Class A vs Class AB vs Class D: What’s the Difference in Hi-Fi Amplifiers?

What Does Amplifier Class Actually Mean?

An amplifier's class describes how its output stage operates to amplify an audio signal. It influences power efficiency, heat generation and circuit design, but it does not automatically determine how good an amplifier sounds.

1. Class A: The Pursuit of Linear Amplification

Class A amplifiers keep their output devices conducting throughout the entire audio waveform. Unlike designs that switch between output devices as the signal changes, Class A operates continuously, helping avoid the crossover distortion associated with Class B operation.

The trade-off is efficiency. A substantial amount of electrical power is converted into heat, even when the amplifier is playing music quietly or sitting idle. This means Class A designs often require substantial heatsinks, robust power supplies and careful thermal management.

What makes Class A appealing?

Class A is valued for its linear operating characteristics and potential for very low distortion. For enthusiasts building a dedicated listening system, it can be an appealing approach to high-fidelity reproduction.

However, the class alone does not guarantee superior sound. The circuit design, implementation and interaction with the connected speakers remain important.

Exploring premium amplification

A useful example from the high-end world is Krell, a brand known for powerful solid-state amplification and high-current designs. Such as the KRELL KSA 100s.

Best suited for: Enthusiasts who value the design philosophy of Class A, have a suitable budget and are comfortable with the additional heat, power consumption and physical size that many such amplifiers entail.

2. Class AB: A Balance Between Performance and Efficiency

Class AB was developed to combine desirable characteristics of Class A and Class B operation.

In a typical Class AB output stage, the output devices conduct simultaneously for a small portion of the signal cycle. This overlap helps reduce crossover distortion while allowing the amplifier to operate more efficiently than a conventional Class A design.

Why choose Class AB?

Class AB offers a practical balance between sound quality, output power, heat generation and cost. It can be found in everything from affordable integrated amplifiers to sophisticated high-end systems.

What about tube amplifiers?

Tube amplification is often associated with a rich, spacious or harmonically distinctive presentation. However, tube versus solid-state and Class A versus Class AB are two different comparisons.

A tube amplifier can operate in Class A or Class AB, depending on its circuit design and operating conditions.

For example, Jadis is a useful brand to explore when learning about valve amplification. Best suited for: Listeners looking for a traditional stereo amplifier design with a broad range of choices, from everyday integrated amplifiers to more elaborate high-end systems. 

Jadis Defy 7 Tube Pre-Amplifier + DPL Tube Power Amplifier SET

3. Class D: Compact, Efficient and Powerful

Class D takes a different approach. Rather than operating its output devices continuously in their linear region, a Class D amplifier rapidly switches them on and off. The switching pattern is controlled to reproduce the incoming audio signal, and an output filter helps reconstruct the amplified waveform.

Despite the name, Class D does not necessarily mean digital audio. Class D amplifiers can accept analogue inputs, digital inputs or both, depending on their design.

Why has Class D become so popular?

Efficiency is its major advantage. Because the output devices spend much of their time fully on or fully off, relatively little power is wasted as heat compared with conventional linear output stages.

This allows manufacturers to create compact amplifiers that deliver substantial output power without the size, weight and cooling requirements often associated with traditional high-power designs.

Modern Class D amplifiers can also achieve excellent measured performance, including low distortion and noise. Their performance depends on the complete implementation, not simply the switching topology.

Class A vs Class AB vs Class D: At a Glance

Which Amplifier Class Should You Choose?

There is no universally superior amplifier class. The right choice depends on your priorities.

• Choose Class A if you are specifically interested in its operating characteristics and are prepared for the associated heat, energy consumption and cost.

• Consider Class AB if you want the flexibility of a long-established amplifier architecture with a wide range of designs and price points.

• Consider Class D if you want an efficient, compact amplifier that can deliver excellent performance without taking up much space.

The most important distinction is that amplifier class describes how the amplifier works, not a ranking of how musical it sounds.