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304 changes: 304 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: Build a Raspberry Pi HAT with a PAM8403-style stereo Class D amplifier, volume control, speaker terminals, and audio bring-up notes.
---

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

## 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.

<TscircuitIframe defaultView="3d" code={`
import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
<RaspberryPiHatBoard name="HAT1">
<chip
name="U1"
manufacturerPartNumber="PAM8403"
footprint="soic16"
pinLabels={{
pin1: "LIN",
pin2: "RIN",
pin3: "VDD",
pin4: "GND",
pin5: "LOUTP",
pin6: "LOUTN",
pin7: "ROUTP",
pin8: "ROUTN",
pin9: "SHDN",
pin10: "MUTE",
}}
pcbX={0}
pcbY={0}
/>
<chip
name="RV1"
manufacturerPartNumber="10k dual audio potentiometer"
footprint="pinrow6"
pinLabels={{
pin1: "L_IN",
pin2: "L_WIPER",
pin3: "L_GND",
pin4: "R_IN",
pin5: "R_WIPER",
pin6: "R_GND",
}}
pcbX={-18}
pcbY={0}
/>
<connector
name="J1"
manufacturerPartNumber="Left speaker terminal"
footprint="terminalblock_1x02"
pinLabels={{
pin1: "L+",
pin2: "L-",
}}
pcbX={18}
pcbY={8}
/>
<connector
name="J2"
manufacturerPartNumber="Right speaker terminal"
footprint="terminalblock_1x02"
pinLabels={{
pin1: "R+",
pin2: "R-",
}}
pcbX={18}
pcbY={-8}
/>
<resistor name="R1" resistance="100k" footprint="0402" pcbX={-7} pcbY={10} />
<resistor name="R2" resistance="100k" footprint="0402" pcbX={-4} pcbY={10} />
<capacitor name="C1" capacitance="10uF" footprint="0805" pcbX={-4} pcbY={-10} />
<capacitor name="C2" capacitance="100nF" footprint="0402" pcbX={2} pcbY={-10} />
<led name="LED1" color="green" footprint="0603" pcbX={10} pcbY={-14} />
<resistor name="R3" resistance="1k" footprint="0402" pcbX={5} pcbY={-14} />

<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" />
<trace from=".HAT1_chip .GPIO_18" to=".RV1 .L_IN" />
<trace from=".HAT1_chip .GPIO_19" to=".RV1 .R_IN" />
<trace from=".RV1 .L_WIPER" to=".U1 .LIN" />
<trace from=".RV1 .R_WIPER" to=".U1 .RIN" />
<trace from=".RV1 .L_GND" to=".U1 .GND" />
<trace from=".RV1 .R_GND" to=".U1 .GND" />
<trace from=".R1 > .pin1" to=".U1 .LIN" />
<trace from=".R1 > .pin2" to=".U1 .GND" />
<trace from=".R2 > .pin1" to=".U1 .RIN" />
<trace from=".R2 > .pin2" to=".U1 .GND" />
<trace from=".U1 .LOUTP" to=".J1 .L+" />
<trace from=".U1 .LOUTN" to=".J1 .L-" />
<trace from=".U1 .ROUTP" to=".J2 .R+" />
<trace from=".U1 .ROUTN" to=".J2 .R-" />
<trace from=".HAT1_chip .V5_1" to=".R3 > .pin1" />
<trace from=".R3 > .pin2" to=".LED1 .pos" />
<trace from=".LED1 .neg" to=".U1 .GND" />
</RaspberryPiHatBoard>
)
`} />

## 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.

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

export default () => (
<RaspberryPiHatBoard name="HAT1">
<chip
name="U1"
manufacturerPartNumber="PAM8403"
footprint="soic16"
pinLabels={{
pin1: "LIN",
pin2: "RIN",
pin3: "VDD",
pin4: "GND",
pin5: "LOUTP",
pin6: "LOUTN",
pin7: "ROUTP",
pin8: "ROUTN",
}}
/>
</RaspberryPiHatBoard>
)
`} />

## 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.

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

export default () => (
<RaspberryPiHatBoard name="HAT1">
<chip
name="U1"
manufacturerPartNumber="PAM8403"
footprint="soic16"
pinLabels={{
pin1: "LIN",
pin2: "RIN",
pin4: "GND",
}}
/>
<chip
name="RV1"
manufacturerPartNumber="10k dual audio potentiometer"
footprint="pinrow6"
pinLabels={{
pin1: "L_IN",
pin2: "L_WIPER",
pin3: "L_GND",
pin4: "R_IN",
pin5: "R_WIPER",
pin6: "R_GND",
}}
/>
<trace from=".HAT1_chip .GPIO_18" to=".RV1 .L_IN" />
<trace from=".HAT1_chip .GPIO_19" to=".RV1 .R_IN" />
<trace from=".RV1 .L_WIPER" to=".U1 .LIN" />
<trace from=".RV1 .R_WIPER" to=".U1 .RIN" />
<trace from=".RV1 .L_GND" to=".U1 .GND" />
<trace from=".RV1 .R_GND" to=".U1 .GND" />
</RaspberryPiHatBoard>
)
`} />

## 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.

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

export default () => (
<RaspberryPiHatBoard name="HAT1">
<chip
name="U1"
manufacturerPartNumber="PAM8403"
footprint="soic16"
pinLabels={{
pin5: "LOUTP",
pin6: "LOUTN",
pin7: "ROUTP",
pin8: "ROUTN",
}}
/>
<connector
name="J1"
manufacturerPartNumber="Left speaker terminal"
footprint="terminalblock_1x02"
pinLabels={{
pin1: "L+",
pin2: "L-",
}}
/>
<connector
name="J2"
manufacturerPartNumber="Right speaker terminal"
footprint="terminalblock_1x02"
pinLabels={{
pin1: "R+",
pin2: "R-",
}}
/>
<trace from=".U1 .LOUTP" to=".J1 .L+" />
<trace from=".U1 .LOUTN" to=".J1 .L-" />
<trace from=".U1 .ROUTP" to=".J2 .R+" />
<trace from=".U1 .ROUTN" to=".J2 .R-" />
</RaspberryPiHatBoard>
)
`} />

## 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.

```tsx
<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:

```ini
dtparam=audio=on
```

Then test with:

```bash
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.
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