Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: intercom
References: 
"Bill Greenley"  writes:

> We have a power supply from a phone key system that we use to ring phones on
> stage.
> 
> It also presents standard phone voltage (I think 48v -- forgot to look prior
> to this post).
> 
> If we used this voltage across a gang of RJ11 jacks and attached cordless
> base units for cordless phones (the kind with headsets and belt clip units),
> couldn't this be used for a simple intercom system?  

This kind of parallel connection ysually works quite well if you 
have just two phone in parallel powered from one current limited 
power source that give enough operating power to telephone and presents 
high impedance to audio signals (feed through current source, 
large inductance coil or gyrator). 

Two telephones in series work well if connected in resies and powered 
with suitable current limited power source that presents low impedance 
to the audio frequencies.

Those work well. Impedance matching etc. are correct. 

If you add more phones to the system, then you can easily go 
to problems because of possibly following problems:
- bad impedance matching ewakens signal and causes ststrong sidetone
  (hard to say how bad it gets with many phones)
- in paralel connection the powering current might not get evenly to all 
  phones etc.

You can find lots of intercom related information including using 
normal telephones in intercom applications at 
http://www.epanorama.net/links/intercom.html

> We generally use between 2 or 4 units in a show.  

Two units of intercom on telephone 

> We're a small community theatre without a
> huge budget.  When the phone is off-hook, wouldn't it be in on the "party
> line" conversation?  tia

Party line of many normal telephones does not generally work well. 
Few phoens might be, but more than few causes problem..

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