This 50 Hz rejector or notch filter was found in a 1975 documentation. Other frequencies can be obtained by changing components. The filter uses a Wien network and must use a symmetrical power supply.
In this circuit, from a 1977 publication, we have the traditional version of a filter tuned to 1...
We find this circuit in an American 1980s documentation. Modern equivalent integrated circuits can be...
Found in an old Popular Electronics this circuit can also be adapted to be used as a shield. In...