I have heard many, ridiculous definitions for class D topologies. I've heard people say, of course, that D stands for digital or even that the class is like a report card grade; A is the best type of amp, B is the second best, and so forth. These people must not know that topologies go far beyond the F grade... Actually, D just means 4. A was the first type of amp recognized by the IEEE, a group of professional electronic engineers. B was second, C was third, D was fourth, blah blah blah blah... Truth is, there is no such thing as a digital amplifier! If digital information is written in 1s and 0s, then what would a digital amp do? That's right, make big ones and zeros!! ;^)

Class D technology is growing in the 12 Volt industry for years now. There are quite a few companies producing these amps and I suspect a few more companies will jump on the wagon for good reasons. The main reason is for dramatically increased efficiency. This is done by "Switching" and is therefore a "switching amplifier" as compared to a non-switching amplifier like a class A amplifier.

A class D amp can be 80%-90% efficient while a Class A/B (which most amps are) will fall into the 50-60% range. Let's say we have two identical vehicles and we are going to put a class A/B sub amp into one of the vehicles and a class D into the other. Both amps, for comparison put out 1,000 watts RMS with 12 Volts. The vehicle with the class A/B amp will draw approximately 160 amperes of current and, being 50% efficient, will put out 1,000 watts of heat into the heat sink! Bummer, huh?! The class D amp will only draw around 98 amperes to yield the same 1,000 watts RMS output. Not only will your vehicle like you better for not draining the power supply, but the amp will like you better as well because now you will only produce about 175 watts of heat into the class D heatsink! For you tweakers, that's 40% less current draw and 82% less heat for the heatsink dissipate...

Class D is great for bigger sub amps only as the benefits do not outweigh the cost for a smaller amp. So, why don't people make a class D amp to play 20Hz to 20kHz?! Not a simple answer, but here goes... The audio signal is used to modulate the shape of the square-looking waves of high frequencies. Then the high frequencies are eliminated, leaving audio output. The type of modulation used is called Pulse-width Modulation, or PWM. PWM, like the power supply in many car audio amps is used. Truth is, a PWM power supply is an amp that has DC output only. If you want to play 20kHz you would have to have the output devices switch at hundreds of kHz which almost reaches the AM radio frequencies. You see, the Pulse Width Modulated (PWM) wave frequency must be a lot higher than the desired playing frequency. To play 500Hz, we only need to switch at a frequency around 60kHz. The problem is that when you modulate the switching frequency back down, it gets a bit dirty. Clean enough for subwoofer frequencies, but not clean enough for high-end mid/high frequencies.

In summation, class D amps put out more power and less heat with less current consumption. But as for now, they are only beneficial as larger subwoofer amplifiers.