This simple audio generator with the 741 was found in a 90sTexas Instruments. The circuit generates sinusoidal signals from 15 Hz to 150 kHz in 4 bands, as shown in the diagram.
This is an old test circuit for integrated TTL counters, feeding a Nixie tube, common at the time....
This traditional crystal radio circuit, but which has a modern version with diode, was found in an...
This headphone amplifier using a pickup coil can be used as an extension for devices such as TVs,...