Amp power supply

[siming.guo]

I'm very new at this, so this question may be very stupid.
While researching, I found many circuits for amps and filters that people made where the power supply provided only a positive voltage in addition to ground. I was under the impression that the sound signal from the computer was composed of sine waves, which have both positive and negative components. How can a circuit or chip that only has a positive rail amplify both the positive and negative parts of the signal?

 

[pacificblue]

By using the ground as the negative rail and creating a virtual ground at half the positive rails voltage.

Nationals application notes usually offer one example of how to do that with a transistor and a few resistors and capacitors. Some ICs, e. g. TDA2030 have that circuitry built in.