This circuit can be used as an amplifier for musical instruments or even with your cell phone. The circuit is old and was found in the 1990 edition of Radio Electronics Experimenter magazine.
Found in documentation from the 90s, this circuit is based on operational amplifiers of the time....
This oscillator, which can be used as a signal injector, was found in a Popular Electronics from the 90s....
This traditional oscillator uses two complementary transistors to produce low frequency signals....