When you look at car audio amplifiers, you will notice that some say, "Class D."  Well Class D is a type of circuit design that is use to amplify the signal and the power sent to your speakers.  There are two types of amplifiers that you see in car audio, Class AB and Class D.  

What's the difference?

If you consider yourself an audiophile, read on.  If you don't consider yourself an audiophile, you will not know the difference.

If you still want to understand the difference, keep reading:

In order to explain a Class AB amplifier, we will briefly explain both Class A and Class B amplifiers.  Then, we will explain how they take the best of both worlds to make a Class AB amplifier.

The output is always on, at full blast.  The only thing that changes is the level of the input signal.  As you increase the level of the input signal, the output will get louder because the input level is higher.  Whenever there is no signal or low-signal, the output is converted into heat.  Class A amps are inefficient, sound amazing, and generate a ton of heat.  Because of the amount of heat generated and the amount of power consumed, these amplifiers are not practical for car or marine audio applications.  They would just need to be WAY too big in order to get rid of the heat.

Class B amps work similar to Class A amps, but instead of having a single transistor, you have two transistors.  One handles the positive signal, and the other handles the negative signal.  Class B amps are super-efficient, but you get a ton of distortion between the switchover between positive and negative.  Because of the amount of distortion, these are not used in car audio.

So that brings us to Class AB.  A Class AB amplifier has two transistors like a Class B amplifier, but instead of them either being on or off, there is some overlap close to 0.  As a result, you get better efficiency than Class A with significantly better fidelity without the distortion that you get with Class B.  Class AB amps still have big, copper transformers and will still generate heat, but it is manageable within the limits of what will fit in a car.  Class AB amplifiers will be heavier than Class D amplifiers, they typically run warmer, and they aren't as efficient.  But, old school audio guys and audiophiles will tell you that they can hear the difference between a Class AB amplifier and a Class D amplifier.  Most home theater amplifiers are Class AB amplifiers because they have the space and the wall outlet of the house for plenty of power consumption.  

Class D amplifiers use microprocessors to very quickly turn a transistor on and off, really fast.  The transistor is either 100% on or 100% off, but it turns on and off faster than our ear can hear.  This is called Pulse Width Modulation or PWM.  The amount of time the processor is on, the widger the wave is, and the stronger the signal will be.  Instead of amplifying the analog signal, it uses computer chips and a power supply to create sine waves by quickly turning on and off.  Then, the low-pass filters are used to clean up the distortion and the signal to create an analog signal to send to the speakers.  Class D amplifiers are super-efficient, they draw less current, they run much cooler than Class AB amplifiers, and they are typically much smaller.

In 2020, when this article is written, it doesn't really mean anything.  In a car which is a noisy environment, with speaker off-axis not pointing directly at your ears, you will probably not be able to tell the difference.  Sure, if we switch back and forth between two amplifiers, a trained ear might notice a difference between a Class AB and a Class D amplifier.  99% of the population wouldn't know a difference, all things being equal.  

All JL Audio amplifiers are Class D and people think they are incredible.  Most of the Rockford Fosgate Punch and Power amps are Class AB amps and people think they are incredible as well.  The technology and the engineers have improved Class D amplifiers so much that the difference is not really discernable.