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Zobel Loudspeaker Impedance Correction Circuit

 This page will help you understand what a Zobel filter does, and how it can be useful in a loudspeaker crossover network design. If you need more help, click here.
 A "Zobel" circuit can correct the rising impedance load presented to the amplifier. This allows a speaker to reproduce high frequencies better because more power is available for the driver due to the fact that the impedance is relatively constant. It also helps to keep the crossover point from shifting due to impedance variations.
 The cause of this impedance rise is the inductance of the voice-coil of the speaker. A voice-coil is a coil of wire wrapped around the former (which is connected to the back of the cone.) It looks like an inductor wound with small gauge wire. The reason inductors are used in crossover design is because the impedance of an inductor increases as the frequency of the signal applied to it goes up. This reduces the power applied to the speaker at higher frequencies. This is how an inductor "rolls-off" the sound.
 The line with an impedance of approximately 37.5 ohms around 2000 Hz is this driver resonant frequency (Fs). The plot with the greatest impedance centered at approximately 1350 Hz is the impedance of the same speaker with a Zobel filter. Notice on this driver the Zobel filter also lowered the impedance at resonance. Since a flat impedance curve is very desirable, I altered the component values so this one filter is could do the work of two (the lowest line). This reduced the component count of the impedance correction portion of this crossover network from five to two!
 The other (unnecessary in this case) filter is a resonance equalization circuit. This speaker is a good example of how useful a crossover design program can be. The crossover design equations are all useful to get in the "ballpark", but if you are going to design advanced crossover networks, you need a good program to see what is really happening in your crossover. Personally I use CALSOD from Audiosoft, L.E.A.P. is generally regarded as the best loudspeaker design program available, but I can't afford it yet. Maybe someday :).
Explanation of Terms
Le is the inductance of your driver's voice-coil (in henries).
Re is the D.C. resistance of the voice-coil (ohms). The design equations for this type of filter are:
Capacitor = Le/Rc2
Resistor (Rc) = 1.25 X Re
Zobel Impedance Correction circuit
Zobel Filter Diagram
The Formulas and the Calculator
 To use this calculator, enter the D.C. resistance and the inductance of your driver, click the "Calculate Values" button and the required component values will appear in the remaining text boxes.
Enter D.C. Resistance Ohms
Enter Voice-Coil Inductance  Millihenries
Capacitor Microfarads
Resistor = Ohms

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