Abstract: A first power supply and output circuit and a second power supply and output circuit for use with a microphone unit having an electrostatic transducer and pre-amplifier transistor is disclosed. DC power to the microphone unit is supplied from the first circuit through a single core cable. An output signal from the microphone unit also passes through the cable to an output transformer in the first circuit. The first circuit has a first battery and the second circuit a DC power source which, when connected to the first circuit, stops drain on the first battery.