Understanding Amplifier Classes
Amplifier "class" refers to the operating point and conduction angle of the output devices. This fundamental design choice affects everything from efficiency and heat dissipation to distortion characteristics and sound quality. Let's explore what each class means and why it matters.
Class A: The Purist's Choice
In a Class A amplifier, the output devices conduct for the entire 360° of the input waveform. The transistors are always "on," biased at a point where they can swing fully in either direction without ever turning off.
- Advantages: Lowest crossover distortion, simplest topology, often considered the most "musical"
- Disadvantages: Maximum 25% theoretical efficiency (typically 15-20% in practice), runs very hot
- Best for: Low-power applications, headphone amps, preamps, and those who prioritize purity over practicality
Why Class A runs hot: The output devices are always conducting, constantly dissipating power as heat—even with no signal present. A 50W Class A amp might draw 200-300W continuously from the wall.
Class AB: The Practical Compromise
Class AB is the most common topology in traditional hi-fi amplifiers. It uses push-pull output stages where each device conducts for more than 180° but less than 360° of the waveform. A small "bias current" keeps both devices slightly on, smoothing the transition between them.
- Advantages: Much better efficiency (50-70%), much less heat, can deliver high power affordably
- Disadvantages: Some crossover distortion where devices hand off, requires careful bias setting
- Best for: General-purpose amplification, most speaker applications
Class D: The Modern Efficiency Champion
Class D amplifiers use pulse-width modulation (PWM) to rapidly switch output devices fully on or off. Because the transistors are either fully conducting or fully off (never in between), they dissipate minimal heat. The switching signal is then filtered to recover the audio waveform.
- Advantages: 85-95% efficiency, runs cool, compact size, high power capability
- Disadvantages: Requires careful output filtering, potential EMI concerns, historically associated with "harsh" sound (though modern designs have largely overcome this)
- Best for: Powered speakers, subwoofers, high-power applications, portable devices
| Class | Efficiency | Heat | Distortion Profile | Typical Use |
|---|---|---|---|---|
| Class A | 15-25% | Very High | Lowest crossover | Headphones, preamps |
| Class AB | 50-70% | Moderate | Well-controlled | Traditional hi-fi |
| Class D | 85-95% | Minimal | Design-dependent | Powered speakers, subs |
Debunking Class D Myths
Early Class D amplifiers earned a reputation for sounding "digital" or harsh. Modern designs from companies like Purifi, Hypex, and ICEpower have changed this dramatically:
- Advanced feedback topologies eliminate the "switching sound"
- Post-filter feedback (PFFB) corrects for speaker impedance variations
- Modern Class D regularly measures as well as or better than traditional designs
- Many golden-ear reviewers can't distinguish them in blind tests
The Bottom Line: Choose based on your needs: Class A for low-power purity, Class AB for traditional high-fidelity, Class D when efficiency and size matter. Modern implementations of all classes can sound excellent.