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Posting-Version: version B 2.10.1 6/24/83; site rabbit.UUCP
Path: utzoo!linus!philabs!cmcl2!floyd!clyde!ihnp4!houxm!mhuxi!eagle!allegra!alice!rabbit!jj
From: jj@rabbit.UUCP
Newsgroups: net.audio
Subject: Re: son of ampzilla bridge
Message-ID: <1989@rabbit.UUCP>
Date: Wed, 28-Sep-83 10:28:30 EDT
Article-I.D.: rabbit.1989
Posted: Wed Sep 28 10:28:30 1983
Date-Received: Fri, 30-Sep-83 02:49:16 EDT
References: <30@tpvax.fluke.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 34


I must beg to differ with the opinion recently offered about
bridging amplifiers on several grounds:
	1)   An amplifier that is conditionally stable, and
ampzilla's certainly qualify, (I know of some that are
not even conditionally stable, grrr)  will have much
more trouble in a bridged configuration.
	2)  Using a single op-amp invertor is reported to
be "far superior".  This opinion is quite wrong, as the
distortion introduced by a reasonably designed voltage follower
is on the order of the distortion introduced by a
voltage invertor, at MAXIMUM, if a good op-amp (and the
Signetics NE5532AFE certainly qualifies) is used.  Using the
inversion in only one channel risks having phase problems
at the output related to the phase shift (mostly transit delay,
if the circuit is designed right) of the op-amp circuit.
While this delay is small, the results, especially when combined with 
1) above, can lead to surprising results, up to and including 
oscillation that would not occur with a different delay/phase shift.

The problem in 1) comes about due to the fact that an amplifier's
output impedence has several poles and zeros.  These poles and zeros
are also (not too suprisingly :->) the system poles of the amplifier.
Combining the two sets of poles additively through another
complex impedence (the speaker) can lead to various kinds of problems.
In some amplifiers that are quite stable, the problem will be that
the phase margin is reduced, leading to undamped ringing at
a VERY high frequency.  In a few, very well designed amps, the
output impedence is nearly enough resistive (at least at
frequencies less than a MHz) that combinations do not matter.


good luck
rabbit!jj (through allegra, harpo, or research)