Building a Class D Audio Amplifier HAT

Overview

This tutorial walks through a compact Raspberry Pi HAT for a PAM8403-style Class D audio amplifier. The HAT takes 5V power from the Raspberry Pi header, accepts stereo line-level audio from a DAC or external audio header, adds a front-panel volume control, and drives left/right speaker terminal blocks.

What you will build

The finished board includes:

• A PAM8403-compatible Class D stereo amplifier for about 3W per channel into suitable small speakers
• A dual-gang 10k audio-taper potentiometer for left/right volume control
• A three-pin stereo line input header (L, R, GND)
• Left and right two-pin speaker terminals
• Local 100nF and bulk 470uF supply capacitors near the amplifier
• A shutdown pull-up so the amplifier starts when the HAT is powered

The Raspberry Pi 40-pin header is used for 5V and ground. Raspberry Pi GPIO pins do not provide line-level analog audio, so feed this HAT from a DAC, USB audio adapter, HDMI audio extractor, or another line-level source.

Component choices

Reference	Part	Purpose
U1	PAM8403-style Class D amplifier	Efficient stereo speaker drive from 5V
RV1	Dual 10k audio-taper potentiometer	Tracks left/right volume together
J_AUDIO	3-pin audio input header	Line-level left, right, and ground input
J_LEFT_SPK, J_RIGHT_SPK	2-pin terminal blocks	Speaker outputs
C1, C2	1uF coupling capacitors	Block DC from the upstream audio source
C3	100nF ceramic capacitor	High-frequency supply decoupling
C4	470uF electrolytic capacitor	Handles speaker-current transients
R1	10k resistor	Pulls shutdown high to enable the amplifier

How Class D amplification works

A Class D amplifier rapidly switches its output devices instead of operating linearly like a Class AB amplifier. The speaker sees a filtered version of this switching waveform, so the amplifier wastes less power as heat. That efficiency is why a small 5V HAT can drive useful speaker volume without a large heatsink.

For layout, treat the speaker outputs as noisy switching nets. Keep them short, route them away from the line-level input traces, and keep the decoupling capacitors close to the amplifier power pins.

Step 1: Add the HAT and amplifier

Start with the Raspberry Pi HAT outline, then place the amplifier near the speaker terminals so high-current output traces stay short.

Step 2: Add line input and volume control

Use a 3-pin header for stereo line input and model the dual-gang potentiometer as a six-pin component. Each channel has an input end, a wiper, and a grounded end of the resistive track.

Step 3: Add power filtering and shutdown control

The PAM8403 runs from 5V. Put the 100nF ceramic close to the amplifier power pins, then place the bulk electrolytic nearby to supply speaker current peaks. A 10k pull-up keeps shutdown deasserted.

Step 4: Add speaker terminals and complete the circuit

Class D bridge outputs are differential. Do not tie either speaker terminal to ground unless the amplifier datasheet explicitly says it is allowed.

import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
<RaspberryPiHatBoard name="HAT1">
  <chip
    name="U1"
    footprint="soic16"
    manufacturerPartNumber="PAM8403"
    pinLabels={{
      pin1: ["LIN"],
      pin2: ["RIN"],
      pin3: ["GND"],
      pin4: ["VDD"],
      pin5: ["SHDN"],
      pin6: ["LOUTP"],
      pin7: ["LOUTN"],
      pin8: ["ROUTP"],
      pin9: ["ROUTN"],
    }}
    pcbX={2}
    pcbY={-8}
  />
  <connector name="J_AUDIO" footprint="pinrow3" pinLabels={{ pin1: ["L_IN"], pin2: ["R_IN"], pin3: ["AGND"] }} pcbX={-20} pcbY={5} />
  <chip name="RV1" footprint="pinrow6" manufacturerPartNumber="Dual 10k audio taper potentiometer" pinLabels={{ pin1: ["L_IN"], pin2: ["L_WIPER"], pin3: ["L_GND"], pin4: ["R_IN"], pin5: ["R_WIPER"], pin6: ["R_GND"] }} pcbX={-8} pcbY={8} />
  <capacitor name="C1" capacitance="1uF" footprint="0603" pcbX={-15} pcbY={1} />
  <capacitor name="C2" capacitance="1uF" footprint="0603" pcbX={-15} pcbY={-3} />
  <capacitor name="C3" capacitance="100nF" footprint="0402" pcbX={8} pcbY={-3} />
  <capacitor name="C4" capacitance="470uF" footprint="radial" polarized pcbX={14} pcbY={-3} />
  <resistor name="R1" resistance="10k" footprint="0402" pcbX={4} pcbY={3} />
  <connector name="J_LEFT_SPK" footprint="pinrow2" pinLabels={{ pin1: ["L+"], pin2: ["L-"] }} pcbX={20} pcbY={4} />
  <connector name="J_RIGHT_SPK" footprint="pinrow2" pinLabels={{ pin1: ["R+"], pin2: ["R-"] }} pcbX={20} pcbY={-8} />

  <trace from=".HAT1_chip .V5_1" to=".U1 .VDD" />
  <trace from=".HAT1_chip .GND_1" to=".U1 .GND" />
  <trace from=".HAT1_chip .V5_1" to=".R1 > .pin1" />
  <trace from=".R1 > .pin2" to=".U1 .SHDN" />
  <trace from=".J_AUDIO .L_IN" to=".C1 > .pin1" />
  <trace from=".C1 > .pin2" to=".RV1 .L_IN" />
  <trace from=".RV1 .L_WIPER" to=".U1 .LIN" />
  <trace from=".RV1 .L_GND" to=".HAT1_chip .GND_2" />
  <trace from=".J_AUDIO .R_IN" to=".C2 > .pin1" />
  <trace from=".C2 > .pin2" to=".RV1 .R_IN" />
  <trace from=".RV1 .R_WIPER" to=".U1 .RIN" />
  <trace from=".RV1 .R_GND" to=".HAT1_chip .GND_3" />
  <trace from=".J_AUDIO .AGND" to=".HAT1_chip .GND_4" />
  <trace from=".C3 > .pin1" to=".U1 .VDD" />
  <trace from=".C3 > .pin2" to=".U1 .GND" />
  <trace from=".C4 > .pin1" to=".U1 .VDD" />
  <trace from=".C4 > .pin2" to=".U1 .GND" />
  <trace from=".U1 .LOUTP" to=".J_LEFT_SPK .L+" />
  <trace from=".U1 .LOUTN" to=".J_LEFT_SPK .L-" />
  <trace from=".U1 .ROUTP" to=".J_RIGHT_SPK .R+" />
  <trace from=".U1 .ROUTN" to=".J_RIGHT_SPK .R-" />
</RaspberryPiHatBoard>
)

PCB layout checklist

• Put C3 within a few millimeters of U1 VDD/GND.
• Keep C4 close to the amplifier and use a short, wide 5V path from the HAT header.
• Route LIN and RIN as quiet traces away from LOUT* and ROUT*.
• Place speaker terminals at the board edge and label polarity clearly.
• Leave mechanical clearance around the potentiometer shaft or panel connector.
• If the speakers are off-board, twist each speaker pair to reduce radiated noise.

Raspberry Pi audio integration

The HAT receives audio on J_AUDIO; it does not synthesize analog audio from GPIO. Common options are:

1. Use a USB audio dongle and wire its line output to J_AUDIO.
2. Add an I2S DAC HAT or breakout and feed the DAC line output into J_AUDIO.
3. Use HDMI audio extraction if your product already has HDMI in the enclosure.

On Raspberry Pi OS, verify the selected audio device before connecting speakers:

aplay -l
speaker-test -c 2 -t wav

Start with the volume low. If the output clips, reduce the software mixer level or the upstream DAC gain before increasing the HAT volume.

Bring-up checklist

1. With the HAT unpowered, check for shorts between 5V and ground.
2. Power the Raspberry Pi without speakers attached and confirm 5V reaches U1.
3. Confirm SHDN is high through R1.
4. Connect speakers, keep the volume low, then play a stereo test file.
5. Check both speaker terminals for correct left/right channel mapping.
6. Let the board play audio for several minutes and confirm U1 and C4 stay cool.

Next steps

• Add an I2S DAC section so the HAT accepts digital audio directly.
• Add an amplifier mute GPIO by driving SHDN from the Raspberry Pi through a small transistor or level-safe buffer.
• Add ferrite beads or an LC filter if your speaker cables are long.
• Add mounting holes and a panel footprint for the volume knob.