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Don D. Wrote: Saw your website regarding RFI. I'm using a radio around printers/laptops/desktops. Anything that would cut ther electrical interference going to the headphones? I've got a decent signal, but get constant buzz, static and ringing. Any suggestions would be great, Don
John Browne wrote: Hi Don, What kind of radio??????????? John

Don D. Just a basic AM/FM/Cassette walkman type.

J.B. wrote: Don, I presume you are listening to am. If so, there's not much you can do about the interference except keep the radio away from the computer equipment. FM should work ok however. Is this your experience???

Don wrote: Yes, I am listening to AM. And yes I am only having interference problems with AM. Can't I do any kind of sheilding around the walkman or find something that would nock out at least the highest buzzing? What about those noise cancelling devices? Like the ones they sell for airplanes?

J. Browne wrote: Don, It would be a very hard technical problem to solve. Shielding the Walkman would not help because the interference and the am station are on the same radio frequency and the shield would block both. The problem is that the computer and especially the monitor are radiating an amplitude modulated (on and off pulses = 100% modulation) signal that is very rich in harmonics especially in the am band. These signals exit from the computer equipment through various routes: cables, ac cords, etc. The signals radiate directly through holes or unshielded places in the metal cabinet, through plastic cabinets and worse of all through the front of the monitor screen. To solve the problem at the source would require completely enclosing the computer equipment in a metal or screen box (a "Faraday Cage") and filtering all incoming and outgoing lines. On am, most portable radios are very directional and the signal can be peaked or nulled by turning or aiming the radio in the correct direction. By keeping the radio as far from the computer equipment as possible and turning it for best reception you could maybe improve your reception. Another solution could be used if you are listening to one particular station. You would need another am radio and an FM wireless microphone or transmitter such as "the sound bridge" sold by Parts Express at 1 800 338-0531. (I think it costs approx. 17 dollars and runs on one AAA battery. You locate one am radio somewhere away from the computers (within a couple of hundred feet), where it gets good reception, you then plug the sound bridge into the earphone jack on the radio. Turn on the sound bridge and turn the other radio to FM. Tune in the sound from the sound bridge on the FM set and you can listen with no problem even when relatively close to the computers. The am or amplitude modulated interference is not picked up very well by the FM radio and the FM station (formerly the am station)comes through loud and clear. Sincerely, John K. Browne

Don wrote: Thanks for the info. I'll try the sound bridge solution.

My sister works for a well known franchise electronics retail store and it seems that their telephone RFI filter (made by K-COM) is inferior to yours.
Recently, she had a customer come into the store seeking something better so I gave her your address and phone number. Because there are so many radio towers in this area(Worcester, Mass.), there could be quite a market for your filters. I saw a tower yesterday that appears to be 700 plus feet in height.
Have a great day.

Click on the picture for RFI filter construction.


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