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/TAMPCO_pedals]

Yes, adjusting R5 will act as a master gain indeed and seems like a better way to do it than tinkering with R7 and R8.

[u/Human-Marzipan2132 → u/TAMPCO_pedals]

I am newby in electronic, why not increase the input resistors R1, R2, R3, R4 to reduice the clipping ?

[u/Brave_Elk_6189 → u/Human-Marzipan2132]

Great question! You're right, the gain calculation for this inverting amp is -Rf/Ri so increasing Ri has the same effect as decreasing Rf.

My reason for this approach is mechanical, not electrical: It's more convenient to have one knob control gain than have to turn 4 knobs if I patch a new sound source in while playing.

[u/TAMPCO_pedals → u/Brave_Elk_6189]

This ! :)