Building a DIY Power Amplifier (Class AB)

Class AB amplifiers offer a great balance of efficiency and audio quality. They’re a popular choice for DIY audio projects and speaker systems. This guide walks you through the basic concepts and practical steps to build one yourself.

1. Why Class AB?

Class AB amplifiers combine the low distortion of Class A with the efficiency of Class B. They operate by biasing the output transistors slightly above cutoff, reducing crossover distortion while keeping power dissipation manageable.

2. Basic Block Diagram

• Input Stage: Differential amplifier (e.g., using A970/C2240)
• Voltage Gain: VAS with compensation (e.g., C2705/A1145)
• Driver Stage: Push-pull configuration (e.g., C4793/A1837)
• Output Stage: Power transistors (e.g., 2SC5200/2SA1943)
• Biasing: VBE multiplier for thermal tracking (e.g., BD139/D669)

3. Key Considerations

• Power Supply: Choose symmetric ±35V to ±65V depending on your target output power
• Heatsinking: Use large aluminum heatsinks for power transistors
• PCB Layout: Keep feedback and input traces away from power rails
• Protection: Add speaker protection relays and inrush limiters

4. Testing and Assembly

Start with a low-voltage power supply and test the bias current. Use a dummy load and an oscilloscope to verify signal integrity. Gradually increase supply voltage while monitoring thermal performance and output swing.

5. Useful Add-ons

• Volume control potentiometer
• Preamp stage with tone control
• Speaker protection circuit

Building a Class AB amp is a rewarding way to learn about analog design, thermal management, and audio quality. Start with a tested schematic, understand each section, and don’t skip safety steps!