Car Amplifier Classes From A to D

The ABDs of car power amps

Out of all the amplifier classes, only four are commonly used in car audio systems, and one of those is a combination type. These four amplifier classes are A, B, AB, and D. Here's a quick primer on these amplifier classes.

Types and Classes

At the basic level, there are only two types of power amplifiers: analog and switching. These basic types are further broken down into more than a dozen lettered classes. Some of these, such as T and Z, are proprietary, trademarked designs, and others, such as A and B, are produced by a variety of manufacturers.

All power amps perform essentially the same function and operate under the same basic principles, but classes are not created equal.

Comparison Chart of Amplifier Classes

Pros Cons
Class A Clean output, high fidelity, low distortion. Large; creates a lot of heat.
Class B Efficient, has a smaller size, creates less heat. Lower audio fidelity and potential signal distortion.
Class A/B More efficient than class A​; less distortion than class B. Less efficient than class B and more distortion than class A.
Class D Extremely efficient. Distortion at high frequencies.

Class A Car Amplifiers

By definition, class A amplifiers are always on. They use internal circuitry designed to always have current passing through the output transistors. This basic design comes with advantages and disadvantages that make class A amps well suited to some applications and poorly suited to others. The biggest issue when it comes to class A amps in car stereo applications is size.

Class B Car Amplifiers

Unlike class A amps, class B power amplifiers are switched. That means they use internal circuitry that allows them to effectively switch off their output transistors when there is no audio signal to amplify. This results in improved efficiency, which makes class B amps well suited to car audio applications, but it also comes with decreased audio fidelity.

Class AB Car Amplifiers

These amps are a hybrid of the traditional A and B amplifier classes. Although their transistors always have current flowing through them, they use circuitry capable of reducing the amount of current when no signal is present. That results in greater efficiency than pure class A amps without as much distortion as class B amps. Because of these benefits, class AB power amplifiers are the most commonly used full-range amps in car audio systems.

Class D Car Amplifiers

Class A, B, and AB amps are examples of analog amplifier classes, which makes class D the only switched amp class commonly used in car audio systems. Unlike classes A, B, and AB, class D amps operate by switching on and off the current to their transistors very quickly. This creates a switched, or pulsed, output signal that's mapped to the analog input signal.

Although class D car amps are extremely efficient, the switching/pulsing method results in a certain amount of distortion in the higher frequencies. This is often removed by a low-pass filter, given that the lower frequencies don't suffer from the same distortion. A lot of mono subwoofer amps are class D, but the size and power benefits make them one of the more popular amplifier classes for full-range speakers as well.

Beyond A, B, and D

Most car audio amplifiers are A/B or D, but variations of these two main types are also available. These other amplifier classes typically pick and choose characteristics from the main types of amps in an attempt to increase performance without sacrificing too much in return.

For instance, in the same way, that AB amplifiers combine the designs of A and B, class BD amps are designed to offer less distortion at high frequencies than class D amps with more of the efficiency you expect from a class B.

Which Amplifier Class Should You Choose?

With the introduction of bD, GH, and other types of amplifiers, choosing the right class can seem more complicated than it ever has been before. If you want good sound, without getting in too deep, the basic rule of thumb is that A/B amplifiers are best for full range and most component speakers, while class D amplifiers are better at driving subwoofers. You can make it more complicated than that if you want, but sticking to that basic plan will put you on the right track.

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