PURPOSE: To protect the complementary circuit, by blocking the audio signal fed to the final stage transistor at load short circuit, through the comparison of the voltage at the output terminal with the voltage in the final stage bias circuit.

CONSTITUTION: The audio amplifier is constituted with capacitors 14, 15, 16, and 52 and transistors 20, 21, 22, and 23, differential circuit 29 using transistors, diodes 24 to 28, and load 53. With this constitution, the comparison circuit 31 is newly added to the junction of diodes 27 and 28 constituting the bias circuit out of the output terminal 2 and the complementary circuit. Further, by using the output signal of the circuit 31, the interruption circuit 30 blocking the audio signal fed to the base of the transistor 18 is newly attached. Thus, the transistors 20 to 23, especially 21 and 23, can be protected at load short circuit and the performance of the audio amplifier can not be deteriorated.