power cords with active shielding

@blindjim

I inserted an isolation transformer (Furman P-2400 IT) into my system, so everything plugs into the isolation transformer. This had no impact on the specific noise I am targeting, but it did have a big positive affect in sound quality - removed any noise I was hearing in the music, and effectively lowering the noise floor quite a bit. But it had no affect on my intermittent noise. I called Furman directly and spoke to one of their reps and he was shocked to hear the Furman had no affect on the noise problem I am experiencing, he then said it was air born, not in the power line. Which I agree with him.

So I powered down my entire system (and apartment) for that matter and left the dedicated circuit on for my system. I put an RCA shorting plug on 1 amplifier’s input, I put a basic power cord on the amp and turned the amp on. I was able to affect the noise when I lifted the power cord and moved it around and it behaved like an antenna. So I am thinking of trying a new power cord to see if that will improve/change the situation.

I was able to identify the 700-900MHz frequency range by using this device:
https://www.amazon.com/gp/product/B007L09DLY/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
It will play the frequencies it’s detecting through the speaker, and I was able to correlate the intermittent noise to the 700-900MHz frequency range which is a dedicated cellular band for emergency first responders. So that made more sense, because when I hear sirens or the subway below, I usually hear the noise.

What are your thoughts?

@kijanki 

Thank you for that clarification.  I too got a Furman device, I got the isolation transformer, the P-2400 IT.  It basically removed the noise floor from the music in my system.  I am still getting the intermittent GSM signal in my system, that is why I am exploring power cables with better shielding.

James1969,  It is possible.  I had for few days CB like transmission loud voice coming thru pretty much everything at home - TV (turned off in stanby), speakerphone, radio.  I noticed every time the same unmarked car with many antennas passing by.  I suspect that his transmitter was out of whack.  FCC intervenes only when somebody complains.  Intensity of electromagnetic field drops in linear fashion with distance. When you are close it can be pretty strong.

@kijanki 

I have a police station down the street, and a fire station down one block from me.  So the public safety GSM band would probably be in high usage during the daytime (like what I experience now).  I'm up at early hours to enjoy my system (when the city is asleep) and there are no sirens to be heard.  The subway still comes and sometimes I hear the noise, we have cell phone service now down in the subway system.

I wonder if the FCC would listen to my case?

@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