From: "Michael A. Terrell"
Subject: Re: PA (Public Address) Systems
Date: Sat, 07 Dec 2002 11:56:53 -0500
Organization: Have you seen my bench? No, really! Where is it?
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George Shaw wrote:
> Question: Are there any good resources to find out how PA systems are
> designed and how they work?
> I need a source to provide basic information such as speakers, why
> 100V, impedance explained, amplifiers, transformers and line drivers.
> As well as the more advanced topics of acoustics and environment
> Web sites, books and sources or explanations would be of value.
> Any calculations and formula would be of value.
> Any ideas of suggestions as to how a simple PA system comprising of a
> microphone, amplifier, 16 ceiling mount speakers and a two wire cable
> system could be specified and designed for optimum audio in an office
> environment would be a good example. Block diagrams or
100 volt systems are constant voltage distribution with a transformer at
each speaker to adjust the sound level at each speaker. Depending on
the background noise and the size of the room you need somewhere from
.25 watts up. If it is a quiet place and the room is 25' X25' .625 or
1.25 watts is plenty.
The transformers have the taps marked in watts. Add up all the
watts, and that is the load on the amp. If you have .625 watt X 16,
that is 10 watts. The smallest amp you can find is usually 25 watts, so
you can use the next higher tap and have a 20 watt load. You are trying
to get a fairly close match between the amp and the load, and it is
better to be a little under the rating on the amp, than over.
Constant voltage line transformers come in 25 volt, 70 volt, or 100
volt. Choose what matches the amp you buy.
Common sizes are:
2 watts, with taps of 2, 1, .5, .25, .125 watts
5 watts, with taps of 5, 2.5, 1.25, .625, .3125 watts
10 watts, with taps of 10, 5, 2.5, 1.25, .625 watts
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. To run the wire to the
speakers, you can use any convenient pattern. If the rooms are off a
hallway on one or both sides you can run a line down the hall, and tap
it for each room. You can run it room to room. One good idea is to run
it in several groups back to the amp and use a terminal block or
switches to turn off groups if something goes wrong. that way you can
isolate the bad section, and find the problem faster. Also, if you
break it into groups record the DC resistance for that group when you
finish, and update it if you change taps. It will give you a starting
point for troubleshooting years later when the details are all fuzzy.
http://www.grainger.com is one source of hardware.
Michael A. Terrell