Large centre channel question

Discussion in 'Archived Threads 2001-2004' started by RichardMA, Sep 19, 2002.

  1. RichardMA

    RichardMA Second Unit

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    I'm in the process of constructing my 4th centre
    channel. I've built all my own speakers since the
    idea of spending $3000-$5000/pair to get the soundI
    want commercially was just too much.
    This one will use twin 10" Kevlar drivers,
    twin 5" drivers (polypro) and one 1" silk dome tweeter.
    All are Hi-Vi Research.
    I'm wondering if first order crossovers are ok to use
    with this speaker. Box volume is about 2.7 cuft
    and the speaker will have a port that can be closed off
    or left open.
    Also, what about making the crossover so it does not
    produce a crossover point at say 400hz for the woofers
    and mids but instead has dedicated crossovers that
    overlap a bit, so 400hz for the woofers and 350hz
    for the mids? Would this help eliminate that dip
    we see at coicident crossover points normally, or would
    it result in too much compounding of the overlap
    frequencies?
    Thanks!
     
  2. Chris Carswell

    Chris Carswell Supporting Actor

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    Are you going to use "text book" filters, 1st order too???
    I think I see why this will be your 4th center........
     
  3. RichardMA

    RichardMA Second Unit

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  4. ThomasW

    ThomasW Cinematographer

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    Richard

    You should measure the drivers individually in the baffle. Then you can see what type of crossover will work best.
     
  5. Chris Carswell

    Chris Carswell Supporting Actor

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    No. I already assumed you would be trying passive filters.
    By "text book" I would be referring to standard basic text book values for a given xover point. For example: A 1st order low pass filter at 400Hz for a 4 ohm woofer would use a 1.59mH inductor.........
    How much do you plan on spending on your drivers?

    Thomas,
    My thoughts exactly, but I doubt seriously he has Clio, JustMLS, or anything similar to do that with.
     
  6. RichardMA

    RichardMA Second Unit

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  7. Chris Carswell

    Chris Carswell Supporting Actor

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    Yep, that's text book. Have fun with it....................
     
  8. RichardMA

    RichardMA Second Unit

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    Yes, I think all the programs out there use the same
    methods to derive values for various system orders.
    But rather than pointing out the obvious, why not offer
    some constructive cricism of the method instead of useless,
    flip answers in a technical forum?
     
  9. ThomasW

    ThomasW Cinematographer

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  10. Will Orth

    Will Orth Stunt Coordinator

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    This one will use twin 10" Kevlar drivers,
    twin 5" drivers (polypro) and one 1" silk dome tweeter.
    All are Hi-Vi Research.


    Is this all just for a center channle??
    would love to see what else you are running?

    Will
     
  11. Chris Carswell

    Chris Carswell Supporting Actor

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    OK Richard, now for my "constructive criticism" you called for.............................
    I can tell you have some interesting ideas. Specially since something this big/different is going to be just for your center. Like what Will Orth said, I'd like to see what other things you got going on.
    I too started off like you. I wanted to build my own from scratch. I thought it would be exciting and "easy". I mean all you have to do is pick out some drivers you like, look at the Mfg. numbers to make a xover, then put it all together in a nice tight box. Easy right? Umm... Err... Not really... Specially since textbook filters generally don't work that well. If you want to build your own just to do it, then feel free to experiment, though I should warn you that the results will not be spectacular.
    If you'd like to get the full benefits / performance of those speakers as well as have something that sounds great, I'd suggest either getting an adequate measurement system and design software (such as LspCAD, which includes the JustMLS measurement software) or build a kit designed by someone with proper tools (Zaltron, GR, etc.) even though it wouldn't be as cool as saying you made these from scratch although your friends will never know anyway [​IMG] .
    I don't mean to sound like a spoilsport, but just a few weeks ago my friend Steve & I measured the results of yet another crossover done without proper design or adequate measurements: + -6dB with pronounced lower midrange, boomy bass, lacking treble, and a shallow crossover an octave higher than the designer intended (he thought he built a 2nd order at 2KHz, but it turned out to be a semi-1st order at 4KHz). Even though the speakers were built with $400 of drivers and AWESOME cabinetry, it sounded no better than any standard piece of mid-fi equipment you could pick up at your local Best Buy or Circuit City costing much less.
    We plugged the speaker in and gave it a listen. The builder was obviously very proud of his creation and I didn't have the heart to tell him how poor I thought it sounded, though I did constructively suggest a few tweaks. Immediately afterward I demonstrated a speaker that Steve built with only $22 of drivers. He remarked how good the midrange and treble sounded........I didn't say much. I figured that the point was made. Even though more expensive drivers typically yeild better sound and are easier to work with having larger frequency responses and smoother roll offs, Xovers are still the key.
    If you have the itch and really want to build them by all means go for it [​IMG] !!! That's how I got started. That's also how I learned that later down the road xover programs and measuring equipment are essential to reach your designs full potential.
    I can tell you are pretty much set on building this [​IMG] so here are a few tips that might help you out:
    -----------------------------------------------------------
    1. Try to use get the response graphs of the drivers used. PE is pretty good at providing them. You can then visually see where the drivers start to roll off or break up. Design your xover points from there. If graphs are not available then you will have to use the Mfg. supplied Frequency Response of the drivers. Usually they are off so be prepared. Remember to xover at least an octive or two from the frequency response of the driver. Ex: if the tweeter has a frequency response of 1000-20,000 Hz then xover at 2000 Hz or higher.
    2. Center-to-center (ctc) driver spacing plays a little part in selecting the highest crossover point. Rule of thumb for some people is to keep the crossover frequency under the wavelength dictated by the ctc distance.
    In terms of inches, the xover frequency = 13585/ctc distance. So, a 7" driver placed .5" above a 3.75" tweeter will = a xover point ~2312 Hz. This would be acceptable if I was shooting for a 2000 Hz point.
    3. You might want to consider an Impedance Equalization Circuits or Zobel circuits like what Thomas suggested. They are used to counteract the rising impedance of a voice coil caused by inductive resistance. This largest rise mainly occurs in woofers or sub woofers, because of their big coils. It is also present in mids and tweeters as well. Using a Zobel will keep a constant 4 or 8 ohm load. It is simply a capacitor and a resistor in series, placed parallel with the driver, usually after the xover circuit.
    4. You might need to design a L-pads to equalize the different driver sensitivities. L-Pads (loss pads or driver attenuation circuit) are used to decrease the output level of a driver. This is useful in a multiple driver system where the drivers do not have the same sensitivity.
    A L-Pad can be used to decrease the output levels of the louder driver to match, or attenuate, the sensitivity of the less sensitive driver. A L-Pad is simply 2 resistors, one in series, and one in parallel with the driver. Ex: a 3-way system where the tweeter has 2db sensitivity over the mid, and the woofer has 1db sensitivity over the mid, resistors will be used to balance out the sensitivity / load problems. A L-pad circuit will be used to lower the
    tweeter output by 2db and the woofer output by 1db (although not the best idea to put one on woofer).
    This L-pad is important to consider when you go from one driver to two in parallel because efficiency is increased +3dB and sensitivity +6dB. When you go from one driver to two in series efficiency is increased +3dB but sensitivity remains constant.
    There are a ton of other things to consider but I'll let you play with this info for now. Hope it helps.
     
  12. Dan Hine

    Dan Hine Screenwriter

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  13. ThomasW

    ThomasW Cinematographer

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    Dan
    You can get ETF 5.0 for $150 is does MLS and quasi-realtime RTA. It has .cal files for the Behringer ECM 8000 and the RS meter.
    The standard $130 version of LspCAD includes a good XO design program and a 'free' JustMLS program.
    Also there's a progam called WinMLS 2000/Pro it can be purchased in modules
    IMO the Harris Tech programs (BassboxPro and CrossoverPro) are good for newbies. I find people like the slick easy to use Windows interface. More experienced users think the programs aren't as sophisticated as LspCAD and are too quickly out grown.
     

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