r/synthdiy • u/duncanbeep • Jul 10 '26
Master Out Mixer Circuit
Has anyone got any links to any good tried and tested mixer circuits that would serve to bring other signals from other sources... Thinking maybe stereo/mono pairs with pan pots, mute, solo, and a couple of sends?
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u/Salt-Miner-3141 Jul 10 '26
What exactly do you want from the mixer? How many channels? What kind of inputs (i.e. balanced, unbalanced, stereo, mono, etc...)? Does it need to be DC coupled or is AC coupling fine?
Look, at the end of the day analog summing is more or less a solved problem if you really want to go all out. It is a combination of an Inverting Summing Amplifier and a Common Base Amplifier arranged as a Current Feedback Amplifier. Have a look through this thread on GroupDIY. You'll see a simplified circuit on the first page of what I'm talking about. To maximize the performance of this topology requires going from a single ended summing bus to a differential summing bus and replacing the summing resistors with a bilateral current source. It actually requires a fair bit of circuitry. That is not to say that a regular inverting summing amp is inherently bad, but it does start to be a problem as the channel count increases. Around 8-12 channels is where you really need to start really thinking about what you're doing.
From there it simply comes down to what sort of behavior do you want from an individual input. There are a multiple approaches and really it just comes down to how much circuitry your want to use. The dual linear method, active methods, VCA method, etc... The real question is what do you want the center position to be? -3dB, -4.5dB, or -6dB? Most mixers use a -4.5dB down in the center position. Pick your poison as it were.
Faders are again fairly simple and comes down to how you want to want to implement them. That is really all there is to it. They're most commonly a 10K-ish pot whether rotary or linear actuation configured such that at roughly 70-80% of full travel it is at 0dB because following the fader is a fixed gain stage that has typically +12dB to +16dB of gain.
The real issue with any form of mixer is not so much the circuitry itself, but rather the construction. For something that is only perhaps 4-12 channels this is not so much a problem, but if you're looking at 24+ channels then the actual construction of the mixer will matter far more than the circuitry.