A Class A amplifier conducts current through its output transistors for the entire 360 degrees of the input signal cycle, whereas a Class AB amplifier conducts for slightly more than 180 degrees, splitting the push-pull workload to balance efficiency and distortion. When you are evaluating a class A class AB amplifier topology for a bench build or a hi-fi purchase, this single difference in conduction angle dictates everything from your power transformer VA rating to the physical mass of aluminum bolted to your output devices.
In a real circuit, this topology choice fundamentally changes your thermal management and power supply sizing. A Class A stage draws maximum current even when idle (playing silence), meaning your power supply must be sized for continuous peak dissipation. A Class AB stage draws minimal idle current, scaling up only when delivering audio to the load. Think of it like a water valve: Class A is a valve that is always fully open, and you modulate the flow by bleeding water out a side pipe (wasteful but perfectly linear). Class AB uses two valves—one for the positive pressure stroke, one for the negative—handing off the work right at the zero-crossing point.
The Math: Quiescent Current and Heatsink Sizing
Theory is useless if it melts your output stage. Let's run the actual bench math to see what these topologies demand from your heatsinks and power supplies. We will compare a 20W RMS Class A design against a 50W RMS Class AB design, both driving an 8Ω speaker load.
Class A: 20W RMS into 8Ω
- Peak Voltage: $\sqrt{2 \times 20W \times 8\Omega} = 17.88V$
- Rail Voltage: $\pm 22V$ (Total 44V across the device)
- Quiescent Current ($I_q$): Must exceed peak load current ($17.88V / 8\Omega = 2.23A$)
- Total DC Power Draw: $44V \times 2.23A = 98.1W$ (constant, even at idle)
- Total Dissipation: $98.1W - 20W (audio) = \mathbf{78.1W}$
To keep a single silicon junction under 150°C in a 25°C room, you need a thermal resistance ($R_{\theta SA}$) that is physically impossible for a single TO-3P package. You must parallel four output transistors (like the ON Semi MJL21193), dissipating ~19.5W each, requiring a massive extruded aluminum sink (roughly 4.2°C/W per device) and a 120VA toroidal transformer just to idle.
Class AB: 50W RMS into 8Ω
- Peak Voltage: $\sqrt{2 \times 50W \times 8\Omega} = 28.28V$
- Rail Voltage: $\pm 35V$ (Total 70V)
- Quiescent Current ($I_q$): Set to ~50mA to 100mA just to eliminate crossover distortion
- Max Sine Wave Dissipation: Occurs at roughly 1/3 power. Formula: $P_{diss(max)} = \frac{2 \times V_{CC}^2}{\pi^2 \times R_L}$
- Total Dissipation: $\frac{2 \times 35^2}{9.87 \times 8} = \mathbf{31W}$ total for the entire amp
78.1W dissipated for 20W of audio (Class A) vs. 31W dissipated for 50W of audio (Class AB). In the Class AB design, that 31W is split between just two output devices (15.5W each). A standard $6.3°C/W$ heatsink handles this easily, and a 60VA transformer is sufficient. This is why Class AB dominates commercial audio.
Where You Meet This in Practice
You will rarely see pure Class A in high-power commercial gear due to the sheer cost of copper and aluminum required. Here is where each topology actually lives in the wild:
- Class A (Audiophile & Niche): High-end headphone amplifiers (like the Schiit Asgard or Pass Labs HPA-1), low-power guitar pedal buffers, and boutique studio monitor pre-stages. The absence of switching artifacts and crossover distortion makes it the gold standard for sub-5W critical listening.
- Class AB (The Workhorse): 90% of traditional AV receivers (Denon, Marantz), powered studio monitors, and classic hi-fi integrated amps. Integrated circuits like the Texas Instruments LM3886 (a 68W Class AB chipamp) have powered millions of DIY and commercial speaker builds because they offer excellent linearity without requiring a dedicated cooling fan.
Common Confusions: Class B, Class D, and Bias Drift
When researching amplifier schematics, builders frequently trip over three specific misconceptions:
- Confusing Class AB with Class B: Pure Class B conducts for exactly 180 degrees. This creates a "dead zone" at the zero-crossing point, resulting in severe crossover distortion. Class AB adds a small bias voltage to keep both transistors slightly "on" during the handoff, eliminating this dead zone. Never build a pure Class B audio stage.
- Confusing Linear (A/AB) with Switching (Class D): Class D uses Pulse Width Modulation (PWM) to switch output MOSFETs fully on or fully off, achieving >90% efficiency. It requires complex output LC filters. Class A and AB are linear topologies; the transistors operate in their active region, acting as variable resistors.
- Bias Current vs. Thermal Runaway: In Class AB, the bias current (idle current) is highly temperature-dependent. As the output transistors heat up, their $V_{BE}$ drops, causing them to draw more current, which makes them hotter—a positive feedback loop called thermal runaway. This is why Class AB designs require a $V_{BE}$ multiplier transistor mounted directly on the heatsink to compensate for temperature changes. Class A is largely immune to this specific failure mode because the current is fixed by the load and rails, not just the bias network.
Decision Tree: Which Topology Should You Build or Buy?
Stop debating the theoretical merits and look at your actual load requirements. Use this decision matrix to lock in your topology and select a concrete starting point.
| Your Application | Power Requirement | Choose Topology | Concrete Part / Design Pick |
|---|---|---|---|
| Headphone Amp / IEM Driver | < 2W into 32Ω | Class A | JLH 1969 discrete topology or Burr-Brown BUF634 buffer |
| Desktop PC Speakers / Bookshelf | 10W - 30W into 8Ω | Class AB | TI LM3886 (Chipamp) or TDA7294 |
| Floorstanding Towers / PA | > 50W into 4Ω/8Ω | Class AB | Discrete EF-LNJL (Cordell) or Leach Amp design using MJL21193/4 |
| Subwoofer / Bass Shaker | > 100W | Class D | Hypex NCORE or ICEpower module (AB is too thermally inefficient here) |
The Default Recommendation: If you are building an amplifier to drive standard passive bookshelf or floorstanding speakers (anything above 5W), build Class AB. The thermal penalties of Class A at speaker-driving power levels require industrial-grade heatsinks and massive power transformers that will blow past a hobbyist budget. A well-designed Class AB amp with a properly compensated $V_{BE}$ multiplier will measure below 0.01% THD+N—entirely inaudible and vastly more practical.
Frequently Asked Questions
Can I convert a Class AB amplifier into Class A?
Technically, yes, by drastically increasing the quiescent bias current so the output devices never turn off. Practically, no. The existing power supply will likely overheat, the output transistors will exceed their Safe Operating Area (SOA) and suffer secondary breakdown, and the stock heatsinks will fail to dissipate the 4x increase in idle heat. Do not attempt this without redesigning the PSU and thermal management from scratch.
Why do some high-end Class AB amps claim "Class A operation for the first 10 watts"?
This is a marketing term for "high-bias Class AB." The manufacturer sets the quiescent current high enough that for low-level signals (under 10W), the output transistors don't cross the zero-point, effectively operating in Class A. Once the signal demands more than 10W, the amp seamlessly transitions into standard Class AB push-pull. It requires massive heatsinks to handle the idle heat of that high bias current.
Does Class A sound "warmer" than Class AB?
"Warmth" in audio is usually a euphemism for even-order harmonic distortion or a rolled-off high-frequency response. A properly engineered Class AB amplifier with adequate slew rate and phase margin will measure flat to 100kHz and produce distortion artifacts below the noise floor of human hearing. If a Class A amp sounds "warmer," it is usually because its power supply is sagging under the massive continuous current draw, introducing subtle compression and harmonic coloring.
