Building a Class D Audio Amplifier HAT

Overview

This tutorial walks through a small stereo Class D amplifier HAT for a Raspberry Pi. The HAT takes low-level left/right audio from the Pi or an external DAC, routes it through a dual-gang audio potentiometer, and drives a PAM8403-compatible 3 W per-channel amplifier into left and right speaker terminal blocks.

Class D amplifiers switch their output transistors at a high frequency and use the speaker load plus output filtering to recover the audio waveform. That makes them efficient and cool-running compared with linear amplifiers, but it also means layout matters: keep the high-current speaker loops short, keep audio input traces away from speaker outputs, and put supply bypassing right next to the amplifier.

Requirements

PAM8403-compatible 3 W per-channel stereo Class D amplifier.
Dual-gang 10 kΩ audio potentiometer for left/right volume control.
Left and right 2-pin speaker terminal blocks.
100 µF bulk capacitor plus 100 nF local bypass capacitor near the amplifier.
Raspberry Pi HAT connector for 5 V, ground, and optional PWM/I2S audio pins.
Optional 3-pin external audio header if you use a small DAC module instead of direct Pi PWM audio.

Audio integration choices

A Raspberry Pi does not expose a high-quality analog line output on the 40-pin header by default. Use one of these approaches:

External I2S DAC module: route the DAC line outputs into the HAT's AUDIO_L, AUDIO_R, and AGND header. This gives the best quality.
PWM audio with RC filter: use PWM pins and a low-pass filter before the potentiometer. This is acceptable for simple beeps or speech, but noisier.
Pi 3.5 mm jack jumper: if your Pi model has analog audio, route the jack output to the HAT input header.

The example below keeps the amplifier section explicit and uses an AUDIO_IN header so the HAT can be paired with any of those sources.

Step 1: Create the HAT outline

import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
  <RaspberryPiHatBoard name="HAT1">
    {/* amplifier circuit goes here */}
  </RaspberryPiHatBoard>
)

Step 2: Add the amplifier and audio controls

The PAM8403 has separate left/right inputs and differential speaker outputs. The dual-gang potentiometer attenuates both channels together.

<chip
  name="U1"
  footprint="sop16"
  manufacturerPartNumber="PAM8403"
  pinLabels={{
    1: "LOUT+",
    2: "PVDDL",
    3: "LOUT-",
    4: "LIN",
    5: "GND",
    6: "RIN",
    7: "ROUT-",
    8: "PVDDR",
    9: "ROUT+",
    16: "VDD",
  }}
/>
<chip name="RV1" footprint="potentiometer_9mm" manufacturerPartNumber="Dual 10k audio pot" />
<chip name="J_AUDIO" footprint="pinrow3" manufacturerPartNumber="Audio input: L/R/GND" />
<chip name="J_LEFT" footprint="terminalblock_2" manufacturerPartNumber="Left speaker" />
<chip name="J_RIGHT" footprint="terminalblock_2" manufacturerPartNumber="Right speaker" />

Step 3: Add supply filtering

Place the 100 nF capacitor as close as possible to the amplifier VDD pin. Place the 100 µF bulk capacitor close to the amplifier power entry so bass transients do not pull the HAT 5 V rail down.

<capacitor name="C1" capacitance="100uF" footprint="cap_0603" />
<capacitor name="C2" capacitance="100nF" footprint="0402" />
<trace from=".C1 > .pin1" to="net.V5" />
<trace from=".C1 > .pin2" to="net.GND" />
<trace from=".C2 > .pin1" to=".U1 > .VDD" />
<trace from=".C2 > .pin2" to="net.GND" />

Schematic and 3D preview

import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
<RaspberryPiHatBoard name="HAT1">
  <chip
    name="U1"
    footprint="sop16"
    manufacturerPartNumber="PAM8403"
    pcbX={0}
    pcbY={0}
    pinLabels={{
      1: "LOUT+",
      2: "PVDDL",
      3: "LOUT-",
      4: "LIN",
      5: "GND",
      6: "RIN",
      7: "ROUT-",
      8: "PVDDR",
      9: "ROUT+",
      16: "VDD",
    }}
  />
  <chip name="RV1" footprint="potentiometer_9mm" manufacturerPartNumber="Dual 10k audio pot" pcbX={-18} pcbY={0} pinLabels={{ 1: "L_IN", 2: "L_WIPER", 3: "AGND", 4: "R_IN", 5: "R_WIPER", 6: "AGND" }} />
  <chip name="J_AUDIO" footprint="pinrow3" manufacturerPartNumber="Audio input: L/R/GND" pcbX={-34} pcbY={0} pinLabels={{ 1: "LEFT", 2: "RIGHT", 3: "AGND" }} />
  <chip name="J_LEFT" footprint="terminalblock_2" manufacturerPartNumber="Left speaker" pcbX={24} pcbY={9} pinLabels={{ 1: "L+", 2: "L-" }} />
  <chip name="J_RIGHT" footprint="terminalblock_2" manufacturerPartNumber="Right speaker" pcbX={24} pcbY={-9} pinLabels={{ 1: "R+", 2: "R-" }} />
  <capacitor name="C1" capacitance="100uF" footprint="cap_0603" pcbX={-8} pcbY={10} />
  <capacitor name="C2" capacitance="100nF" footprint="0402" pcbX={-4} pcbY={10} />

  <trace from=".HAT1_chip > .V5_1" to="net.V5" />
  <trace from=".HAT1_chip > .GND_1" to="net.GND" />
  <trace from="net.V5" to=".U1 > .VDD" />
  <trace from="net.V5" to=".U1 > .PVDDL" />
  <trace from="net.V5" to=".U1 > .PVDDR" />
  <trace from=".U1 > .GND" to="net.GND" />
  <trace from=".C1 > .pin1" to="net.V5" />
  <trace from=".C1 > .pin2" to="net.GND" />
  <trace from=".C2 > .pin1" to=".U1 > .VDD" />
  <trace from=".C2 > .pin2" to="net.GND" />

  <trace from=".J_AUDIO > .LEFT" to=".RV1 > .L_IN" />
  <trace from=".J_AUDIO > .RIGHT" to=".RV1 > .R_IN" />
  <trace from=".J_AUDIO > .AGND" to="net.GND" />
  <trace from=".RV1 > .L_WIPER" to=".U1 > .LIN" />
  <trace from=".RV1 > .R_WIPER" to=".U1 > .RIN" />
  <trace from=".RV1 > .AGND" to="net.GND" />

  <trace from=".U1 > .LOUT+" to=".J_LEFT > .L+" />
  <trace from=".U1 > .LOUT-" to=".J_LEFT > .L-" />
  <trace from=".U1 > .ROUT+" to=".J_RIGHT > .R+" />
  <trace from=".U1 > .ROUT-" to=".J_RIGHT > .R-" />
</RaspberryPiHatBoard>
)

Bill of materials

Ref	Part	Notes
U1	PAM8403 stereo Class D amplifier	5 V, 3 W per channel into suitable speakers
RV1	Dual-gang 10 kΩ audio potentiometer	Audio/log taper preferred
J_AUDIO	3-pin audio input header	Left, right, analog ground
J_LEFT/J_RIGHT	2-pin terminal blocks	Speaker outputs
C1	100 µF capacitor	Bulk 5 V storage
C2	100 nF capacitor	Local high-frequency bypass

Raspberry Pi audio configuration

For an I2S DAC source, enable I2S in Raspberry Pi OS and select the DAC overlay required by your module. A typical /boot/firmware/config.txt setup looks like this:

dtparam=i2s=on
# Example only: use the overlay for your actual DAC board.
dtoverlay=hifiberry-dac

Then test a quiet audio file first:

speaker-test -t wav -c 2
aplay /usr/share/sounds/alsa/Front_Left.wav

Layout guidance

Keep the audio input header and potentiometer on the quiet side of the board.
Route speaker outputs as short, paired traces to the terminal blocks.
Do not run speaker outputs under the potentiometer or audio input header.
Use a solid ground return from the amplifier to the HAT ground pins.
Put C2 directly beside U1; put C1 close to the 5 V entry path.

Bring-up checks

With no speakers connected, confirm there is no short between 5 V and ground.
Power the Pi and verify 5 V appears at the amplifier supply pins.
Start with the potentiometer fully down and play a low-volume stereo test.
Connect inexpensive test speakers before connecting final speakers.
If there is hiss or PWM noise, use an I2S DAC or improve the analog low-pass filtering before the HAT input.

Overview​

Requirements​

Audio integration choices​

Step 1: Create the HAT outline​

Step 2: Add the amplifier and audio controls​

Step 3: Add supply filtering​

Schematic and 3D preview​

Bill of materials​

Raspberry Pi audio configuration​

Layout guidance​

Bring-up checks​