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Thread: bi amp help

  1. #1
    Senior Member bldozier's Avatar
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    bi amp help

    hello all thank you for viewing & replying,
    in a scenario where the 2202 is used as a mf in a 4 way configuration,
    is the MF wired with the HF, and UHF leaving the LF by itself.

    im going full active, and i noticed no attenuation for the MF in a 4350,
    what is the roll point off of the 2202 in the large format JBL its purely vocal but
    why no attenuation?

    i would like to quad amp this particular system using the c20,
    and tube gear,
    do i ned to take into account output impedance when using an active

    thanks
    can't wait for your reply's
    brian

  2. #2
    Senior Member Ian Mackenzie's Avatar
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    The way to do it is look at the voltage drive of the passive network.

    Tune you active crossover to the voltage drive curve (not the spl curve)


    http://www.audioheritage.org/vbullet...php?10614-4355


    The 2202is not as sensitive as the horn so the only is attenuated.

  3. #3
    Senior Member SteveJewels's Avatar
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    So you are looking to quad-amp?

    You are a man after my own heart. I would tri-amp my espresso maker if I could figure out where to hook up all the wires.

    If you are going full active, there are crossovers that will give you 4 outputs such as the Marchand XM44.

    They are superb for sound authenticity and can do anything you need them to do such as roll off rate, frequency and attenuation.

    The disadvantage is that 'tuning' requires a hardware modification. It is not too difficult but it is not just turning a knob or pushing a button.

    An active crossover also eliminates the considerable power waste in the passive components and eliminates cross talk between transducers and allows the amplifier to have direct control of the transducer.

  4. #4
    Senior Member Ian Mackenzie's Avatar
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    That a possibility but as you say the filters would need to be tuned specifically for that system.

    But more important is the fact that only a true measurement of the voltage drive in the form of a trace with the passive crossover network connected will give the correct voltage drive required for any active system conversion. That means a measurement at the driver terminals.

    Cone drivers in particular but compression drivers can also act as a reactive load due to inductance and impedance variations due to horn loading. The filters also incorporate passive EQ that tilts the response of the drivers so the overall response is fast in the passband.

    This means that design of new active system would require the voltage drive data for approximate design of the filters.

    The Jbl voltage drives published are into a pure 8 ohm resistance.

    When the active system is connected the filters need final adjustment to match the passive network voltage drives taken at the driver terminals.

    If that sounds like a lot of work and probably something difficult to do with suitable test equipment you are correct.

    This is why aside from simple bi amping most diy situations are best left passive.

    Here is the thing.

    Way back when Jbl designed and built these systems they designed the crossovers network empirically so it not only measured the way they wanted but also voiced subjectively by golden ears from a panel of experts within the company. Some of theses adjustments they make are subtle but have an profound impact on the final sound of the system.

    That’s what makes the Jbl sound.

    This does not mean to say that a diy attempt at quad amping won’t work. You can use stock text book filters like LR 24 dB slopes.

    But to get it to function exactly like the passive JBL network is a far more precise and requires engineering expertise.

    This applies to either analogue or digital (dsp) crossover networks.

  5. #5
    Senior Member SteveJewels's Avatar
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    Quote Originally Posted by Ian Mackenzie View Post
    .....The Jbl voltage drives published are into a pure 8 ohm resistance........

    I didn't follow this part. Could you please expand on this?

  6. #6
    Senior Member Ian Mackenzie's Avatar
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    http://www.jblproservice.com/pdf/Net...%20Fixture.pdf

    This is how Jbl test the crossovers in production. So they are measured with a standard jig to be within a specified tolerance.

    The actual crossover is designed around real drivers and the measurements under those conditions can be quite different.

    As l mentioned earlier itís the same deal with a dsp based crossover. But be aware despite how clever the dsp engine may appear with Wizards and what not itís not a highly skilled Jbl systems engineer like Greg Timbers and what might look good on paper may not actually be subjectively successful.

    Dsp can overcome certain limitations in passive network but it takes a solid understanding of the existing system and how to use dsp correctly to obtain those benefits. Not everyone accepts dsp is perfect particularly where a $2.00 cheap chip is used in the output buffers so there are compromises to be considered. Some add hi end (and expensive) AD and DA converters to minimise losses in resolution and transparency. My concern is that dsp can too easily take the thinking and decisions out of the hands of the user like auto pilot. You really need to know how to fly a plane before engaging auto pilot.

    My preference at the moment is to use a simple discrete class A analogue active crossover where it will have the most impact in terms of passive versus active benefit. Itís sounds better and costs less than most full dsp solutions and can be organised with the correct voltage drives so all you have to do is plug it in. Below is a discrete pure analogue bi amp and a tri amp unit I commissioned recently.

    If of course you are not interested in using the original Jbl crossover voltage drives you can come up with your own recipe but it wonít be the Jbl design or possibly the Jbl sound.

    Itís nothing to get too excited about unless you are planning a serious journey into a full active system.

    If anyone reading this is interested in more in this subject you can
    Pm me. Will be in LA from 18-23 Jan so if you would like to talk to me and pick my brain about your own bi amp or active project let me know. There is a welcome feast happening on or around the 18th under the Forum Announcements thread so go to that if you can.

    Being an American foodie l am looking forward to my first meal in LA! I love spicy, modern American, anything with pulled pork or Ribbs. I also like Sierra Nevada Pale Ale.
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  7. #7
    Senior Member SteveJewels's Avatar
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    Thanks Ian!

  8. #8
    Senior Member bldozier's Avatar
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    Quote Originally Posted by Ian Mackenzie View Post
    The way to do it is look at the voltage drive of the passive network.

    Tune you active crossover to the voltage drive curve (not the spl curve)


    http://www.audioheritage.org/vbullet...php?10614-4355


    The 2202is not as sensitive as the horn so the only is attenuated.
    so if bi-amping i would connect the 2202 with the HF & UHF
    leaving the K series in my case, alone. i understand the 15" are
    playing down to 4ohm

    i have been looking at an ashly sc80 as an active
    i did not understand if input impedance was matched pre or post the active

  9. #9
    Senior Member Ian Mackenzie's Avatar
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    I donít think 12 dB slope in the two woofers is quite enough. It will work but itís not ideal. The impedance should not be a problem. You can use the unbalanced inputs if required or the outputs unbalanced for that matter.

    If you can get a good on this active crossover itís s good place to start.

  10. #10
    Senior Member bldozier's Avatar
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    Quote Originally Posted by Ian Mackenzie View Post
    I don’t think 12 dB slope in the two woofers is quite enough. It will work but it’s not ideal. The impedance should not be a problem. You can use the unbalanced inputs if required or the outputs unbalanced for that matter.

    If you can get a good on this active crossover it’s s good place to start.
    the sc80 is available in 12 and 18 db
    i read for the 4343/4344 i should use the 12db slopes
    the 4350/55 is not mentioned. but i have read if using a active filter then one might consider the 18db slope

    again for the the 4343/4344 the 12db is recommended.

    i will ask again if bi-amping, should i connect the 2202 with the HF and UHF
    or connect it with the 2231a
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  11. #11
    Administrator Robh3606's Avatar
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    i will ask again if bi-amping, should i connect the 2202 with the HF and UHF
    or connect it with the 2231a
    This is an original bi-amp design so the same as the original speaker. You should also be able to find the crossover slopes in the 5234 or earlier JBL active crossover manuals. So woofer then the top so 2202 2440 2405 up top.

    http://www.jblpro.com/pub/obsolete/4350b.pdf


    Rob
    "I could be arguing in my spare time"

  12. #12
    Administrator Robh3606's Avatar
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    I donít think 12 dB slope in the two woofers is quite enough.


    Hello Ian

    Yeah its a little weird figure 12db sealed box roll-off plus 12db electrical for the 2202 with just 12db on the paired woofers?? Asymmetrical for sure!

    Rob
    "I could be arguing in my spare time"

  13. #13
    Senior Member Ian Mackenzie's Avatar
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    In the 5234 manual JBL refer to the 5140 special card for the 4350, but in the 4350 cut sheet they refer to 250 hertz 12 db slope.

    The 5140 card had spread crossover points of something like 220 hertz and 270 hertz looking at the RC time constants.

    In the real world what happens is not always what one expects with acoustics and there is a certain amount of measured mutual coupling with the woofers and the mid driver when using those baffles and butter-worth 12 db crossover slopes. JBL usual put the monitor in a pit on the roof of the factory near the barbecue and strung a mic 30 feet above the baffle. That is how they did the measurements and avoided office politics...

    They overcame this effect by spreading the crossover points. Really awkward any todays standards but that is how they did it then.

    I really doubt you would notice this effect at home because the domestic room is a jungle of peaks and dips all over the place.

    BTW the 5145 18 db card was used for the 4345 and the 4355 monitor.

    Use the 12 db active crossover. With the exception of the uhf driver the other slopes are close to 12 db

  14. #14
    Senior Member Ian Mackenzie's Avatar
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    Rob could just about drive over and figure it out but he is very parochial and somewhat argumentative ...........LOL

  15. #15
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    52-5140 - For JBL 4340, 4343 et 4350
    12dB with frequencies a bit more than 250Hz, ButterworthIf you don't succeed in getting a 5140 use the 250Hz card or any 12dB/oct 250Hz active crossover







    Card 51-5145 for JBL 4344/4345/4355
    18dB 290Hz (but frequencies are a bit shifted, ButterworthBe careful with component tolerances to get the right responses






    Many details about 5234/5234A/5235 crossover boards are available here: http://www.jbl-vintage.com/t3066-fil...-jbl-urei-5235
    It is in French, and I never add enough time to translate it into English. Feel free to ask questions here.

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