If you’ve shopped for a guitar amp, headphone amp, or power amp you may have seen the specs list include an amplifier’s class.

Class A, Class B, but what do these terms really mean? Do they impact tone? Is it just tech mumbo jumbo?

There are some tonal impacts, but there are also practical implications. The two main factors impacted by an amplifier’s class are an amp’s linearity and efficiency.

Linearity refers to an amplifiers ability to properly reproduce the input signal in an accurate, transparent manner.

Efficiency is just what it sounds like, it’s the amps ability to properly power itself versus how much energy is wasted through things like heat dissipation.

These two factors are a balancing act, as you cannot have an amplifier that is both perfectly efficient and perfectly linear, you are going to have to make trade-offs between the two.

Amplifier Class Types

Class A

Class A amplifiers are generally considered by audiophiles to be the cream of the crop. They are highly inefficient but generate incredible fidelity and accuracy by operating well across the entire range of frequencies being captured by the input.

Because of the lack of efficiency, they need to be designed to properly ventilate themselves from a large amount of heat buildup, requiring a larger size and therefore an increased cost. They’ll also require far more power than other classes due to this lack of efficiency.

This audio quality is the reason many high-end headphone amplifiers or power amplifiers employ a Class A design, they are very truthful and cast a sonically wide net. You’ll generally have a bigger amp, but at least you’ll build some extra muscle carrying it around.

Class B

In response to the lack of efficiency of Class A, Class B attempts to bring balance to that by splitting the job of a Class A between multiple output devices. These multiple devices can spread the workload to increase efficiency, merging together before the output. The downside here is that crossover distortion can occur during the merging period which can be an issue when going for a linear signal.

As any guitarist will tell you, distortion isn’t necessarily a bad thing, but it’s highly situationally dependent.

Crossover distortion is not a desirable trait. We’re not necessarily trying to get a perfect input to output ratio – and we often change our tone with pedals or even the preamplifier section of these amps – but there’s a difference between wanted and unwanted tonal changes.

This becomes much more of an issue for things like headphone amps, power amps, or any studio-grade hardware, which is why they tend to lean towards Class A. Low distortion lets you trust what you’re hearing is 100% accurate.

Class AB

Merging the best of both worlds, a Class AB amplifier is a modification of a Class B design where each output device attempt to cover more than just half of the workload, giving them a much more accurate profile when the output is merged between the two devices. This significantly improves crossover distortion.

Class AB is by far the best balance of linearity and efficiency, although is still not quite as linear as Class A, which is why it hasn’t taken the entire industry by storm. At lower wattages, however, it operates very much like Class A, it’s only at higher wattages that the linearity may start to diminish.

Many Marshall’s, Rectifier’s, Lone Stars etc. employ a Class AB design because it gives you the best of both worlds in many ways.

Class D

In this case, our only factor is efficiency. Class D uses what’s called “Pulse Wave Modulation,” and while you don’t need all the nerdy details, this essentially means we’re using a digital operation rather than an analog one.

They are highly efficient and therefore can be put into a much smaller package and require far less power consumption. They’re small, cheap, and can even be battery powered in some cases.

By the nature of the process, Class D is not the best of the best by any means in terms of linearity as they do have to separate some of the sound of the PWM process from the input signal in order to maintain tonal integrity. The good news is that this also lowers the price. Some high-end Class D’s do exist, but they don’t come cheap.

Tonal Differences

While this may be a lot of information to process, the good news is that in terms of guitar amplification there isn’t a lot to worry about.

When it comes to size and form factor there’s a notable difference, and price is certainly a factor, but the tone isn’t worth the stress.

While something like a Class AB amp might be a little better for getting the most clean headroom possible due to the merging of power and low distortion versus a purely Class A amp, that doesn’t mean you can’t get incredibly clean and loud tones from another class, the differences are not worth selling your amp for.

The majority of amps are going to be either Class AB or Class A due to their low crossover distortion, so there really isn’t much of a choice to stress about when the higher distortion designs are so much less prevalent.

While tonal details are important, this is a good lesson in marketing: just because something is on a spec sheet or in an ad that doesn’t mean it’s going to be important to your tone or your decision.