The Zobel Circuit Uncovered

[Michael Rhodes]

(Forget the T-line. It's a distraction from the Zobel circuit, part
of the BIG conspiracy.)

This discussion refers to any 2- or 3-way speaker which uses a passive
crossover. That includes most in general use. A simple circuit
should be added to the driver(s) in the speaker. It will improve the
response of any driver which uses a low-pass filter, an inductor.
Those include the woofer, and the midrange if it has one. The tweeter
does not require it.

The problem is that crossovers are designed for speakers which are
generally 4-8 ohms, and constant at that rating, as if they were
resistors. But speakers use a coil over a magnet to drive the cone.
Coils are inductors, and their impedance will increase with frequency.
That effects the crossover's response. The higher impedance of the
driver coil will force more highs to the driver from the crossover
coil, muddying the sound. It isn't simple to understand why that is,
but the simple Zobel circuit will clean it up.

The suggested circuit is an series RC circuit placed electrically
between the driver's contacts, but preferably mounted to the crossover
to protect it from vibration. One side is connected to the hot wire
going to the speaker, the other to ground. It does not matter whether
the resistor or capacitor lead goes to the hot wire.

The circuit is an impedance compensation network, sometimes called a
Zobel circuit. Unfortunately, it is not often included in speakers,
though it isn't difficult to install, nor is it expensive; and
speakers REALLY need it.

If you decide to take on the project, first check to see that it is
missing from yours before adding another. It probably isn't there.
The reasoning for it's deletion is simple, if not illegal and immoral.
Audio companies prefer that you continue your never-ending search for
the 'perfect' system by perpetually selling you a flawed product.

There is a procedure for determining the values of the resistor and
capacitor. The standard values are Rz = 1.25 * driver dc resistance,
and Cz = driver inductance/(Rz^2). Another source I've found suggests
that Rz should equal driver resistance. BUT ANYTHING HELPS!

---->
Lacking information about driver characteristics should not prevent
you from installing something. I would suggest...

Rz=8 ohms (or 4 for 4 ohm speakers.)
Cz=30-40 uf, at least, for the woofer.
Cz=10 uf for the midrange.
---->

The ratings should be reasonably large for the amplifier being used,
at least 10 watt resistors, and 50 volt caps. I would put two
16-ohm/10 watt resistors in parallel for woofers using large amps, and
100-volt caps. Bipolar capacitors will work. I use them. But the
experts suggest polyester, polypropylene or oil-filled paper. The
common polarized capacitors cannot be used with speakers.

One source: http://sound.westhost.com/lr-passive.htm
--
Michael