A small bit of Radio History, Lee De Forest Audion, Marconi
Electronics: Vacuum Tubes (Valves): Triode & Multipurpose Tubes ~ 1943 US Army Training Film TF1-471
An audio power amplifier is an electronic amplifier that reproduces low-power electronic audio signals such as the signal from radio receiver or electric guitar pickup at a level that is strong enough for driving loudspeakers or headphones. This includes both amplifiers used in home audio systems and musical instrument amplifiers like guitar amplifiers. It is the final electronic stage in a typical audio playback chain before the signal is sent to the loudspeakers and speaker enclosures.