Building a Class D Audio Amplifier HAT

Overview

This tutorial will walk you through building a Class D audio amplifier HAT for Raspberry Pi. Using the PAM8403 chip, we'll create a stereo amplifier delivering 3W per channel, perfect for desktop speakers or portable audio projects.

Objectives

Building a Class D amplifier HAT teaches essential audio electronics:

• Class D Operation - Understanding pulse-width modulation amplification
• Audio Signal Path - AC coupling and input filtering
• Bridge-Tied Load - Maximizing power from low voltage supplies
• HAT Integration - Interfacing with Raspberry Pi audio output

Use Cases

• Desktop Speakers - Power small speakers from Pi audio
• Media Centers - Audio for Kodi or Pi-based media players
• Voice Assistants - Speaker output for Pi voice projects
• Portable Audio - Battery-powered Bluetooth speakers

Bill of Materials

Component	Value	Footprint	Purpose
PAM8403	-	SOIC-16	Class D stereo amplifier
C1, C2	1µF	0603	Input DC blocking
C3	10µF	0805	Power supply filtering
C4	100nF	0402	High-freq decoupling
RV1	10kΩ	Potentiometer	Volume control
J1	-	3.5mm jack	Audio input
SP_L, SP_R	2-pin	5.08mm	Speaker terminals
J2	40-pin	2.54mm	Pi GPIO header

Understanding Class D Amplifiers

Class D amplifiers use pulse-width modulation (PWM) for high efficiency:

Amplifier Class	Efficiency	Heat Generation	Use Case
Class A	20-30%	High	High-end audio
Class AB	50-60%	Moderate	General audio
Class D	85-95%	Low	Portable/efficient

The PAM8403 achieves over 90% efficiency, enabling operation without heatsinks.

Circuit Design

Step 1: PAM8403 Power Supply

The PAM8403 operates from 2.5V to 5.5V. Using the Pi's 5V rail provides maximum output power:

Step 2: Audio Input with DC Blocking

Capacitors block DC offset while passing audio frequencies:

Capacitor selection:

• Target frequency: 20Hz (low end of audio)
• Input impedance: 20kΩ typical
• C = 1 / (2π × f × R) = 0.4µF minimum
• Using 1µF provides margin

Step 3: Volume Control

A potentiometer provides analog volume adjustment:

Step 4: Bridge-Tied Load Outputs

The PAM8403 uses BTL configuration, doubling voltage swing across speakers:

Complete Schematic

Raspberry Pi Audio Configuration

Configure the Pi to output audio to the 3.5mm jack:

# Set audio output to headphone jack
amixer cset numid=3 1

# Test with a sound file
aplay /usr/share/sounds/alsa/Front_Center.wav

# Adjust volume
alsamixer

For I2S audio (higher quality), modify /boot/config.txt:

dtparam=audio=on
audio_pwm_mode=2

Speaker Selection

The PAM8403 works best with 4-8Ω speakers:

Speaker Impedance	Max Power (5V)	Recommended Use
4Ω	3W per channel	Desktop speakers
8Ω	1.5W per channel	Small speakers

Higher impedance speakers reduce power but also reduce noise.

Ordering the PCB

Export the fabrication files and upload to JLCPCB. See Ordering Prototypes for detailed instructions.