[Object]To downsize a power amplifier, while keeping isolation between a transmission line and ground potential.[Means to Realize Object]Two terminals (2a, 2b) of an audio signal source (2) are connected to an input of an isolation input circuit (6). An audio signal developed, being isolated from the input-side, between two output-side terminals (6a, 6b) of the isolation input circuit (6) are inputted to a power amplifying stage (12). The power amplifying stage (12) amplifies the audio signal and outputs it at a high voltage from two output terminals (12c, 12d) to a plurality of loudspeakers (22) without using an insulation transformer. Operating power is supplies to the power amplifying stage (12) from two power supply terminals (20c, 20d) of a DC power supply (14). One (6d) of the terminals of the insulation input circuit (6), one (12d) of the output terminals of the power amplifying stage (12) and one (20d) of the power supply terminals of the DC power supply (14) are connected together to provide a common potential point different from the ground potential.