Newsgroups: rec.audio.tech
Subject: Re: Telephone recording to sound card - 60 Hz hum problem.
References: <3a2aeb11.30223987@nntp>
[email protected] (Scott Ledford) writes:

> I have a unique problem, I guess, since I can't find a reference to it
> anywhere.
> 
> I am using a Radio Shack device that connects to the telephone line to
> record phone calls. ("DuoFone cat no 43-236B")
> It has 2 output cables - one mono "mini" plug for the sound, one mono
> "mini-mini" to trigger some tape recorders to start.
> 
> I am trying to hook up the sound into my PC sound card so I can use a
> program like Scanner Recorder to record all my calls, without worrying
> about fillng up tapes, etc.
> 
> When I hook this up to the MIC input I get a hum that's louder than
> the voice on the phone.
> When I hook this up to the LINE input it's not loud enough, plus the
> hum is there, too.
> 
> I never notice this hum using microphones or an amplifier hooked up to
> either input.
> 
> I've done this on several phone line/PC's with similar results.
> 
> One thing I've noticed - I tried this with a laptop as well. I can get
> that to work with no hum by running the laptop on battery, but the hum
> pops up as soon as the laptop is plugged into 120V. 
> 
> Can anyone give me a way to fix this?

YES! GEt a better sound recording adapter which is properly designed
to work with somethign else than just fully floating small tape deck.
The folling information is based on feeback I have received (no personal
experience): The telephone recording adapters do not provide
a galvanic isolation between the input and output.

Galvanic isolation is a needed feature to avoud ground loop situation
which would be created between the telephone line and the computer
if you use this kind oadapter without isolation. Galvanic isolation 
between the telephone line is also a safety feature which sould be
there in proerly designed recording adapter (demanded in many
Eurioean countries by telecommunication regulations)!

According to the feedback and number of questions on this kind
of problems I have received my view is that RADIO SHACK TELEPHONE
RECORDING ADAPERS ARE UNSUITABLE TO BE CONNECTED TO COMPUTER
and such connection should not be attempted! My advice is
DO NOT BUY A TELEPHONE RECORDING ADAPTER FROM RADIO SHACK IF YOU
WANT TO USE IT WITH COMPUTER! By following this advice you will
save lots of headaches and potential damage to sound card.

 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/