Build a High-Quality Class AB Audio Amplifier
The Class AB amplifier is the most widely used audio power amplifier design today. It combines the excellent sound quality of a Class A amplifier with the high efficiency of a Class B amplifier, making it ideal for Hi-Fi systems, professional PA amplifiers, active speakers and home theatre equipment. This project introduces every major stage of a modern power amplifier, from the input differential amplifier to the complementary output transistors.
Project Overview
A Class AB amplifier uses two complementary output transistors that each conduct for slightly more than half of the audio waveform.
This greatly reduces crossover distortion while achieving efficiencies of approximately 60–70%, making it the preferred design for most audio power amplifiers.
Project Difficulty
| Item | Value |
|---|---|
| Difficulty | ⭐⭐⭐⭐ Advanced |
| Build Time | 1–3 Days |
| Supply Voltage | ±25 V to ±80 V DC |
| Output Power | 50–500 Watts |
| Soldering | Essential |
Typical Circuit Stages
| Stage | Purpose |
|---|---|
| Differential Input Pair | Amplifies the input signal and compares it with the feedback signal. |
| Current Mirror / Active Load | Improves gain and balance. |
| Voltage Amplifier Stage (VAS) | Provides most of the voltage gain. |
| Bias Circuit (VBE Multiplier) | Eliminates crossover distortion. |
| Driver Stage | Supplies current to the output transistors. |
| Output Stage | Delivers high power to the loudspeaker. |
| Negative Feedback Network | Reduces distortion and stabilises gain. |
Typical Components
| Component | Examples |
|---|---|
| Differential Pair | 2SA970 / BC556 |
| VAS | 2SC2240 / BC546 |
| Bias Transistor | 2SD669 / BD139 |
| Drivers | 2SC4793 / 2SA1837 |
| Output Transistors | 2SC5200 / 2SA1943 |
| Emitter Resistors | 0.22 Ω – 0.47 Ω, 5 W |
| Filter Capacitors | 4,700 µF – 22,000 µF |
How the Amplifier Works
The input signal enters the differential amplifier, where it is compared with the feedback signal from the output.
The Voltage Amplifier Stage (VAS) increases the signal voltage to a level suitable for driving the output section.
The bias circuit sets a small idle current through the output transistors, ensuring smooth transitions between the positive and negative halves of the waveform.
The driver transistors provide the current needed to control the power transistors, which finally deliver high power to the loudspeaker.
Power Supply
A dual-polarity power supply is normally required.
| Amplifier Power | Typical Supply |
|---|---|
| 50 W | ±25 V |
| 100 W | ±35 V |
| 200 W | ±50 V |
| 400 W | ±70 V |
The supply usually consists of a transformer, bridge rectifier and large electrolytic filter capacitors.
Heat Management
Although more efficient than Class A amplifiers, Class AB amplifiers still produce significant heat at high output power.
- Use large aluminium heat sinks.
- Apply thermal grease between transistors and heat sink.
- Use insulating washers where required.
- Add cooling fans for high-power amplifiers.
- Ensure good airflow inside the enclosure.
Testing Procedure
- Inspect all solder joints carefully.
- Verify transistor orientation.
- Check capacitor polarity.
- Measure resistance between the supply rails before applying power.
- Power the amplifier through a dim-bulb tester or current-limited supply.
- Measure the DC offset at the speaker output.
- Adjust the bias current according to the design.
- Connect an 8 Ω dummy load or test speaker.
- Apply a low-level audio signal.
- Gradually increase the volume while monitoring temperature.
Troubleshooting
| Problem | Possible Cause |
|---|---|
| No output | Power supply fault or incorrect wiring. |
| Output transistors become hot immediately | Bias current too high or shorted transistor. |
| Large DC voltage on speaker output | Fault in the differential input stage or output transistors. |
| Distorted audio | Incorrect bias adjustment or defective driver transistor. |
| Hum | Poor grounding or insufficient power supply filtering. |
| Amplifier oscillates | Layout problems or missing compensation components. |
Protection Circuits
A high-quality amplifier should include protection features such as:
- Speaker protection relay.
- Power-on delay.
- DC offset protection.
- Short-circuit protection.
- Thermal protection.
- Over-current limiting.
- Soft-start for large transformers.
Applications
- Hi-Fi stereo amplifiers.
- Professional PA systems.
- Subwoofer amplifiers.
- Studio monitors.
- Guitar and keyboard amplifiers.
- Home theatre systems.
- Powered loudspeakers.
Project Improvements
- Add a preamplifier with tone controls.
- Include a VU meter.
- Add Bluetooth audio input.
- Install a speaker protection board.
- Use toroidal transformers for reduced hum.
- Fit temperature-controlled cooling fans.
- Add clipping and power indicators.
Skills Learned
- Differential amplifier operation.
- Voltage amplifier stage design.
- Bias adjustment.
- Driver and output transistor operation.
- Power supply construction.
- Heat sink selection.
- Audio amplifier testing and troubleshooting.
Safety Notes
- Large power supplies contain dangerous voltages.
- Always use mains fuses of the correct rating.
- Discharge filter capacitors before servicing.
- Use a dim-bulb tester during the first power-up.
- Never connect expensive loudspeakers until the amplifier has been fully tested.
Key Points
- Class AB amplifiers provide an excellent balance between sound quality and efficiency.
- Proper bias adjustment is essential to minimise crossover distortion.
- A stable power supply and effective heat management are critical for reliable operation.
- Protection circuits greatly improve amplifier safety and reliability.
- The Class AB design remains the most common choice for high-quality audio amplification.