The Two Dominant Amplifier Technologies
When shopping for an amplifier — whether for home audio, car audio, or a portable rig — you'll inevitably encounter two dominant topologies: Class AB and Class D. Understanding their differences helps you make a smarter buying decision rather than just going by brand name or price tag.
How Class AB Amplifiers Work
Class AB amplifiers are the traditional workhorse of the audio world. They operate by keeping two transistors (or sets of transistors) partially conducting at all times — one handles the positive half of the audio waveform, the other handles the negative half. The overlap between the two (the "AB" region) eliminates the crossover distortion found in pure Class B designs while maintaining reasonable efficiency.
Key characteristics of Class AB:
- Efficiency typically between 50–70%
- Warm, natural sound character valued by audiophiles
- Runs warm or hot — requires adequate heatsinking
- Larger, heavier designs due to toroidal transformers and heatsinks
- Proven, well-understood technology
How Class D Amplifiers Work
Class D amplifiers take a radically different approach. Instead of linearly amplifying a signal, they switch transistors on and off at very high frequencies (hundreds of kHz to MHz), producing a pulse-width modulated (PWM) signal. A low-pass filter at the output reconstructs the original audio waveform. The result: extremely high efficiency with minimal heat generation.
Key characteristics of Class D:
- Efficiency often above 90%
- Runs cool — small form factor possible
- Ideal for battery-powered and portable devices
- Modern designs have closed the sound quality gap significantly
- Sensitive to speaker load impedance and output filter design
Side-by-Side Comparison
| Feature | Class AB | Class D |
|---|---|---|
| Efficiency | 50–70% | 85–95% |
| Sound Character | Warm, traditional | Clean, neutral (modern designs) |
| Heat Output | Moderate to high | Low |
| Size / Weight | Larger, heavier | Compact, lightweight |
| Cost (entry level) | Moderate | Low to moderate |
| Best Use Case | Home audio, hi-fi listening | Car audio, portable, powered speakers |
Sound Quality: Has Class D Caught Up?
For years, Class D had a reputation for sounding "clinical" or harsh at high frequencies due to switching noise artifacts. Modern designs using advanced feedback loops, better output filters, and higher switching frequencies have largely resolved these issues. High-end Class D amplifiers from brands like Hypex, Purifi, and others are now considered reference-grade by many audiophile reviewers.
That said, some listeners still prefer the harmonic distortion profile of Class AB, which tends to produce even-order harmonics that many ears find pleasant and "musical."
Which Should You Choose?
Choose Class AB if:
- You're building a dedicated home hi-fi setup
- Sound character and warmth are priorities
- You're not constrained by space or power consumption
Choose Class D if:
- You need a compact, lightweight amplifier
- Efficiency matters (portable, battery-powered, car audio)
- You want modern, neutral performance at a competitive price
The Bottom Line
Both technologies deliver excellent sound when well-implemented. The "which is better" debate is largely settled: the quality of the design matters far more than the class. Focus on reputable manufacturers, check measurements, and always try to audition before buying if possible.