Very old amp (still working though), Elektor design "Ekwin" from 1976 (Semi-complementary, using BD139/140 and 2N3055).
I know it looks lousy, but remember I was 14 or so when I
built it.
Amp, based on the National Semiconductor LM12 application note (not my favorite amplifier).
I like this one: "Micromite", sophisticated low power (2 x 12 W eff) design by Randy Slone,
from his book "The Audiophile's Project Sourcebook: 80 High-Performance Audio Electronics Projects"
In an effort to make the simplest and most energy-friendly amplifier possible, I used an Adafruit class-D module (from electronics celebrity Lady Ada) based on a MAX9744 chip: very cheap, enough power for home use (2 x 20W), and to my ears, quality is flawless... I know the housing could be nicer!
This is a low-power version of the well-known (for those who know it well) p101 MOS-FET amplifier ( link). Since I did not need the potential of high power, I used a low supply voltage, starting from a recycled toroidal transformer (2 x 18V, 120VA). If you're interested, the output transistors are 2SJ162/2SK1058 which I found in the Reichelt online shop. The only extra is a switch-on delay and DC output protection. The case, including the much-too-big heatsinks, was bought as a kit from Alibaba.