This single transistor audio mixer is used in an amplifier circuit design with base driven transistor and with its emitter being current controlled, most of the driving current flows through the collector away. Using the values in the audio mixer circuit shown in the diagram, the collector current will be about 1 mA. At 15 volts power supply, the input resistors should be 33K. Additional input lines can be connected to the emitter line. Each added input must be series limited by the 33K resistor.