PURPOSE: To obtain a high-quality reproduced sound, by controlling the working current flowing to the transistor of output stage via the load with a negative feedback circuit which forms a feedback loop separated from the load.

CONSTITUTION: The working current I flowing to the transistor TR1 of output stage via the load Z is controlled by a negative feedback circuit which forms a feedback loop separated from load Z. The negative feedback circuit gives a negative feedback control to the current I by the audio input signal voltage V1 and the working voltage V2 that is applied to the output side of transistor TR1, i.e. applied between the collector and the emitter. Then the instant value of the working current, the instant value of the input signal voltage and the instant value of the working voltage are referred to as I, V1 and V2 respectively within the working range set previously to an audio amplifier. Thus I=K1V1+K2V2+K3 (K1, K2 and K3: constants) is satisfied to enhance the quality of reproduced sound.