This circuit for frequencies of 20 to 2 kHz was obtained in English documentation from 1966. The transistor can be any general purpose and powered by batteries.
This filter, from an audio publication from 1977, passes the frequency range corresponding to the...
In the figure we have three voltage doubler configurations that can be used in high and low...
This circuit was found in Motorola documentation from 1966. The lamps are according to power and...