What Is an Amplifier Class?
The "class" of an amplifier describes how its output transistors (or tubes) are biased and operated — specifically, what portion of the audio waveform cycle each output device handles. This has profound effects on efficiency, heat dissipation, distortion characteristics, and ultimately, sound quality. Understanding amplifier classes helps you make better purchasing decisions and understand why some amps run hot while others stay cool.
Class A
In a Class A amplifier, the output devices conduct current for the entire 360° of the audio cycle. There is no switching — the transistor or tube is always on.
Characteristics:
- Sound quality: Widely considered to sound the best — lowest crossover distortion, smooth and linear
- Efficiency: Very low — typically only 15–30% efficient. Most power is wasted as heat
- Heat: Runs very hot, always — even at idle
- Power output: Typically modest — high-wattage Class A designs are expensive to build and run
- Best for: Listeners who prioritize sonic purity and are willing to accept the heat and cost
Class B
In Class B, two output devices each handle half of the audio cycle (180° each). When one transistor handles the positive half-wave, the other is off, and vice versa.
Characteristics:
- Efficiency: Much higher than Class A — up to ~78% theoretical efficiency
- Distortion: Introduces crossover distortion at the zero-crossing point where one device hands off to the other
- Sound quality: Less refined due to crossover distortion; rarely used alone in modern hi-fi
- Best for: High-efficiency applications where fidelity is less critical
Class AB (The Most Common)
Class AB is the dominant design in modern audio amplifiers. It biases the output devices to conduct for slightly more than 180° each, overlapping slightly to eliminate the crossover distortion of Class B while being far more efficient than Class A.
Characteristics:
- Efficiency: Moderate — typically 50–70%
- Distortion: Low — crossover distortion is largely eliminated
- Heat: Moderate — warms up but not nearly as hot as Class A
- Sound quality: Very good — the majority of well-regarded hi-fi amplifiers use Class AB
- Best for: The vast majority of home audio applications; an excellent balance of performance and practicality
Class D (Digital/Switching)
Despite the name, Class D is not strictly "digital" in the audio signal sense. It works by switching the output transistors on and off at very high frequencies (typically several hundred kHz to over 1 MHz), using pulse-width modulation (PWM) to encode the audio signal. A low-pass filter at the output reconstructs the audio waveform.
Characteristics:
- Efficiency: Very high — often 85–95%+. Runs cool even at high power
- Size and weight: Extremely compact — ideal for portable and space-constrained applications
- Sound quality: Has improved dramatically in recent years; modern Class D designs rival Class AB in audible performance
- Distortion: Modern designs have very low THD; older or budget designs can exhibit high-frequency noise
- Best for: Subwoofer amplifiers, car audio, portable gear, multi-channel home theater, and anyone who prioritizes efficiency and small size
Comparison Summary
| Class | Efficiency | Sound | Heat | Typical Use |
|---|---|---|---|---|
| A | 15–30% | Excellent | Very High | Audiophile, headphone amps |
| B | ~78% | Fair | Low | Rarely used alone |
| AB | 50–70% | Very Good | Moderate | Most hi-fi stereo amps |
| D | 85–95%+ | Good–Excellent | Very Low | Subwoofers, compact amps, AVRs |
Which Class Is Right for You?
For most listeners building a home audio system, Class AB offers the best combination of sound quality, reliability, and reasonable running costs. If you're a dedicated audiophile with efficient speakers and budget to match, Class A has a genuine sonic case. And if you need compact, cool-running, high-efficiency amplification — especially for subwoofers or multi-room audio — Class D has matured to the point where it deserves serious consideration.