This is a Eurorack Mixer module with four inputs in 6HP. It's quite simple and only uses an op-amp and a handful of passive components. There are four inputs and one output. The inputs pass through independent potentiometers for attenuation and the all the inputs are summed and output. Here's what it looked like on the breadboard:
There's also a simulation. This type of mixing allows a strong input to effect other inputs (crosstalk) so it's not optimal for an input that needs to be routed to some other place pre mix. Other than that limitation it seems to work just fine. The potentiometers used to attenuate the signal can be audio (logarithmic) or linear depending on how it's getting used.
I'm glad the design works because after panelizing these I got 30 in the first order which is more than enough for me. I haven't found any issues yet with the v1 circuit, but I haven't used them much either. Time will tell.
If I make another mixer it will probably have a buffer for each input to avoid the above mentioned crosstalk and an output stage amplifier/gain adjustment.
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