About the circuit.

This is a low cost amplifier circuit using 4 transistors and one op amp. The op amp uA 741 produces the required gain and the transistors wired in complementary Darlington configuration produces the drive for the speaker.

The input for the Darlington pairs are derived from the power supply as a voltage drop across R2 and R3.The voltage drop across these resistors will be proportional to the input signal because the supply current to the op amp varies according to the input signal .The negative feedback from the junction of Q4 and Q2 stabilizes the amplifier. This circuit will produce a decent 12W out put power on a 4 Ohm speaker .

Circuit diagram and parts list.

12-w-transistor-amplifier_circuit_al.JPG

Tips. 

  1. Use a  +12 , -12 , 1A  dual power supply for powering the circuit.The power supply must be well regulated and filtered to reduce the noise.
  2.  Wire the board on a good quality PCB or common board.
  3. Do not give more than +15 , -15   volts to the circuit.