Mixer 4 – Version 1.0 | David Haillant's electronic stuff

The Mixer 4 is a straightforward 4 AC-coupled mono input mixer for Eurorack systems.

Circuit description

The circuit itself is very simple and has a relatively low component count.
There are only two op-amps in a single 8-pin package, U1.

Mixer 4, schematics — Schematics of the Mixer 4 module.

The two op-amps are configured as inverting amplifiers.
More precisely, the first stage, U1B, is configured as a summing amplifier. It combines the four input signals, mixing, inverting and slightly attenuating them.

Each input has the exact same circuitry:
First, the signal passes through a low-pass filter to remove high-frequency noise, then though a low-pass filter to remove the 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” logarithmic potentiometer, which can globally attenuate the mixed signals.

Finally, the mixed signal is inverted and amplified by the second op-amp stage (U1A) before being sent to the output jack.
This last stage includes a short-circuit protection resistor (R21), and its amplification gain is set by R10.

Components

All the components are SMT, except for the controls (potentiometers and jacks), and the Eurorack power connector.

Value changes

R10 should be 47k (it was originally 22k). Note that if you’re satisfied with the current global gain of the mixer, there’s no need to replace the resistor. With a value of 22k, the global gain is approximately 0.47, meaning that a single signal will be attenuated at the output even if all potentiometers are set to maximum. Replacing R10 with a 47k resistor achieves a gain of 1, though this will, of course, saturate the output op‑amp more easily.