“mini” audio amplifier referring to its small dimension and small output. It is intended for earpiece/earphone use. The characteristics are consume power less than 3mA, using push-pull output which can drive the earpiece to very loud level when audio is detected, whole circuit DC coupled, therefore makes it difficult to set up. Due to its size, be careful when assemble the circuit.
The 8k2 across the 47u sets the emitter voltage on the BC 547 and this turns it on. The collector is directly connected to the base of a BC 557, called the driver transistor. Both these transistors are now turned on and the output of the BC 557 causes current to flow through the 1k and 470R resistors so that the voltage developed across each resistor turns on the two output transistors. The end result is mid-rail voltage on the join of the two emitters. The two most critical components are 8k2 between the emitter of the first transistor and 0v rail and the 470R resistor.
