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From: jeh@cmkrnl.uucp
Newsgroups: sci.electronics
Subject: Re: Design idea: PA and Music Amp questions
Message-ID: <1991May14.164451.17@cmkrnl.uucp>
Date: 14 May 91 23:44:51 GMT
References: <72314@microsoft.UUCP>
Organization: Kernel Mode Consulting, San Diego CA
Lines: 30

In article <72314@microsoft.UUCP>, gordonl@microsoft.UUCP (Gordon LETWIN)
 writes:
> I'm building a house which will have good quality ceiling speakers in 
> most rooms.  They'll play music at low volumes and act as alarm/PA/Intercom
> speakers at higher volumes.
> 
> Hooking say 20 pairs of speakers in parallel provides too low an impedance
> for the amplifier.  

Gordon, you need to talk to someone who knows about PA systems... NOT sound-
reinforcement systems, and NOT home audio... and ask about 25-volt and 
70-volt lines.  I don't know much about them myself... but I do know that 
that is how this problem is addressed in, for example, school and office
paging systems.  

As I understand it, the PA amp has a speaker output terminal labelled
"70-volt".  You run this to all of your speakers, all at once.  Each speaker
picks power off the line via a 70-volt-to-8-ohm transformer.  Your L-pad goes
between the transformer and the speaker, just as you would expect.  You can add
as many speakers as you like (each with its own xformer) without worrying about
impedance matching.  

Of course, the amp must still have enough power to run all those speakers at 
once.  No free lunch.  

But I said "as I understand it".  Ask someone who works in the field.  

	--- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA
Internet:  jeh@dcs.simpact.com, hanrahan@eisner.decus.org, or jeh@crash.cts.com
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