Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!cs.uoregon.edu!ns.uoregon.edu!milton!whit
From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Design idea: PA and Music Amp questions
Keywords: high current amp
Message-ID: <1991May15.052708.2456@milton.u.washington.edu>
Date: 15 May 91 05:27:08 GMT
References: <72314@microsoft.UUCP>
Organization: University of Washington, Seattle
Lines: 63

In article <72314@microsoft.UUCP> gordonl@microsoft.UUCP (Gordon LETWIN) writes:
>I'm building a house which will have ... speakers in most rooms.

>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 

>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.  

	The high power output stage of an amplifier is just as you
state, a follower.  In fact, a push/pull AB amplifier has two of 
'em, one to source current and one to sink it (so the output impedance
is HALF the output impedance of a simple follower).  What
does your scheme really gain?  Good power amplifiers aren't terribly
expensive (especially if you get 'em as modules from one of the
kit-building houses).
	The main problem with using a MOSFET is that as a follower,
its output impedance varies with drive (so you'll get distortion
unless you use some sort of feedback).  A minor problem is that
the output impedance is the inverse of the transconductance; for
a reasonable power MOSFET, that's about 10 Ohms (i.e. worse
than just driving the speaker).
	As for using a bipolar transistor, the output impedance
there is low; it's 

	Rout = (kT/q)/Ie = (.020 Volts)/Ie   at room temperature

For 1A quiescent current, this will give you a very low output
impedance, all right; 0.02 Ohms.  That's too low (among
other problems, you will likely get RF oscillation due to
stray wiring inductances).  The transistor soon burns up,
because this is now a class A amplifier (the careful biasing
in most amps is intended to keep waste power to a minimum).

	Last problem: a high-quality high-power audio amplifier
has at least +/- 30V power supplies (and can deliver undistorted
output to about 30V on peaks); it's going to be cheaper to
get amplifiers than to buy batteries that can handle that voltage.
This assumes, of course, that your speaker choice is similar
to the 4/8 Ohm units most common in home audio gear.

	That said, if your speakers ARE IDENTICAL, you can just put
L-pads on 'em and put 'em in series in pairs, THEN hook the pairs
up in parallel.  Any stereo amp would drive eight speakers that
way, easily.  L-pads are a two-potentiometer attenuator that
have the same input impedance at all settings (so the other
speaker in series won't react to the attenuation of its sibling).
L-pads are easy to find (Radio Shack #40-977 or #40-980).

	If the speakers are non-identical, series connection
cannot be recommended because the frequency dependence of the
impedances will not match.


	John Whitmore