Class A vs. Class AB vs. Class D: What’s the Difference in Sound?

Letters That Matter (But Maybe Not the Way You Think)

If you’ve spent any time browsing hi-fi forums or amp spec sheets, you’ve probably seen references to Class A, Class AB, and Class D amplifiers. These designations sound technical — and they are — but they also come loaded with strong opinions. That actually may be the understatement of the week.

Fun fact: As we kick off this “class” on amplifier classes, here’s a surprise — those letters don’t actually stand for anything. They’re not acronyms. For instance, Class D doesn’t mean “digital” — it’s just the fourth letter engineers used when labeling different circuit designs. More on this a little later.

Some audiophiles swear by the purity of a Class A circuit, regardless of the temperature in their location. Others will tell you Class D design is the future. And plenty of manufacturers have made decades-long pursuits out of refining Class AB. So who’s right? 

The truth is, each amplifier class has its own set of strengths, trade-offs, and ideal use cases. This guide breaks down what those letters really mean, how each class affects sound and performance, and which might be the best match for your system.

Spoiler: It’s not just about “better” or “worse” — it’s about what fits.

What Is an Amplifier Class, Anyway?

In simplest terms, an amplifier’s class refers to how it handles the flow of electrical current to amplify a signal. It’s about the circuit topology — how the amp turns a small musical signal into a powerful one that can drive your speakers.

Each class does this a bit differently:

• Class A amplifiers continuously run the signal through the output transistors. This means the full waveform is always being amplified, leading to excellent linearity and low distortion, but also heat and inefficiency.
• Class AB amplifier designs split the workload, using each output device for a portion of the waveform. That reduces power draw and heat, while maintaining good fidelity.
• Class D amplifiers take a very different approach — rapidly switching the signal on and off (like a light switch flickering thousands of times per second). This makes them extremely efficient, compact, and cool-running, but more reliant on filtering to smooth out the signal.

What does this mean for sound?

We’ll get into each in a moment. But understanding this basic framework helps explain why some amps run hot, weigh 80 lbs, or sound “sweet,” while others are featherweight powerhouses that fit on your desk.

A Quick Note on Class Naming

Contrary to popular belief, amplifier classes aren’t acronyms — they’re simply labels assigned in alphabetical order as new designs emerged. For example, Class D doesn’t mean “digital” (even though it often involves digital-like switching). Here’s a look at what those letters really mean — and don’t.

Class	Misconception	Reality
A	“Analog”	Just the first class identified and known for always-on operation and high fidelity. Class A was the first recognized topology.
B	“Balanced” or “Better”	Came second. Uses two devices, each handling half the waveform. Class B came next.
AB	“Analog-Balanced”	A hybrid, combining A and B for better efficiency and lower distortion. Class AB is a hybrid of A and B.
C	“Communications”	Not audio — used in RF; very efficient but high distortion. Class C came later (primarily used in RF applications).
D	“Digital”	It’s not digital, it’s a switching amplifier. The letter just came next in line. Class D followed — and no, the “D” does not stand for “Digital” — even though it often uses digital-like switching
G	“Gain” control	Uses multiple voltage rails for efficiency.
H	“Hybrid”	Similar to G but with smoother rail switching (sometimes called modulated rails).

Class A – The Purist’s Choice

If amplifier classes were cuts of steak, Class A would be wagyu: rich, indulgent, and uncompromising. These amps amplify the entire waveform using a single output stage that’s always “on,” even when there’s no signal. The upside? Incredibly linear, low-distortion output that’s beloved for its natural, effortless tone. Many listeners describe Class A sound as warm, fluid, or “organic” — especially in tube-based designs.

But all that purity comes at a cost. Class A amps are wildly inefficient. They convert much of their energy into heat (even at idle), making them heavy, hot-running, and power-hungry. They’re often limited in wattage for that reason, but what you get per watt is special.

Best for:

• When you crave warmth, detail, and don’t mind the heat (literally)
• Listeners chasing tonal magic, not maximum power
• High-efficiency speakers (to compensate for lower wattage)
• Systems where heat and space aren’t dealbreakers
  

Popular Examples:

• Pass Labs INT-25 / XA30.8  
• Luxman L-595ASE
• Sugden A21SE
• Triode and SET tube amps
  

Class AB – The Sweet Spot

Most hi-fi amplifiers, especially integrated amps and affordable separates, use Class AB amplification. It blends the best of A and B designs: the first part of the waveform is handled in Class A mode for finesse, then the bulk is handled in a more efficient Class B style.

This hybrid approach offers a great balance of musicality, power, and efficiency. You still get smooth mids and decent warmth, but with less heat and greater dynamic headroom than pure Class A. It’s why Class AB is the workhorse of home hi-fi — and a favorite for people who want high performance without the fuss.

Best for:

• Audiophiles seeking a balance of musicality, power, and affordability
• General-purpose listening
• Systems needing more power output
• Listeners who want great sound without overheating the room
  

Popular Examples:

• Parasound Halo series 
• McIntosh solid-state amps (many are AB with autoformers)
• Rotel Michi series  
• Rega Elicit Mk5

Class D – The Modern Minimalist

Class D amplification has come a long way. Once dismissed as harsh and “digital-sounding,” today’s best Class D amps are clean, dynamic, and highly resolving. Instead of using continuous current flow, Class D works by rapidly switching the signal on and off — a technique called pulse-width modulation. The result? Huge efficiency, very little heat, and impressive power from a small chassis.

While Class D can still be a bit lean or clinical in the wrong setup, recent designs have significantly improved, especially when paired with effective output filtering and a well-designed power supply. Some Class D designs now rival Class AB in musicality — and beat everything in terms of footprint and power density.

Best for:

• Compact or minimalist systems
• Tech-forward setups and efficiency-first thinking.
• High-wattage needs (like driving tough loads or subwoofers)
• Systems where energy efficiency or space is a concern
  

Popular Examples:

• NAD M33 
• Mola Mola Perca / Kula  
• Bel Canto e.One
• Cambridge Audio Evo 150  

What About Class G, H… and Tubes?

Not every amp fits neatly into the A/B/D framework. Over the years, particularly in the last decade, there have been innovative designs that have reached new territory with great success in the audio realm. A few others worth knowing:

Class G & H:

• These use multiple power rails or variable voltage rails to increase efficiency. Think of it as a smart version of Class AB that dynamically adjusts the power supply.
• Found in amps by Arcam, Cambridge Audio (like the Edge A), and some pro gear.
• Sound? Often indistinguishable from good Class AB, but more efficient.
  

Tube Amplifiers:

• Tubes are typically configured in Class A or AB topologies, but they have a sound all their own: lush, spacious, and harmonically rich.
• Known for “soft clipping” and a forgiving character, they’re adored by analog lovers and vinyl-heads.
• Downsides: heat, maintenance, and lower damping factor — not ideal for all speaker loads.

So Which One Is Best for You?

There’s no “best” amplifier class — only the one that fits your system, space, and ears. Here’s a quick cheat sheet:

You Prioritize…	Look For…
Warmth and musicality	Class A or tubes
Balance and versatility	Class AB
Power and efficiency	Class D
Compact, all-in-one solutions	Class D or G
Vintage or analog-style tone	Class A/B tubes

Ultimately, system synergy matters more than letters on a spec sheet. A modest Class AB amp with great speakers can beat a giant Class A monster in the wrong room.

Listen First, Label Later

The amplifier class is just one piece of the puzzle. Yes, Class A can sound sublime. Class D can punch above its weight. And Class AB might strike the perfect balance for your setup. But these are just tools—what matters most is how the whole system works together to deliver music that moves you.