Class D Audio Amplifier HAT

Overview

This tutorial walks through designing a Class D audio amplifier HAT using tscircuit. We'll use the PAM8403 — a popular 3W+3W stereo Class D amplifier — to drive a pair of speakers from a line-level audio input. Class D amplifiers are perfect for battery-powered and portable audio projects because of their high efficiency (often above 90%).

What is Class D Amplification?

Class D amplifiers work fundamentally differently from traditional linear amplifiers (Class A, B, or AB). Instead of using transistors in their linear region, Class D amps use transistors as switches — rapidly turning them on and off.

How Class D Works

PWM Modulation: The incoming audio signal is compared to a high-frequency triangular wave (typically 250kHz–1MHz), producing a Pulse Width Modulated (PWM) signal
Switching Stage: This PWM signal drives MOSFETs that switch the power supply voltage on and off at the PWM frequency
LC Filter (or Filterless): A low-pass filter (or the speaker's own inductance in "filterless" designs) removes the high-frequency switching noise, leaving only the amplified audio

Why Class D?

Feature	Class AB	Class D
Efficiency	50–70%	85–95%
Heat	High (needs heatsink)	Low (often no heatsink)
Size	Large	Compact
Battery life	Short	Long
Audio quality	Very good	Very good (modern ICs)

Class D is the dominant technology for portable speakers, soundbars, car audio, and any battery-powered audio device.

The PAM8403

The PAM8403 is one of the most widely used Class D amplifier ICs. Key features:

3W per channel into 4 speakers at 5V
Filterless operation — no output LC filter needed
Stereo output — two independent channels
>90% efficiency
SOP-16 package — easy to hand-solder
Built-in shutdown, mute, and pop suppression

Circuit Requirements

Our Class D amplifier HAT needs to:

Accept stereo line-level audio input (left and right)
Amplify both channels via the PAM8403
Output to two separate speakers through pin headers
Include power supply decoupling for clean audio
Support shutdown and mute control
Be powered by a 5V supply

Building the Circuit Step by Step

Step 1: Add the PAM8403 Amplifier IC

The PAM8403 is a 16-pin chip with distinct left and right channels. The pin arrangement groups related functions together: power on the top, inputs in the middle, and control on the bottom.

Step 2: Add Input Coupling Capacitors

Audio input signals often have a DC offset. Input coupling capacitors (C3, C4) block this DC offset, passing only the AC audio signal to the amplifier. Use 1 F ceramic capacitors for good low-frequency response.

Step 3: Add Input Bias Resistors

Input bias resistors (R1–R4) set the input impedance and provide a DC path to ground for the amplifier inputs. The 10k resistors form a voltage divider that establishes the correct DC bias point. LIN- and RIN- are tied directly to ground for single-ended input configuration.

Step 4: Add Power Supply Decoupling

Class D amplifiers draw current in short, high-amplitude pulses at the switching frequency. Large bulk capacitors (100 F) supply these current pulses and prevent voltage sag on the power rail. Place them as close as possible to the VDD pins.

Step 5: Add Bypass Capacitor

The BYPASS pin requires a 1 F capacitor to ground. This capacitor filters the internal reference voltage, reducing noise and improving Power Supply Rejection Ratio (PSRR).

Step 6: Wire Everything Together

Now we assemble the complete circuit — amplifier IC, input coupling, bias resistors, power decoupling, control pins, and speaker outputs.

export default () => (
<board width="50mm" height="35mm">
  <chip
    name="U1"
    manufacturerPartNumber="PAM8403"
    footprint="sop16"
    pinLabels={{
      pin1: ["LOUT-"],
      pin2: ["PGNDL"],
      pin3: ["LOUT+"],
      pin4: ["VDD"],
      pin5: ["LIN+"],
      pin6: ["LIN-"],
      pin7: ["BYPASS"],
      pin8: ["SHUTDOWN"],
      pin9: ["MUTE"],
      pin10: ["VREF"],
      pin11: ["RIN+"],
      pin12: ["RIN-"],
      pin13: ["VDD"],
      pin14: ["ROUT+"],
      pin15: ["PGNDR"],
      pin16: ["ROUT-"],
    }}
    schPortArrangement={{
      leftSide: { pins: ["LOUT-", "PGNDL", "LOUT+", "VDD", "LIN+", "LIN-", "BYPASS", "SHUTDOWN"], direction: "top-to-bottom" },
      rightSide: { pins: ["ROUT-", "PGNDR", "ROUT+", "VDD", "RIN+", "RIN-", "VREF", "MUTE"], direction: "top-to-bottom" },
    }}
    pcbX={0}
    pcbY={0}
  />

  <capacitor name="C1" capacitance="100uF" footprint="0805" pcbX={-8} pcbY={-8} />
  <capacitor name="C2" capacitance="100uF" footprint="0805" pcbX={0} pcbY={-8} />
  <capacitor name="C3" capacitance="1uF" footprint="0603" pcbX={-8} pcbY={8} />
  <capacitor name="C4" capacitance="1uF" footprint="0603" pcbX={0} pcbY={8} />
  <capacitor name="C_BYPASS" capacitance="1uF" footprint="0402" pcbX={-5} pcbY={-12} />

  <resistor name="R1" resistance="10k" footprint="0402" pcbX={-10} pcbY={3} />
  <resistor name="R2" resistance="10k" footprint="0402" pcbX={-6} pcbY={3} />
  <resistor name="R3" resistance="10k" footprint="0402" pcbX={6} pcbY={3} />
  <resistor name="R4" resistance="10k" footprint="0402" pcbX={10} pcbY={3} />

  <pinheader name="J1" pinCount={4} pcbX={-15} pcbY={0} />
  <pinheader name="J2" pinCount={2} pcbX={15} pcbY={-5} />
  <pinheader name="J3" pinCount={2} pcbX={15} pcbY={5} />

  <trace from=".J1 .pin1" to="net.V5" />
  <trace from=".J1 .pin2" to="net.GND" />
  <trace from=".J1 .pin3" to=".R1 .pos" />
  <trace from=".J1 .pin4" to=".R3 .pos" />

  <trace from=".R1 .neg" to=".U1 .LIN+" />
  <trace from=".R2 .pos" to=".U1 .LIN+" />
  <trace from=".R2 .neg" to="net.GND" />

  <trace from=".R3 .neg" to=".U1 .RIN+" />
  <trace from=".R4 .pos" to=".U1 .RIN+" />
  <trace from=".R4 .neg" to="net.GND" />

  <trace from=".C3 .pos" to=".U1 .LIN+" />
  <trace from=".C3 .neg" to=".R1 .neg" />
  <trace from=".C4 .pos" to=".U1 .RIN+" />
  <trace from=".C4 .neg" to=".R3 .neg" />

  <trace from=".U1 .LIN-" to="net.GND" />
  <trace from=".U1 .RIN-" to="net.GND" />

  <trace from=".U1 .VDD" to="net.V5" />
  <trace from=".U1 .PGNDL" to="net.GND" />
  <trace from=".U1 .PGNDR" to="net.GND" />

  <trace from=".U1 .SHUTDOWN" to="net.V5" />
  <trace from=".U1 .MUTE" to="net.GND" />

  <trace from=".C_BYPASS .pos" to=".U1 .BYPASS" />
  <trace from=".C_BYPASS .neg" to="net.GND" />

  <trace from=".C1 .pos" to="net.V5" />
  <trace from=".C1 .neg" to="net.GND" />
  <trace from=".C2 .pos" to="net.V5" />
  <trace from=".C2 .neg" to="net.GND" />

  <trace from=".U1 .LOUT+" to=".J2 .pin1" />
  <trace from=".U1 .LOUT-" to=".J2 .pin2" />
  <trace from=".U1 .ROUT+" to=".J3 .pin1" />
  <trace from=".U1 .ROUT-" to=".J3 .pin2" />
</board>
)

Connector Pinout Reference

Input Header (J1) — 4-pin

Pin	Signal	Description
1	V5	5V power input
2	GND	Ground
3	LIN	Left channel audio input
4	RIN	Right channel audio input

Speaker Outputs (J2, J3) — 2-pin each

Pin	Signal	Description
1	LOUT+ / ROUT+	Positive speaker terminal
2	LOUT- / ROUT-	Negative speaker terminal

Important: PAM8403 outputs are bridged (BTL). Do NOT connect the negative speaker terminals to ground, and do NOT connect them together. Each speaker connects only to its own + and - outputs.

Control Pins Explained

The PAM8403 provides two logic-level control pins:

Pin	Active	Behavior
SHUTDOWN	LOW	Disables the amplifier entirely (typical `<1 µA` current draw)
MUTE	HIGH	Mutes the audio output while keeping the amplifier powered

In our circuit, SHUTDOWN is tied to V5 (always enabled) and MUTE is tied to GND (never muted). For more control, connect these to GPIO pins on a microcontroller:

<trace from=".U1 .SHUTDOWN" to="net.SHDN" />
<trace from=".U1 .MUTE" to="net.MUTE" />

Then drive them from your microcontroller — pull SHUTDOWN LOW to power down, pull MUTE HIGH to mute.

Choosing Speakers

The PAM8403 is optimized for 4 speakers at 5V, delivering up to 3W per channel. Here's the power vs. impedance relationship at 5V:

Speaker Impedance	Power per Channel	THD
4	3W	10%
8	1.5W	10%

For best results, use 3W–5W rated 4 speakers. The amplifier can drive 8 speakers at reduced power, which may be suitable for headphone-level volumes.

PCB Layout Guidelines

export default () => (
<board width="50mm" height="35mm">
  <chip
    name="U1"
    manufacturerPartNumber="PAM8403"
    footprint="sop16"
    pinLabels={{
      pin1: ["LOUT-"],
      pin2: ["PGNDL"],
      pin3: ["LOUT+"],
      pin4: ["VDD"],
      pin5: ["LIN+"],
      pin6: ["LIN-"],
      pin7: ["BYPASS"],
      pin8: ["SHUTDOWN"],
      pin9: ["MUTE"],
      pin10: ["VREF"],
      pin11: ["RIN+"],
      pin12: ["RIN-"],
      pin13: ["VDD"],
      pin14: ["ROUT+"],
      pin15: ["PGNDR"],
      pin16: ["ROUT-"],
    }}
    schPortArrangement={{
      leftSide: { pins: ["LOUT-", "PGNDL", "LOUT+", "VDD", "LIN+", "LIN-", "BYPASS", "SHUTDOWN"], direction: "top-to-bottom" },
      rightSide: { pins: ["ROUT-", "PGNDR", "ROUT+", "VDD", "RIN+", "RIN-", "VREF", "MUTE"], direction: "top-to-bottom" },
    }}
    pcbX={0}
    pcbY={0}
  />

  <capacitor name="C1" capacitance="100uF" footprint="0805" pcbX={-8} pcbY={-8} />
  <capacitor name="C2" capacitance="100uF" footprint="0805" pcbX={0} pcbY={-8} />
  <capacitor name="C3" capacitance="1uF" footprint="0603" pcbX={-8} pcbY={8} />
  <capacitor name="C4" capacitance="1uF" footprint="0603" pcbX={0} pcbY={8} />
  <capacitor name="C_BYPASS" capacitance="1uF" footprint="0402" pcbX={-5} pcbY={-12} />

  <resistor name="R1" resistance="10k" footprint="0402" pcbX={-10} pcbY={3} />
  <resistor name="R2" resistance="10k" footprint="0402" pcbX={-6} pcbY={3} />
  <resistor name="R3" resistance="10k" footprint="0402" pcbX={6} pcbY={3} />
  <resistor name="R4" resistance="10k" footprint="0402" pcbX={10} pcbY={3} />

  <pinheader name="J1" pinCount={4} pcbX={-15} pcbY={0} />
  <pinheader name="J2" pinCount={2} pcbX={15} pcbY={-5} />
  <pinheader name="J3" pinCount={2} pcbX={15} pcbY={5} />

  <trace from=".J1 .pin1" to="net.V5" />
  <trace from=".J1 .pin2" to="net.GND" />
  <trace from=".J1 .pin3" to=".R1 .pos" />
  <trace from=".J1 .pin4" to=".R3 .pos" />

  <trace from=".R1 .neg" to=".U1 .LIN+" />
  <trace from=".R2 .pos" to=".U1 .LIN+" />
  <trace from=".R2 .neg" to="net.GND" />

  <trace from=".R3 .neg" to=".U1 .RIN+" />
  <trace from=".R4 .pos" to=".U1 .RIN+" />
  <trace from=".R4 .neg" to="net.GND" />

  <trace from=".C3 .pos" to=".U1 .LIN+" />
  <trace from=".C3 .neg" to=".R1 .neg" />
  <trace from=".C4 .pos" to=".U1 .RIN+" />
  <trace from=".C4 .neg" to=".R3 .neg" />

  <trace from=".U1 .LIN-" to="net.GND" />
  <trace from=".U1 .RIN-" to="net.GND" />

  <trace from=".U1 .VDD" to="net.V5" />
  <trace from=".U1 .PGNDL" to="net.GND" />
  <trace from=".U1 .PGNDR" to="net.GND" />

  <trace from=".U1 .SHUTDOWN" to="net.V5" />
  <trace from=".U1 .MUTE" to="net.GND" />

  <trace from=".C_BYPASS .pos" to=".U1 .BYPASS" />
  <trace from=".C_BYPASS .neg" to="net.GND" />

  <trace from=".C1 .pos" to="net.V5" />
  <trace from=".C1 .neg" to="net.GND" />
  <trace from=".C2 .pos" to="net.V5" />
  <trace from=".C2 .neg" to="net.GND" />

  <trace from=".U1 .LOUT+" to=".J2 .pin1" />
  <trace from=".U1 .LOUT-" to=".J2 .pin2" />
  <trace from=".U1 .ROUT+" to=".J3 .pin1" />
  <trace from=".U1 .ROUT-" to=".J3 .pin2" />
</board>
)

Key layout tips for Class D amplifiers:

Keep power traces wide: Use wide traces (at least 0.5mm) for VDD and output paths to minimize voltage drop
Separate power and signal grounds: Connect PGNDL and PGNDR with a star-ground approach to avoid ground loops
Place decoupling close: C1 and C2 should be as close as possible to the VDD pins — within 5mm
Keep input traces away from outputs: Route audio inputs away from the high-current switching outputs to prevent coupling
Use a ground pour: A solid ground plane on the bottom layer helps with both noise and thermal performance

Powering the Amplifier

The PAM8403 operates from 2.5V to 5.5V. For best performance:

5V USB power: The simplest option. Use a USB power bank or wall adapter. At 5V you get the full 3W+3W
3.7V Li-Ion battery: Lower output power (~1.5W per channel) but excellent for portable use
Pair with a USB PD Trigger HAT: For higher voltage systems, use a PD trigger to negotiate 9V or 12V, then regulate down to 5V with a buck converter

Ordering the PCB

You can order this PCB by downloading the fabrication files and uploading them to JLCPCB. Follow the instructions from Ordering Prototypes.

Next Steps

Add a potentiometer for volume control on the input
Add a 3.5mm audio jack instead of (or in addition to) pin headers
Integrate a Bluetooth audio module (e.g., MH-M18) for wireless streaming
Add an I2S DAC (e.g., PCM5102A) for higher-quality digital audio input
Design a speaker enclosure and use the 3D CAD model from tscircuit for fit-checking
Combine with a USB PD Trigger HAT to build a complete portable speaker system

Overview​

What is Class D Amplification?​

How Class D Works​

Why Class D?​

The PAM8403​

Circuit Requirements​

Building the Circuit Step by Step​

Step 1: Add the PAM8403 Amplifier IC​

Step 2: Add Input Coupling Capacitors​

Step 3: Add Input Bias Resistors​

Step 4: Add Power Supply Decoupling​

Step 5: Add Bypass Capacitor​

Step 6: Wire Everything Together​

Connector Pinout Reference​

Input Header (J1) — 4-pin​

Speaker Outputs (J2, J3) — 2-pin each​

Control Pins Explained​

Choosing Speakers​

PCB Layout Guidelines​

Powering the Amplifier​

Ordering the PCB​

Next Steps​