Newsgroups: sci.electronics.misc
Subject: Re: Low Level Line to Telephone
References: <[email protected]> <[email protected]>
[email protected] (Dbowey) writes:

> RE:  "I don't have a strong background in electronics, but I can't imagine that
> much would be required to do this."
> 
> There's the problem.  You need to imagine that much IS required to do this.
> 
> If you want to use line in and line out (4 wires) to a phone or a phone line (2
> wires) you will need to configure it electronically so that the signal from the
> line out does not get to the line in without lots of attenuation.  At the same
> time, you want very little attenuation from the phone or phone line to the line
> in, and the line out to the phone or phone line.  This is best done with a pair
> of transformers wired in a "hybrid" configuration.

You are right.
 
> If you don't want to buy transformers, and if you can find an old phone, remove
> the mike and earpiece and then connect line out to where the mike was, and
> connect line in to where the earpiece was.  Older phone contain a very good
> hybrid.  It will work well.

That's true. One fact which needs to be taken in account that normal
telephones do not geenrally provide galvanic isolation between
the microphone inut and the line (and not even fron line to earpiece
also). If you connect this thing to an external circuit, the external
circuit is quite surely directly or indirectly electrically connected
to telephone line which is not good for safety and noise reasons.

If you are planning to connect this mind of circuit to any audio
or computer system and publec telephone netwwork, 
the galvanic isolation (isolation transformers) between the telephone
itself is an essential safety feature which should be there!


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