difference across the first capacitor 206 is generated across the second capacitor 212 and an electrical level at the second terminal of the second capacitor 212 thus becomes u2212 vcc at last the second voltage source having the electrical level u2212 vcc is thus generated as shown in fig3 with both the abovementioned first stage and second stage the first voltage source having the electrical voltage vcc and the second voltage source having the electrical voltage u2212 vcc are generated simultaneously in the power supply circuit 200 illustrated in both fig2 and fig3 for driving each sub audio amplifier 400 in the sub audio amplifier set 300 shown in fig1 please refer to fig4 which is a diagram of the sub audio amplifier 400 shown in fig1 according to a preferred embodiment of the present invention as shown in fig4 the sub audio amplifier 400 includes a comparator 402 a preceding driver 404 a transistor set 406 an inductor 408 a third capacitor 410 and a speaker 412 the comparator 402 has a positive input terminal coupled to a sawtooth signal source and a negative input terminal coupled to an audio signal source audio in as shown in fig4 with the aid of the sawtooth signal source audio signals from the audio signal source audio in are transformed into pulse width modulation signals at an output terminal of the comparator 402 the preceding driver 404 has an input terminal coupled to the output terminal of the comparator 402 for receiving the pulse width modulation signals the transistor set 406 has an input terminal coupled to an output terminal of the preceding driver 404 and includes a first transistor 414 and a second transistor 416 the first transistor 414.