This simple circuit mixes two or more channels into one channel (eg. stereo into mono). The circuit can mix as many or as few channels as you like and consumes very little power. The mixer is shown with two inputs, but you can add as many as you want by just duplicating the "sections" which are clearly visible on the schematic.
100K 1/4 W Resistor
6.8K 1/4 W Resistor
C1, C2, C3
2N3819 Junction FET
Wire, Shielded (Metal) Case, Phono Or Other Plug For Output
1. As many or as few channels as are required can be added to the mixer. Do this by just duplicating the input "sections" which are clearly shown on the schematic. One version of this mixer I saw had 25 inputs!
2. A shielded case is probably needed to reduce hum and help stop oscillations.
3. The circuit can be powered by a single 9 volt battery.
Source: Electronics Lab