Impedance Equalization Circuit (Zobel)

An Impedance Equalization Circuit is used to counteract the rising impedance of a voice coil caused by inductive reactance. The cause of this impedance rise is due to the speaker's voice coil inductance (Le).

For a midwoofer you calculate a Lowpass Butterworth 2nd order filter with a cutoff frequency at 4.5 kHz with the nominal impedance Re (from datasheet) of 6.0 ohm

The calculated inductor is 0.21 mH for 4.5 kHz cutoff frequency.

Impedancecurve Fullrange driver without Zobel compensation.

Take a look at the impedance plot of this fullrange speaker. The impedance at 4.5 kHz is not 6.0 ohm, but 9.3 ohm!

At this impedance you inductor has to be 0.33 mH and not 0.21 mH
This wrong inductor of 0.21 mH would create a crossover frequency of 2.9 kHz and not as assumed a crossover point of 4.5 kHz

The importance to use impedance compensation should be clear now...
Re = Ohms Rz = Ohms
Le = mH Cz = uf (film type capacitor preferred)
  • Re is the DC Resistance of the driver [ohms]
  • Le is the Voice coil Inductance of the driver [mH]
  • Rz is the calculated Zobel resistor [ohms] and
  • Cz is the calculated Zobel capacitor [uF].

Adding a Zobel to a woofer will allow the passive crossover to work more effectively.
The impedance of the woofer will also be restored to the driver’s nominal impedance throughout the high frequency range. This technique allows the use of crossovers designed for resistive loads to operate as intended.

Impedancecurve Fullrange driver with Zobel compensation.

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