Someone here in the echo from Europe somewhere was asking about Zobel
networks a few weeks ago.  Well, below an excerpt from a message I
found in a local BBS rec.audio Usenet conference. (I did not write
this, but reformatted its line length and spacing;-)
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(cut)
One of the things on the list that had yet to be done was to install a
zobel network on the spkr terminals.  A particular value of cap was on
order as were some resistors; the resistors came in but the cap was
still a long way off.  Anyway, found some Wima caps in the junk box and
decided to build the networks with them as they were pretty close to
the needed value.  A couple of banana plugs were used to make assembly
and installation easy and it worked out rather well.  One of the most
interesting things about this was the result of some simulations on the
overall network.  Using an equivalent model of the output Z of the
Krell (in closed loop) with the RLC parameters of the cable and the
tweeter's RL impedance as a load indicated a very high Q resonance
around 700KHz.  This wouldn't seem to matter much but the Krell has a
relatively high open loop bandwidth and thus it looked possible that
near oscillation could occur, or at the very least extensive ringing,
without the added shunt impedance of the zobel due to the increased
open loop gain at this resonant frequency.  .  .  not a good thing. 
Well, it got installed Friday night and the results were instantly
obvious, even from another room.  Apparently the simulation results
were close to correct and whatever the amp was doing, it stopped doing
it right away.  The upper octaves are now much smoother than they had
been, sort of like they were before the mods but without that haze and
glare that had been apparently obscuring the problem.  Now, the
tweet/mid resonance is more clearly audible but not nearly as bad as I
thought it would be given its relatively high Q.  'Course, the
replacement of the mids (in house already) and the mid/tweet xover is
next on the list, after the completion of the UJB mods, as the custom
air core inductors shipped yesterday.  One of these days, it just might
be finished.  .  .  well, maybe not.  .  .  Anyway, if you have any
doubts about whether something might help tame a particularly nasty
interaction between amp, cable, and spkr, get the data sheet on the
tweeter and make up a zobel with a resistance equal to the voice coil
resistance and a capacitor equal in value to the voice coil inductance
(add 10%) divided by the square of the resistance.  the R and C should
be placed in series and the network placed across the terminals of the
spkr.
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The person asking about these originally did so about amps, but, it
seems they might be even more useful -- customised for the intended
load!
 
I've the author's e-mail address.  Mine is .
Thanks to -Zeitgeist BBS- (Free 24 hours at (927) 403-9406).   
...
 * ATP/qwk 1.42 * TV advertising is the rattling inside a swill bucket.
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 * KMail 3.10m  1996:  The year of the BBS!
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