Building a Class D Audio Amplifier HAT

Overview

This tutorial walks through a Raspberry Pi HAT that uses a PAM8403-style Class D amplifier to drive a pair of small speakers. The HAT includes a stereo audio input, dual-channel volume control, speaker terminal blocks, power filtering, and a shutdown pin that can be controlled from Raspberry Pi GPIO.

Circuit Requirements

The amplifier HAT needs to:

• Accept a stereo audio signal from the Raspberry Pi or an external DAC
• Drive left and right speakers from a 5 V Class D amplifier
• Provide a physical volume control
• Expose speaker outputs on terminal blocks
• Add enough local power capacitance to handle audio current peaks
• Let Raspberry Pi GPIO mute or enable the amplifier

The PAM8403 is a common choice for small HATs because it can drive 4 ohm or 8 ohm speakers from the Raspberry Pi 5 V rail. It is still an amplifier, so layout, grounding, and speaker wiring matter.

Component Choices

Reference	Part	Purpose
J1	Stereo input header	Left, right, and ground audio input
RV1	Dual 10k audio taper potentiometer	Shared volume control for both channels
C1, C2	1 uF coupling capacitors	Block DC before the amplifier inputs
U1	PAM8403-style Class D amplifier	Drives the speaker outputs
C3	100 nF bypass capacitor	High-frequency local decoupling
C4	470 uF bulk capacitor	Handles speaker current peaks
R1	10k pull-up	Enables the amplifier by default
J2, J3	Speaker terminal blocks	Left and right speaker outputs

Building the Circuit Step by Step

Step 1: Add the Stereo Audio Input

Start with a simple three-pin input header for left audio, ground, and right audio. You can connect this to a Raspberry Pi audio DAC, a small I2S audio module, or a filtered PWM audio output.

Step 2: Add the Volume Control

A dual-gang potentiometer changes the left and right channels together. In this schematic, each channel enters the top of the potentiometer, the lower side goes to ground, and the wiper feeds the amplifier input through an AC-coupling capacitor.

Step 3: Add the Class D Amplifier

The amplifier takes the attenuated left and right input signals and drives bridge-tied speaker outputs. Do not tie either speaker output to ground; each speaker connects across its positive and negative output pair.

import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
<RaspberryPiHatBoard name="HAT1">
  <capacitor name="C1" capacitance="1uF" footprint="0603" pcbX={0} pcbY={-10} />
  <capacitor name="C2" capacitance="1uF" footprint="0603" pcbX={0} pcbY={-2} />
  <chip
    name="U1"
    footprint="sop16"
    manufacturerPartNumber="PAM8403 Class D amplifier"
    pinLabels={{
      pin1: ["LIN"],
      pin2: ["RIN"],
      pin3: ["SD"],
      pin4: ["VDD"],
      pin5: ["GND"],
      pin6: ["LOUTP"],
      pin7: ["LOUTN"],
      pin8: ["ROUTP"],
      pin9: ["ROUTN"],
    }}
    schPortArrangement={{
      leftSide: { pins: ["LIN", "RIN", "SD"], direction: "top-to-bottom" },
      rightSide: { pins: ["LOUTP", "LOUTN", "ROUTP", "ROUTN", "VDD", "GND"], direction: "top-to-bottom" },
    }}
    pcbX={8}
    pcbY={-6}
  />

  <trace from=".C1 > .pin2" to=".U1 .LIN" />
  <trace from=".C2 > .pin2" to=".U1 .RIN" />
  <trace from=".HAT1_chip .V5_1" to=".U1 .VDD" />
  <trace from=".HAT1_chip .GND_1" to=".U1 .GND" />
</RaspberryPiHatBoard>
)

Step 4: Add Power Filtering and Shutdown Control

The 100 nF capacitor belongs close to the amplifier power pins. The larger bulk capacitor can sit near the amplifier and speaker terminals. The shutdown pin uses a pull-up so the amplifier turns on by default, while GPIO 22 can pull it low to mute the output.

import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
<RaspberryPiHatBoard name="HAT1">
  <chip
    name="U1"
    footprint="sop16"
    manufacturerPartNumber="PAM8403 Class D amplifier"
    pinLabels={{
      pin1: ["LIN"],
      pin2: ["RIN"],
      pin3: ["SD"],
      pin4: ["VDD"],
      pin5: ["GND"],
      pin6: ["LOUTP"],
      pin7: ["LOUTN"],
      pin8: ["ROUTP"],
      pin9: ["ROUTN"],
    }}
    pcbX={8}
    pcbY={-6}
  />
  <capacitor name="C3" capacitance="100nF" footprint="0402" pcbX={8} pcbY={5} />
  <capacitor name="C4" capacitance="470uF" footprint="radial_cap_6.3mm" pcbX={18} pcbY={5} />
  <resistor name="R1" resistance="10k" footprint="0402" pcbX={-2} pcbY={8} />

  <trace from=".HAT1_chip .V5_1" to=".U1 .VDD" />
  <trace from=".HAT1_chip .GND_1" to=".U1 .GND" />
  <trace from=".U1 .VDD" to=".C3 > .pin1" />
  <trace from=".C3 > .pin2" to=".U1 .GND" />
  <trace from=".U1 .VDD" to=".C4 > .pin1" />
  <trace from=".C4 > .pin2" to=".U1 .GND" />
  <trace from=".HAT1_chip .GPIO_22" to=".U1 .SD" />
  <trace from=".U1 .SD" to=".R1 > .pin1" />
  <trace from=".R1 > .pin2" to=".HAT1_chip .V3_3_1" />
</RaspberryPiHatBoard>
)

Step 5: Add Speaker Terminals

Each speaker gets two terminals because the outputs are bridge-tied. Mark the terminal blocks clearly and keep the left and right pairs separated on the board.

import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
<RaspberryPiHatBoard name="HAT1">
  <chip
    name="U1"
    footprint="sop16"
    manufacturerPartNumber="PAM8403 Class D amplifier"
    pinLabels={{
      pin1: ["LIN"],
      pin2: ["RIN"],
      pin3: ["SD"],
      pin4: ["VDD"],
      pin5: ["GND"],
      pin6: ["LOUTP"],
      pin7: ["LOUTN"],
      pin8: ["ROUTP"],
      pin9: ["ROUTN"],
    }}
    pcbX={8}
    pcbY={-6}
  />
  <chip
    name="J2"
    footprint="terminalblock_1x02"
    manufacturerPartNumber="Left speaker"
    pinLabels={{
      pin1: ["L+"],
      pin2: ["L-"],
    }}
    pcbX={26}
    pcbY={-10}
  />
  <chip
    name="J3"
    footprint="terminalblock_1x02"
    manufacturerPartNumber="Right speaker"
    pinLabels={{
      pin1: ["R+"],
      pin2: ["R-"],
    }}
    pcbX={26}
    pcbY={4}
  />

  <trace from=".U1 .LOUTP" to=".J2 .L+" />
  <trace from=".U1 .LOUTN" to=".J2 .L-" />
  <trace from=".U1 .ROUTP" to=".J3 .R+" />
  <trace from=".U1 .ROUTN" to=".J3 .R-" />
</RaspberryPiHatBoard>
)

Raspberry Pi Audio Hookup

The Raspberry Pi GPIO header does not expose a universal analog line output on every model, so choose one of these input paths:

Input path	When to use it
I2S DAC module	Best audio quality and predictable Linux support
USB audio adapter wired to J1	Quickest bring-up path
Filtered PWM audio on GPIO18 and GPIO19	Useful for compact builds where audio quality is less critical

For an I2S DAC, wire the DAC left output, right output, and ground to J1. Enable the DAC overlay recommended by the DAC vendor, then play a WAV file:

aplay -l
speaker-test -c2 -t wav

For PWM audio experiments, enable a stereo PWM overlay in /boot/firmware/config.txt, then add a proper RC filter before J1 so high-frequency PWM energy does not go directly into the amplifier inputs.

PCB Layout

Keep the amplifier, speaker terminals, and bulk capacitor close together. Route the speaker output pairs as matched, short traces and keep them away from the audio input lines. Use a solid ground return for the input section and connect the audio input ground near the amplifier ground pin.

import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
<RaspberryPiHatBoard name="HAT1">
  <chip
    name="J1"
    footprint="pinrow3"
    manufacturerPartNumber="Stereo audio input"
    pinLabels={{
      pin1: ["LIN"],
      pin2: ["GND"],
      pin3: ["RIN"],
    }}
    pcbX={-22}
    pcbY={-6}
  />
  <chip
    name="RV1"
    footprint="pinrow6"
    manufacturerPartNumber="Dual 10k audio taper potentiometer"
    pinLabels={{
      pin1: ["LIN"],
      pin2: ["LWIPER"],
      pin3: ["LGND"],
      pin4: ["RIN"],
      pin5: ["RWIPER"],
      pin6: ["RGND"],
    }}
    pcbX={-10}
    pcbY={-6}
  />
  <capacitor name="C1" capacitance="1uF" footprint="0603" pcbX={0} pcbY={-10} />
  <capacitor name="C2" capacitance="1uF" footprint="0603" pcbX={0} pcbY={-2} />
  <chip
    name="U1"
    footprint="sop16"
    manufacturerPartNumber="PAM8403 Class D amplifier"
    pinLabels={{
      pin1: ["LIN"],
      pin2: ["RIN"],
      pin3: ["SD"],
      pin4: ["VDD"],
      pin5: ["GND"],
      pin6: ["LOUTP"],
      pin7: ["LOUTN"],
      pin8: ["ROUTP"],
      pin9: ["ROUTN"],
    }}
    pcbX={8}
    pcbY={-6}
  />
  <capacitor name="C3" capacitance="100nF" footprint="0402" pcbX={8} pcbY={5} />
  <capacitor name="C4" capacitance="470uF" footprint="radial_cap_6.3mm" pcbX={18} pcbY={5} />
  <resistor name="R1" resistance="10k" footprint="0402" pcbX={-2} pcbY={8} />
  <chip
    name="J2"
    footprint="terminalblock_1x02"
    manufacturerPartNumber="Left speaker"
    pinLabels={{
      pin1: ["L+"],
      pin2: ["L-"],
    }}
    pcbX={26}
    pcbY={-10}
  />
  <chip
    name="J3"
    footprint="terminalblock_1x02"
    manufacturerPartNumber="Right speaker"
    pinLabels={{
      pin1: ["R+"],
      pin2: ["R-"],
    }}
    pcbX={26}
    pcbY={4}
  />

  <trace from=".J1 .LIN" to=".RV1 .LIN" />
  <trace from=".J1 .RIN" to=".RV1 .RIN" />
  <trace from=".J1 .GND" to=".RV1 .LGND" />
  <trace from=".J1 .GND" to=".RV1 .RGND" />
  <trace from=".RV1 .LWIPER" to=".C1 > .pin1" />
  <trace from=".C1 > .pin2" to=".U1 .LIN" />
  <trace from=".RV1 .RWIPER" to=".C2 > .pin1" />
  <trace from=".C2 > .pin2" to=".U1 .RIN" />
  <trace from=".HAT1_chip .V5_1" to=".U1 .VDD" />
  <trace from=".HAT1_chip .GND_1" to=".U1 .GND" />
  <trace from=".U1 .VDD" to=".C3 > .pin1" />
  <trace from=".C3 > .pin2" to=".U1 .GND" />
  <trace from=".U1 .VDD" to=".C4 > .pin1" />
  <trace from=".C4 > .pin2" to=".U1 .GND" />
  <trace from=".HAT1_chip .GPIO_22" to=".U1 .SD" />
  <trace from=".U1 .SD" to=".R1 > .pin1" />
  <trace from=".R1 > .pin2" to=".HAT1_chip .V3_3_1" />
  <trace from=".U1 .LOUTP" to=".J2 .L+" />
  <trace from=".U1 .LOUTN" to=".J2 .L-" />
  <trace from=".U1 .ROUTP" to=".J3 .R+" />
  <trace from=".U1 .ROUTN" to=".J3 .R-" />
</RaspberryPiHatBoard>
)

Bring-Up Test

Test the amplifier before mounting it on a Raspberry Pi:

1. Inspect the board for solder bridges around U1 and the speaker terminals.
2. Connect 5 V power with no speakers attached and check that U1 receives 5 V.
3. Confirm that the shutdown pin is high when GPIO 22 is not actively driving it low.
4. Connect two inexpensive 8 ohm speakers.
5. Start with the volume at minimum and play a low-volume test file.
6. Raise the volume slowly while checking for heat, distortion, or resets.

You can control the shutdown pin from Python:

from gpiozero import DigitalOutputDevice
from time import sleep

amp_enable = DigitalOutputDevice(22, active_high=True, initial_value=True)

sleep(5)
amp_enable.off()

Troubleshooting

Symptom	Check
No sound	Confirm the DAC or audio adapter is selected by aplay -l
Sound only on one side	Swap the left and right input wires at J1
Harsh noise	Keep speaker traces away from J1 and add input shielding
Raspberry Pi resets	Add bulk capacitance or use a supply with more current margin
Speaker gets hot	Make sure neither speaker terminal is tied to ground

Next Steps

• Add an I2S DAC directly on the HAT
• Add a headphone detect switch that mutes the speakers
• Add an EEPROM so Raspberry Pi OS can identify the HAT
• Add mounting holes and silkscreen labels for 4 ohm and 8 ohm speaker ratings