I made this module in July 2016... basically my first after moving country. The reason I did this, although there are hundreds of modules which mix signals, is that almost non of them have a switch to turn it on or off. They always come with a volume button. After getting a few rough ideas in terms of design, I created the plan myself. It's nothing world-shaking. I think it turned out quite good though. I've added some gain amplification (1:3) on the total summed output to get some distortion after 2 o'clock roughly and I added 1+2 summed output, 3+4 summed output and all the summed outputs.

It mixes audio and cv, although keep in mind that the first 2 summed outputs (1+2 and 3+4) are inverted due to the mixing into the negative op amp input. You could add another op amp, but I was lazy ;)

I know it's not perfect and there is a little bleed. An engineer would definitely argue on many points, but it works and I'm pleased with my own result. By the way, it serves as an attenuator as well by having an output after every poti...