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Can someone explain this old Polk speaker crossover I came across

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  • Can someone explain this old Polk speaker crossover I came across

    Just asking for educational purposes, but part makes not a lot of sense. L4 and C2 ?? If one removes those 2 components it makes TOTAL sense.


    Click image for larger version  Name:	ezgif.com-gif-maker.webp Views:	0 Size:	26.3 KB ID:	1490278

  • #2
    I used Xsim to model. So weird. MTM or TMM. Woofers are damped second order crossing around 2K. L2 and C-4 notch the second woofer. L-4 and C-2 add matching notches in the Mid and Tweeter responses. Kind of cool but I can't see the value in the small notches.
    John H

    Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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    • #3
      There's more to that schematic off to the right, the Polk RTA 11

      Click image for larger version

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      • #4
        I thought the RTA line sounded good back in the day

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        • #5
          It looks like they are pulling a narrow frequency signal from the tweeter and feeding it into the woofers instead of shunting it to ground. Possibly to keep the impedance from dropping too low. Or maybe it just sounds better this way.
          Last edited by Billet; 09-07-2022, 11:43 AM.

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          • #6
            It's literally amazing how wonky you can wire up a crossover sometimes and still have it work to some degree. I'm really bad at reading schematics when they get too complicated and I've had my alligator clip wires crossed all kinds of weird ways to where I was sure it should have tripped my amp's protect circuit, but didn't. Amazingly, some of those mistakes didn't sound bad at all.

            The guys at Polk were pretty sharp back in the day.

            My friend has a pair of 25-year-old Polk TMM mini towers that STILL sound excellent to my ears.

            TomZ
            Zarbo Audio Projects Youtube Channel: https://www.youtube.com/channel/UCEZ...aFQSTl6NdOwgxQ * 320-641 Amp Review Youtube: https://youtu.be/ugjfcI5p6m0 *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
            *Part 2 *Gluing multiple curved laminations of HDF

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            • #7
              I can NOT see that .webp (or whatnot).

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              • #8
                Originally posted by Chris Roemer View Post
                I can NOT see that .webp (or whatnot).
                better!
                Click image for larger version

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                • #9
                  Maybe they are filtering a spike from the woofer and feeding it to the tweeter instead of shunting it to ground.

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                  • #10
                    To expand on John's observation ...

                    The notch freq. in the woofers' filter is fairly narrow (in the 500-600Hz range), and NOT load dependent.
                    The "mystery notch" applies to all 3 drivers around 3.3kHz and is very narrow (interesting topology, though).

                    The depth of the notches IS load dependent.
                    The summed output (of all 3 drivers) is down about -5dB (at a nominal 8ohms), or -3dB @ 4ohms.
                    A nom. 8ohm tweeter is notched about -4dB, and -3 @ 4n(ohms).
                    The midbass pair is notched about -7dB (@ 8n nom.), but only -3dB @ 4n.

                    Doubtful that the the same 3.3k anomaly exits in both woofers AND tweeter.
                    We'd probably just notch whichever one was guilty - if necessary.
                    Given the size of that "extra" coil (nearly 5mH), I'd definitely try running w/out that leg in the filter.

                    (thanks for the pic, kev)

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                    • jhollander
                      jhollander commented
                      Editing a comment
                      Adding on, my thought was that there was some baffle or grill issue that would drive the mystery notch. Still it's a lot parts/ cost for something so small
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