Newsgroups: rec.audio.tech
Subject: Re: Telephone recording to sound card - 60 Hz hum problem.
References: <3a2aeb11.30223987@nntp> 
"Jerry G."  writes:

> The switching supply in the computer is giving you an AC ground loop
> back to the tele line.

Properly designed telpehone recording adapter should not cause
a ground loop because they have an isolation transformer between the line
to both provideo isolation, surge resitance and safety.
Proper isolation between telephone line and computer sound card
is an essentia feature to have when doing connections to public
telephone network.
 
> You would most likely get good results if you use an audio isolation
> tranformer.   An impdedance of 10 kohm primary and 10 kohm secondary
> should work okay with this...

Anythign from 600 ohms to 10 kohms will propably work acceptably on
this. The isolation shoud be preferably inside the adapter itself.

> There are a number of companies that make these transformers.  They
> should be available from most major parts suppliers.   Hammond is
> one manufacture of quality transformers of most all types.

You can find losts of better recoding adapter designs than this 
RADIO SHACK model at:
http://www.hut.fi/Misc/Electronics/docs/telecom/telerecord.html and
and http://www.hut.fi/Misc/Electronics/circuits/teleinterface.html

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