Today computer all comes with sound card on it, bad part, on many models (DG31PR intel mobo is one of them) its really low power output (probably only a few mWatt), unable to drive even a small speaker, in other words, it is designed only for headphone, or need additional audio amplifier for louder sound.
after looking around on the semiconductor chipset handbook, I found that TDA7053 is decent enough for this purpose, it has 2 lines with 1W each, and only need 5Volt to drive the circuit, so, yes, correct everyone, I can power it using USB port from computer.
another bad news after another, I cant find TDA7053, only TDA7052, its a 7053 cousin, only with 1 line, so to make it stereo, I need 2.
if we look on the schematic below, its really easy to make only need couple additional component to build this small audio amplifier.
and here is my build, nasty and spartan :D, black cable is for 5V DC coming from computer USB port, and copper color cable is the audio input from sound card, I will try to clean up this and put the circuit on small box later, but for now, I am more than happy, as my DG31PR intel mobo now capable to drive medium speaker with this.