Building a Class D Audio Amplifier HAT

Overview

This tutorial builds a small Raspberry Pi audio HAT around a PAM8403-style stereo Class D amplifier. The HAT takes left and right audio from the Pi, routes it through a dual-gang volume control, drives two speaker terminal blocks, and includes local power filtering for the amplifier.

What You Are Building

The HAT contains:

• PAM8403-style 3 W per channel stereo Class D amplifier
• Dual-gang 10 kOhm volume control
• Left and right speaker terminal blocks
• Power filtering near the amplifier
• Pull-down resistors on both audio inputs
• Power LED for quick bring-up feedback

The PAM8403 is commonly used from a 5 V supply. Use speakers that match the amplifier board and power source you choose.

Bill of Materials

Reference	Part	Value or package	Notes
U1	PAM8403 stereo amplifier	SOIC/QFN module-friendly footprint	3 W + 3 W Class D output
RV1	Dual audio potentiometer	10 kOhm, logarithmic preferred	Shared volume control
J1, J2	Speaker terminals	2 pin, 5.08 mm	Left and right speaker outputs
R1, R2	Resistors	100 kOhm	Input pull-downs
R3	Resistor	1 kOhm	LED current limit
C1	Capacitor	10 uF or larger	5 V bulk filtering
C2	Capacitor	100 nF	Local high-frequency bypass
LED1	LED	Green, 0603	Power indicator

Step 1: Add the Amplifier

Start with the amplifier and label the audio inputs, speaker outputs, supply, and control pins clearly.

Step 2: Add Volume Control

A dual-gang potentiometer keeps the left and right channels at the same volume. The wipers feed the amplifier inputs, and both low sides connect to signal ground.

Step 3: Add Speaker Terminals

Class D outputs are usually bridge-tied load outputs. Do not connect either speaker terminal to ground unless the amplifier datasheet explicitly says it is safe.

Step 4: Add Power Filtering

Place the bulk and bypass capacitors close to the amplifier. Audio boards are sensitive to supply bounce, especially when both speakers draw current at the same time.

<capacitor name="C1" capacitance="10uF" footprint="0805" />
<capacitor name="C2" capacitance="100nF" footprint="0402" />
<trace from=".HAT1_chip .V5_1" to=".U1 .VDD" />
<trace from=".HAT1_chip .GND_1" to=".U1 .GND" />
<trace from=".C1 > .pin1" to=".U1 .VDD" />
<trace from=".C1 > .pin2" to=".U1 .GND" />
<trace from=".C2 > .pin1" to=".U1 .VDD" />
<trace from=".C2 > .pin2" to=".U1 .GND" />

Raspberry Pi Audio Setup

For quick testing, route PWM audio to GPIO18 and GPIO19, or feed the HAT from an external DAC module. A simple /boot/firmware/config.txt test setup can enable the Pi audio path:

dtparam=audio=on

Then test with:

speaker-test -t sine -f 440 -c 2

Start with the volume low, confirm left and right channels separately, then raise the volume slowly.

PCB Layout Notes

• Keep speaker output traces short and wide.
• Keep input traces away from the switching speaker outputs.
• Put C1 and C2 near the amplifier power pins.
• Route left and right input traces as similar lengths when practical.
• Place the volume control on a board edge so the knob is reachable.
• Add silkscreen labels for left and right speaker polarity.

Bring-Up Checklist

1. Inspect for shorts between 5 V and GND.
2. Power the HAT without speakers and check that the LED turns on.
3. Confirm the amplifier supply pin is near 5 V.
4. Connect one speaker at low volume and play a sine wave.
5. Repeat for the other channel.
6. Check that the amplifier stays cool after several minutes at normal listening volume.

Common Fixes

• Hum or hiss: shorten the input wiring and keep it away from the speaker outputs.
• One channel silent: check the potentiometer wiper and speaker terminal polarity.
• Distortion at low volume: confirm the audio source is not clipping before the amplifier.
• Amplifier overheats: use a lower volume, higher-impedance speakers, or a stronger 5 V supply.