PURPOSE: To prevent an abnormality from spreading from a part of circuit to other parts by a opening a power supply relay based on an abnormality detection signal upon occurrence of abnormality in the circuit of a set thereby entirely interrupting the current supply to the set.

CONSTITUTION: A power ON signal is delivered on a control signal line from a microprocessor unit 11 through resistors 13, 14 to the gate of a transistor Tr 15 thus turning the Tr 15 ON. Consequently, a relay coil 17 is excited to close a contact 19 thus feeding a circuit 22 with DC power obtained by converting the power from an AC power supply through a rectifying circuit 20. When an abnormality signal, e.g. an overvoltage or an undervoltage detected by a sensor, is delivered from the circuit 22 to a protective circuit 21, the contact is turned ON to ground the control signal line CL. Consequently, the Tr 15 is tune OFF and the relay coil 17 is deenergized to close the contact 19 thus interrupting the DC current entirely for the circuit 22. This constitution prevents a local abnormality from spreading to other sound circuitry.