From: "Michael A. Terrell"
Subject: Re: PA (Public Address) Systems
Date: Sat, 07 Dec 2002 14:36:01 -0500
Organization: Have you seen my bench? No, really! Where is it?
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"Michael A. Terrell" wrote:
> John Woodgate wrote:
> > I read in sci.electronics.design that Michael A. Terrell
> > wrote (in <3DF22855.14115CC2@mfi.net>) about 'PA
> > (Public Address) Systems', on Sat, 7 Dec 2002:
> > > Constant voltage line transformers come in 25 volt, 70 volt, or 100
> > >volt. Choose what matches the amp you buy.
> > >
> > [snip]
> > >
> > > Depending on where you are, you may need to have an electricians
> > >license to do 70 or 100 volt systems, but 25 volt is considered low
> > >voltage, and rarely requires a license.
> > This is about US practice and does not apply in UK. It's 100 V or
> > nothing. No way are you going to cope with 16 loudspeakers in a low-
> > impedance (8 ohm) system.
> > --
> > Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
> > Interested in professional sound reinforcement and distribution? Then go to
> > http://www.isce.org.uk
> > PLEASE do NOT copy news posts to me by E-MAIL!
> 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.
> Michael A. Terrell
> Central Florida
Sorry, bad math. A 1500 foot run of #22 AWG is 49.38 ohms, so with a
1250 ohm tap at .5 watts it puts a .48 watt load on the amp. This
allows for .02 watt loss in the wire and .46 watt to the transformer
input. The wire drops about 4% of the power from the amp.
Michael A. Terrell