This low cost amplifier is simple and give you good result. Here the op-amp used is uA 741 which produces the required gain.The four transistors are wired as complementary Darlington’s which produces the drive for the speaker.
The voltage drop across the resistors R2 and R3, are used as the input of the Darlington pairs . As the input current to the op-amp depends on the level of the signal op amp is amplifying the voltage drop across the resistors R2 and R3 will be proportional to the input signal. These voltage drops are given to the base of Darlington pairs. The amplification is stabilized as a result of the negative feedback from the junction of collectors of Q2 and Q4. The theory may seem little awkward for you. But its working good.cSuch a simple but stable circuit as this can produce a reasonable output of 12W on a 4 Ohm speaker.