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From: ee5391aa@hydra.unm.edu (Duke McMullan n5gax)
Newsgroups: sci.electronics
Subject: Re: radio interference while recording from phone line
Message-ID: <1508@ariel.unm.edu>
Date: 6 Feb 90 16:47:53 GMT
References: <1067@ux.acs.umn.edu>
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Reply-To: ee5391aa@hydra.unm.edu.UUCP (Duke McMullan n5gax)
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Organization: University of New Mexico, Albuquerque, NM
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In article <1067@ux.acs.umn.edu> keith@ux.acs.umn.edu (Keith MaloneyHuss)
writes:
>A while back, someone described a circuit that activated a recorder
>or other device when a phone was "off hook".  Sorry, I can't remember
>who posted this, but my question is:
>When recording from the line using this circuit, I get a recording of
>a local radio station on top of the conversation being recorded.  Is there
>any way to remove this radio interference?  Basically, the recording tap
>goes through two 0.01 uF caps and two resistors from line to microphone
>input.  The phone line is not shielded, but I never hear the radio
>interference on the phone itself.

Keith, this is probably due to the RF being rectified by junctions in the
recorder. To swallow that RF, shunt the phone line with another capacitor --
no larger than 1 nF, and probably more like 100 pF, on the line side of your
interface circuit. Additionally, it may prove necessary to ground the "ground"
side of your recorder input circuit. The phone line will be dc-isolated by the
10 nF caps that you're already using, so that should be no problem. Still, be
careful. I've not seen any tape recorders in the past ten years that have a
"hot" (from the pwr. line) side...they all use power transformers these days...
but Murphy being the way he is, put a voltmeter on there first, just to pro-
tect your fuses. ;^)

If it's still persistent, try installing a couple of RF chokes in series with
your two 10 nF caps. If that doesn't work, gather wolfbane and mistletoe, wait
until the next full moon and....


							Hope it helps,
								d


    "Don't reinvent the wheel -- steal the plans instead."   - Don Lancaster
      Duke McMullan n5gax nss13429r phon505-255-4642 ee5391aa@hydra.unm.edu