A third terminal of a five-electrode plug is connected to a power supply terminal of a first differential amplifier and a power supply terminal of a second differential amplifier in an earphone. The first differential amplifier and the second differential amplifier supply amplified voltages to a first piezoelectric element and a second piezoelectric element, respectively. A battery is connected to a third terminal of an earphone jack. A processor is connected to a sixth terminal of the earphone jack and determines insertion of a plug into the earphone jack.