power cords with active shielding

@kijanki,

Back in the late 1980s I had the same problem with a Ham Radio Operator just a few houses away. I bought a 31" console TV that had a synthesized surround sound amplifier. When I hooked up a pair of remote rear speakers to the TV it didn’t sound all that bad. That was until the Ham operator decided to dust off his radio and start transmitting again. Every time he keyed the mic and spoke I could hear his garbled voice through not only the two rear speakers but the front speakers in the Console TV. Even with the TV turned off I could still hear the guy when he was using his radio, though not as loud when the TV was turned on. The guy’s garbled voice did not change with the sound volume setting of the TV.

Problem ended up being the two speakers cables that ran around the wall of the room to the rear speakers. The two cables were working as an antenna and picking up the Ham Operator’s transmitted signal.

I tried everything. Ferrite beads on the power ac cord. Ferrite beads on the two rear speaker cables at the two speakers. Filter caps, at the speaker terminals. A factory made 95% shielded twisted pair cable for the speaker cables. Nothing helped.

I finally disconnected the rear speaker cables from the TV. With the rear speaker cables disconnected, the front speakers in the TV worked fine. Not a hint of the Ham’s garbled voice.

In james1969 other running thread I mentioned he might try wrapping some aluminum foil around the speaker cables feeding his 2 speakers. Not sure if he tried it or not. I am not sure the aluminum foil will even help.
https://forum.audiogon.com/discussions/high-frequency-intermittent-noise

As for ferrite beads and filter caps. Don’t they have to be selected for the frequencies you are trying to block, reject?

Jim

@jea48 

Not yet on wrapping the speaker cable.  I need to get more aluminum foil, then I will try that. 👍🏻🤓

Jim, speaker cables are antennas for the electrical noise since amplifier has low output impedance only for low frequency signals.  Induced noise enters amps input thru the feedback loop.  

As for the ferrite - on one hand you need highest permeability, but on the other high frequency response.  Self Resonant Frequency (SRF) specification might be useful since above this frequency impedance of the choke for common mode signals will drop like a rock.  Read this:
https://www.coilws.com/index.php?main_page=page&id=128

I've been once in the company that had so much of the noise pollution (multiple high frequency seem welders) that nothing worked - not even telephones.  Technicians there told me that common rules, like grounding shield on one end only don't even work. At the end they had to put every power cable in conduit.  We had to put our equipment, we sold them, in the metal NEMA box with metal mesh screen on the front door glass.

@kijanki ,

Thank you for your response.

If you find the time would you please read through this thread of james1969.
https://forum.audiogon.com/discussions/high-frequency-intermittent-noise
Maybe you can think of something to try.

Jim

I did read everything but unfortunately cannot think of anything else, especially since Al and Ralph already commented.  It could be the power cable, speaker cables or both.  I listen to recorded noise and watched youtube video showing sound of different type of interference.  Judging by >1s gap between occurrences I would suspect that it might be intelligent power meter.  They repeat readings from each other for each household or apartment on "Mesh Network" at 900MHz few times a day.
Where is it located?  It could be also that it picks up all the sources - cell tower, trains, power meter etc.   What about getting metal conduit in Home Depot (it is cheap) and cut it to the size of power cable and speaker cables.  After running cables thru it ground them well.  Look for alternative grounds like faucet or radiator.  It is only to check what it is.  If noise goes away then we can start removing conduits one by one.  If it doesn't then either filtering from the Furman is too weak or amp itself is picking up noise and needs better grounding.  Most of shields are not perfect Faraday cage and have to be grounded in order to work.  For instance to make high frequency current flow on the outside because of skin effect (cable shield or metal box) it needs place to flow to - a ground.  Amplifier box should be grounded with separate wire directly to good ground.  I would experiment with different grounds.  If faucet pipes work, than running long ground wire is better than suffering from audible noise.  Perhaps speaker box also needs shielding? (Shadorne suggested it).  If it is difficult to do/check replace it with very small speaker (even plain individual) and shield it.