Amplifier

Class D amplifiers — popular kings of efficiency

Class D amplifiers operate in a unique fashion. Onboard circuitry creates very high-frequency (often over 100K Hz) pulses of DC current. The width of each pulse is then modified by the input signal — the wider the pulse, the louder the signal. This is called "pulse width modulation" or PWM. These DC pulses are run through the amplifying output transistors creating the high-power output.  Because they are getting DC pulses, not analog signals, the transistors, also called MOSFETs, are either on full power or off with no power. This is the most efficient way of running these transistors — as much as 90% efficient in some cases.

D does not stand for digital

Although making a signal by rapidly switching transistors on and off resembles digital processing that uses zeros and ones, Class D amplifiers are not digital devices. Some of them might have digital control circuits, but the amplifier circuits will be strictly analog.

Less-than-highest fidelity

After amplification, a low-pass filter smooths the output signal so the amp won't put out pulses of power but rather, a continuous analog power output. It also removes the interference generated by those high-frequency DC pulses. Because of this, most audiophiles won't use Class D amplifiers in their systems, citing that need for filtering out generated distortion.

Class D has become the go-to choice

On the other hand, in professional PA systems and car audio applications where perfect fidelity isn't as important, Class D amplifiers have become much more popular because they're smaller, lighter, and run cooler than the other Classes of amplifiers with the same amount of power. And these are big advantages when you have to fit an amp in a vehicle or carry one around for gigs.