Car Amplifier Classes

The ABDs of car power amps

All power amps perform essentially the same function and operate under the same basic principles, but that doesn’t mean all car amplifier classes are created equal. Some amps are better suited to specific uses than others, and the easiest way to tell which type you need is to look at the class. Each class is referred to by a letter of the alphabet and is delineated pretty clearly, although there are also combinations and hybrids that include characteristics of more than one class.

The Head of the Class

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

Out of all of the different amplifier classes, there are only four that 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.

Comparison Chart of Amplifier Classes

Pros Cons
Class A

Clean output, high fidelity, and low distortion.

Large size and creates a lot of heat.

Class B

Efficient, has a smaller size, and creates less heat.

Lower audio fidelity and potential signal distortion.

Class A/B

More efficient than class A​ and 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.” These amps are grouped together due to the fact that they use internal circuitry that is designed to always have current passing through the output transistors. This basic design comes with both 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 vastly 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 effectively a hybrid of the traditional A and B amplifier classes. Although their transistors always have current flowing through them, they use circuitry that is capable of reducing the amount of current when no signal is present. That results in a much higher degree of efficiency than pure class A amps without as much distortion as a class B amp. Due to 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 all examples of analog amplifier classes, which makes class D the only “switched” amp class commonly used in car audio systems. Unlike class A, B and AB, class D amps operate by switching on and off the current to their transistors very quickly. This effectively creates a switched, or pulsed, output signal that’s mapped to the analog input signal.

While 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 since 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 either 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 a lot more complicated than it ever has been before. If you just 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 a lot more complicated than that if you want, but sticking to that basic plan will put you on the right track.

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