This circuit employs an old germanium transistor. The circuit is from a 1950s publication and uses RC coupling.
This circuit was found in a 1992 publication, using an operational amplifier not very common...
This 13.8 V transistorized power supply with a current capacity of 20 A has a protection system...
This amplifier with 6 or 9 V power supply was sold in Japan in the 1960s. Its power was fractional watt...