From: henry@ginger.sri.com (Henry A. Pasternack)
Newsgroups: rec.audio
Subject: Amplifier output impedance and its effect on sound.
Date: 15 Apr 92 07:05:43 GMT
Reply-To: henry@ginger.sri.com (Henry A. Pasternack)
Organization: SRI International


   Sneering, cynical, anti-audiophile types have displayed their
ignorance by questioning the value of tube amplifiers, noting that
these units typically have relatively low damping factors (high output
impedance).  In truth, the system Q of a loudspeaker is dependendent
both on its construction and the series impedance through which it is
driven.  It is perfectly possible to optimize a loudspeaker design to
deliver ideal performance when driven by an amplifier having quite
high output impedance.  The only compromise is a loss in system
efficiency.  This fact runs counter to the widespread and wholly
erroneous audiophile notion that an amplifier "controls" woofer cone
motion through some sort of servo feedback effect interacting with
voice coil back EMF.  A previous poster has noted that the typical DC
resistance of the voice coil of an eight Ohm loudspeaker driver is on
the order of five Ohms.  Thus, a 0.75 Ohm output impedance, while
significant enough to have an audible effect, is not necessarily a
recipe for sonic disaster.

   It is true that popular solid-state amplifiers have very low output
impedances.  I believe that the sonic value attributed to this
property (very solid, powerful bass) is related only indirectly, is
not a direct consequence, of the low output impedance.  A frequently-
cited cause of "transistor sound" is the potentially harsh clipping
behavior of solid-state devices.  To minimize or eliminate entirely
the possibility of clipping under most conditions of signal and load,
high-end designers have taken to designing amplifiers that incorporate
extremely robust output stages consisting of very many devices wired
in parallel.  This topology allows the amplifier to source very high
output currents, on the order of fifty Amperes or more, so that
clipping is quite unlikely to occur.  In one design (Threshold stasis),
the paralleling of many devices is a way of reducing static distortions
in the output stage.  The very low output impedance is a natural 
consequence of this type of circuit topology, a means to an end, but
not necessarily an end in itself (except to satisfy the demands of
fashion).

   Contrary to popular audiophile culture, extremely high output
current capability may not, in itself, be a necessary condition for
high-quality sound.  Instead, it is a by-product of an expensive
band-aid solution to the problem of harsh solid-state clipping.
Fashion being what it is, a well-designed amplifier that deals with
the clipping problem such that a huge power supply is unnecessary is
unlikely to come into audiophile favor.  This is a sad thing, for
it is the power supply, the chassis, and the heat sinks that make
big amplifiers expensive to produce.

   Tube amplifiers, it is claimed, exhibit clipping behavior that is
much less objectionable, sonically.  Although they usually lack the
huge current capabilities of their solid-state counterparts, tube amps
can play loud in the bass without producing obnoxious high-frequency
distortion under conditions of modest clipping.  Given that
solid-state monster amplifiers have dominated the high-end scene for
some time (although tube amps are reasserting their position), speaker
designers no doubt optimize their products to achieve proper bass with
low-impedance drive.  A speaker designed for solid-state (low
impedance) amplification may sound "warm", "loose", or "muddy" when
driven by a solid-state amplifier.  On the other hand, a speaker
designed for higher-impedance drive may sound unsatisfyingly "dry" or
"tight" when driven by a low-impedance source.  There is no voodoo to
these descriptions; they are simply qualitiative ways of describing
variations in loudspeaker system Q.  The art of matching components
consists, in part, of picking amplifiers and speakers that synergize
well in the frequency range near woofer resonance.

   Perhaps a good solution would be for designers to standardize on
a fixed source impedance, say, one Ohm.  Systems could be supplied with
a selection of series resistors to be chosen by the user to achieve
the most satisfying sound.  There are some technical reasons relating
to the compromises inherent in bass enclosure design why such a solution
may not be desirable.  Perhaps the most compelling reason that such
a standard would never stick is that audiophiles could never give up
their belief in the necessity for welding-cable gauge speaker cable.

   I would like to remind readers that my position is to straddle the
middle ground between objectivity and subjectivity.  The ear is the
final arbiter of good sound.  From time to time, I have been quite
surprised by sonic effects that are not easily explained by conventional
wisdon.  More often than not, the smug engineers and mathematicians who 
dismiss these subjective findings with their explanations and their
equations are utterly full of themselves, possessing sufficient knowledge
to form an opinion, but lacking the expertise to appreciate *facts*.  
The same is true of the subjectivists, who twist their simplified view 
of audio engineering to support erroneous theories that have absolutely 
no grounding in *facts*.  Quite often, a thoughtful, balanced application 
of basic principles will reveal a plausible explanation for surprising
audible observations.  Thus, I strive to be both a "golden ear" and an
engineer.

-Henry


Xref: sparky rec.audio:7530 rec.video:6303