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From: gordonl@microsoft.UUCP (Gordon LETWIN)
Newsgroups: sci.electronics
Subject: Design idea: PA and Music Amp questions
Keywords: high current amp
Message-ID: <72314@microsoft.UUCP>
Date: 13 May 91 17:17:26 GMT
Organization: Microsoft Corp., Redmond WA
Lines: 37

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.  I can't series/parallel them because each has it's own
PAD for volume management, so their effective impedance may vary.

Contractors have proposed buying N expensive hifi amps, each amp driving just
a few speakers in parallel.  I've come up with another idea that should work
well, but as an amateur EE I'd like an opinion on the scheme from more
knowledgable people.

My idea is basically to take any amplifier that I please, but just feed it's
output into a 8 ohm dumy load and build a high current voltage follower
that follows the amp output.  The follower would use some very high current
MOS transistor.  The theory is that this would impress the "high musical
quality" signal produced by the hifi amp onto all of the speakers in parallel;
the high current transistor would gate however many amperes this took.  

As for getting that current, I'm thinking of a big gell battery on a voltage 
regulated power supply, with caps, chokes, etc., as needed.  The power
supply would provide enough amps for the normal low-level background music,
and when the alarm system wanted high volume the battery would provide the
necessary current for the necessary minutes.  So the battery acts as a giant
power supply "capacitor" so that the power supply only provides average, not
peak, current needs.

Is there any reason that it's not just as simple as this?  I.E., use a
high current transistor and a car battery to provide a very high current
voltage follower; use that to "amplify" the current capacity of an audio amp
so that it can drive 20 or 30 speakers in parallel.

Pls advise

	thanks
	gordon letwin