This audio mixer is built with transistors. This audio mixer circuit is unique, not based on low input impedance amplifier. Most of audio mixer circuit is based on low impedance amplifier circuit, so a high resistance signal source point can be summed together without disturbing each other. This mixer circuit is different, since the circuit is based on something like open collector circuits that sink the current from a pull-up resistor together.
This circuit avoid crosstalking the inputs with the isolation characteristic of collector current to base current, where any variation in collector current is only affect a much smaller variation of the base current. There are two outputs (upper and lower), which have opposite phases. [Circuit’s schematic diagram source: lh-electric.4t.com]