I want to mix together two signals. The signals have a DC bias but i'm only interested in the AC portion. Capacitors C1 and C2 are used to strip out the DC from the input. Potentiometers R1 and R2 can be used to attenuate the signals independently. OpAmp U1 then combines the signals as an inverting summer. The OpAmp has a bias at half battery voltage so that it can produce AC with a single rail supply. The DC bias of the OpAmp is stripped out with C3. The input signal is between 100Hz and 40kHz.

The problem i'm having is that the potentiometers affect the DC bias of the OpAmp.

How can I modify this circuit so that the potentiometers don't affect the bias point of the OpAmp?

My best guess is to place capacitors between the potentiometers and the summing resistors.


simulate this circuit – Schematic created using CircuitLab


1 Answer 1


As you say, you can put the capacitors between the pot wipers and the summing resistors. You can then lose C1/C2 and R9/R10 do nothing in the above circuit.

Note that the resistance the C sees varies with the pot setting somewhat (about +/-12.5%) so the low frequency cutoff will not be completely constant, but it will vary less than it does with your present circuit (varies by ~2:1).

  • \$\begingroup\$ R9 and R10 are representing the DC bias that the sources have. C1 and C2 block out that DC. \$\endgroup\$
    – vini_i
    Jan 17, 2017 at 23:53
  • \$\begingroup\$ @vini_i Sure, but they do nothing since the voltage sources V1, V2 are zero impedance. If you want to simulate the offset, add it into the voltage sources by editing V1, V2 (but it won't be apparent from the schematic). \$\endgroup\$ Jan 17, 2017 at 23:58

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