A power amplifier comprising: a class-D amplifier that comprises an input portion and a switching device (121; 122), wherein the switching device is configured to be switched in accordance with an input signal (AlN) that is input to the input portion so that a current is supplied to a load from a power source via the switching device; and an input signal supplier configured to supply the input signal to the input portion of the class-D amplifier, to calculate a virtual output voltage (V 0) to be output from a virtual power source having a prescribed internal impedance characteristic when a current output from the virtual power source flows through the load, and to adjust an amplitude of the input signal in accordance with the virtual output voltage, wherein the input signal supplier comprises a signal processing circuit which is configured to perform a signal amplification processing based on the virtual output voltage (V 0) of the virtual power source that is a simulation of a power source for a vacuum tube amplifier to adjust the amplitude of the input signal; the power amplifier being characterized in that in the signal amplification processing, the input signal supplier is configured to multiply the input signal (AlN) by a coefficient (α) corresponding to a ratio of the virtual output voltage (V 0) and a voltage of a power source connected to the switching device and the virtual output voltage (V 0) being calculated based on an internal DC voltage source (Vi) of the virtual power source, an internal impedance of the virtual power source and a load current flowing through the load; and wherein the internal impedance of the virtual power source is obtained from an internal inductance, an internal resistance (Rs), and an internal capacitance of the power source for the vacuum tube amplifier and a square of a turn ratio (n2/n1) 2, wherein numbers n1 and n2 correspond respectively to turns of a primary and secondary sides of an output transformer of the vacuum tube amplifier. a class-D amplifier that comprises an input portion and a switching device (121; 122), wherein the switching device is configured to be switched in accordance with an input signal (AlN) that is input to the input portion so that a current is supplied to a load from a power source via the switching device; and an input signal supplier configured to supply the input signal to the input portion of the class-D amplifier, to calculate a virtual output voltage (V 0) to be output from a virtual power source having a prescribed internal impedance characteristic when a current output from the virtual power source flows through the load, and to adjust an amplitude of the input signal in accordance with the virtual output voltage, wherein the input signal supplier comprises a signal processing circuit which is configured to perform a signal amplification processing based on the virtual output voltage (V 0) of the virtual power source that is a simulation of a power source for a vacuum tube amplifier to adjust the amplitude of the input signal; the power amplifier being characterized in that in the signal amplification processing, the input signal supplier is configured to multiply the input signal (AlN) by a coefficient (α) corresponding to a ratio of the virtual output voltage (V 0) and a voltage of a power source connected to the switching device and the virtual output voltage (V 0) being calculated based on an internal DC voltage source (Vi) of the virtual power source, an internal impedance of the virtual power source and a load current flowing through the load; and wherein the internal impedance of the virtual power source is obtained from an internal inductance, an internal resistance (Rs), and an internal capacitance of the power source for the vacuum tube amplifier and a square of a turn ratio (n2/n1) 2, wherein numbers n1 and n2 correspond respectively to turns of a primary and secondary sides of an output transformer of the vacuum tube amplifier.