When an output voltage from the amplifying circuit includes a positive DC voltage, an electric current continuously flows in a power source voltage, a load, and a reference voltage in this order. As a result, the reference voltage increases so as to be a first threshold voltage or more . The detecting section detects that the reference voltage is the first threshold voltage or more. When the output voltage from the amplifying circuit includes a negative DC voltage, an electric current continuously flows in the reference voltage, the load, a grounding potential in this order. As a result, the reference voltage reduces so as to be a second threshold voltage or less. The detecting section detects that the reference voltage is the second threshold voltage or less.