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  1. #1
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    Default Zobel calculation for woofers in series

    I have 2 woofers wired in series and would like to know if the following calculation for a zobel circuit is correct.

    2x woofers in series.
    Revc - 3.4679 ohms x 2 = 6.9385
    Le - 0.3484 mH (at 1 kHz.) = .6968

    Zobel

    C1= 14473.61302015319 F
    R1= 8.67312 Ohms

  2. #2
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    Default Re: Zobel calculation for woofers in series

    Uh, that cap value is a weee bit high. Try 14uF.

  3. #3
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    Default Re: Zobel calculation for woofers in series

    My calculation is more like 9.3 uF for the cap.

  4. #4
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    Default Re: Zobel calculation for woofers in series

    I used the following calculator for the zobel. Is the reason my calc came out different due to the calculator or does it have to do with the configuration fo the drivers?

  5. #5
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    Default Re: Zobel calculation for woofers in series

    Quote Originally Posted by Hackomatic View Post
    My calculation is more like 9.3 uF for the cap.
    I didn't actually look too close. I just saw the size of that cap and figured it to be a rounding error. But looking at the Le of those drivers, ya, the 9.3uF is probably more correct.

  6. #6
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    Default Re: Zobel calculation for woofers in series

    Quote Originally Posted by brianpowers27 View Post
    I used the following calculator for the zobel. Is the reason my calc came out different due to the calculator or does it have to do with the configuration fo the drivers?

    Not sure what you used . . I used this one . . .

    http://diyaudioprojects.com/Technical/Speaker-Zobel/

  7. #7

    Default Re: Zobel calculation for woofers in series

    I calculate 18.6uf/4.34 Ohms for a single driver. For two in series, reduce C by half and double R.

  8. #8
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    Default Re: Zobel calculation for woofers in series

    I just realized what I missed. In order to get the 9.2 I needed to multiply the re * 1.25.

    Question: Can individual resistors be added together in series to get the desired resistance? EG: 7.5 + 1.2 = 8.7

    Is the same true for capacitors?

  9. #9
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    Default Re: Zobel calculation for woofers in series

    Resistors placed in series add, but caps should be done in parallel.

  10. #10
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    Default Re: Zobel calculation for woofers in series

    Quote Originally Posted by brianpowers27 View Post
    I just realized what I missed. In order to get the 9.2 I needed to multiply the re * 1.25.

    Question: Can individual resistors be added together in series to get the desired resistance? EG: 7.5 + 1.2 = 8.7

    Is the same true for capacitors?

    A better way to arrive at Zobel values, is to use the woofers' actual impedance profiles in ANY kind of XO simulator. You can vary the resistor and cap values and "see" how and where the rising impedance gets flattened (even though Zobel component values are not "critical", i.e. - you can vary their values by a large margin and still have a considerable effect on the otherwise continually rising impedance profile).

    What drivers are you using?

  11. #11
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    Default Re: Zobel calculation for woofers in series

    I have 2 polk audio woofers that my friend donated to me. I am working with Jeff Bagby's passive crossover designer for the first time and am not entirely sure what I am doing.

    I ordered some passive crossover components to design a 3k 4th order LR filter based upon an impedance of 8.35(per the zobel). Does this seem correct?

  12. #12
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    Default Re: Zobel calculation for woofers in series

    Whoah! I just modeled the crossover in the PCD. The zobel values it gives for 2 woofers wired in series are dramatically different. I used the same values. re 3.45 le .345. WHich one is right?

  13. #13
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    Default Re: Zobel calculation for woofers in series

    Quote Originally Posted by brianpowers27 View Post
    Whoah! I just modeled the crossover in the PCD. The zobel values it gives for 2 woofers wired in series are dramatically different. I used the same values. re 3.45 le .345. WHich one is right?
    For two 4 ohm woofers, wired in series (yielding 8 ohms), your Zobel should most likely use an 8 or 9 ohm resistor (no big deal), and a cap that's in the 10uF to 14uF range. As I said before, the BEST way to determine these values is to run an actual impedance sweep on your woofers and determine the Zobel component values using some crossover simulator.

    Chris

  14. #14
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    Default Re: Zobel calculation for woofers in series

    Thank you for your answer. I do not have any impedance testing equipment. I was surprised when I used the PCD and it calculated zobel values of 1.87uf and 20ohms. I expected the calculation to produce a slight variance but that is way out of the ballpark.

  15. #15
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    Default Re: Zobel calculation for woofers in series

    If you don't have an impedance sweep (and resulting .zma file), I would not be surprised if the PCD got a little wonky.

  16. #16
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    Default Re: Zobel calculation for woofers in series

    Quote Originally Posted by brianpowers27 View Post
    Thank you for your answer. I do not have any impedance testing equipment. I was surprised when I used the PCD and it calculated zobel values of 1.87uf and 20ohms. I expected the calculation to produce a slight variance but that is way out of the ballpark.
    Let's see if we can make some sense out of this. When you say that PCD calculated these values what had been entered into the program for it to work with?

    PCD is pretty powerful for a spreadsheet and contains a lot of features that I think many people are unaware that it has. For example it has several other worksheets filled with textbook calculators, including more than one for Zobels. These are accessed via the buttons near the top and to the right of the charts on the main page. I am assuming that you did not use these textbook calculators, but if so, let me know.

    If you instead used the button on the main page that auto-calculates the Zobel values, then you need to understand that it works by analyzing the actual impedance curve of the imported zma file. If you don't have impedance data for the driver, then what did you use?

    This feature doesn't use the textbook formula for a Zobel either, because I found that the textbook formula doesn't result in a very flat impedance curve - and it shouldn't either. That's one of my gripes with this formula. First, voice coil inductance is not a pure inductance, as it only approaches 60 degrees of phase rather than the 90 degrees you would have with an inductor. Second, it is constantly changing with frequency. A typical driver that has a rated Le value of .65 mH at 1kHz may have an inductance of 6 mH at 200Hz and .15 mH at 10kHz. Because of this, I developed my own proprietary formula based on curve fitting analysis that results in a much flatter impedance.....if there is an actual zma file imported.

    Let me know what you have, and I will try to help.

    Jeff B.

  17. #17
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    Default Re: Zobel calculation for woofers in series

    Quote Originally Posted by Jeff B. View Post
    Because of this, I developed my own proprietary formula based on curve fitting analysis that results in a much flatter impedance......
    I always wondered (well assumed) that you were using something a little different. Nice work.

    Aside from Jeff's PCD "auto-zobel", I have had the best luck with this formula:

    http://www.the12volt.com/caraudio/crosscalc3.asp


    Again, it requires the actual impendence curve. For nominal, I enter the lowest impendence, then the freq. at which it doubles.
    Thanks,
    Zach Tripp
    http://home.comcast.net/~zach_beth/diy.html
    FOLLOWMY529.COM - Subscribe to get monthly updates on performance.

  18. #18
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    Default Re: Zobel calculation for woofers in series

    If you think you are serious about this, you can't go wrong with the Woofer Tester 3 (free shipping right now). You get the actual impendence and the actual T/S parameters (for box modeling).

    http://www.partsexpress.com/pe/showd...number=390-804
    Thanks,
    Zach Tripp
    http://home.comcast.net/~zach_beth/diy.html
    FOLLOWMY529.COM - Subscribe to get monthly updates on performance.

  19. #19
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    Default Re: Zobel calculation for woofers in series

    Quote Originally Posted by Hackomatic View Post
    My calculation is more like 9.3 uF for the cap.
    Based on his values, that's what I get too with the textbook formulas. That's also what PCD delivers in it's Zobel Calculator.

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