If an amplifier has more than one voltage rail then it is designated Class H. Its operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required. For most musical program material the output stage is connected to the lower supply voltage, and automatically switches to the higher rails for large signal peaks. If fact the input signal actually modulates the higher supply voltage. This allows the power supply to track the audio input and provide just enough voltage for optimum operation of the output devices. Using multiple power supplies improves efficiency enough to allow significantly more power for a given size and weight.