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Raspberry Pi HATs

Building a Class D Audio Amplifier HAT

Overview​

This tutorial will guide you through designing a Raspberry Pi Audio HAT featuring the PAM8403, a highly efficient Class D audio amplifier. It provides 3W per channel stereo output, making it perfect for driving small speakers in arcade cabinets, media centers, or voice assistants.

Our HAT will feature:

  • A PAM8403 Class D amplifier chip
  • A dual potentiometer for hardware volume control
  • Speaker terminal blocks for easy connection
  • RC low-pass filters to convert the Raspberry Pi's PWM audio into a smooth analog signal

Understanding Class D Amplifiers​

Unlike traditional Class AB amplifiers that waste excess energy as heat, Class D amplifiers act like high-speed electronic switches. They convert the incoming analog audio into a high-frequency PWM (Pulse Width Modulation) signal.

Because the internal transistors are either fully "ON" or fully "OFF", they dissipate very little heat, allowing the PAM8403 to achieve up to 90% efficiency. This is why a chip smaller than a fingernail can output a massive 3W per channel without needing a heatsink!

Circuit Requirements​

To build this audio HAT, we need:

  • PAM8403 Chip: The core amplifier.
  • RC Low-Pass Filter: The Raspberry Pi outputs audio as a digital PWM signal on GPIO 18/19. We use resistors (270Ω) and capacitors (33nF) to smooth this high-frequency PWM into a clean analog sine wave.
  • Input Coupling Capacitors: 1µF capacitors are placed before the PAM8403 inputs to block any DC voltage bias from entering the amplifier, preventing speaker popping.
  • Potentiometer: A dual-gang 10k potentiometer acts as a physical volume knob, dividing the voltage before it reaches the amplifier.

Building the Circuit Step by Step​

Step 1: Add the Raspberry Pi Board and Amplifier​

First, we set up the board and drop in the PAM8403 IC using a standard SOIC-16 footprint.

import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
  <RaspberryPiHatBoard name="HAT1">
    <chip
      name="U1"
      footprint="soic16"
      manufacturerPartNumber="PAM8403"
      pinLabels={{
        pin1: ["OUTL+"], pin2: ["PGND"], pin3: ["OUTL-"], pin4: ["PVDD"],
        pin5: ["MUTE"], pin6: ["VDD"], pin7: ["INL"], pin8: ["VREF"],
        pin9: ["NC"], pin10: ["INR"], pin11: ["GND"], pin12: ["SHDN"],
        pin13: ["PVDD"], pin14: ["OUTR-"], pin15: ["PGND"], pin16: ["OUTR+"]
      }}
      pcbX={0}
      pcbY={0}
    />
  </RaspberryPiHatBoard>
)

Step 2: Input Filtering and Volume Control​

We must filter the Pi's digital PWM output into an analog signal. We'll add our RC filter, the volume potentiometers (represented here as SIP3 chips), and the DC-blocking capacitors.

Schematic Circuit Preview

Step 3: Add Speaker Terminals and Trace Routing​

Finally, we connect the power lines (using the Pi's 5V rail since the PAM8403 runs perfectly on 5V) and route the high-power outputs to screw terminals (SIP-2) for easy speaker wire connection.

import { RaspberryPiHatBoard } from "@tscircuit/common"

export default () => (
<RaspberryPiHatBoard name="HAT1">
  <chip name="U1" footprint="soic16" pinLabels={{pin1: ["OUTL+"], pin3: ["OUTL-"], pin14: ["OUTR-"], pin16: ["OUTR+"]}} pcbX={0} pcbY={0} />
  
  <chip name="J_SPK_L" footprint="sip2" pcbX={15} pcbY={-10} />
  <chip name="J_SPK_R" footprint="sip2" pcbX={15} pcbY={10} />

  <trace from=".U1 .OUTL+" to=".J_SPK_L > .pin1" />
  <trace from=".U1 .OUTL-" to=".J_SPK_L > .pin2" />
  <trace from=".U1 .OUTR+" to=".J_SPK_R > .pin1" />
  <trace from=".U1 .OUTR-" to=".J_SPK_R > .pin2" />
</RaspberryPiHatBoard>
)
Schematic Circuit Preview

Raspberry Pi Audio Configuration​

By default, modern Raspberry Pi operating systems try to route audio through HDMI. To route audio through the GPIO pins (PWM audio), you need to configure your Pi's device tree.

  1. Open a terminal on your Pi and edit the boot config file: ```bash sudo nano /boot/firmware/config.txt

    On older OS versions: sudo nano /boot/config.txt

    ```

  2. Add or uncomment the following line to enable PWM audio on GPIO 18 (Left) and GPIO 19 (Right): ```text dtoverlay=audremap,pins_18_19 ```

  3. Save the file (Ctrl+O, Enter, Ctrl+X) and reboot: ```bash sudo reboot ```

  4. Test your speakers using the terminal! ```bash speaker-test -c2 -t wav ```

Next Steps​

  • Add an I2S DAC (like the PCM5102A) instead of using PWM audio for audiophile-grade high-fidelity sound.
  • Add an LED to indicate when the amplifier is powered on.
  • Experiment with the `MUTE` and `SHDN` pins of the PAM8403 to disable audio programmatically via a separate GPIO pin.