Building a Class D Audio Amplifier HAT

Overview

This tutorial shows how to build a Raspberry Pi HAT around a PAM8403-based stereo Class D amplifier stage. The design focuses on the parts that matter in practice: line-level stereo input, volume control, speaker terminals, and solid power decoupling for the 5V rail.

Why Class D?

The PAM8403 is a 3W stereo Class D amplifier. In practice, that means:

• high efficiency compared with linear audio amplifiers
• low heat for a compact HAT enclosure
• direct drive for small speakers without a bulky output filter
• enough power for desk speakers, media boxes, and audio add-ons

The official Diodes page for the part is here: PAM8403.

Circuit Requirements

Our HAT needs to:

• accept stereo line-level audio from the Raspberry Pi or an external DAC
• provide independent or shared volume control
• drive left and right speakers from the amplifier outputs
• stay stable on the Pi's 5V rail with nearby decoupling capacitors

Building the Circuit Step by Step

Step 1: Place the HAT board and amplifier stage

The base of the design is the Raspberry Pi HAT form factor plus the PAM8403 amplifier block.

Step 2: Add the stereo input and volume control

For a clean layout, the tutorial uses one volume trimmer per channel. The audio source enters through a 3-pin connector, then each channel passes through a coupling capacitor and a potentiometer before reaching the amplifier input.

Step 3: Add the speaker terminals

Class D outputs should go to the speaker terminals directly. Do not tie either speaker lead to ground.

Step 4: Add decoupling and power

The amplifier needs a clean 5V supply. Put a small ceramic capacitor and a larger bulk capacitor close to the amplifier power pins.

import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
<RaspberryPiHatBoard name="HAT1">
  <chip
    name="U1"
    footprint="soic8"
    manufacturerPartNumber="PAM8403"
    pinLabels={{
      pin1: "LIN",
      pin2: "L_OUTP",
      pin3: "L_OUTN",
      pin4: "GND",
      pin5: "R_OUTN",
      pin6: "R_OUTP",
      pin7: "RIN",
      pin8: "VDD",
    }}
    schPinArrangement={{
      leftSide: {
        direction: "top-to-bottom",
        pins: ["LIN", "L_OUTP", "L_OUTN", "GND"],
      },
      rightSide: {
        direction: "top-to-bottom",
        pins: ["R_OUTN", "R_OUTP", "RIN", "VDD"],
      },
    }}
    pcbX={0}
    pcbY={-6}
  />

  <capacitor name="C3" capacitance="1uF" footprint="0603" pcbX={10} pcbY={-20} />
  <capacitor name="C4" capacitance="100uF" footprint="1206" pcbX={16} pcbY={-20} />

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

Understanding the Amplifier

Class D amplifiers work by switching their output stage rapidly instead of running it in a linear region. The speaker and output network average those switching pulses back into audio, which is why the part is efficient and runs cooler than many linear designs.

The practical takeaway is simple: use short speaker traces, give the chip good decoupling, and keep the speaker outputs floating. The outputs are not meant to be shorted to ground.

Raspberry Pi Integration

The HAT itself uses the Pi's 40-pin header for power and mounting. For audio, the safest assumption is line-level stereo input from the Pi's audio path or an external DAC. If you want a self-contained product, you can swap the input header for a jack or DAC module without changing the rest of the amplifier section.

Practical connections to keep in mind:

• 5V and ground come from the HAT header
• the audio source should stay line-level, not speaker-level
• the amplifier output goes only to speakers
• a mute or enable GPIO can be added later if you want software control

Audio Configuration Guide

On Raspberry Pi OS, make sure audio output is enabled and test the source before you blame the amplifier.

1. Open raspi-config and enable the relevant audio output.
2. Confirm the device tree and firmware audio settings are active.
3. Test playback with a sine wave or a short WAV file.
4. If the output sounds weak, check the source level before increasing gain in the amplifier path.

For a production board, double-check the exact PAM8403 package pinout against the datasheet for the chip variant you source.

Next Steps

• add a mute pin or GPIO-controlled enable line
• swap the input header for a 3.5 mm jack footprint
• add silkscreen labels for the left and right speaker channels
• pair the HAT with a small enclosure and panel-mounted terminals