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Help with crossover calculations: GRS 8FR-8 "BOFU" + Goldwood GT-510

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  • Help with crossover calculations: GRS 8FR-8 "BOFU" + Goldwood GT-510

    Hi,

    I'm looking for help with the design of a 1st order 2-way crossover for a non-critical application (large suitcase boombox)

    Fullrange driver: GRS 8FR-8 (Pioneer "BOFU" clone)
    Tweeter: Goldwood GT-510
    Crossover freq: 6000 Hz

    I found an online crossover calculator and came up with the following:
    - 3.3uF cap for tweeter HPF
    - 0.2 mH coil for the LPF

    Would that work? Also do I need to reverse the tweeter polarity?

    Thanks in advance!

  • #2
    Re: Help with crossover calculations: GRS 8FR-8 "BOFU" + Goldwood GT-510

    Won't you have to attenuate the tweeter (doesn't it play "louder" than the full-range)?
    A "1st order" coil won't roll off the full-range due to its rising impedance curve (unless you add a "Zobel" to it).

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    • #3
      Re: Help with crossover calculations: GRS 8FR-8 "BOFU" + Goldwood GT-510

      I think you're right, Chris. Thanks for pointing that out. I can't find the FR curve for the GRS BOFU clone, but my guess is that tweeter is about 4-5dB more sensitive at 1W/meter.

      I really don't know what i'm doing with these calculators - a little knowledge is a dangerous thing in my case. Could I add an L-pad using the following values to attenuate the tweeter by 5Db?

      * R1 (series): 3.5 Ohm
      * R2 (parallel): 10.3 Ohm

      Would that L-pad come before or after the crossover?

      I've seen people use the BOFU with no crossover (just an HPF on the tweeter), essentially running the driver full range. Thoughts on that approach?

      Comment


      • #4
        Re: Help with crossover calculations: GRS 8FR-8 "BOFU" + Goldwood GT-510

        1) Yes, LPad and a simple cap, assuming a high XO point should be sufficient for this application.
        2) You'll have to Zoebel the impedance of the Bofu wannabe if you want it to roll off as intended with a true first-order filter. If not, the tweeter should be crossed very high indeed (15kHz), which is consistent with point 1.

        I'd just put an Lpad and a small cap on the tweeter and call it a day- as you say, boombox, and SQ isn't critical. The tweeter will fill in things and smooth out the generally terrible off-axis performance of (any) whizzer cones.

        FWIW I use a cap-and-resistor setup on my modded bofus (pioneer originals) with a coaxially mounted Dayton soft dome. Sometimes it's better to do KISS when working with wideband drivers.

        I use a

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        • #5
          Re: Help with crossover calculations: GRS 8FR-8 "BOFU" + Goldwood GT-510

          OK. So we put a "Zobel" across the 8" terminals (an 8ohm resistor and a 10uF cap - an "npe" is fine here) which pulls its impedance at 20kHz down from 160ohms to 8ohms. That lets you get by w/a relatively small (1st order) coil. Use a 0.60mH #20.

          The tweeter needs to go 2nd order (a big reason why is that it does NOT have "ferrofluid" and hits 27ohms at Fs). You need a 3uF series cap and then a 0.40mH "shunt" coil (to gnd). Between that filter and the tweeter we need an "L-pad": SR = 4ohms, PR = 12ohms.

          This crosses around 3.5kHz. Shouldn't sound too bad. Looks like the worst "artifact" is about a +3dB "hump" around 800-1000 cycles which is inherent in the 8"FR's response.

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          • #6
            Re: Help with crossover calculations: GRS 8FR-8 "BOFU" + Goldwood GT-510

            Thank you so much! Very helpful.

            Comment


            • #7
              Re: Help with crossover calculations: GRS 8FR-8 "BOFU" + Goldwood GT-510

              And, it SEEMs like the tweeter should go reverse polarity. Try it both ways and go w/your ears.

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