PURPOSE: To prevent destruction of an output transistor (TR) of an amplifier or a load speaker by detecting an abnormal negative voltage fluctuation of a neutral potential of a full direct couple DC amplifier by means of a small detecting current and interrupting a switch when the current exceeds a prescribed range.

CONSTITUTION: A voltage across a diode 11 placed at a higher potential side in a constant voltage circuit using the forward constant voltage characteristic of two diodes 11 and 12 is divided by resistors 21 and 22, and a base of an NPN TR15 is connected to said voltage dividing point. An emitter and a collector of an input PNP TR16 are connected respectively to an emitter of said TR15 and ground, a diode 13 forming a current mirror circuit is connected between a positive power supply and a collector of the NPN TR15, a collector of other TR14 of the current mirror circuit is used as an output terminal and a base of the input TR16 is used as an input terminal.