Cab configuration and impedance related to xover

Discussion in 'Amps [BG]' started by GustavFL, Feb 13, 2013.


  1. GustavFL

    GustavFL

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    Hi folks,

    Now I understand drivers behavior with a crossover regarding impedance. First, it varies with frequency. As a filter works by elevating impedance as high as a frequency goes or as low as a frequency goes, when a crossover is connected youre separating drivers' operation, therefore nominal impedance at each frequency band will be of the driver(s) operating at that band.

    Usually people crossover the same number of drivers at each side of the crossover, or try to keep nominal impedance equal in low, mid and high bands. In my case I have 3 woofers and 1 mid driver, all of them 8 ohms drivers and a GK MB 500 amp. I can use the following configurations:

    1. 210/6 and 1 woofer stays unused for now.

    If I wire the woofers in parallel I get 4 ohms at the low band which is not good, considering the amp gives 500W@4ohms. So, the way would be to wire them in series and get a 16 ohms cab in the low band and 8 ohms at mid drivers band. The matter is, this variation in impedance from low to mid frequency bands would be sound defective? Or maybe give more relative power to the mid driver?

    2. 110/6 and 210.

    In the 110/6 I can only get 8 ohms through all frequency range, since both drivers are 8 ohms. However using a 200W cab with an amp that produces 350W@8ohms is a bit too much, huh? I'd like to use this cab in case I don't need too much power, in other cases I would wire the 210(16 ohms if woofers are in series) in parallel with it, producing 5.33 ohms load, which would give me a ~375 W limited stack with the amp giving something between 400 and 450 W.

    I couldn't find anywhere a graph of power output vs impedance for my amp, or the information of power @ 16 ohms and 5.33 ohms. If one can please inform me MB500's output @16 and @5.33 ohms (already mailed GK support and no answer) or even if there's a formula to get it.

    Waiting to hear from your experience guys! :D
  2. bgavin

    bgavin Supporting Member

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    You can avoid all this complication by using an active crossover.
    It is immune to all impedance variations, but requires a 2-channel amp.

    With a passive crossover and head arrangement, you can use a commercial crossover, or build your own.
    Most commercial types are nominal 8-ohm loads for both Low and High outputs.
    They don't care how you get to an 8-ohm nominal load.

    If you roll your own, you should take an impedance measurement of the desired combination, at the desired crossover point.
    Do this for both Low and High drivers.
    There are numerous internet design programs that will get you to the right set of components for the precise impedance you desire.

    Passive crossover components are pricey.
    Especially in the 100~ 250 Hz range, due to the size of them.
    You can easily pay more in components than you will pay for an active crossover.
    Tinkering will be required. Lots of tinkering.

    Why not try a simple high pass for just the High driver?
    Let the Low run full range, and protect the High with a high-pass filter.
  3. CL400Peavey

    CL400Peavey Supporting Member

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    Pretty much your choice of drivers, and the amp you want to use are not going to work well. I know thats not the answer you want, but using three 8 Ohm woofers sucks with a 4 Ohm stable amp. To get equal power distribution you need to have them all wired in parallel which puts you at 2.66 Ohms.

    Your options are to build a 210 with the mid driver, or get a different amp.
  4. Bmorefoozler

    Bmorefoozler

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    or buy an additional 10" for a series parrallel 8 ohm load.
  5. B-string

    B-string Supporting Member

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    There is no standard formula for output power vs load. The head would have to be actually tested at 16 ohms and 5.33 ohms. Not something the standard test sheet would likely include.
  6. GustavFL

    GustavFL

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    Thanks for the answers.

    I'm tempted to do the 210/6 only with a high-pass to protect the mid driver as b-gavin suggested. How would be the correct way to wire them using a 2nd order L-R filter? Would I have any problem with phase between drivers?
  7. GustavFL

    GustavFL

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    Bump.

    Can anyone help me on this one?

    How should I wire these 2 woofers and the mid driver to get nominal 16 ohms load using only a high pass to protect the mid driver and leting the woofers roll off by themselves?
  8. will33

    will33

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    If this is for your bp102's I would start by doing that. They don't run real high anyway and the contribution of the small speaker will be glaringly obvious. That will get you going with a cab to play, then tinker, add more as you learn more.
  9. will33

    will33

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    You can wire your woofers to 4ohms when playing the cab by itself, 16 when stacking it with any other speakers. Eventually, if you pit a lowpass filter in there, you're going to have to pick one impedance and stick with it. The LPF parts will not work correctly at impedances other than the one it is designed for. You can see this by running an online calculator and see how much the part valuea change for a 4ohm vs. 16ohm woofer. The highpass protecting you mid driver can stay the same regardless of woofer impedance.
  10. Passinwind

    Passinwind Charlie Escher Supporting Member

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    What woofers, specifically? As Bruce says, the first step is to measure their actual impedance at your desired crossover frequency. Failing that, a factory graph of the woofer's impedance would at least put you in the ballpark of understanding how to do what you want.
  11. Bmorefoozler

    Bmorefoozler

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    He's using 8 ohm BP102's and a LA6CBMR
  12. will33

    will33

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    OP has had another thread or two on this. Prices and availability of drivers, etc. is not that great in Brazil and he is not made of money, but wants to learn and get something workable with what he has, which is a total of three 8ohm bp102's and one LA6cbmr. Was struggling with the odd impedances vs. off-the-shelf xover stuff and wiring in the first thread but is catching on.
  13. Bmorefoozler

    Bmorefoozler

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    I think the 16ohm 210/6 while waiting to add an additional 16ohm 210 is the sollution for him at this time. Obviously if another BP102 could stretch his budget an 8 ohm 410/6 would make it easier, but it is what it is.
  14. GustavFL

    GustavFL

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    I think running this cab by itself at 16 ohms would more reasonable so I could stack it with a similar 16 ohms or 8 ohms and pull more power from the amp, making this 400W cab 4 ohms fto use with a 500W@4 ohms amp is kind of overkilling, isn't it?

    Take a look at this wiring guys, just a 1st order to protect the mid driver, probably around 1500 hz. I would put 2 parallel inputs, for daisy chain and connection to amp. Is the polarity ok?

    [​IMG]
  15. Passinwind

    Passinwind Charlie Escher Supporting Member

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    Not at all, IMHO and IME.
  16. GustavFL

    GustavFL

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    Well, considering cab combination(in parallel), power and nom impedance here are some options:

    - 210/6 16 ohms 400W + 210 16 ohms 400W = 800W at 8 ohms (not very useful, as I can just use 350W of it)
    - 210/6 16 ohms 400W + 210 4 ohms 400W = 800W at 3.2 ohms (bad for my amp)
    - 210/6 4 ohms 400W + 210 16 ohms 400W = 800W at 3.2 ohms (bad for my amp)
    - 210/6 16 ohms 400W + 210 8 ohms 400W = 800W at 5.33 ohms (nice, but how to make that second cab with 8ohms..?)

    Anyway I'm just gonna build the 210/6 for now, and when I build the 210 it will be a backup cab for power. So in many situations I'm gonna take only the 210/6, that said I think I must priorize that in it's own impedance I can pull all it has to offer. If a 16 ohms configuration can deliver it's ~250W real power then the first or last option is acceptable. Making this 210/6 4 ohms is not good for adding other cabs. Geting 800W@3.2 ohms isn't secure for my amp.

    What about wiring diagram, is it alright?
  17. Bmorefoozler

    Bmorefoozler

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    I wouldn't worry about the power handling all that much.

    Build your 16 ohm 210/6 now, and build another identical one when you have enough money for another identical BP102. OR, if you decide you need 410's most of the time build a new 410 cabinet for all 5 drivers, and keep the 210 cabinet around incase you want to add 2 more 10's and another 6" to get you to 5.33 ohms.

    More speakers trump more watts every time. Worrying about "getting all the power you can" to a certain cabinet will get you in to trouble more often then not.
  18. GustavFL

    GustavFL

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    Thanks Bmorefoozler, I'll consider both your suggestions. I think regarding what I have that will be a good way to use it.

    P.S. You guys responded while I was correcting miscalculation of impedance xD

    When I'm finished I'll post another thread of cab construction process with the results. :bassist:
  19. Passinwind

    Passinwind Charlie Escher Supporting Member

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    Deleted my last post. It's very simple to try both 4 and 16 ohms, and your proposed 1st order high pass. I didn't have much luck with 1st order with my particular 6 inch drivers, but it sounds like what worked for me is way out of your budget for now anyway. Best of luck, I'll be watching to see how you make out. ;)
  20. GustavFL

    GustavFL

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    GK technician just told me amp's output at 16 ohms would be around 170W.

    Just had an idea. What do you think about this configuration?

    - 110/6 with high pass, 8 ohms 200W + 210 with 16 ohms, 400W

    This combination would give me a light 110/6 cab and when stacked would be 600W@5.33 ohms, matching well with amp's power with no sacrifices AND using all drivers I dispose. Would 200W cab be too forced by 350W amp?

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