Solid State Amplifier Classes

Class A vs. Class AB vs. Class D — how they work and where they fit

What the classes mean

An amplifier's "class" describes how its output transistors behave over each cycle of the audio signal — specifically, how much of the waveform each device is actively amplifying. That single choice cascades into efficiency, heat, weight, cost, and (arguably) sound character.

Class A

The purist's choice

How it works

Output devices conduct through the entire 360° of the signal cycle. Transistors are always on, always drawing current — even at idle.

Pros

  • Lowest distortion of the three
  • No crossover distortion (only one device handles the signal at a time in single-ended designs)
  • Smooth, linear transfer characteristic
  • Favored by many audiophiles for its sonic character

Cons

  • Very inefficient — most input power becomes heat
  • Runs hot; needs massive heatsinks
  • Heavy, large, expensive
  • Impractical for high power output
Typical efficiency
15–30%
Heat output
Very high

Class AB

The practical sweet spot

How it works

Two output devices each handle slightly more than half the cycle, with a small bias current keeping them both active through the crossover region. Class A behavior at low levels, Class B at high levels.

Pros

  • Much more efficient than pure Class A
  • Proper biasing largely eliminates crossover distortion
  • Well understood; mature, proven designs
  • Dominant topology in mainstream hi-fi for decades

Cons

  • Still generates significant heat at higher power
  • Heavier and larger than Class D
  • Requires careful bias management to sound its best
Typical efficiency
50–70%
Heat output
Moderate

Class D

The modern powerhouse

How it works

Switch-mode operation. Output transistors are either fully on or fully off at very high frequencies (hundreds of kHz to MHz), using pulse-width modulation. A low-pass filter at the output reconstructs the analog signal.

Pros

  • Extremely efficient — very little wasted as heat
  • Small, light, cool — great for compact or portable gear
  • Scales to high power cheaply
  • Modern designs (Hypex, Purifi, ICEpower) measure superbly

Cons

  • Output filter design is critical and load-sensitive
  • Older/cheap implementations can sound harsh in the treble
  • Switching noise requires careful layout and shielding
Typical efficiency
85–95%
Heat output
Low

Side-by-side

Attribute Class A Class AB Class D
Conduction angle360°180–360°Switching (on/off)
Efficiency15–30%50–70%85–95%
Heat generatedVery highModerateLow
Size / weightLarge, heavyMediumSmall, light
Distortion (modern designs)Very lowLowLow to very low
Cost per wattHighModerateLow
Typical power range5–50 W20–500 W20–2000+ W
Common usesAudiophile hi-fi, studio referenceHi-fi, pro audio, guitar ampsCar audio, powered speakers, subs, PA, modern hi-fi

Which one for which job?

None of these is universally "best" — the right pick depends on the room, the speakers, the budget, and what you value.

Choose Class A if you're chasing the lowest possible distortion on a moderately sensitive speaker, don't mind the heat and weight, and the cost isn't a concern.
Choose Class AB if you want proven, well-rounded hi-fi performance with flexibility for a range of speakers and don't want to think about trade-offs.
Choose Class D if you need high power in a small package, care about efficiency, or want the best measured performance per dollar — the modern implementations have closed the audible gap.