On a line level input some RF noise can forward to PREamp. Nearly all of the manufacturer protect just the hot pin from this kind of signal. But the shield of line (connector) can work like antenna. For example if I plug the tuner at power off state to amp the the CD sound quality is go down. The problem is the same if I plug the LP player (with MC transformer) to amp.
The only reason is the RF interference go trough the shield of line input and make garbling in the internal signal of the amp.

So anybody know some magic box to filter RF signal from hot and shield point of RCA line connector?