Above schematic is for a three channel audio
mixer. That will provide an overall gain of one
between the output and each input channel. Each
input channel includes a single 0.1uf capacitor
and 100-Kilohms resistor to provide an output
impedance of 100K. The number of input channels
to this audio mixer can be increased by adding
more capacitors and resistors with same value as
capacitor(0.1uf) and resistor(100K). The mixer
should be located between the output of tone
control circuitry and the input to the power
amplifier. One input should be taken from the
output of the tone control circuit, and the
other inputs should either be grounded or taken
from desired source.