> Anybody out there have any good ideas on how to eliminate a 60 cycle > hum from a telephone line. The hum is directly related to the phone > wire being run with 500 feet of power cable in the same trench. Shielded cable might not help. Shielding does little to protect against magnetic coupling, unless it is a more expensive type with a mu-metal shield. The fact that the hum increases when he draws power, implies that there is some magnetic coupling.
How the pair is twisted is the key. Most unshielded data cables are designed with good twist characteristics to control crosstalk. Many are designed so that they can be laid in the same trays with power cables in computer data centers (though electrical codes might prohibit this). But on the other hand, twisted pair Ethernet isn't bothered much by 60 Hz. CAT5 is probably overkill for voice and won't be any better than good twisted phone pairs. The other two factors affecting crosstalk, are separation (the more the better), and how well the equipment on both ends maintains an electrical balance. Common-mode pickup is unavoidable; you don't want to do something that turns common-mode hum into differential hum. A defective electronic phone or answering machine anywhere on the line can do this. So can a leaky connection somewhere to ground. In the worst case, grounding one wire on a phone line is a sure way to get gobs of hum. If this weren't for a telephone, I'd advise using good transformers on both ends. Andy -- Author: Ingraham, Andrew INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
