If you’re shopping around for an amplifier, powered speaker, or subwoofer, you may come across terms such as Class A, Class AB, and Class D. At first, these names probably sound like quality ratings, as though Class A must be better than Class D.
Spoiler alert! That’s not what they mean.
An amplifier’s class describes how its output stage handles and amplifies an electrical signal. Different amplifier classes use different methods to increase the power of an audio signal, and those methods affect things such as efficiency, heat, size, distortion, and power output.
Class A, Class AB, and Class D amplifiers can all produce excellent sound. The main difference is how they achieve it.
Let’s start with the basics. An audio amplifier takes a relatively small electrical signal and makes it powerful enough to drive a speaker or another load.
For example, the signal coming from a preamplifier or audio source may contain the shape of the music, but it does not necessarily have enough power to move a loudspeaker cone. A power amplifier increases the signal's voltage and current so that the speaker can reproduce the music at the desired volume.
The challenge is doing this accurately while wasting as little energy as possible. And that’s where amplifier classes come in.
In traditional linear amplifiers, the class is closely related to how long the output transistors remain conducting during each cycle of the waveform.
A complete waveform can be thought of as a 360-degree cycle.
In simplified terms:
Class A: the output device conducts for the full 360-degree cycle.
Class B: each output device conducts for approximately 180 degrees.
Class AB: each output device conducts for more than 180 degrees but less than 360 degrees.
Class D: uses a different approach in which the output transistors operate mainly as high-speed switches.
Class A is one of the simplest amplifier operating principles to understand. In a Class A amplifier, the output device remains active throughout the entire audio waveform. In other words, it is always conducting current, even when there is little or no audio signal being reproduced.
It’s similar to leaving a car engine running continuously so that it is immediately ready whenever you press the accelerator. The response can be smooth, but fuel is being consumed even when the car is not moving.
ADVANTAGES
One of the biggest advantages of Class A is linearity. Because the output device remains active throughout the waveform, the amplifier does not have to switch between separate devices at the point where the waveform crosses zero.
This means Class A naturally avoids the type of crossover distortion associated with some other linear amplifier designs. For this reason, Class A amplifiers are often associated with simple circuit designs and strong audio performance.
DISADVANTAGES
The major disadvantage is efficiency. Since current flows continuously through the output stage, a substantial amount of electrical energy is converted into heat rather than useful audio output. As a result, Class A amplifiers often require large heat sinks, substantial power supplies, good ventilation, and larger enclosures.
For this reason, pure Class A operation is more common in specialist hi-fi equipment, smaller amplifiers, headphone amplifiers, and certain low-power stages than in extremely powerful compact amplifiers.
Class AB is designed to provide a compromise between the linearity of Class A and the improved efficiency of Class B. Many traditional audio power amplifiers use Class AB because it offers a useful balance of sound quality, efficiency, cost, and heat production.
To understand Class AB, let’s briefly discuss Class B.
In a typical Class B output stage, one transistor handles half of the waveform while another handles the opposite half. Each transistor therefore operates for about 180 degrees of the signal cycle. This is good for efficiency, but when the signal moves from one transistor to the other near the zero-crossing point, there can be a small region where neither transistor responds perfectly. This creates crossover distortion.
To solve this problem, Class AB was introduced. It works by applying a small amount of bias so that both output devices remain slightly active around the crossover point. Instead of switching completely off at exactly 180 degrees, each device conducts for a little longer.
ADVANTAGES
Class AB offers several practical benefits, including better efficiency and less heat than Class A, low crossover distortion, excellent audio performance, and suitability for relatively high output power.
Because of these features, Class AB became especially common in home audio, professional audio, guitar amplifiers, and integrated amplifiers.
DISADVANTAGES
Class AB is still a linear amplifier, so it continues to dissipate a meaningful amount of power as heat. At high output levels, for example, substantial heat sinks are typically required.
The biasing of the output stage must also be carefully controlled. Too little bias can increase crossover distortion, while too much bias can increase heat and reduce efficiency. Even so, a well-designed Class AB amplifier can achieve extremely low distortion.
Class D takes a very different approach.
Instead of asking the output transistors to continuously reproduce the shape of the audio waveform, a Class D amplifier controls its output devices primarily as switches. The transistors rapidly turn on and off at frequencies much higher than the audio signal itself.
The audio information controls this switching process. An output filter then removes much of the high-frequency switching component, leaving the amplified audio signal to drive the speaker.
This switching approach is the key reason Class D amplifiers can be so efficient.
FUN FACT: The “D” in Class D does not stand for “Digital.” This is a pretty common misunderstanding, but it’s helpful to remember that the letters used for amplifier classes are simply labels. The defining feature of Class D is the switching output stage, not whether it is used for analog or digital.
ADVANTAGES
The main advantage of Class D is efficiency. This high efficiency brings several added benefits like smaller heat sinks, smaller enclosures, more compact power supplies, and less cooling. This makes Class D particularly attractive for portable and high-power equipment.
Their efficiency is especially valuable in subwoofer amplifiers, where very large amounts of output power may be required.
DISADVANTAGES
Class D designs are not without challenges. Because the amplifier switches at high frequencies, engineers must carefully control switching noise, electromagnetic interference, output filtering, and feedback. And the quality of the switching transistors, gate drivers, power supply, feedback system, circuit layout, and output filter can all influence performance.
Early Class D amplifiers sometimes developed a reputation for inferior sound quality compared with high-quality linear amplifiers. But they’ve improved with time, and modern designs can achieve very low distortion, low noise, wide frequency response, and excellent overall audio performance.
Which is best? Well, Class D has become the modern go-to choice and is generally considered the greatest of the three. But no single amplifier class is best for every situation.
Class A is a solid go-to when simplicity and linear operation are more important than efficiency, size, or heat. Class AB is a strong all-around solution for both linear performance and reasonable efficiency. And Class D is especially useful when high efficiency, high output power, or low heat production.
They solve the same basic problem (making an audio signal powerful enough to drive a load), but they approach the problem in different ways. That’s why different amplifier classes continue to exist.
And while knowing the differences is important and helps you understand what a manual means when it lists the amplifier class as “AB,” it’s also important to remember not to judge an amplifier solely by its class. The quality of the overall design matters far more than the letter printed on the specification sheet.
Thanks for reading! If you’re interested in purchasing an amplifier, help yourself to our large collection of gear for sale here.
Have a great day!
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