From: "Michael A. Terrell"
Subject: Re: PA (Public Address) Systems
Date: Sun, 08 Dec 2002 00:33:44 -0500
Organization: Have you seen my bench? No, really! Where is it?
X-Mailer: Mozilla 4.61 [en]C-CCK-MCD (Win95; U)
John Woodgate wrote:
> I read in sci.electronics.design that Michael A. Terrell
> wrote (in <3DF249B6.937B12BD@mfi.net>) about 'PA
> (Public Address) Systems', on Sat, 7 Dec 2002:
> > A 25 volt system at 25 watts is 25 ohms, and in a typical office area
> >will work just fine. I prefer 70 volt systems, but serviced a number of
> >25 volt ones over a 15 year period School intercoms use 25 volt wiring
> >and i have serviced systems with 200 speakers and a 100 watt amplifier
> >that worked quite well, after I serviced it. Some of the speaker lines
> >were 1500 feet from the office, but every speaker was a home run. A .5
> >watt tapped 25 volt transformer is 1250 ohms, so 3000 feet of 22 AWG is
> >16.46 ohms/1000' *3, or 49.38 ohms. 26.31% power loss is quite
> >acceptable for occasional paging, and emergency announcements. At 25
> >volts, the output impedance is 6.25 ohms, but this was a safety
> >requirement to prevent shocks if someone tampers with the system.
> I'm not criticizing 25 V or 70 V systems, just saying that we don't have
> the parts for them here.
> Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
> Interested in professional sound reinforcement and distribution? Then go to
> PLEASE do NOT copy news posts to me by E-MAIL!
The National Electrical Code classes 25 volt systems as low voltage,
and some places are required to use them. Both 70 and 100 volt are used
elsewhere, with 70 volt being the most used of all three types. It is
simple, if you're used to it. :)
Michael A. Terrell