Audio Amp Classes Explained: Efficiency, Heat, and THD

Amp classes categorize how an amplifier's output transistors are biased and conduct current relative to the input audio signal waveform, determining the fundamental trade-off between power efficiency and signal fidelity. In a real circuit or installation, the chosen class directly dictates the physical mass of the heat sink required, the amperage rating of the DC power supply, and the baseline Total Harmonic Distortion (THD) before negative feedback loops are applied.

The Core Trade-Off: Conduction Angle vs. Efficiency

Every linear audio amplifier relies on transistors (BJTs or MOSFETs) acting as variable resistors to modulate current from a DC power supply into an AC audio waveform across a speaker load. The "class" of the amplifier simply defines the conduction angle—the portion of the 360-degree input waveform cycle during which the output device is actively conducting current.

Think of the conduction angle like scheduling shift workers to move boxes (the audio signal). Class A is a single worker on a 24/7 shift; they are always drawing power and sweating, even when no boxes arrive, resulting in massive heat waste but zero hand-off errors. Class B uses two workers on exact 12-hour shifts (one for the positive half-wave, one for the negative). It is highly efficient when idle, but the workers inevitably drop the baton during the handoff at the zero-crossing point, creating crossover distortion. Class AB solves this by overlapping their shifts slightly, keeping both workers just barely awake during the handoff. Class D abandons the linear shift model entirely, using ultra-fast on/off switching (Pulse Width Modulation) to move boxes in discrete, high-frequency bursts.

Spec Sheet Breakdown: Comparing the Major Amp Classes

Before selecting an amplifier topology for a DIY build or evaluating a commercial receiver, you need to understand the hard limits of each class. The table below outlines the theoretical and practical realities of the most common topologies you will encounter on the bench.

Note: Practical efficiency varies based on the crest factor of the audio signal and the specific load impedance. Data sourced from Analog Devices Audio Amplifier tutorials and standard semiconductor application notes.

Worked Example: Sizing the Power Supply and Heat Sink

To see how amp classes change a physical installation, let’s calculate the power supply and thermal requirements for a stereo amplifier delivering 100W RMS per channel into an 8-ohm load (200W total continuous audio output).

Scenario 1: Class AB Design (Practical Efficiency ~55%)

• DC Power Required: 200W (audio out) / 0.55 (efficiency) = 363W DC input.
• Heat Dissipated: 363W (input) - 200W (output) = 163W of waste heat.
• Hardware Impact: You need a massive, finned aluminum extrusion heat sink (likely requiring forced-air cooling via a 120mm fan) to keep the silicon junction temperatures below 125°C. The power supply must deliver roughly 15A at ±24VDC continuously, necessitating a heavy, expensive 400VA toroidal transformer and large filter capacitors (e.g., 10,000µF per rail).

Scenario 2: Class D Design (Practical Efficiency ~92%)

• DC Power Required: 200W (audio out) / 0.92 (efficiency) = 217W DC input.
• Heat Dissipated: 217W (input) - 200W (output) = 17W of waste heat.
• Hardware Impact: The 17W of heat is easily managed by a small, stamped aluminum heat sink or even just the PCB copper pours. The power supply only needs to deliver ~9A at 24VDC, allowing you to use a lightweight, cheap Switch-Mode Power Supply (SMPS) brick. This is why modern active PA speakers and car audio subwoofers are so compact despite massive wattage ratings.

Where You Meet This in Practice (and Common Confusions)

Understanding these topologies helps you select the right commercial gear or design the right circuit for your specific application.

Real-World Applications

• Class A: High-end audiophile headphone amplifiers and boutique guitar pedal buffers. The massive heat and low power output are acceptable because the zero-crossover-distortion "warmth" is highly prized, and headphone drivers require very little actual wattage.
• Class AB: Traditional home theater AV receivers, studio monitor amplifiers, and mid-tier guitar amps. It offers the best compromise of low distortion and manageable heat for multi-channel setups.
• Class D: Car audio subwoofer amplifiers, portable Bluetooth speakers, and modern active studio monitors (like the Genelec 8000 series). When battery life, physical weight, or extreme wattage (1000W+) are the priority, Class D is the only logical choice.
• Class G/H: High-power professional PA amplifiers. They use a Class AB output stage but dynamically switch between multiple voltage rails (e.g., ±35V and ±70V) depending on the signal peaks, drastically improving efficiency during normal musical dynamics.

Common Confusions on the Bench

1. "Class D means Digital."
This is the most persistent myth in audio electronics. Class D is an analog switching topology. The input analog signal is compared against a high-frequency triangle wave to generate a Pulse Width Modulated (PWM) analog signal. There is no digital logic, no binary code, and no DSP involved in the core amplification stage, though modern Class D chips often include digital control interfaces.

2. Confusing Classes with Channels.
Beginners often conflate "Class" with "Channel." A "2-channel Class D amplifier" means a stereo amplifier (Left and Right) that uses switching topology. It does not mean it is "twice as advanced" as a Class A amp.

3. "Class A is always superior for audio."
While Class A eliminates crossover distortion, it introduces severe thermal drift. As the massive heat soak changes the operating temperature of the silicon, the bias point shifts, which can actually increase THD over time unless complex thermal compensation circuits (like Vbe multipliers) are implemented. A well-designed Class AB or modern Class D amp with proper global feedback will routinely out-measure a basic Class A circuit in THD+N (Total Harmonic Distortion plus Noise).