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Impedance Phase Question

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  • Impedance Phase Question

    Hey Guys

    I'm working on my first design - a tiny, low budget MTM. It's too early in development to have a thread yet, but I've been playing with PCD and I've got a question.

    In a recent design thread I think I remember reading that it's recommended your system impedance phase never go outside about +/-30deg limits. Am I remembering correctly? Is that a reasonable rule of thumb?

    I've currently got a dip down to about -60degs near woofer resonance. Magnitude is about 10Ohms at this point. Lowest magnitude is ~3.88Ohms in a span near the xover point, with phase very near 0degs.

    The phase is an amplifier concern, right? I'm not going to be running a tube amp or anything.

    I haven't posted the curves because they are essentially useless - I did some quick free-air FR measurements so I could start playing in PCD... I have no idea what the in-box curves will look like and the entire design is likely to change. But in principle, I'd like to know how critical the phase swing is.

    Thanks a lot,

  • #2
    Re: Impedance Phase Question

    what crossover are you using and at what freq ?

    " Voicing is often the term used for band aids to cover for initial design/planning errors " - Pallas


    • #3
      Re: Impedance Phase Question

      The 30 degree limit is excessively tight. I would never give that advice. You should be concerned if you are at 60 degrees and your impedance is low. For a bass peak high phase angle is usually coupled with high impedance, and that will be fine to drive. None of the numbers you gave concern me.
      Click here for Jeff Bagby's Loudspeaker Design Software


      • #4
        Re: Impedance Phase Question


        Both high and low pass are second order electrical. I'm at work at the moment and I can't remember the frequencies... I want to say they are both in the 2.5K area.

        There was a bump in the measured woofer response near the XO frequency. For budget reasons I don't want to add a notch filter. I fiddled with the crossover components A LOT until eventually the summed response was pretty flat (not sure about the phase). Then I noticed the impedance phase was outside of the +/-30 envelope I had in the back of my head and I started wondering how much I have to worry about that.

        As I mentioned - the details of the project are bound to change once I get in-box measurements... just looking for some guidance on the impedance phase issue.

        Thanks Jeff. Perhaps I read +/-60 was the envelope to shoot for and I'm remembering wrong. Thanks for the feedback


        • #5
          Re: Impedance Phase Question

          Originally posted by lowpolyjoe View Post
          Then I noticed the impedance phase was outside of the +/-30 envelope I had
          I'm probably where that came from, but I got it from Curt C as his rule of thumb back when I was starting out. I agree that +/-15 is really tight, but I rarely have an issue meeting the +/-40 spot. If I hit -60 (neg is capacitive) I tweak to improve that. There've only been 1 or 2 builds I've simulated that I could not get away from -60 deg, but it was not more neg than that, and it wasn't drastically low in impedance magnitude in that range.

          Usually the woofer's Fs resonance and the corresponding peak/dip phase is nothing to worry about. It's the xover range that should be watched more carefully. Usually placing the series padding resistor for the tweeter before the xover network resolves the typical issues. You could try that....

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          • #6
            Re: Impedance Phase Question

            Thanks Wolf - I will keep your comments in mind as I move forward.

            I'll have a design/build thread posted eventually with curves and measurements, etc... Then any issues I have will probably be easier to diagnose. For now I was just curious in a general sense.



            • #7
              Re: Impedance Phase Question

              I like to use guidelines that are somewhat concrete, albeit a rough estimate :rolleyes:. I use the formula Impedance * COS(angle*PI()/180) and then if the resulting Impedance is too low for the amplifier I plan to use (for example, lower than 3 or 4 ohms or so), I re-work the crossover.
              Our greatest glory lies not in never falling, but in rising each time we fall.


              • #8
                Re: Impedance Phase Question

                I hadn't really thought of it that way, jmb, but maybe that's the point.... how big is the resistive component of the impedance.

                Cosine of 60degs is 0.5, so an envelope of +/-60deg phase limits results in resistive part of the impedance being half the magnitude. About 10*0.5 = 5 Ohms in my particular case, which meets your conditions.