Understanding Amplifier Classes: What They Mean and Why They Matter

If you've spent any time researching audio amplifiers — whether for a hi-fi system, a home theater, or a guitar rig — you've encountered the terminology of amplifier classes. Class A, Class B, Class AB, Class D (and increasingly Class H, Class G, and others) appear constantly on specification sheets and in marketing materials. These terms are often invoked to justify premium pricing or explain sonic differences, but they're rarely explained clearly.

Here's a plain-language breakdown of what each major amplifier class actually means, how it affects performance, and what you should care about as a listener.

The Basic Principle: Output Stage Conduction Angle

Amplifier classes are fundamentally about how the output transistors (or valves/tubes) conduct electricity during a cycle of the audio signal. Specifically, they describe the conduction angle — what fraction of the waveform cycle each output device is active for. This has direct implications for efficiency, heat generation, and distortion characteristics.

Class A

In a Class A amplifier, the output transistors conduct for the full 360° of the waveform cycle — meaning they're always "on", even when not technically needed. This has a significant consequence: the amplifier draws the same current from the power supply regardless of whether there's any audio signal at all.

Characteristics:

  • Efficiency: Very low — typically 15–30%. The rest of the energy is dissipated as heat
  • Distortion: Primarily low-order harmonic distortion, which many listeners find more pleasing
  • Heat: Substantial — Class A amplifiers run very hot and require significant heatsinking
  • Sound: Widely regarded as exceptionally smooth and musical, with a characteristic "liquidity"

Class A amplifiers are common in high-end audiophile products and headphone amplifiers. Their inefficiency makes high-power Class A designs impractical for home theater applications.

Class B

Class B takes the opposite approach. Two complementary transistors each handle exactly half the waveform (180° each) — one for the positive half, one for the negative. When one is conducting, the other is switched off entirely.

Characteristics:

  • Efficiency: High — theoretically up to around 78%
  • Distortion: Prone to crossover distortion at the point where one transistor hands off to the other
  • Heat: Much less than Class A

Pure Class B is rarely used in high-quality audio applications due to the crossover distortion issue. It's more common in radio frequency applications.

Class AB

Class AB is the practical compromise that dominates the audio industry. Both output transistors conduct slightly more than 180° of the cycle each — there's a small overlap around the zero-crossing point that eliminates Class B's crossover distortion while maintaining much better efficiency than Class A.

Characteristics:

  • Efficiency: Moderate — typically 50–70%
  • Distortion: Low, especially with negative feedback applied
  • Heat: Moderate; manageable with reasonable heatsinking
  • Sound: Excellent when well-designed; the vast majority of quality hi-fi and home theater amplifiers use Class AB

Class AB amplifiers represent the mature, refined technology base for most serious audio equipment. A well-designed Class AB amplifier is transparent and accurate, and there is no audible reason to prefer any other class solely on the basis of marketing claims.

Class D

Class D (sometimes misleadingly called "digital amplification") is a switching amplifier topology. The output transistors switch rapidly between fully on and fully off states — not between linear on/off states like the previous classes. The audio signal is encoded as a pulse-width modulated (PWM) signal, then decoded by a low-pass output filter.

Characteristics:

  • Efficiency: Very high — often 90% or above
  • Heat: Very little; Class D amplifiers can be small and light
  • Distortion: Historically higher in certain measurements; modern designs have closed the gap significantly
  • Sound: Early Class D amplifiers had a distinct sonic character; premium modern Class D designs (from manufacturers like Hypex and Purifi) are considered reference-class performers

Class D is now dominant in powered speakers, subwoofer amplifiers, and increasingly in high-end audiophile amplifiers. The technology has matured dramatically in the past decade.

Which Class Should You Choose?

The honest answer: judge amplifiers by their measured performance and sound quality, not their class designation. A well-designed Class D amplifier will outperform a poorly designed Class A amplifier. Class AB remains the dominant choice for good reason — it's a mature, well-understood technology that can be optimised to exceptional performance levels. Class A has genuine sonic merits but practical limitations. Class D is the future for many applications, and that future has largely already arrived.