This very low frequency circuit was found in a 1964 Electronic Design. Transistors may be modern equivalents. The filter is low pass type.
With this circuit it is possible to vary the resistance by adjusting the mark/space reaction of...
This circuit is from a 1967 US publication. The configuration can serve as a basis for designs...
This analog TV circuit can be used on a small transmitter. The circuit is from an 80's American...