Class D amplifier
A power stage whose transistors are only ever fully on or fully off, with the music encoded in how long — which makes the amp small, cool and efficient.
A classic class AB amplifier runs its transistors in their linear middle range and turns a good share of the power into heat. Class D converts the audio into pulses, often pulse-width modulation at several hundred kHz, and slams the transistors between fully conducting and fully off. Neither state wastes much power, so efficiency is often above 90 per cent. A low-pass filter at the output strips the switching frequency so only the music reaches the speaker.
The idea is old, but early attempts in the 1960s earned a poor reputation. Fast MOSFETs and modules such as Denmark's ICEpower are what made the technology properly credible. Today class D sits in almost everything with a built-in amplifier: phones, smart speakers, active studio monitors. And no, D does not stand for digital. It was just the next letter.