PURPOSE: To have such a construction of the protective circuit that will not adopt the method of detecting only either one of output voltage or output current of the amplifier, but will detect both of them simultaneously, so that fail-proof protective operation of the circuit is ensured, even if shorting occurs during the state of small output load.

CONSTITUTION: Excess current detecting resistances R11 and R12 are serially connected to the common emitter circuit of complementary transistors Q11 and Q12 which constitute the final stage of amplifier 21, and the center point between the resistances is connected to speaker RL which is the load of amplifier 21. Next, the center point between these resistances R11 and R12 is connected to output voltage detection terminal P3, and both ends of the resistances are connected to output current detection terminals P1 and P2. Protective circuit 22 which is connected to these terminals P1 to P3 is composed of transistors Q13 to Q18, capacitor C11, diode D11 for protective use, relay K11 and resistances R13 to R23. Suppose speaker RL has shorted, excess voltage will appear at terminals P1 and P2 owing to excess current, and the voltage at terminal P3 will become O. As a result, circuit 22 is actuated to shut off power supply to the amplifier.