I have heard many, ridiculous definitions for class D
topologies. Ive 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? Thats right, make big ones
and zeros ; )
Class D technology is growing in the 12 Volt industry.
There are 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 more than 80% efficient while a
Class A/B (which most amps are) will fall into the 50-60% range. Lets 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, thats 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 as 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.
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