This circuit synchronized by the power grid can be used as a shield. We found it in an 80's Electronic Engineering. R1 can typically be 4k7.
In this interesting circuit from Popular Electronics, the output voltage is the square root of the...
We found this circuit in a documentation from the 90s. It lets frequencies around 1 kHz pass,...
This is one of many configurations with a 4-input mixer transistor that can be found on this...