Abstract: The output signal of an l-f (low-frequency) power amplifier is supplied to an electroacoustic transducer, through an inductive element for preventing parasitic oscillation, to cause the inductive element to generate a magnetic flux whose density depends on the output current signal of the power amplifier while a current according to the magnitude of the output voltage signal of the power amplifier is supplied to the current input terminals of a Hall-effect element disposed in the magnetic field induced by the inductive element, so that a voltage according to the output power of the l-f power amplifier is taken out of the output terminals of the Hall-effect element. When the voltage exceeds a predetermined level, the l-f signal supplied to the electroacoustic transducer is attenuated by a signal attenuating circuit so that the transducer is prevented from being damaged.