Filter to remove AM radio from telephone lines

The 'pigtail' type filters are only two wire connections for a single phone line. If you use the other two wires, like a 2 line phone, they don't work on that extra line

I've seen two types of pigtail filters: both cost around 10 to 12

1) simple single cap across the line - don't work well 2) common mode choke and cap - work very well

the choke looks like one of those 1/4 inch 600 ohm audio transformers, actually has laminated core in it

Sometimes radio stations change their beam pattern to satisfy FCC requirements. Depending on the time of day, you may see different influences from that AM station. Like dusk, 6pm or so.

In Aptos, California near the 10KW AM radio station, the Station's Engineer kindly supplied the excellent pigtail line filters to nearby residents, who asked for them, at no charge, as a 'public relations courtesy'

You mentioned that the effect gets worse with time OFF HOOK. Very interesting, because ON HOOK you have high voltage and ON HOOK there is lower voltage. Perhaps there is corrosion that is detecting the AM signal.

Even using pigtails, keep in mind that *if* your lines are unbalanced, the audio can get into them because the protection block is nonlinear and detects AM. Once the audio is on your lines, it cannot be blocked at any phone, because it is now legitimate audio.

- Robert -