A friend has asked me to make a module so he can send a bunch of Korg Volca and Bastl stuff through his rack, and this is what I came up with:
theres a couple more copies of the preamp circuit off the image (lifted from MI ears / MTM mikrophonie) and I added a simple opamp mixout.
Main thing is, did I make any stupid mistakes?
Secondly, any suggestions for the spare opamp?
I would add an 1k output resistor at the TL072.
As for the spare op amp, if you don’t have any plans, it would be nice to specify a couple of placehold resistors from ground to + and from - to the output. If you end up not using it, you can just jumper those. Otherwise, one can hack some use out of it by replacing the jumpers with resistors and whatnot.
As for the spare op-amp you can go hi-fi and use the spare as a parallel physical overlay, ( the signal is wired to both inputs and then - and this is important - you need to use small resisters to balance the outputs and allow them to share the current load and they must be in place before any load or the blue smoke will appear)
It’s called paralleling and there are many good examples and use cases for sucking the marrow from that chip and using everything it’s got!
You can also use the spare as a voltage follower of the first and again use resisters to tie the outputs together.
Attack!
thanks, added the output resistor.
I think I’l just leave the opamp as is, yours, and Farabides suggestion are certainly better but i think I risk breaking it while trying to add it.
Ok, the idea of the unused opamp is giving me sleepless nights, so I decided to go with the best possible option, an LED. I’ve stolen the values from another mixer. Any issues with this?
That won’t work. I am pretty sure you need a resistor in the op amp loop. Otherwise you get a gain of 0.
i seem to have buggered up the layout now, so it’s going to be a mega pita to edit it.
The opamp will have to stay redundant
If you loop the spare as a zero gain stage you’ll keep the ic thermally stable….. or just ignore it 
Yes it does work and no you don’t want a resistor in the feedback loop. The point of the circuit is to convert a voltage over R18 into a current through LED1, so voltage gain isn’t relevant.
There are some minor issues though. When the output of U1A goes negative, the output of U1B will crash to the positive rail because LED1 doesn’t pass current in reverse. It can be fixed by adding a diode (or another LED) in reverse-parallel with LED1.
Also to reduce the load on the output of U1A I would connect R18 to ground and the output of U1A to the noninverting input of U1B.
