written by
Bojan Šernek

Guide to “Class A” for audio engineers

Gear 1 min read

When looking at gear and deciding on your next purchase, there are many things to consider. For example, who is making the gear, what it does and what it’s supposed to excel at.

It’s also customary for manufacturers to publish detailed specifications and measurements for the more technically inclined buyer. These tell us more about the internal design and performance characteristics. It sounds boring, but this information is helpful in the real world if you know how to interpret it.

For example, when buying a preamp. We can look at the noise level measurement and compare that to a preamp we already have. We can use a recording of silence made by that preamp and adjust its volume in the DAW by the difference.

Neve Preamps

If the expensive preamp you are considering has a 10dB better noise floor, you can decrease the fader volume by 10dB. If it is 10dB worse, increase the level by 10dB. This will give us a good idea of how much noise the preamp we are considering will have. A good noise floor can shave hours off of editing work and save recording that would otherwise not be salvageable to be an important consideration for your work.

But some of the other specifications can be pretty cryptic. One that occurs quite frequently is the “class” of the gear. Usually, it’s labeled as “Class A” or “Class D” or some other letter or combination of letters like “Class AB.”

What buyers tend to believe (and marketers bank on) is that “Class A” is the best class with the most prestige and best sound. But how true is that? Does this label come from a particular component, and if so, could we change them out to upgrade a “Class B” to a “Class A” device?

What is an amplifier class, exactly?

Most audio gear - equalizers, compressors, and preamps are made out of signal amplifiers and other components. It’s the amplifiers that have “class,” and when all of them inside a device is of the same class, we usually say that the gear is of this “class” as well.

In simple terms, an amplifier is an electronic device that is used to increase (or amplify) the current, voltage, or power of an analog electrical signal. In the recording studio, preamplifiers are used to increase the signal captured by a microphone to a line-level signal that can be routed through a mixer or digitized with an analog-to-digital converter.

What makes an amplifier a particular class? Contrary to popular belief, it’s not the quality of components, purity of materials, or a manufacturing process (like hand-made) that makes amplifiers a specific class. Instead, it’s a way of organizing components inside the amplifier a certain way. You can take the same type of components in a Class B amplifier and turn them into a Class A amplifier.

The way components are organized can have a massive impact on the way the amplifier behaves. For example, it can define how much maximum amplification the amplifier can produce (power capability) and how much of the power it needs to operate is used for actual audio work instead of heating its surroundings. This is also known as power efficiency.

The table below lists the various classes and their properties. Remember, if a device claims to be of a certain class, all of its internal amplifiers need to be of this type, which means that these properties generally extend to the whole device.

Amplifier Alphabet Soup - George Slade

We will focus mainly on comparing the revered Class A to the by far most common Class AB.

Pure but hot - Class A

Audio engineers revere and seek out Class A the most. It has the least distortion, outstanding fidelity out of the box, and is easy for the amplifier designer to implement. This translates into ease of repair when something goes wrong and a high likelihood that the device won’t act up during its lifetime.

However, there are drawbacks; realistically, a Class A amplifier only converts about a good 20% of the energy it uses into actual audio work. The rest of this energy is expelled as heat, making Class A amplifiers very hot and requiring big heatsinks and a lot of space to be practical. All this energy must also come from somewhere, so the power supplies feeding class A amplifiers must be big and robust to handle the power-hungry amplifiers.

Since most power is consumed as heat, the actual power capability is low. This means that for the same amount of amplification power, usually more than one Class A amplifier or output component is used in parallel, further increasing stress on the power supply and cooling.

The way the components are organized in Class A, they are always fully on and amplify the whole signal simultaneously regardless of the gain or shape of the signal. This is a good thing with regards to signal integrity and not so good when it comes to utilizing the components efficiently. The Class A amplifier works and heats full-on even when no signal passes through or its volume is fully down.

Efficient but compromising - Class B

This is not a typical class to see these days as better designs have emerged since its invention. However, it’s good to discuss it as a basis for other classes of amplifiers that work on the same idea and improve on it.

The class B amplifier uses components in pairs. One part of the pair only amplifies the positive part of the signal while the other amplifies the negative part. This arrangement allows the components to rest when they are not in use, including when there is no signal to amplify. That, in turn, means less stress on the power supply and less power to waste as heat.

The downside of Class B is that switching between components is neither instant nor entirely seamless. Even with the best components, slight gaps or jumps will occur. These artifacts translate into an audible and notable distortion that is also not dependent on the level and can’t be controlled or dialed in.

Sometimes this is what a sound needs if it is too clean. Still, generally as audio engineers, we are looking to add distortion in more controlled, level-dependent ways like with transformers, saturation, or gentle clipping.

Best of both worlds? - Class AB

Class AB amplifiers take the Class B idea of using pairs of components that each amplify its own part of the signal but extend the handoff such that one of the components is still working while the other one is getting ready to take over.

This significantly reduces hand-over artifacts to (nearly) zero while still retaining Class B'sFor example; some exceptions are power and heat benefits. The downside is that the design of these amplifiers is slightly more complex, and the amplifier designer has to tune the handoff to hit a particular sweet spot where one component stops working when the other starts working.

This is the most common class of amplifiers, and unless the device explicitly mentions which class it is, this is the one it is using. Class AB has proven itself countless times over as a good balance between heat, power, and fidelity.

However, even when tuned very precisely, the handover will never be as clean as Class A.

Can the class of a device be changed?

That’s a simple question that tackles a complex problem. It depends. What can be guaranteed is that it’s seldom easy. The whole idea of mass-produced electronic devices is that precisely the same components are placed in exactly the same spot and connected to precisely the same other components in the same organization every time.

As we now know, the class of an amplifier is determined not by components but by the way they are organized. A change from one class to another is completely different from changing a vacuum tube inside a guitar amplifier or another user's replaceable part like a fuse.

There are some exceptions - for example in a hand-wired guitar amplifier, all connections between elements are on simple wires. It’s not very hard to reroute some of these connections and change a Class B to a Class A amplifier - though it’s still a much more complex and error-prone process compared to changing a tube.

When faced with devices that are assembled out of circuit boards with chips on them, rerouting enough connections will be much harder, if not downright impossible. Additionally, some chips will be full-fledged amplifiers. It’s tough to open up a chip and change its internal wiring.

The additional thermal stress and power consumption must also be considered, as they will increase when changing to Class A from other classes. Failing to do so could damage the device or even ignite it.

Does it matter?

A class of a device does determine its behavior, the amount of baseline distortion and heat generated, power used, and so on. It does not, however, replace good design practices and knowledge.

Boutique gear designers such as Jakob Erland of Gyraf Audio or Ruben Tilgner, or Elysia are passionate, knowledgeable engineers who will go to great lengths to optimize their designs. As with most things, the basic principle (like amplifier class) is just a part of the story that can be taken miles further with skill and dedication.

At mix analog, we are delighted to host such gear for people to experience first hand and have selected BURL Audio as our audio converters of choice, which are implemented exclusively in Class A. It will probably not surprise you that you hear Class A amplifiers throughout when selecting to run, for example, an Elysia Museq.

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