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From: jgk@osc.COM (Joe Keane)
Newsgroups: sci.electronics
Subject: Re: Design idea: PA and Music Amp questions
Summary: Build an amplifier.
Keywords: high current amp
Message-ID: <4785@osc.COM>
Date: 15 May 91 22:12:12 GMT
References: <72314@microsoft.UUCP>
Reply-To: jgk@osc.COM (Joe Keane)
Organization: Versant Object Technology, Menlo Park, CA
Lines: 39

In article <72314@microsoft.UUCP> gordonl@microsoft.UUCP (Gordon LETWIN)
writes:
>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.  

I think this is a good idea.  The only part i don't like is that you're not
using the old amplifier to do much of anything.  Especially in hi-fi, simpler
is better.  Having a power stage that doesn't do anything is just asking for
more distortion and noise.

Even if you just build a current booster, i'd say you want feedback anyway to
keep the output voltage accurate.  At this point it's only a couple more
components to add some voltage gain.

So, i think that you might as well just build a high-power amplifier, which
works directly from line level.  If your old amplifier has features like tone
control that you don't want to do yourself, you can tap into your old
amplifier _before_ the power stage.

I've seen lots of schematics for high-power amplifiers.  Typically they use a
current feedback configuration, and there is one stage of bipolar transistors
between the op-amp output and the output MOSFETs.

One nice feature they note is that, unlike bi-polar transistors, you can
parallel MOSFETs arbitrarily to get as much power as you want.  So if you're
running out of power and you want to add more speakers, you can just drop in
some more output transistors, with heat sinks of course.

Note that even though MOSFETs have very high input resistance, power MOSFETs
have can have a hefty input capacitance, so it takes a decent amount of
current to drive a bunch of them in parallel.  As long as the previous stage
is designed with this in mind, it shouldn't be a problem.
--
Joe Keane, amateur electronics hacker
jgk@osc.com (...!uunet!stratus!osc!jgk)