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  1. #1

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    Question X-Over Modification Guidance

    Ok folks, I'm gonna chime in here with a technical request. When I was in high school, I purchased an old pair of JBL monitors (circa 1985-86) with one blown tweeter. At the time, they were a good deal so I installed some piezo horns from Radio Shack in both units for timbre matching. Recently, thanks to har, I became a new owner of some Polk LSi (Vifa) ring tweeters from Ebay. I'm looking at pulling out the piezo horns and installing the ring tweeters, but am a bit concerned about the existing x-over that's in the system and how adding a 4 ohm tweeter to a "speaker marketed as a 8 ohm speaker" would affect the performance of the Vifa. Attached below is a picture of the X-Over, and the post below this will be the tech specs on the speaker that JBL customer service provided me with.

    I need help on the following items:
    • if I install the 4 ohm tweeter in place of the orginal 8 ohm tweeter (the original woofer and midrange are rated for 4 ohms), would I now have a speaker enclosure rated for 4 ohms?
    • i know caps and inductors used to select the x-over frequency are selected based off of speaker load. What changes would I have to make to the orginal x-over to keep my high-pass frequency at 3.4 KHz.
    • looks like the mid-range driver is swapped phase. I though 1st order x-overs weren't supposed to have this?
    • anyone here have a circuits program and could build this x-over to confirm if the phase shift will change from the orginal design and what the new nominal speaker load would be?

    From the attached x-over network, it looks like JBL used 3 seperate 1st order crossovers for their network, with the mid-range having both a high pass and low pass filter to keep it in the range of 1.1 - 3.4 kHz.

    Thanks for any advice guys.

    Tony
    Attached Images  
    Last edited by TonyPTX; 05-23-2003 at 11:05 AM.
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  2. #2

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    Default Part II - Tech Spec Sheet

    Here is the technical spec sheet I got from JBL customer service.
    Attached Files
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  3. #3

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    Default Re: X-Over Modification Guidance

    Originally posted by TonyPTX
    • if I install the 4 ohm tweeter in place of the orginal 8 ohm tweeter (the original woofer and midrange are rated for 4 ohms), would I now have a speaker enclosure rated for 4 ohms?
    • i know caps and inductors used to select the x-over frequency are selected based off of speaker load. What changes would I have to make to the orginal x-over to keep my high-pass frequency at 3.4 KHz.
    • looks like the mid-range driver is swapped phase. I though 1st order x-overs weren't supposed to have this?
    • anyone here have a circuits program and could build this x-over to confirm if the phase shift will change from the orginal design and what the new nominal speaker load would be?
    1. No.

    2. Don't know. The capacitance value in the tweeter's filter does not match up with the combination of an 8ohm tweeter and a frequency of 3400Hz. The tweeter was NOT 8ohms at the crossover frequency if the frequency is 3400Hz. You'll have to increase the capacitance assuming 4 ohms and 3.4k. C2 and C4 are probably electrolytics... yes?

    3. 1st-order crossovers induce a 90' phase shift. The mid has two 1st-order filters attached to it. Usually, 3-way crossovers are at least 2nd order, certainly on the mid.

    4. Go to the Parts Express forum. There is a wealth if DIY expertise there, and the guys are as willing to help as the polkies here are.

  4. #4

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    Default

    1.) Didn't think so.

    2.) You got me here bud. I'm just reading the specs provided to me. I got the 8 ohms nominal for the tweet from the Tech Spec sheet.

    3.) Using two first orders on the mids may have been easier to fabricate and model rather than the typical 2nd order. Don't forget the age of these things. They pre-date decent computers. Either way, the info you provided makes sense now, however, if the woofer and tweet are shifted by 90, and the mid is shifted by 180, then isn't the mid still off by 90 even if you swap the phase?

    4.) I'm posting everywhere I can think of.

    Attached is a pic of the x-over. The yellow guys are labeled:
    0.01M
    100 CP


    I don't know enough about these thigns to call them electrolytics.

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  5. #5

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    Default Re: Re: X-Over Modification Guidance

    Originally posted by burdette
    2. Don't know. The capacitance value in the tweeter's filter does not match up with the combination of an 8ohm tweeter and a frequency of 3400Hz. The tweeter was NOT 8ohms at the crossover frequency if the frequency is 3400Hz. You'll have to increase the capacitance assuming 4 ohms and 3.4k. C2 and C4 are probably electrolytics... yes?
    I think C2 and C4 are bypass capacitors. As for JBL's choice in caps for the 3400Hz cross-over, industry choice of ~5 uF wasn't that off from JBL's 4 uF selection. You gotta consider that the cap was probably +/- 10% plus consider that the tweet may have had increased impedance at the xover frequency cause it approached the tweeter Fs and resulted in the need to select a smaller capacitor.

    4. Go to the Parts Express forum. There is a wealth if DIY expertise there, and the guys are as willing to help as the polkies here are.
    Thanks for the advice here. I had someone reply back with some recomendations with 2 hours of posting!!! They definately have some sharp folks over there.
    Damn....8 lines...I've gotta put my sig on a diet now....

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