An audio signal amplifying device and method for reducing a size and corresponding power consumption of demodulation filters in the signal amplifying device. In the device and method, carrier waves having phase shifts are generated from a received audio signal. The shifted carrier waves are compared to the audio signal to generate modulation signals, which are amplified based on a power supply voltage. The amplified modulation signals or a power supply signal are output based on a comparison between the carrier waves and the audio signal. The output signals are synthesized to produce a synthesized signal having a lower average voltage and current value, thus reducing power consumption of the signal amplifying device.