Afrotechmods shows us how a Class D Audio Amplifier works and how to build one. A TPA3122 is a class D amplifier on a chip. There are no external power components needed since class D amplifiers use mosfets for switching and thus the heat dissipation is low enough that it can be squished into the DIP package with no heatsinking needed.
“The TPA3122D2 is a 15-W (per channel) efficient, Class-D audio power amplifier for driving stereo single ended speakers or mono bridge tied load. The TPA3122D2 can drive stereo speakers as low as 4.”