PCBs are available for this project from PCBWay as a shared project.

Power output: 40W @ 8 ohm; 51W @ 4 ohm
Frequency response: 15Hz - 25kHz
THD: (not measured)
Stability: Unconditional

Fig. 1: the schematic diagram to the amplifier

The schematic in Fig. 1 is a fully discrete Class-AB audio power amplifier operating from ±35V supply rails. The amplifier follows a traditional linear amplifier architecture consisting of a differential input stage, a voltage amplification stage, and a complementary push-pull output stage. Global negative feedback is used throughout the design to reduce distortion, stabilize gain, and improve overall linearity.

The audio signal enters through connector J1 and passes through coupling capacitor C1, which blocks any DC voltage from the signal source while allowing the AC audio signal to pass into the amplifier. Resistor R1 sets the input impedance, while R2 establishes the ground reference for the input stage. Resistor R3 and capacitor C2 form a small input low-pass filter that helps improve high-frequency stability and reduce radio-frequency interference.

The input stage itself is formed by transistors Q2 and Q3, which operate as a differential amplifier pair. The base of Q2 receives the input signal, while the base of Q3 receives the global feedback signal from the amplifier output through resistor R12. The differential pair continuously compares the input signal with the feedback signal, and the resulting difference becomes the error signal that drives the rest of the amplifier.

The emitter current for the differential pair is supplied by Q1, which acts as a constant current source. Resistor R4 feeds current into Q1, while diodes D1 and D2 establish a relatively stable reference voltage at its base. This arrangement forces Q1 to supply an approximately constant current into the joined emitters of Q2 and Q3. Maintaining a constant tail current improves linearity, gain stability, and common-mode rejection within the differential stage.

The collectors of Q2 and Q3 connect to the current mirror formed by Q4 and Q5. In this arrangement, Q4 is diode-connected, with its collector tied to its base, establishing a reference current. Q5 mirrors that current and acts as an active load for the differential amplifier. Compared to ordinary collector resistors, the current mirror greatly increases the voltage gain of the input stage while also improving symmetry and linearity. The current mirror additionally converts the differential signal into a single-ended signal capable of efficiently driving the next stage.

The offset adjustment network consisting of RV1, R5, and R6 allows the DC offset at the amplifier output to be trimmed close to zero volts. Because the amplifier is direct coupled from input to output, small transistor mismatches could otherwise create unwanted DC voltage at the speaker terminals.

The amplified signal from the differential stage drives transistor Q7, which forms the voltage amplification stage, commonly called the VAS. This stage provides most of the amplifier’s open-loop voltage gain. The collector of Q7 is capable of swinging across a large portion of the supply rails, generating the large voltage swing required to drive the output stage to full output power. Resistor R14 establishes the operating current for the VAS.

Capacitor C7 is the dominant-pole compensation capacitor connected around the voltage amplification stage. This capacitor intentionally reduces the amplifier’s high-frequency open-loop gain to ensure the global feedback loop remains stable. Without this compensation, the amplifier would likely oscillate at high frequencies.

The bias and driver section is built around Q8, Q9, Q10, and the bias adjustment potentiometer RV2. This section establishes the correct Class-AB idle bias current for the output stage while also providing additional current gain between the voltage amplification stage and the power output transistors.

The output stage itself is a complementary triple-emitter-follower Class-AB arrangement. The upper half uses Q11, Q13, and Q15, while the lower half uses Q12, Q14, and Q16. The BD139 and BD140 transistors function as pre-drivers, while the TIP36C and TIP35C devices act as the main power output transistors responsible for delivering current to the loudspeaker.

The output transistors operate primarily as emitter followers, meaning they provide very high current gain with a voltage gain close to unity. Their purpose is not voltage amplification but power amplification. During positive output swings, the upper transistor chain sources current from the positive supply rail into the speaker load. During negative swings, the lower transistor chain sinks current from the speaker into the negative supply rail.

The bias spreader network formed by diodes D3 through D6 together with potentiometer RV2 establishes the voltage between the upper and lower driver transistor bases. This keeps both halves of the output stage slightly conducting even when no signal is present, reducing crossover distortion around the zero-crossing region of the waveform.

The small emitter resistors R21, R22, R24, R25, R26, and R27 improve thermal stability and ensure proper current sharing between the parallel output devices. Since individual power transistors never have perfectly matched characteristics, these resistors help prevent one transistor from carrying excessive current.

Capacitor C8 provides additional high-frequency stabilization within the driver and output stage, helping to prevent parasitic oscillation.

Transistors Q9 and Q10 form the VI limiter, also called the current limiting or safe-operating-area protection circuit. This section monitors the voltage developed across the low-value emitter resistors R21, R22, R24, R25, R26, and R27. Under normal operating conditions, the voltage across these resistors is small and Q9 and Q10 remain off.

As output current increases, the voltage drop across the emitter resistors also increases. When this voltage becomes large enough, Q9 or Q10 begins conducting. These transistors then steal drive current away from the driver stage, reducing the output transistor base drive and limiting the output current. This protects the output transistors against excessive current and secondary breakdown conditions.

Resistors R16, R17, R18, and R19 determine the VI limiting threshold and operating characteristics. Because the limiter senses both current and output voltage conditions, it provides more intelligent protection than a simple current limiter. The limiting threshold effectively changes with output voltage, helping maintain the output transistors within their safe operating area.

However, there is some consensus that VI limiters introduce distortion - especially when operating close to full power, and I tend to agree.

Diodes D5 and D6 are part of this VI limiter network and provide directional steering and biasing for the protection transistors. They ensure that the appropriate limiter transistor responds during positive or negative output current conditions. This is only to prevent output stage instant destruction due to an accidental short-circuit. You really should never purposely short the output of an amplifier when it's working - no short-circuit protection is fool proof.

At the amplifier output, resistor R28 and capacitor C10 form a Zobel network, also known as a Boucherot cell. This network stabilizes the amplifier when driving inductive speaker loads and helps maintain a predictable load impedance at high frequencies. Diodes D7 and D8 provide protection against reverse voltage conditions and inductive transients generated by the speaker load.

The supply rails are protected by fuses F1 and F2, while capacitors C9, C11, C12, and C13 provide local supply decoupling and filtering close to the amplifier circuitry.

Overall, the amplifier uses a classic high-performance linear topology combining a differential input stage, active-load current mirror, high-gain voltage amplification stage, Class-AB biasing network, and complementary emitter-follower output stage. The design achieves high voltage gain, substantial output current capability, and relatively low distortion through the use of negative feedback, constant-current biasing, and carefully stabilized transistor stages.

I haven't simulated this circuit in TINA, so I can't provide a total harmonic distortion estimated figure - but, it should be relatively low. Frequency response is flat 15Hz to 25kHz and stability in to a capacitive load is unconditional.