A first amplifier amplifies a first audio signal. A synthesizer circuit synthesizes an output signal of the first amplifier with a second audio signal. The first amplifier is arranged such that a first state in which the first amplifier amplifies a signal with a set gain and a second state in which the first amplifier outputs a fixed bias voltage can be seamlessly switched. A signal processing unit is disposed upstream of the first amplifier, subjects a third audio signal to a predetermined signal process, and outputs it as the first audio signal. When a state of the signal processing unit is switched, the first amplifier is changed from the first state to the second state, the state of the signal processing unit is switched in the second state, and subsequently the first amplifier is switched from the second state to the first state.