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Thread: crossover thoughts for JBL 2405 compression tweeter

  1. #1
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    crossover thoughts for JBL 2405 compression tweeter

    I need some advise on a passive crossover for the JBL 2405 and 2470 mid:

    Just got a pair of JBL 2405h tweeters! They are to be setup to work with my JBL 2470 mid horns, with a passive crossover. Also, what is the appropriate baffle dimentions to mount the 2405 tweeters? Can it be mounted without a baffle?

    I started playing with the crossover settings on my digital crossover DBX Driverack PA that let me try and switch between different settings quickly and easily. And I came to this conclusion:
    JBL recommends a 12db/octave crossover at 7khz or higher on the 2405 tweeter, but...
    I found that a 6db/octave slope sounded more seamless with the 2470 mid. In fact the steeper the slope was (12, 18, 24 db/octave) the more detached the 2405 tweeter sounded.
    The 2470 mid horns roll off naturally after 8 khz. So they could be used with or without a low pass filter.

    digital crossovers observations:
    2470 6 db/octave at 9 Khz and 2405 6 db/octave at 9 Khz -- best sound
    2470 wide open and 2405 6 db/octave at 9 Khz -- almost as good, slightly harsh on top.

    JBL 2470 is 16 ohms
    JBL 2405 is 8 ohms

    I am interested in making a passive version. Is a series crossover an option? How do I do it?
    Simple version:
    What is the kind and value of tweeter capacitor for the, high pass, 6 db/octave at 9 Khz? (mid plays wide open)

    Better version:
    What is the kind and value of inductor for the 2470 low pass 6 db/octave at 9 Khz?
    As an option, what is the network for the 2407 low pass 12 db/octave at 9 Khz?

    It will be interesting to compare the passive and active crossovers, soundwise.

    -Herman

  2. #2
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    Starter XO

    Fellow amature tweaker from the other side of the lake, went through that project recently. Check this 3106 schematic. Can be tweaked by removing elements, changing caps, etc. All purpose toy! Also has a nice fixed L-pad on the Low freq side to help get the horn under control. If you want to go with a simple 6 db for the tweeter only, try a 1.0 mF for starters - that's what I use with my 375/077. You can find original 3106 assemblies on eBay occasionally but not that difficult to build from scratch either.

    http://www.jblproservice.com/pdf/Net...%20Network.pdf

    All this stuff is available under the Technical Reference section at the bottom of the site intro page. Look under links/JBL pro/system networks. I'd say you're off to a great start! Oh yeah, check out the search engine too. Tons of stuff. Enjoy the site! Mike

  3. #3
    Senior Member Baron030's Avatar
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    Hi Herman,

    You talk about a 2470 compression driver and 2405 slot driver, but you forgot to say which horn you are using? And horns do make a big difference.

    If you are using a 2350 horn, then your comments about the mid naturally rolling off after 8 kHz would be correct, and from the chart pictured below, it looks like it's rolling off at a good 12db/octave rate.

    So, running the 2470 "wide open" without a low pass filter would be one option. In a way, you could say that this horn/driver combo has a 12db/octave low pass filter built right in.

    You said that you tried the "2470 wide open and 2405 6 db/octave at 9 KHz" combination and it sounded "slightly harsh on top". This harshness might be coming from a peak being created right at the crossover point.

    If it was me, I would try running the 2470 wide open and then run the 2405 with a 12db/octave high pass filter. Start by testing with the 2405's high pass filter set a 10 KHz and then look for a peak or hole at the crossover point. If there is a peak then raise the 2405 high pass filter to a higher frequency or set it to a lower frequency should a hole be present. Acoustically, they should sum together nicely with 12db/octave slope rates.

    Another thing that might be going on, particularly with your comment about how "the steeper the slope was (12, 18, 24 db/octave) the more detached the 2405 tweeter sounded."

    If the 2405 is mounted flush with the mouth of the horn, then signal from the 2405 may not be acoustically in phase with the signal coming from the 2470 driver. At 9 KHz, the wave length is so very short and with a long horn, the 2470 driver's signal will be trailing behind the 2405's by several wave lengths. It's not the whole number of wave lengths that is an issue here. It's that remaining fractional part that really matters.

    For example, in my own system, the 2446/2382a driver's signal trails behind my 2405's by a full 5 1/2 wave lengths @ 8 KHz. So, if the two drivers are electrically wired in phase with each one another, then there is a big deep hole at the crossover point. But, as soon as I reversed the wires on one of the drivers, the hole disappeared and the system sounds right. And I am using an electronic crossover with 24 db/octave slopes. So, I don't think slope rates are an issue here.

    So, try reversing a few wires to acoustically bring the drivers into phase. Or try moving the 2405 back until it time aligns with 2470 driver. As it could be a phasing issue that is giving you "detached tweeter sound".



    Baron030

  4. #4
    Senior Member Baron030's Avatar
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    Here's the chart that i was referring to:
    Attached Images Attached Images  

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    The Baron is right,

    You need to measure what is happening.

    Be careful od low order networks on the 2405 or you will blow the driver and that driver sounds horrible without a steep filter imho.

    JBL normally specifies 18 db slopes on the Pro Series monitors for the 2405.

    The reason is it protects the driver and the phase relationhip is much better. They also use 12 db low pass on the horn. Although in your has I would, start with 6db for an acoustic slope of 18db. If you use too shallow a filter serve comb filtering will occurr and you will have a lot of vertical beaming at the crossover point

    Try 1 uf from the amp and 0.12 mH DCR 0.75 ohms to ground and 1.5 uf to the driver with a 8 ohm L pad and go from there.. The green curve is with the values increased 25 %. Divisions are 5 db . Std JBL test jig for voltage drives


    Ian
    Attached Images Attached Images  

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    Baron,

    I tried 2 horns on the JBL 2470:

    Altec 811b
    http://picasaweb.google.ru/noviygera...46838059017634


    Community SRH 90
    http://picasaweb.google.ru/noviygera...46878584092290





    The Altec horn sounded, bright, harsh, but overall natural.
    The Community horn's sound was full, delicate, dark, dynamic but a definitely "nasal" sound, like if people spoke with their nose pinched.

    Neither of the horns sounded "right" but my girlfriend helped me choose the overall winner: Community SRH 90. It was just more enjoying to listen to.

    And at this point I'm trying to figure out what other horns would sound proper on the JBL 2470.
    Maybe conical?
    Last edited by Herman Chigrin; 08-24-2008 at 11:41 AM. Reason: add images

  7. #7
    Senior Member Baron030's Avatar
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    Entering Uncharted Territory, “Here be dragons”

    Hi Herman,

    It looks like you might be breaking some very new ground with that driver and horn combinations. So, I would suggest getting some test equipment to measure the frequency response of these combinations. Only then it would it be possible to come up with a crossover circuit that will give you a flattest overall system response.

    You realize that you are heading into a really advanced area of speaker design here.
    Crossover design is very much an art form unto itself.
    And without any test equipment, you might as well be shooting in the dark.

    Baron030

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