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This is a virtual earth mixer.
Most analogue mixers are built like this these days…
The circuit mixes the incoming signals at a zero volt point, effectively meaning that all signal sources will sink current into this point. It's called virtual earth, since the zero volt point at the inverting input of the op amp is not connected to earth (and must NOT be connected to earth). Rather, this point in the circuit is a current sink that effectively is more earthy than any of the input sources….
Why is this good? It means that there is NO cross-talk between channels AND the pots do NOT affect each other as is the case with passive mixing. This avoids a real total nightmare.
This circuit is shown with only two channels, but more may be added.
It is normal to put switches in the path at each input channel; this is done so that unused channels do not generate unwanted noise.
In Practice, you can add up to six channels per low noise op amp before you start seeing unwanted, nasty problems.
So if you use a TL072, you get 12 channels in total from one 8 pin DIL device!
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