To save troubles when changing the connection of parallel/series bi-amplifiers.

A printed-wiring board 3A is fixed to the lower portion of first input terminals IN1(+), IN1(-) and second ones IN2(+), IN2(-) fixed to a terminal block 2A. A fuse holder 3A and a rotary switch 40 are attached to the printed-wiring board 3A. The fuse holder 3A has first voice coil terminals VC1(+), VC1(-) and second voice coil terminals VC2(+), VC2(-) connected to both the ends of first and second voice coils VC1, VC2 and two pairs of terminals HT11, HT12 and HT21, HT22 for fuses. The rotary switch 40 has first and second switch circuits 41, 42 including movable contacts X, Y and first-third fixed contacts X1-X3, Y1-Y3 corresponding to three positions of parallel, series, and bi-amplifiers. The IN1(+) is connected to VC1(+) and Y. The IN1(-) is connected to the HT11 and X. The HT12 is connected to the VC1(-). The IN2(+) is connected to the Y1 and HT21. The HT22 is connected to the VC2(+). The X2 is connected to the VC2(+). The IN2(-) is connected to the VC2(-) and X1.