They are shown as conventional transistors in figure for ease.So don’t get confused.Even though each of them have 2 transistors ,2 resistors and 1 diode inside ,only three pins ,base emitter and collector are coming out.Rest are connected internally.So its quite OK to assume each of them as transistor for ease.

Remember TIP 142 and 147 are Darlington pairs.

We shall upload a sample PCB file (for download) shortly. PCB’s for this project can be ordered through PCBWay. Transistor Q3 drives the push-pull pair and its base is directly coupled to the collector of Q5. Diodes D1 and D2 biases the complementary pair and ensures Class AB operation. A complementary Class AB push-pull stage is built around the transistors Q1 and Q2 for driving the loud speaker. Feedback voltage is applied to the base of Q4 from the junction of 0.33 ohm resistors through the 22K resistor. Input signal is applied to the base of Q5 through the DC decoupling capacitor C2. Thus overall performance of the amplifier is improved.

The use of a differential amplifier in the input stage reduces noise and also provides a means for applying negative feedback.

The preamplifier section of this circuit is based around Q4 and Q5 which forms a differential amplifier. Here two BC 558 transistors Q5 and Q4 are wired as pre amplifier and TIP 142 ,TIP 147 together with TIP41 (Q1,Q2,Q3) is used for driving the speaker.This circuit is designed so rugged that this can be assembled even on a perf board or even by pin to pin soldering.The circuit can be powered from a +/-45V, 5A dual power supply.You must try this circuit.Its working great! TIP 147 and 142 are complementary Darlington pair transistors which can handle 5 A current and 100V ,famous for their ruggedness. This is the cheapest 150 Watt amplifier circuit you can make,I think.Based on two Darlington power transistors TIP 142 and TIP 147 ,this circuit can deliver a blasting 150 W Rms to a 4 Ohm speaker.Enough for you to get rocked? then try out this. In this project, we’re building a simple 150 watt amplifier circuit.

Power amplifier diy