PROTECTING CIRCUIT OF AMPLIFYING DEVICE

PURPOSE:To protect an amplifying device and a loudspeaker when a ground- fault is generated, by providing a control circuit for cutting off an electric power source side driving transistor (TR) of one SEPP amplifying circuit in case when ground-fault of an output terminal of its amplifying circuit, and also giving an electric current for making a ground side driving TR of the other SEPP amplifying circuit conduct. CONSTITUTION:When an output terminal 10 becomes a ground level, TRs T1, T3 are turned on, the base potential of TRs T4, T5 is raised, and they are made to conduct. As a result, an electric current from a constant-current circuit connected to idling circuits I1, I2 flows through the TRs T4, T5, and the potential of terminals 13, 23 drops. A TRQ15 is turned off, accordingly, a TRQ17 is also turned off, and its breakdown can be prevented. On the other hand, as the potential of the terminal 23 drops, a TRQ24 is turned on, and a TRQ25 is also turned on, therefore, an output terminal 20 becomes a ground level. In this way, both terminals of a loudspeaker SP become almost equal potential and the loudspeaker SP is protected.