Re: Soundcard/POTS interface question

"Andrew Holme" wrote:
> Jon Danniken wrote:
> > Hello,
> >
> > I have built one of the basic isolated POTS interface circuits, using it
> > record a call (into the mic jack of my soundcard), or using it to play
an
> > audio file into the line (from the output of my soundcard).
> >
> > Currently, I just plug into either the microphone jack or the output
jack,
> > but I would like a way to not have to keep plugging and unplugging every
> > time I change mode from play to record.
> >
> > So, would there be anything inherently wrong with connecting *both* the
> > microphone input and the output together at the same point in the
circuit?
> > I would naturally mute the output while recording, to avoid feedback.
> >
> > Will this work, or is there a more elegant solution to isolate the two
> > points on the soundcard?
>
>
> Well, if you're literally moving the jack plug from one socket to the
> other, without changing anything else, it might work - the levels
> should be similar. There's a slight risk of clipping due to
> over-loading, but it shouldn't cause permanent damage; however, what
> about when AC ringing voltage is present on the line? That could blow
> the soundcard. I take it your POTS interface contains an isolating
> transformer?

Yes, indeed. The one I have built (breadboarded) is based on this model;
there are zeners on the output to limit the back end voltage:

http://www.rane.com/note150.html

> What voltage do you see on the jack plug when the 'phone
> is ringing? Have you looked at it on a 'scope?

Don't know, and my scope is AWOL presently.

> Have you already had it plugged into *both* sockets at different times,
when the 'phone has
> rung?

No, currently I just plug it into one or the other.

Jon









.

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