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Mix driver impedance with xover impedance

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  • Mix driver impedance with xover impedance

    What happens when you mix driver impedance with crossover impedance, say a 8 ohm driver with a 4 ohm crossover..
    I have read that if you use a 4 ohm driver with a 8 ohm xover that it can either double or halve the xover freq, depending on type of driver used.
    But what I want to know is about an 8 ohm driver with a 4 ohm xover..
    Also how does it effect the over-all impedance that the amplifier sees..?

  • #2
    Not sure how to answer that question other than, it's not that simple. Maybe download some free xover modeling software like WinPCD, xSim, or SoundEasy and play with that.
    http://techtalk.parts-express.com/fo...khanspires-but
    http://techtalk.parts-express.com/fo...pico-neo-build
    http://techtalk.parts-express.com/fo...ensation-build

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    • #3
      Speaking in textbook formulae....
      It doesn't work like you think. The xover meant or designed 'ideally' for a 4 ohm resistive driver or set of drivers is optimized for a certain Q. The resonance of the xover components that results in the impedance change and resulting rolloff in frequency, and is terminated by the driver impedance. The driver impedance is the damping for the resonance, and results in the target Q for the best result.

      If you use an 4 ohm driver on a 4 ohm targeted xover, the Q is (for example) 0.5 for the Linkwitz Riley xover with the intended load. If you swap in an 8 ohm load, the Q will double and result in a peak before rolloff, or yield an underdamped rolloff. If you swap a 4 ohm for an intended 8 ohm network, the Q will halve.

      Many will incorrectly state that your xover point will double or halve with the change of a driver impedance when using 12dB or higher slopes. This is not true, and only should be applied for 6dB or first order network applications in idea textbook criteria. In fact, the Q will change as previously described, and the responses will slope as expected. While this may change the response of the drivers, the inherent Fc of the xover has not changed, only the Q has shifted.

      Now- speaking in the real-world applications of the hobby- you won't really even have to worry about this. The measurements will dictate what you do with the design, and nothing is as ideal as described above since drivers are not ideal things.

      Hope that answers your question,
      Wolf
      "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
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      • #4
        Okay thank you Wolf for that fine forensic analysis but according to Eminences website which is where I got this info, "If you use a 4 ohm high frequency driver with an 8 ohm crossover, the crossover point will double. If you use a 4 ohm woofer with an 8 ohm crossover, the crossover point will halve. The result of using 4 ohm components with a 2-way 8 ohm crossover will leave a hole in the response"
        https://www.eminence.com/faqconc/can...ur-crossovers/.


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        • #5
          Originally posted by RedK9 View Post
          Okay thank you Wolf for that fine forensic analysis but according to Eminences website which is where I got this info, "If you use a 4 ohm high frequency driver with an 8 ohm crossover, the crossover point will double. If you use a 4 ohm woofer with an 8 ohm crossover, the crossover point will halve.
          That's true of first order filters, but not with second order or higher. You wouldn't think Eminence would get it wrong, but in this case they did.

          www.billfitzmaurice.com
          www.billfitzmaurice.info/forum

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          • #6
            And I'll add that 1st order (electrical) filters nearly never work due to:
            On tweeters - the impedance peaks at resonance (Fs - like 1500Hz) to 2X - 4X the driver's "nominal" impedance (although typically tweeters that use "ferrofluid" in the gap can have quite a low peak), which messes up the expected rolloff curve.
            On woofers - the impedance "rises" typically above about 300Hz to sometimes MANY TIMES the driver's nominal impedance in the top registers. Drivers with a lower "Le" value will have less of a rise. (You CAN add a simple RC network across the voice coil (sometimes called a "Zobel") to flatten this rise - although some on here don't seem to like them - sc.) This effectively keeps a "textbook" derived coil from rolling off the top end effectively. Peerless actually used to give specs for a recommended Zobel right on their datasheets.

            A good book to help learn all this stuff is "SpeakerBuilding 201" by Ray Alden. (sc)
            Last edited by Chris Roemer; 05-29-2020, 08:52 AM.

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