schematics - Making a Simple Audio Mixer - Electrical Engineering Stack Exchange
Basic audio mixer with tone control
DIY Passive Audio Mixer – Scott Brown | Audio Visual Artist
Simple Audio Mixer Circuits under Repository-circuits -24861- : Next.gr
Mono Audio Mixer #2 Article by Ray Wilson This project is an older design and has most likely been superceded by a newer one.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 ...