This was originally published in the Maplin Electronics magazine in 1979. I updated the circuit to use modern commonly available transistors and made a few tweaks from the original. Power supply should not exceed +/-40V DC. The schematic is shown below:

This circuit is a classic 50-watt class-AB Hi-Fi audio power amplifier based on a traditional discrete transistor design. It uses several amplification stages working together to provide voltage gain, current gain, low distortion, and sufficient output power to drive a loudspeaker.

The input signal enters through connector J1 and passes through coupling capacitor C1, which blocks any unwanted DC voltage from the source equipment. Resistor R1 establishes the input impedance and references the input stage to ground, while R3 and C3 help filter high-frequency noise and improve stability. R2 and C2 provide a “ground-lift” circuit, which attempts to prevent mains-hum ground loops being introduced by interconnected equipment.

Q1 and Q2 form the differential input stage of the amplifier. This stage compares the incoming audio signal with the feedback signal taken from the amplifier output. The differential amplifier is one of the most important sections because it largely determines the amplifier’s linearity, gain accuracy, and distortion performance. RV1 allows adjustment of the DC offset at the output by balancing the differential pair.

Q3 acts as a constant current source for the differential pair. The zener diode ZD1 establishes a stable reference voltage, while R6 sets the current flowing through Q3. Using a current source instead of a simple resistor greatly improves the gain and stability of the differential stage.

The signal from the differential amplifier drives Q4 and Q6, which form the voltage amplification stage (VAS). This stage provides most of the amplifier’s voltage gain. Q5 operates as a current sink for the VAS, improving linearity and allowing the stage to swing voltage more effectively across the supply rails.

Capacitors C7 and C16 provide frequency compensation and high-frequency stabilization. These capacitors help prevent oscillation and ensure stable operation when negative feedback is applied.

RV2 is the bias adjustment control for the output stage. Together with Q6, Q7, Q8, and the surrounding resistors, it establishes the correct quiescent current for class-AB operation. Proper biasing is essential because it reduces crossover distortion that occurs when the output signal transitions between positive and negative halves of the waveform.

Q7 and Q8 act as the driver transistors for the output stage. They provide enough current gain to drive the large power transistors efficiently.

The final output stage uses complementary power transistors arranged in a parallel emitter follower configuration. Q9 and Q11 form the positive output half using TIP36C PNP transistors, while Q10 and Q12 form the negative half using TIP35C NPN transistors. Using parallel output devices improves current handling capability and distributes heat more evenly. Emitter resistors R21 through R24 help balance current sharing between the transistors and improve thermal stability.

Negative feedback is returned from the output node back to the differential input stage through resistor R10. This feedback stabilizes the amplifier gain, lowers distortion, widens bandwidth, and improves DC operating conditions.

At the output, R25 and C12 form a Zobel network, which helps maintain amplifier stability when driving reactive speaker loads. L1 and R26 form the output inductor network, preventing instability caused by highly capacitive speaker cables or difficult loudspeaker loads.

The power supply uses dual rails, Vcc and Vee, allowing the amplifier output to swing symmetrically around ground without needing a large output coupling capacitor. Capacitors C11 through C15 provide local power supply decoupling and filtering to reduce noise and maintain stable operation under heavy load conditions.

Overall, this is a fairly traditional high-fidelity discrete transistor amplifier design. It combines a differential input stage, current sources, voltage amplification stage, adjustable class-AB biasing, and a robust complementary output stage to achieve good audio performance with relatively low distortion and solid output power capability.

Where I have built and tested this amplifier on breadboard and confirm it to work, I have not simulated it. Therefore I cannot tell you what the total harmonic distortion (THD) would be; but I suspect it would be less than 0.05%. Frequency response hasn't been simulated or tested, but it should be standardly flat between 20Hz and 20kHz. Bias should be set to around at total current draw from the supply of around 40mA. This should provide the output stage an Iq of around 25mA. Q6 should be in contact with the main heatsink to track the output stage and adjust the bias current accordingly to maintain thermal stability.

The original amplifier article can be found in this archived magazine here; page 285.