Mixing console - single opamp audio mixer | Elektor Magazine
Audio Mixer Circuit
Multi-channel audio mixer circuit using LM3900 | Projects | CircuitMaker
Mono Audio Mixer #2 Article by Ray Wilson This project is an older design and has most likely been superceded by a newer one. However, it is still a viable design and useful for your home-brew synth. Sorry but most of these do not have circuit descriptions ...