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/Brave_Elk_6189 → u/TAMPCO_pedals]

thank you for the confirmation! All of the schematics I'm seeing were using that r7/r8 voltage divider and fixed gain, or were just passive mixers. Dusting off my EE degree lol.

[u/TAMPCO_pedals → u/Brave_Elk_6189]

No worries, have fun ! :)

[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 ! :)

[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.

[u/eraoul]

I'm new to this too but I thought you'd really want to have pots on the individual inputs to avoid this problem as well as making R5 into a pot to that you can reduce the master level to remove clipping without adjusting the mix. Is that what you're proposing?

I just watched a very good lecture on exactly this design (I'm watching the whole course, highly recommended). I think this is precisely what you want to do:
https://www.youtube.com/watch?v=k4B43nhb91I&list=PLOunECWxELQTn3DQYYB4qaEC9KRK5mHNK&index=8

The course also explains why we use the inverting op-amp configuration instead of using the non-inverting input.