Class D Amplifiers: Why Your Next Amp Should Probably Be One

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TL;DR: Class D amps use smart switching instead of brute force, run way cooler, save power, and sound great when done right.

What’s the Big Deal?

Traditional amplifiers (Class A, AB, B) keep transistors running in “linear mode” – basically constantly burning power as heat. It’s like keeping your car’s engine revving at 3000 RPM all the time, even when parked.

Class D amps are different. They switch the output transistors rapidly between fully on and fully off, creating pulses that average out to your audio signal. Since the transistors are either fully on (low resistance) or fully off (no current), they barely heat up.

The Efficiency Advantage

Here’s where it gets interesting:

  • Class A: 25% efficient at full power, 3% at normal listening levels (ouch!)
  • Class B: 78.5% efficient at full power, 28% at normal levels
  • Class D: 90% efficient at full power, 78% at normal levels

Real world example: A 10W amp at typical 1W listening volume:

  • Class D: 282mW wasted
  • Class B: 2.53W wasted
  • Class A: 30.2W wasted (needs a heatsink the size of a brick)

This means no massive heatsinks, longer battery life, and you can cram more channels into smaller spaces. Perfect for flat-screen TVs, car audio, and portable devices.

The Challenges

Nothing’s perfect. Class D has to deal with:

  • EMI/Interference: All that high-frequency switching can cause radio interference
  • LC Filters: Need inductors and capacitors to smooth out the pulses (adds cost/size)
  • Design complexity: Getting low distortion requires good feedback loops
  • Switching noise: The modulation creates high-frequency content you don’t want

How They Actually Work

The magic happens in the modulator – usually PWM (Pulse Width Modulation) or Sigma-Delta. It converts your audio signal into precisely-timed pulses. The LC filter then smooths these back into audio frequencies for your speaker.

The amp needs protection for overcurrent, overheating, and shoot-through (when both transistors conduct at once – bad news). Modern designs handle all this automatically.

Sound Quality?

Early Class D amps had a reputation for sounding harsh. Not anymore. Modern designs with proper feedback achieve:

  • THD < 0.01% (basically distortion-free)
  • SNR > 100dB (dead quiet)
  • PSR > 60dB (power supply noise rejection)

Some audiophiles still prefer Class AB for critical listening, but well-designed Class D is closing that gap fast.

Bottom Line

For most applications – especially anything portable, space-constrained, or multi-channel – Class D is the smart choice. You get solid sound quality with way better efficiency. The only real downsides are the need for LC filters (or filterless designs for low-power stuff) and potential EMI issues if poorly designed.

Best for: Car audio, home theater, portable speakers, anything battery-powered
Skip it for: Maybe high-end audiophile gear if you’re really picky (and have unlimited power/cooling)

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