This simple scheme for mixing two or more channels in one channel (e.g. stereo to mono, etc.). The scheme can mix, this number of channels as you need and consumes very little power. The mixer is shown with two inputs, but you can add as many as you want, just duplicating the "sections" which are clearly visible in the diagram.
List of components used:
R1, R3 - 10K Pot
R2, R4 - 100K 1/4 watt,
R5 - 6.8 K 1/4 W,
C1, C2, C3 0.1 μf
Q1 - 2N3819 Junction FET
For this scheme it is better to use shielded housing for noise reduction and induced harmonics.