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.Such a simple but stable
circuit as this can produce a reasonable output of 12W on a 4 Ohm speaker.
