Class A, A/B and D amplification: The letters don’t tell the whole story
People sometimes think guitar amplifier classes describe the whole personality of an amp.
They might say that:
Class A is warm.
Class A/B is classic.
Class D is digital.
But that’s not the reality.
Amplifier class doesn’t tell you whether an amplifier is valve, solid-state, analogue or digital. It describes how the power stage of the amplifier works. In other words, how the amplifier takes a small signal and turns it into something strong enough to drive a speaker.
That distinction matters.
Class D amplifiers can still be analogue. Class A amplifiers can still sound bad. Class A/B amplifiers can be beautifully responsive. The class tells you something about the method of amplification, but it doesn’t tell you about the sound, the feel or the quality of the amplifier.
That distinction matters.
Class D amplifiers can still be analogue. Class A amplifiers can still sound bad. Class A/B amplifiers can be beautifully responsive. The class tells you something about the method of amplification, but it doesn’t tell you about the sound, the feel or the quality of the amplifier.
What is a power amplifier?
In a guitar amplifier, the preamp does a lot of the early shaping. It takes the small signal from the guitar, adds gain, controls tone, and helps to give the amp a lot of its character.
The power amplifier comes later. Its job is to increase the signal enough to move the speaker.
That sounds simple, but it’s a huge part of how an amplifier feels. The way the power section responds, compresses, holds together or starts to give way can change the whole playing experience and the sound.
So, when people talk about Class A, Class A/B or Class D, they are usually talking about the power stage.
The power amplifier comes later. Its job is to increase the signal enough to move the speaker.
That sounds simple, but it’s a huge part of how an amplifier feels. The way the power section responds, compresses, holds together or starts to give way can change the whole playing experience and the sound.
So, when people talk about Class A, Class A/B or Class D, they are usually talking about the power stage.
Amplifier class is not the same as analogue or digital
Class A, Class A/B and Class D describe how the power stage works, they don’t tell you whether an amplifier is analogue or digital.
In guitar amps, Class A and Class A/B are usually found in analogue designs. Class D can be used in analogue amplifiers too, but it’s also often used in digital modelling systems, powered speakers and compact modern equipment.
That’s where some of the confusion comes in. As Class D is often used in digital or modern-looking gear, people sometimes assume Class D means digital. It doesn’t.
As I covered in the analogue vs digital article, the real distinction between analogue and digital is whether the guitar signal stays intact as a continuous flowing analogue current, or whether it’s converted into data packets and processed by software.
In guitar amps, Class A and Class A/B are usually found in analogue designs. Class D can be used in analogue amplifiers too, but it’s also often used in digital modelling systems, powered speakers and compact modern equipment.
That’s where some of the confusion comes in. As Class D is often used in digital or modern-looking gear, people sometimes assume Class D means digital. It doesn’t.
As I covered in the analogue vs digital article, the real distinction between analogue and digital is whether the guitar signal stays intact as a continuous flowing analogue current, or whether it’s converted into data packets and processed by software.
Class A amplifiers
In a Class A amplifier, the output device is working all the time.
Valve or transistor, the amplifying device is always conducting signal. It’s not switching off for part of the waveform. It stays constantly awake, so to speak. This can give Class A amplifiers a very immediate and harmonically rich feel. They are often described as smooth, touch-sensitive and lively under the fingers.
But Class A is also inefficient. A lot of energy is wasted as heat, and the design has limits when it comes to clean power. That’s one reason many Class A guitar amps are lower wattage, usually no more than 5 watts.
Class A can be wonderful, but it doesn’t guarantee a great amplifier.
Valve or transistor, the amplifying device is always conducting signal. It’s not switching off for part of the waveform. It stays constantly awake, so to speak. This can give Class A amplifiers a very immediate and harmonically rich feel. They are often described as smooth, touch-sensitive and lively under the fingers.
But Class A is also inefficient. A lot of energy is wasted as heat, and the design has limits when it comes to clean power. That’s one reason many Class A guitar amps are lower wattage, usually no more than 5 watts.
Class A can be wonderful, but it doesn’t guarantee a great amplifier.
Class B and Class A/B
In a Class B power stage, the work is split between two sides. One side handles one half of the waveform, and the other side handles the other half.
That’s more efficient than Class A, but it can introduce a problem at the point where one side hands over to the other. That little handover moment can create crossover distortion if it’s not handled well.
So, Class A/B is a way of softening that handover.
In a Class A/B amplifier, each side conducts for a little more than half of the waveform. The two halves overlap slightly, which helps smooth out the transition while still being much more efficient than pure Class A.
This is one of the most common power amp designs in guitar amplifiers, and for good reason. It can be powerful, musical, practical and responsive.
A lot of the classic amplifier sounds people love come from Class A/B designs.
That’s more efficient than Class A, but it can introduce a problem at the point where one side hands over to the other. That little handover moment can create crossover distortion if it’s not handled well.
So, Class A/B is a way of softening that handover.
In a Class A/B amplifier, each side conducts for a little more than half of the waveform. The two halves overlap slightly, which helps smooth out the transition while still being much more efficient than pure Class A.
This is one of the most common power amp designs in guitar amplifiers, and for good reason. It can be powerful, musical, practical and responsive.
A lot of the classic amplifier sounds people love come from Class A/B designs.
Class D amplifiers
Class D is where people often get confused.
The “D” does not stand for digital.
Class D is not automatically a digital amplifier. It’s a type of switching power amplifier. Instead of the output devices staying partly on in the same way as Class A or Class A/B designs, they switch on and off very quickly. That switching is then filtered into a signal that can drive the speaker.
The result is efficiency. Class D amplifiers can be small, light and powerful, and they produce far less heat than many traditional designs.
That’s why Class D is used in a wide range of modern music equipment, from bass amplifiers and PA systems to guitar amplifiers and powered speakers. But Class D still has a reputation problem among some guitar players. Part of that is fair, and part of it is confusion.
A badly designed Class D amplifier can feel stiff, flat or disconnected, but that’s usually down to the circuits around the power stage. How the signal is shaped, how the speaker is controlled, and how the whole amplifier is voiced.
A good Class D power section can still be a solid part of an analogue amplifier. The guitar signal doesn’t have to be converted into digital data. No modelling software has to be involved. The signal can remain analogue, shaped by real components, then amplified by a Class D power stage.
This is one reason I use Hughes & Kettner’s Spirit Tone Technology in some Colson builds. It’s a superb example of how a Class D power section can be made to feel more organic and responsive. It brings in the feel of sag, where the amplifier gives slightly under the player’s attack instead of staying rigid and flat.
That kind of response matters. Sag is one of the things players often associate with nostalgia and older valve amplifiers, but the important part is the way it allows an analogue amplifier to breathe with you as you play.
The “D” does not stand for digital.
Class D is not automatically a digital amplifier. It’s a type of switching power amplifier. Instead of the output devices staying partly on in the same way as Class A or Class A/B designs, they switch on and off very quickly. That switching is then filtered into a signal that can drive the speaker.
The result is efficiency. Class D amplifiers can be small, light and powerful, and they produce far less heat than many traditional designs.
That’s why Class D is used in a wide range of modern music equipment, from bass amplifiers and PA systems to guitar amplifiers and powered speakers. But Class D still has a reputation problem among some guitar players. Part of that is fair, and part of it is confusion.
A badly designed Class D amplifier can feel stiff, flat or disconnected, but that’s usually down to the circuits around the power stage. How the signal is shaped, how the speaker is controlled, and how the whole amplifier is voiced.
A good Class D power section can still be a solid part of an analogue amplifier. The guitar signal doesn’t have to be converted into digital data. No modelling software has to be involved. The signal can remain analogue, shaped by real components, then amplified by a Class D power stage.
This is one reason I use Hughes & Kettner’s Spirit Tone Technology in some Colson builds. It’s a superb example of how a Class D power section can be made to feel more organic and responsive. It brings in the feel of sag, where the amplifier gives slightly under the player’s attack instead of staying rigid and flat.
That kind of response matters. Sag is one of the things players often associate with nostalgia and older valve amplifiers, but the important part is the way it allows an analogue amplifier to breathe with you as you play.
What about Class C?
Class C exists, but it’s not normally used for guitar amplification.
In Class C, the amplifying device conducts for less than half of the waveform. That makes it very efficient, but it also creates a lot of distortion if you’re trying to reproduce a full audio signal.
Class C is useful in things like radio-frequency circuits, where the design can be tuned around a particular frequency. But for a guitar amplifier that needs to reproduce a complex audio signal, it’s not usually appropriate.
So, if you’re talking about guitar amps, Class C is mostly something to understand by contrast. It shows that amplifier class is really about conduction and efficiency, not about tone labels.
In Class C, the amplifying device conducts for less than half of the waveform. That makes it very efficient, but it also creates a lot of distortion if you’re trying to reproduce a full audio signal.
Class C is useful in things like radio-frequency circuits, where the design can be tuned around a particular frequency. But for a guitar amplifier that needs to reproduce a complex audio signal, it’s not usually appropriate.
So, if you’re talking about guitar amps, Class C is mostly something to understand by contrast. It shows that amplifier class is really about conduction and efficiency, not about tone labels.
Class does not equal character
It’s also tempting to turn amplifier classes into simple personalities.
Class A equals warm.
Class A/B equals classic.
Class D equals modern.
There may be some truth in those associations, but they are too simple. The class of the power stage is only one part of the amplifier. The circuit design, components, preamp, tone stack, power supply, speaker, cabinet and overall voicing all matter.
A guitar amplifier is not just a power stage. It is a complete system. And that’s why two Class A amplifiers can feel completely different. Two Class A/B amplifiers can behave nothing alike. And a Class D amplifier can be either sterile or musical, depending on how it’s designed and what circuitry surrounds it.
Class A equals warm.
Class A/B equals classic.
Class D equals modern.
There may be some truth in those associations, but they are too simple. The class of the power stage is only one part of the amplifier. The circuit design, components, preamp, tone stack, power supply, speaker, cabinet and overall voicing all matter.
A guitar amplifier is not just a power stage. It is a complete system. And that’s why two Class A amplifiers can feel completely different. Two Class A/B amplifiers can behave nothing alike. And a Class D amplifier can be either sterile or musical, depending on how it’s designed and what circuitry surrounds it.
The Drew Colson approach
At Colson of York, I’m not interested in labels for their own sake. I care about how the amplifier responds to the player.
I use Class A, Class A/B and Class D designs in different builds, depending on what the amplifier is meant to do. Each approach has its place. Class A can feel immediate and harmonically rich. Class A/B can be powerful, familiar and expressive. Class D can be compact, efficient and, when designed well, far more responsive than many players expect.
Some Colson amplifiers use traditional Class A or Class A/B power stages. Others use Hughes & Kettner Spirit Tone Technology, which is a Class D design with a more musical response, fully analogue, including sag-like behaviour. That allows certain builds to stay compact and practical without giving up the feeling of an amplifier that reacts to the player.
What’s important to me is whether the amplifier feels alive, whether the speaker and cabinet are allowed to add their character, and whether the whole thing feels like part of the instrument rather than a piece of computer equipment sitting in the way.
An amplifier doesn’t have to be a valve amp to be analogue. It doesn’t have to be Class A to be musical. And it doesn’t become digital just because it uses a Class D power stage.
For me, the important questions are:
Does it feel connected?
Does it respond to the guitar, the pedals, the speaker and the room?
Does it make you smile and want to keep on playing?
That matters way more than the letter that’s printed on the power stage circuit description.
I use Class A, Class A/B and Class D designs in different builds, depending on what the amplifier is meant to do. Each approach has its place. Class A can feel immediate and harmonically rich. Class A/B can be powerful, familiar and expressive. Class D can be compact, efficient and, when designed well, far more responsive than many players expect.
Some Colson amplifiers use traditional Class A or Class A/B power stages. Others use Hughes & Kettner Spirit Tone Technology, which is a Class D design with a more musical response, fully analogue, including sag-like behaviour. That allows certain builds to stay compact and practical without giving up the feeling of an amplifier that reacts to the player.
What’s important to me is whether the amplifier feels alive, whether the speaker and cabinet are allowed to add their character, and whether the whole thing feels like part of the instrument rather than a piece of computer equipment sitting in the way.
An amplifier doesn’t have to be a valve amp to be analogue. It doesn’t have to be Class A to be musical. And it doesn’t become digital just because it uses a Class D power stage.
For me, the important questions are:
Does it feel connected?
Does it respond to the guitar, the pedals, the speaker and the room?
Does it make you smile and want to keep on playing?
That matters way more than the letter that’s printed on the power stage circuit description.
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