PURPOSE: To improve the operability by switching the function of an amplifier with the start operation of a source device and starting the source device with the operation of a function switching circuit of the amplifier.

CONSTITUTION: When, for example, the start button of a record player 4 out of source devices is depressed, an high-level signal is applied to a start circuit 14, and an FF9b is set through an input circuit 8. An output circuit 10b is set to the high level by setting of the FF9b, and a gate 11b is made conductive, and therefore, the signal applied to a terminal 26 of a function switching circuit 7 from an output terminal 24 of the player 4 is applied to a speaker 13 through an amplifier 12. When function changeover switches 6a∼6d are operated, function switching is performed through the input circuit, and start circuits 14, 16, 18, and 19 of corresponding source devices 21∼24.