The following circuit uses LM1875 audio amplifier capable of delivering up to 30W RMS power into 8 ohm speakers with a total harmonic distortion (THD) 1%.

Note : The LM1875 integrated circuit should be installed on the heatsink to dissipate heat properly.

Range Vcc = 16V to 60V (60V 4A for maximum power)
Output = 30W RMS
THD (Pout = 20W, fo = 1kHz, 8 ohms) = 0.015%
THD (Pout = 20W, fo = 20 kHz, 8 ohms) = 0.05%
THD (Pout = 20W, fo = 1kHz, 4 ohms) = 0.022%
THD (Pout = 20W, fo = 20kHz, 4 ohms) = 0.07%
Gain = 20 (26dB)
Idle power consumption = 70mA

Parts list :
R1,R2,R3 : 22K
R4 : 1M
R5 : 10K
R6 : 200K
R7 : 1 ohm
C1 : 1.0uF
C2,C3 : 10uF
C4 : 0.1uF
C5 : 0.22uF
C6 : 2200uF
C7 : 100uF
IC : LM1875

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As seen in the design, the only active component is an integrated circuit TDA7386, SGS-Thomson. It provides four channels of amplification from a 12v source. The DC input is blocked from 0.1μF capacitor. STBY mute control terminal and may or may not be implemented, is up to you. Output is symmetrical, so there is no speaker terminal is grounded (both amplified).

Cooling:
Heatsink should be enough to keep the chip in the appropriate temperature. You can use a Pentium III with a cooling fan.It is sufficient to connect to Vcc as used 12v like the amplifier.

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