Audio Amplifier 100W

This audio amplifier uses a push-pull output stage with complementary power transistors: BDX66B (NPN) and BDX67B (PNP). It delivers clean, high-fidelity audio suitable for driving speakers at moderate power levels.

Power Supply

The amplifier requires a symmetrical ±40V DC supply. A suitable transformer is a 220V/110V to 30-0-30V AC unit, which is followed by rectification and filtering to obtain the ±40V DC rails. Proper filtering capacitors and a bridge rectifier ensure stable DC voltage for low distortion operation.

Schematic Overview

The circuit is composed of the following stages:

• Input stage: Differential or single-ended amplifier to accept audio signals.
• Driver stage: Provides sufficient current and voltage gain to drive the output transistors.
• Output stage: Complementary push-pull BDX66B/BDX67B transistors delivering the final amplified signal to the speaker.
• Power supply: Transformer 30-0-30V AC → bridge rectifier → smoothing capacitors → ±40V DC rails.

Construction Notes

• Mount BDX66B and BDX67B output transistors on a large heatsink with thermal paste to prevent overheating.
• Keep input signal traces away from high-current output paths to avoid oscillations.
• Use high-quality electrolytic capacitors rated at least 50V for filtering the power supply.
• Include fuses and proper wiring for safety when working with mains voltage.
• Ensure symmetrical layout for the ±40V rails to maintain balanced operation of the push-pull stage.

Power Supply Schematic diagram

PCB Layout 1

PCB Layout 2