The following audio mixer electrical circuit doesn’t use a low impedance input to mix no ideal sources, but use many amplifiers to offer ideal sources before mixed through simple resistors. Ideal source implies a sources with low impedances, make the fundamental interaction between signals in the output does not impact the input.
Take note that this mixer circuit has high impedance output, so you will need a next processing equipment with suitable input specs. This audio mixer circuit is configured around an LM3900 quad op amp and blends 2 line and 2 mike inputs and sums them at the output terminal. To alter the gain (around +23 dB), we can change the R7 through R10.
Here is the circuit :