# Potentiometer action in an audio mixer

I found these schematics for an audio mixer.

I reproduce the mainpart of the schematics. A first stage of input buffers before each in line consisting of an opamp is not reproduced. I am only interested in unipolar mode.

I recognize a summing inverting op amp circuit. After the first op amp we have

$$V = - \sum_i V_i \frac{1}{1 + R_i / 100k}$$

Where $$\R_i\$$ is the resistance value of the i-th potentiometer.

If the potentiometer is on the far right, we then have a coefficient of $$\1\$$ in front of $$\V_i\$$. It is ok.

When you turn the potentiometer on the far left, for a potentiometer of 25k, you then have a coefficient of $$\ \frac{1}{1 + 25k / 100k} = 0.8\$$.

The documentation says:

When in unipolar mode, all pots feeding that output behave in regular fashion, that is when the knob is fully counter-clockwise, no signal from the associated input passes into the mix. As the knob is advanced clockwise, a greater portion of the signal passes into the mix.

I don't understand how 80% of the signal can be considered no signal.

It seems that I am missing something.

• Note that the schematics read "* multiple decoupling caps, as needed". You might get away with it as drawn, but I'd suggest adding (the pads at least) for 0.1µF caps near each TL072 + and - pin and ground. That way, if it is built and squeals badly or generates high-frequency oscillations, just add the caps. Jun 14 at 12:09
• Do you mean decoupling capacitors between power supply pins and ground or between inverting and noninverting inputs and ground ? Jun 26 at 11:23
• Power supply and ground. Adding caps to the inputs will essentially add a low-pass filter to the input and block all but the lowest audio signals. Jun 27 at 12:07