Amplifier Classes Explained: A, A/B, D, and More

Amplifier "class" refers to the topology or design of the output stage—how the transistors are configured to amplify the signal.

Different classes offer different tradeoffs between:

• Efficiency (how much power becomes sound vs. heat)
• Sound quality (distortion characteristics)
• Size and cost
• Power output capability

In car audio, you'll primarily encounter Class A/B and Class D, with some niche applications using other classes.

Class A amplifiers conduct current through the output transistors for the full 360° of the input signal cycle—they're always fully "on."

Characteristics

• Efficiency: ~20% (extremely low)
• Distortion: Lowest possible (no crossover distortion)
• Heat: Massive (dissipates ~80% as heat regardless of signal)
• Size: Requires enormous heatsinks

Car Audio Use

In the automotive context, where power is derived from a limited alternator and battery, Class A's ~20% efficiency is prohibitive. A Class A amplifier capable of delivering significant wattage would require massive heatsinks and place an unsustainable current draw on the vehicle's electrical system.

Verdict: Relegated to niche applications—low-power tweeters or exotic builds where efficiency is irrelevant.

*Source: Verified efficiency ratings per amplifier topology analysis*

Class A/B is a hybrid design where output transistors are biased to conduct for slightly more than 180° (typically 181°–200°). This slight overlap ensures both transistors are conducting during the crossover point, significantly reducing distortion.

Characteristics

• Efficiency: 50-65%
• Distortion: Very low (eliminates crossover distortion)
• Heat: Moderate to high
• Sound: Often described as "warm" or "musical"

The Thermal Reality

An amplifier generating 1000W output in Class A/B dissipates approximately 500-600W of heat, requiring a chassis size often incompatible with modern vehicle interiors.

Car Audio Use

For decades, Class A/B was the standard for full-range automotive amplification:

• Component systems where sound quality is priority
• SQ (Sound Quality) competition setups
• Mid-range and high-frequency reproduction

Popular Examples

• JL Audio HD series
• Mosconi AS series
• Zapco reference amplifiers
• Arc Audio SE series

Verdict: Excellent for mids/highs and SQ-focused systems where efficiency is less critical.

*Source: Amplifier topology analysis, MECP certification materials*

Class D uses high-frequency switching (PWM) rather than linear amplification. The output transistors rapidly switch between fully-on and fully-off states, modulated by the audio signal.

A Brief History

The Class D concept originated with Bernard Oliver's 1958 patent for pulse-width modulation audio amplification. The technology remained niche until 1996, when Tripath Technology introduced their "Class T" amplifiers with proprietary digital processing that dramatically reduced distortion.

Modern car audio was revolutionized by innovations like Rockford Fosgate's "bd" (bipolar delta-sigma) topology, which brought audiophile-grade Class D to the automotive market. Today's designs use GaN (Gallium Nitride) FETs in high-end units, enabling even higher switching frequencies and lower distortion.

Characteristics

• Efficiency: 85-95% (dramatically better than linear classes)
• Distortion: Low—modern Class D is indistinguishable from Class A/B in blind listening tests
• Heat: Minimal (only 5-15% wasted as heat)
• Size: Extremely compact
• Power: High output in small packages

Car Audio Use

Class D dominates modern car audio:

• Subwoofer amplifiers (mono blocks) — nearly universal
• High-power full-range (modern Class D)
• SPL competition (efficiency = more power from the alternator)
• Multi-channel systems (compact 5-channel units)

The "Class D Sounds Bad" Myth

Early Class D (1990s-early 2000s) suffered from audible switching noise and poor high-frequency reproduction. This is ancient history.

Modern Class D amplifiers: - Use sophisticated output filtering - Achieve THD+N figures comparable to Class A/B - Pass double-blind listening tests against reference amplifiers

The persistence of this myth is a case of outdated forum wisdom refusing to die.

Popular Examples

• Sundown Audio SFB/SCV series
• Taramps MD/HD series
• Crescendo BC series
• DC Audio amplifiers
• JL Audio VXi (full-range Class D)

Verdict: The undisputed standard for subwoofers; fully competitive for all applications.

*Source: Historical data from IEEE archives; modern THD comparisons from independent amplifier testing*

Class G & H

These are variations of Class A/B with rail switching—they use multiple power supply rails and switch between them to improve efficiency.

• Class G: Discrete voltage steps
• Class H: Continuously variable rails

Some high-end car audio amps use these techniques.

Class T (Tripath)

A branded Class D variant that was popular in budget audiophile circles. Tripath went bankrupt; the technology lives on in various forms.

Class GH, BD, etc.

Marketing terms combining techniques. Focus on actual specifications rather than class designations.

For Subwoofers

Class D is the clear winner. Efficiency matters when you're pushing 1000+ watts. Modern Class D mono blocks are the industry standard.

For Mids and Highs

• SQ Priority: Class A/B offers excellent performance
• Power Priority: Modern Class D full-range amps are competitive
• Budget: Class D often provides more watts per dollar

For Competition

• SPL: Class D mono blocks (efficiency = more power)
• SQ: Often Class A/B for scoring, though Class D is increasingly accepted

Real-World Recommendation

For most enthusiasts: Class D mono for subs + Class A/B or Class D 4-channel for speakers. Don't overthink it—modern Class D is excellent across the board.