Years ago, in a store, (retail electronics), we used a probe to hear the dirty line noise in an AC typical shop circuit. Then a small cheap device about the size of a wall wart, got plugged into an outlet in this circuit, and the dirty noise & hum was gone.

I can't find this kind of device these days. It really worked where other noise suppressors did nothing.

Where might one find a noise filter like that.

 

Was it a pass-through or just a plug-in?

 

As I remember, it was just a plug in.

 

It sounds like nothing but a capacitor. A nerd would hide one for each phase in the circuit breaker box.

As for the retail version? I haven't got a clue.

 

Sounds like it was a scam to sell line filters.

Ken