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From: versatc!VERSATEC!peter@ames.arc.nasa.gov (Peter Tapscott)
Newsgroups: rec.audio.high-end
Subject: Re: crossover design / bi-amping
Message-ID: <9400@uwm.edu>
Date: 7 Feb 91 13:57:08 GMT
Sender: news@uwm.edu
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Approved: tjk@csd4.csd.uwm.edu
Originator: tjk@csd4.csd.uwm.edu


In article <9319@uwm.edu> mike@brl.mil (Michael John Muuss) writes:
>
>In article <9230@uwm.edu>, leconte@irisa.fr (Thierry Leconte) writes:
>|> And how about a true digital amp?
>|> Why not  modulate a high level ouput signal which will
>|> be send directly to the speakers  ?
>|> I think that a simple passive output filter will suffice.
>|> 
>
>This has actually been done as long ago as 1979.  Infinity produced
>a 1-bit switching amplifier (code-named the "SWAMP") that operated
>at a multi-megahertz rate.  200 W/ch.
>	Best,
>	 -Mike

Even before '79, a friend made one.  

We were in grad school at Syracuse, and had access to pretty nice
labs and instrumentation.  We used to make a lot of amplifiers,
mostly hybrids like op amps driving class-A current amps,
phono-preamps that sounded better than those in our amps, etc.

We wondered why switching power supply concepts couldn't be applied
to audio amps, so one of the more clever among us decided to
design one.  (Steve Zahorian, where are you now?)

He used a resonant circuit to create the switching frequency,
as was common on switching power supplies of the time.  This
requires some pretty clever use of feedback theory, and the amp
itself was the first application I knew of sampled data feedback 
systems.

It provided about 40 watts, but needed only a couple of small
pieces of aluminum sheet for heatsinks since the output transistors
operated in digital mode.  The simple passive output filter
that Thierry suggests was a handmade 2" long by 1/2" diameter
inductor.

When Infinity came out with theirs, I said "Oh, Rats, now
everyone will be using switching power amps, and I have missed
the window on the market."  So I started taking computer classes,
and the rest is misery.... er ..., I mean history.

They still seem to make sense.  200W switching power supplies are
quite common.  Rather than having a power supply and a switching
amp (like Steve's above), doesn't it make sense to have the
switching on the line (mains to UK readers) side, like a switching
power supply?
-- 
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