From: DPierce@world.std.com (Richard d Pierce)
Subject: Re: Bi-Amp vs. Bridged Bi-Wire
Organization: The World Public Access UNIX, Brookline, MA
Date: Sat, 5 Sep 1992 11:45:15 GMT

SOmebody asked...
>>>>What is a "Zobel" or response compensation network?  I'd guess its
>>>>an E.Q. type device to compensate for speaker non-linearities??

Chris Christensen answers...
>>>A Zobel is a simple network that helps to compensate for the imprdance
>>>bump of a driver at resonance.  It's selection is somewhat iterative
>>>based roughly on the frequency of the bump and it's Q.

Alex Mitaru chimes in...

>>  I'm sorry Chris, but that is wrong. A Zobel is a RC network to compensate
>> for the increase in impedance at higher frequencies. A uncompensated woofer
>> can present an impedance of 20 to 40 ohms to the low pass filter at 2-3 KHz;
>> hardly what you have computed your filter. Using a RC network in paralel with
>> the woofer you can maintain a stable impedance and therefore your low pass
>> will behave as expected.

And the Chris retorts...

>I have seen L's used in conjunction with an RC to control the Zobels
>characteristics.  This is a nit to pic but Zobels are more than a
>simple RC,  that's why I refer to them as networks.  It also serves to
>make me sound smarter than I am  :-)
>
>As I have viewed speaker system design I have seen  designers use a
>Zobel for LF impedance compensation and cross over tha driver before
>the rising response becomes an important term in the equation.
>

Yes it is! No it isn't! Yes it is! NO IT ISN'T! YES IT IS, DAMMIT! MOMM!!! :-)

Please, please....

Sorry Alex, it is you that is wrong, sort of...

Strictly speaking, a "Zobel" is any sort of conjugate network whose intent
is to linearize the impedance of a network. It's original intent (as outlined
in Mr. Zobel's original patent application back in the '20's (Otto J. Zobel,
"Complementary Filter," U. S. Patent #1,557,230, filed 30 Apr 1920, granted
13 Oct 1925, assigned to AT&T) was to provide a method of linearizing the
impedance of long telephone transmission lines as a method of reducing the
extreme frequency dispersion that was seriously reducing speech intelligibility
(this was because of the fact that the impedance effects of the line were
causing low frequancy information and high frequency information to propogate
at slightly different velocities, over a long enough line, they would arrive
sufficiently far apart to scramble the audio).

There is no rule that says a Zobel network (I much prefer the term "conjugate")
can only be used for linearizing the high frequency impedance rise due to the
voice coil inductance. Chris is correct in saying that they can also be
effective in controlling the impedance bump at resonance as well.

Systems such as the KEF 104/2 use extensive conhugate networks to correct for
all impedance variations in the drivers: the result is that the passive
crossover sees damn near pure resistive loads at all frequencies, and the
amp ends up looking at a near pure 4 Ohm resistor as a result. I think this
approach is a little extreme. My own preference has been to use conjugate
compensation where it was necessary to tame the impedance for the crossover.

The standard formula for calculating the conjugate capacitor to compensate for
the voice coil inductance is bogus because it assumes that the impedance
effect if due to a simple series inductance. That assumption is seriously
flawed. The phenomenon is skewed because in addition to a simple inductor, we
have an inductor which is capable of doing work on a frequency-dependent basis,
thus the inductor is a little less than "pure". Further, it also behaves like
a poorly coupled transformer, whose secondary is a frequency-dependent
partially shorted somewhat lossy turn due to the generation of eddy currents
in the metal parts surrounding the voice coil. It's real complicated, and if
you use the standard formula, you're almost sure to be wrong.

-- 
|                Dick Pierce                |
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