Mixer troubles - Hot opamp - no output

This time I tried to make a schematic… Because pictures are pretty hard to see.
Problem is… It’s my first try and the schematic isn’t pretty to look at. (It’s horrible working with those programs, but I need to take it a bit slower.)

So I tried this schematic. Tried to make something out of the Doepfer pages.


But have I made a terrible mistake in the schematic? Or did something go wrong with my perfboard?
Breadboarding a single tl072 worked fine… 2volts in. Could amplify it perfectly.

However, my opamps are getting quite warm and they’re not giving any voltage out. Not hot as in I mistakenly switched the voltage. I’ve done that… Just warm… Touchable… But… Something feels off.

I’m going to look over all the traces again. But as always, I’m hoping someone is going to point out a blatant mistake.

I already notice it’s good to post. As it’s a great way to reflect on what I’ve been doing… Maybe if I look at it again now, I’ll see what I’ve done wrong. That’s my wishful thinking.

Thanks for taking a look.


PS: You should have made Sam’s simple mixer! But no! You wanted to have gain on your pots… Pff! Look at you now… Scratching your head.


Ok, a quick pass over the circuit:

  • The input amplifiers use the standard non-inverting configuration, where gain is given by 1 + R1/R2 (so max 1+100/15=7.6 in your case). There may be some subtle issues here (more below), but I don’t think that’s what you’re seeing.
  • Next you seem to connect all the outputs to each other? This isn’t great, because now the amplifiers will fight with each other; if say three go high and one goes low, the latter will have to sink almost all the current that the others can source, without any limit, which isn’t healthy. If the idea is to mix things up, you need resistors on the outputs to turn things into a mixing node (summing up currents).
  • The lower TL072 has a floating input.
  • You have a non-inverting input stage and an inverting output stage, so the output will be out of phase with the input. Consider using inverting amplifiers also on the way in.
    • They also have the advantage that the opamp inputs are kept at 0 V (since one of them is grounded, and the opamp strives to keep both inputs the same) which keeps you far away from the subtle issue I mentioned earlier, which is that most opamps don’t like it when the inputs get too close to the supply rails. This is especially important for the TL07x series, which cannot handle inputs below 4 V above the negative supply (i.e −8 V if you use a −12 V supply).
  • (also, not sure what tool you used, but the editor probably has a snap setting that you can set to a larger value to make it easier to keep the wires strictly horizontal or vertical.)

See e.g. this page for a bit more on summing amplifiers. @Caustic also did a bit of detailed circuit analysis here:


Thanks so much! I will read up.

Edit: Ok… I read it and remember already having read it. Which obviously isn’t a bad thing… I can see where things go wrong here. I’m going to start fresh and work from Sam’s schematic and adjust it to taste… But will have to study up on opamps. Because it’s pretty passive knowledge. Once you try it out in real life… It becomes obvious it’s pretty passive. Thanks a lot fredrik!


This is a nice intro video too a simple mixer using op amps. https://www.youtube.com/watch?v=0kC-33YV7oM&t=111s hope it helps


Thank you! Taking a look right now…