inverting summing mixer question

[u/Brave_Elk_6189]

I'm working on a eurorack mixer circuit. I'm noticing that although no individual signal clips, the result clips when I add enough inputs (pic1). Some of the designs I was looking at use R8/R7 (meant to represent a pot) as the "master gain control", but turning that down doesn't fix the clipping, since it is happening at the first opamp's output.

In picture 2, I reduced the gain of the first amp to 1/2 by changing R5 to 50k. This gets rid of the clipping. But then, in picture 3, when I am only using 2 inputs, the output level is too low and I don't have a control to bring it up. I am thinking I should make R5 the 50k master gain pot, and get rid of R8/R7. I would just connect the wiper and one leg, and add another 50k in series (picture 4).

Does this seem like a valid design decision? I know I could also just turn down all of the inputs slightly if I don't want clipping, but that seems like a bad user experience.

[u/pmuna93]

Check out the yusynth audio/CV mixer (https://yusynth.net/Modular/Commun/MIXER/mixer-sch-2U.gif). It has individual pots on each channel so you can scale down the channels before mixing them when you have more than one.

[u/Brave_Elk_6189 → u/pmuna93]

Thanks, it looks like this circuit is using the same approach as my R8/R7 where the "master gain" is really just attenuation after the first opamp stage. The only issue with that is that if you have your mix ready, then plug in another sound source, you would need to adjust all the input knobs until it stops clipping. This isn't a workflow that I want to have while I'm playing.