sometimes turn the vol pot to 9-12 o'clock should be enough. When I playing with multiple gain stage distortion, I realize that turn the vol pot all the way up will give different tone output character than the 12 o'clock and down that.

Then I just have some idea,


Just imagine that the 1st opamp stage is the last distortion stage(which in some practice could be filter or tone control). Then the second opamp is our 'volume' control.
volume pot has been moved to the feedback loop of the 2nd opamp to eliminate the tied-to-ground vol pot that make different tone character as mention above.

How does it works?
All(which is full/max vol pot position) output from the last stage of dist pedal is go to inverting opamp.
The Inverting opamp input R is set to be have the same value of 'volume' pot value (Im using 10K for R and B10K for volume).
With the 'volume' pot at max(10k) the gain of inverting opamp is 1. Making the output the same as input but the phase is reversed.
As the 'volume' pot set lower and lower than Input R, the gain is also lower than 1.
And after the 'volume' pot set to min, the gain is 0 (Im not sure) which make no output.

The idea of this application is to get better volume control for distortion pedal that have the same tone character at min and max pot position.
How did you thing guys?

And this is the simulation using android app.
Input                    : sine wave 1Khz 500mV
First opamp gain: 21
Blue: Input signal
Orange: output after 1st opamp
Green: final output go directly to output jack

Clean signal:

1. Volume on min position


2. Volume on middle


3. volume on max


More details:
1. Volume at min


2. Volume at middle


3. Volume at max

[PRR]

> How did you thing guys?

You have re-invented a very old idea.

It generally works good, with one flaw. When the pot wiper loses contact, the gain goes to "infinity". VERY LOUD.

But I've used consoles worked this way and been quite happy.

[Agung Kurniawan]

You have re-invented a very old idea.

ahahah, yeah I know this one should be very old.

[amptramp]

There is another form of inverting op amp stage volume control and that is one where the input to the stage goes to one end of a pot, the output of the op amp goes to the other end and the inverting input goes to the slider.  This gives you something approaching a hyperbolic control with a linear pot since the gain is R2 (output to slider)/R1 (input to slider).

[Agung Kurniawan]

There is another form of inverting op amp stage volume control and that is one where the input to the stage goes to one end of a pot, the output of the op amp goes to the other end and the inverting input goes to the slider.  This gives you something approaching a hyperbolic control with a linear pot since the gain is R2 (output to slider)/R1 (input to slider).

Did we need fixed R after the output that goes to end of the pot?

[ElectricDruid]

If you're feeding one distortion pedal into another distortion pedal, changing the volume of the first pedal will change the *tone* of the second pedal, no matter what method you use to control the volume. That's just the way it works - more signal is more distortion in the second pedal.

Tom