If you disconnect R1, R2 and R3 from the minus input of the opamp and connect them to the plus input of the opamp (and remove R4), then you should have a working mixer which does not invert the signals, if I’m not mistaken. You can then leave out RV4 by shortening that (the minus input is then only connected to the opamp’s output). I’m not sure why R1, R2, R3 have a low value, I’d change them into 10k Ohm each.
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Good ideas, I will probably try that with your adjustments (at least those that I can do without making a new board). I just remembered that I got the schematic from this Modular in a Week https://www.youtube.com/watch?v=5TNLjV3xJjU
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You can go for a less complicated design:
First change the 10kOhm resistor in 1k, then the aplification is 1.
The first opamp sums everything up and inverts the summed signal, the second inverts it back.
So with one TL074 you have 2 opamps left.
[edit] The input resistors may of course be preceded by potentiometers. Choose the same value for all of them and give the feedback resistor (between pin 1 and 2) the same value.
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That’s pretty similar to the schematic for the AI Synthesis mixer module, of which I have one:
Note it includes feedback capacitors (these are commonly used, though I haven’t yet got around to understanding what they’re for), and filter capacitors on the power rails. The input resistors are pots, the feedback resistor on the first stage is variable, for an overall master amplitude control, and there is a 330R resistor on the output. You can buy the PCB for $10
though it has no cool drawing on it.
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Right, an important bit is shown in this AI0002-Mixer-Schematic that I overlooked. The input resistors are not replaced by potentiometers, they are preceded by potentiometers. This is to prevent that one pot when turned all the way down will short all signals coming into the opamp (OOPS) to ground. The capacitors on the powerline are there to get rid of HF signals and should be as close to the chip as possible,
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They prevent ringing/oscillation under certain circumstances, caused by e.g. by stray capacitances on opamp inputs, PCB tracks, and for output buffers long cables. Not always needed, because modern opamps usually have some built in. NCS has a detailed article here:
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Seeing two schematics here, one for three inputs and one for four, could we extend the design for five inputs? by adding another resistor?
You can add as many as you like. Just copy the part that is repeated for every input part and add it to the schematic.
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I just found this video and with this and your explanations I now understand a little bit, how this all works 
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would it work if i connect a jack cable out of the volume pot like a cv input? for and env gen or a lfo?
How did your stereo version differ? I’m trying to figure this out, and basically built it twice, one for left, and one for right, but I can’t figure out pan
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I don’t believe there is a pan. You basically adjust left and right to compensate. Otherwise you need a panning circuit as well.
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Yeah. 2 fully separate 8 channel Simple mixers sharing one power plug. One is your left channel, other is your right. Turn volume up on left, right, or both depending on where you want to place the audio.
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Nice, that’s what I’m doing, but the actual pan potentiometer is what I’m hung up on. On mine, I’ve done 4 mono Chanel’s and two stereo channels. All I seem to get for output is the left though. I did stereo pots as the volume knobs, treating each as a stereo feed to both super simple mixers. I have my mono input going to a couple different pot examples per channel, one of which was marketed as a panning pot, but I’m stumped on wiring. All I get is left output
Oh ok now that i re-read your comment, so you just have two volume knobs per channel then, and not a pan knob?
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Yup. No pan, just two volumes for each channel. To be honest…when i build another, it will have a pan. Much easier keeping things straight on the fly.
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Hey all,
I just built myself a modified version of the super simple mixer, and it works great so far. I’m considering trying to make one with additional buffered outputs, and I’m wondering if I could do the following:
Take the TL072 and replace it with a TL074. Take everything after the first op amp stage and simply copy/paste it going into the three additional op amp stages of the '74 (instead of the 1 remaining stage of the '72) and bam, three outputs instead of one.
Does that make sense, or am I overlooking something problematic?
My plan is for one of the buffered outputs to get normalled back to the switched signal pin on one of the input jacks - that way, in the absence of a cable plugged in, I could have a feedback loop to fatten things up. It seems to kind-of work when done passively, but I’m wondering if I will get better results with a buffered copy of the signal rather than a passively split one.
Cheers,
-Wesley
Surely the feedback loop would just do that. It would feed back the amplified signal, and that would be amplified, and that would be amplified until saturation. Just like when you leave your mic in front of the speaker…
.
Rob
Just to make sure we are on the same page, there would be a passive attenuator in the feedback loop so you can control the level of saturation. My hope is that it would be somewhat musical, but it more experimentation is needed.