Understanding Amplifier Classes
Amplifier class describes how the output stage operates and how its output devices are controlled. These design choices affect efficiency, heat generation, power capability, distortion behavior, and implementation requirements. No amplifier class is inherently superior in sound quality. The finished result depends on the complete design and how it behaves with the load it is asked to drive.
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: Avoids the conventional crossover region between output devices and can achieve very low distortion with relatively straightforward output-stage operation.
- Disadvantages: Maximum 25% theoretical efficiency (typically 15-20% in practice), runs very hot.
- Commonly used for: Lower-power amplifiers and some small-signal stages where efficiency and heat are less important constraints.
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: Greater efficiency and lower heat generation than Class A while still allowing high output power and very low distortion when properly implemented.
- Design challenge: The transition between output devices must be carefully biased and controlled to minimize crossover distortion.
- Commonly used for: A very wide range of traditional hi-fi power amplifiers and integrated amplifiers.
Class G and H: Extending Efficiency
Class G and Class H amplifiers build on conventional linear amplifier operation but improve efficiency by changing the power-supply voltage available to the output stage.
Class G typically uses two or more supply-voltage levels and switches to a higher rail when the signal requires greater output.
Class H varies or tracks the supply rails more continuously with the signal.
Both approaches reduce the amount of excess voltage that must be dissipated as heat, allowing higher power with better efficiency than a conventional fixed-rail Class AB design.
Key point: Class G and H are not indicators of sound quality. Their performance still depends on the quality of the amplifier’s complete implementation.
Class D: The Modern Efficiency Champion
Class D amplifiers use rapidly switched output devices rather than operating the output transistors continuously through the audio waveform as linear amplifiers do. The switching waveform is then passed through an output filter to recover the amplified audio signal. Because the output devices spend much of their time either fully on or fully off, Class D can achieve very high efficiency with relatively little heat.
- Advantages: Very high efficiency, relatively low heat generation, compact size, and high power capability.
- Design challenges: Switching behavior, output filtering, feedback implementation, electromagnetic interference, power-supply interaction, and speaker-load behavior must all be properly controlled.
- Commonly used for: Everything from compact integrated amplifiers and powered loudspeakers to subwoofers and very high-power hi-fi amplifiers.
| Class | Efficiency | Heat | Distortion Profile | Typical Use |
|---|---|---|---|---|
| Class A | 15-25% | Very High | No conventional crossover region | Headphones, preamps |
| Class AB | 50-70% | Moderate | Design-dependent | Traditional hi-fi |
| Class G / H | Higher than conventional Class AB | Moderate to Low | Design-dependent | Higher-power linear amplification |
| Class D | 85-95% | Minimal | Design-dependent | Powered speakers, subs |
Debunking Class D Myths
Early Class D amplifiers developed a reputation for load-dependent frequency response, higher distortion, switching artifacts, and sometimes less refined performance than good linear amplifiers. Modern implementations have improved dramatically through better output stages, control loops, feedback, filtering, power supplies, and semiconductor technology.
- Modern feedback and control-loop designs can greatly reduce distortion and other errors produced by the switching output stage.
- Post-filter feedback (PFFB) can reduce the influence of the output filter and speaker impedance on frequency response and distortion behavior.
- Well-designed modern Class D amplifiers can achieve exceptionally low noise and distortion and highly load-independent performance.
The Bottom Line: Amplifier class describes how the amplifier operates, not how good it will sound. Class A, AB, G, H, and D each involve different engineering tradeoffs in efficiency, heat, complexity, size, and power capability. Excellent amplifiers can be built using several of these approaches. What ultimately matters is the quality of the implementation and how the finished amplifier behaves with the loudspeaker load and listening demands placed upon it.