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377 changes: 377 additions & 0 deletions docs/tutorials/pi-hats/class-d-audio-amplifier-hat.mdx
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---
title: Building a Class D Audio Amplifier HAT
description: >-
Design a Raspberry Pi HAT that powers a PAM8403-style stereo Class D amplifier,
adds volume control, and exposes screw-terminal speaker outputs.
---

## 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.

import CircuitPreview from "@site/src/components/CircuitPreview"
import TscircuitIframe from "@site/src/components/TscircuitIframe"

<TscircuitIframe defaultView="3d" code={`
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"],
}}
schPortArrangement={{
leftSide: { pins: ["LIN", "RIN", "SHDN"], direction: "top-to-bottom" },
rightSide: { pins: ["LOUTP", "LOUTN", "ROUTP", "ROUTN"], direction: "top-to-bottom" },
topSide: { pins: ["VDD"], direction: "left-to-right" },
bottomSide: { pins: ["GND"], direction: "left-to-right" },
}}
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>
)
`} />

## 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.

<CircuitPreview splitView={false} hidePCBTab hide3DTab defaultView="schematic" code={`
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}
/>
</RaspberryPiHatBoard>
)
`} />

## 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.

<CircuitPreview splitView={false} hidePCBTab hide3DTab defaultView="schematic" code={`
import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
<RaspberryPiHatBoard name="HAT1">
<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} />
</RaspberryPiHatBoard>
)
`} />

## 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.

<CircuitPreview splitView={false} hidePCBTab hide3DTab defaultView="schematic" code={`
import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
<RaspberryPiHatBoard name="HAT1">
<chip
name="U1"
footprint="soic16"
manufacturerPartNumber="PAM8403"
pinLabels={{ pin3: ["GND"], pin4: ["VDD"], pin5: ["SHDN"] }}
pcbX={2}
pcbY={-8}
/>
<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} />

<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=".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" />
</RaspberryPiHatBoard>
)
`} />

## 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.

<CircuitPreview hidePCBTab hide3DTab defaultView="schematic" code={`
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:

```bash
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.
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