Class-D Amp Reference Design
A compact 2×20W amplifier board built around a Class-D output stage, with layout choices aimed squarely at keeping switching noise out of everything nearby.
Overview
Class-D amplifiers work by rapidly switching the output fully on or off (PWM) rather than running output devices in their linear region, which is why they run cool and efficient enough to put real power on a small board. The tradeoff is that the switching edges themselves are a significant EMI source if the layout doesn't respect them.
The output filter — an LC low-pass per channel — reconstructs the audio waveform from the PWM signal and keeps switching harmonics off the speaker leads. Layout choices (short high-current loops, a solid ground plane, ferrite beads where cables leave the board) do as much for real-world noise performance here as the schematic does.
Block Diagram
Key Components
- Integrated Class-D amplifier IC (PWM modulator plus H-bridge output stage)
- LC output filter per channel
- Input coupling capacitors and input anti-alias filtering
- 12–24V DC power input with adequate decoupling
- Heatsink or thermal pad under the amplifier IC
- Ferrite beads and shielding at cable exit points
- Speaker output terminals
Bill of Materials
| Ref | Part / Value | Rating | Package |
|---|---|---|---|
| U1 | TPA3116D2 — Class-D amplifier IC, 2-channel | 2×20W @ 14V/4Ω | HTSSOP-32 (thermal pad) |
| L1, L2 | Output inductor, 10µH | 5A saturation | Shielded, 10×10mm |
| C1, C2 | Output filter capacitor, 0.1µF film | 50V | Radial |
| C3, C4 | Input coupling capacitor, 1µF X7R | 50V | 0805 |
| FB1, FB2 | Ferrite bead, speaker leads | 600Ω @ 100MHz | 0805 |
| PSU | DC input supply | 12–24V, 3–5A | — |
Build Requirements
- Layout discipline around the switching stage — short, wide high-current loops and a solid ground plane
- Some EMI awareness, ideally pre-compliance scanning if this is headed toward a product
- Basic audio measurement — even a simple THD or scope check on the output filter
- Care with input/output trace separation to avoid coupling switching noise into the audio path