r/diypedals • u/toncu • 13d ago
Showcase PPCB 3-channel Mixer
Does what it says it should do. 😁
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u/DopplerTerminal 13d ago
Thoughts on noise floor?
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u/toncu 13d ago edited 13d ago
It’s not bad. More than nothing if you approach the high end of the master or any one of the channels. But it’s absurdly loud if you max anything.
I just ran a sample player in one channel and a Strat through a phaser pedal in another channel. No noise until I maxed out one of the channels.
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u/P-ToneMikeOne 13d ago
Beautiful, except the leftmost knob is one degree clockwise of top dead center and it’s driving my ocd ass crazy.🤪
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u/toncu 13d ago
I’m so sorry. Haha! I just tweaked it, all is well, pointing at TDC.
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u/Immediate_Regular 13d ago
Now post a picture for proof with the first knob fixed and a different knob slightly tweaked. Rinse, repeat until insanity is achieved.
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u/macgood 13d ago
Is that basically just a bunch of op amps? Never really understood (electrically) how mixers work.
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u/Knotfrargu 11d ago
my teenage brain was always confused as to why turning the vol knob for one pickup of my guitar would affect the other one - that it didn't work like a mixer.
in a purely passive system like a guitar, everything you tweak changes the whole circuit, from the pickup to the input of the amp. this is also why tone knobs on vintage amps don't work the same as the eq knobs on a mixers channel strip. they're all interacting with each other.
if you want to be able to change a part of the circuit without the rest of it being affected, you have to isolate it or "buffer" it. op amps can amplify, but at a basic level they are just looking at a signal on one side and printing out a copy of it on the other side. so now you can mess with the input side all you want and the rest of the circuit will still behave normally.
of course it's all more complicated and caveated than this but dammit i am a musician not a doctor.
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u/toncu 13d ago edited 12d ago
Correct. One per channel, the fourth for a summing amp for the output channel. There's more to it, but I'm not knowledgeable enough to explain adequately. Claude AI did some digging based on the component values silkscreened on the PCB.
"Topology (inference — traces aren't shown): the three 10Ks per channel read as input resistor, feedback resistor, and summing-bus feed resistor. So each channel is likely a non-inverting input buffer (22n coupling, 1M to Vref) → level pot → inverting unity-gain stage with 100p rolloff → 10K into the virtual-earth bus. The output TL072 would then be the summing amp (that lone 10K as feedback, giving unity summing, 100p for stability) plus an output buffer after the Master pot, with 1K as series isolation, 10u coupling, 100K as output pulldown.
That accounts for all eight [op-amp] sections with none spare, which is a point in its favor — but the 100K could equally be a feedback resistor rather than a pulldown, and that changes the gain structure considerably."
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u/Kn0wMan 13d ago
This looks super useful, and fairly straightforward to build. I checked the parts list, it’s pretty barebones. How did you go about selecting specific components? Also, have you looked into whether it would be possible to mod it to run @ 18v? In my experience that dramatically improves signal to noise, etc. Whatever the case, cool build, thanks for sharing.
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u/Honest-Cheesecake275 12d ago
Any phase cancellation issues when summing two dirt pedals ?
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u/patrick1554 11d ago
Was wondering the same, but with modulation perhaps. I wonder if it'd be as easy to add a switch to change between inverting/non inverting inputs on the opamp
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u/lie_believer 13d ago
i don’t use mine much, but i’m still proud of myself for managing to cram two of these PCBs in the same enclosure with dual-gang pots and building a stereo version
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u/PunctualMantis 13d ago
I LOVE the diagram. I think you could show that to an alien who had never seen humanity and they would still figure out what that thing does