About Zobels
This simple circuit consists of a single resistor and
capacitor. It is commonly referred to as a Zobel network,
named for the Bell Laboratories engineer whom invented
it. An excellent starting point can be calculated by using
this formula.
R = 1.25 x Re
C = Le / R squared
R is in ohms
C is in farads
The 6-1/2" woofer parameters were measured using the
LMS Loudspeaker Measurement System (a PC based
package for measuring loudspeaker systems), and the Le
was found to be 1.87 millihenries. Using an accurate ohm-
meter, the Re was 6.5 ohms. Using the above formula:
R = 1.25 x 6.5
R = 8.125 ohms
so
C = .00187 / 66
C = .0000283 farads or 28.3 uF
Although these values will give a good result, actual in box impedance measurements indicated an even better com-
pensation can be had with a 47uF capacitor and a 8 ohm resistor.
As can been seen on the graph above, the impedance is now very flat through the crossover region. From 250 Hz to
10 KHz, the impedance is 7 ohms +/- 0.5 ohm.
Page 4
6.5" Impedance w/ Zobel Network
Re is measured voice coil DC resistance in ohms.
Le is measured voice coil inductance in henries