The Mixer 4 is a straightforward 4 AC-coupled mono input mixer for Eurorack systems.
The circuit itself is very simple, and the component count pretty low:
There’s only 2 op-amps in a single 8 pin package U1.
The two op-amps are set as inverting amplifiers.
More precisely, the first stage, U1B, is set as a Summing amplifier. It combines the 4 input signals and mix, invert and slightly attenuate them.
Each input signal has the exact same circuit: The signal goes first through a high pass filter in order to remove high frequency noise, then though a low pass filter to remove DC component. Then each signal is individually attenuated by a logarithmic potentiometer.
The output of the first op-amp stage is fed to the “master volume” log potentiometer which can also globally attenuate the mixed signals.
Finally, the mixed signal is again inverted then amplified by the second op-amp stage (U1A) and sent to the output jack. This last stage has a short circuit protection resistor (R21). Its amplification gain is set by R10.
R10 should be 47k (it was originally 22K).