A plurality of main amplifiers amplifies input audio signals with an adjustable gain. A first selector selects any of the input audio signals. An auxiliary amplifier receives an output of the first selector and amplifies this output with an adjustable gain. A second selector receives n outputs from the main amplifiers and selects any of the outputs. A soft switching circuit receives an output of the second selector at a first input terminal, receives an output of the auxiliary amplifier at a second input terminal, and makes the output transition gradually from one of the input terminals to the other of the input terminals. Output switches are respectively provided for the main amplifiers, an output of a corresponding main amplifier is received at a first input terminal, an output of the soft switching circuit is received at a second input terminal, and one of the outputs is selected to be outputted.