Tech question on bridging amp

Discussion in 'Live Sound [BG]' started by xring, Jul 24, 2013.


  1. xring

    xring

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    Hello. I haven't tried this particular configuration yet. So I have a Crown Drivercore 1500. It bridges into 4 ohms. I plan on daisy chaining 2 - 8 ohm passive 15" PA cabs for a 4 ohm load. I'm ok so far. But we just purchased 2 powered subs. The connection instructions are to run speaker cable to the sub, then out of the sub to the passive main. Each side if running stereo of course. So going bridged, I plan to run the 1 bridged cable like this: Amp>Sub in>Sub Out>Speaker Main In>Speaker Out>Sub#2 In>Sub#2 Out>Main Speaker #2 In.
    My question is will the Crown ignore the ohms (unknown) of the powered subs so that it is seeing a 4 ohm load? I don't want to blow my amp or the drummers PA.
    My guitarist has an idea of utilizing the sub outs on the Mackie board, but not sure if we'll go there this gig...maybe next time.
    Thank You!
     
  2. Bass_Pounder

    Bass_Pounder

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    If it is a POWERED sub, I would NOT connect it to any kind of power output.

    They are usually connected with shielded (instrument) cable, from a line level output - NOT speaker outputs.
     
  3. xring

    xring

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    After researching the sub, it says that a line level, or speaker level input can be used via flipping a switch. I presume ohms will also be disregarded...
     
  4. Zooberwerx

    Zooberwerx Gold Supporting Member

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    What subs are we talking about? From your description, it appears that you may use line level signal (via XLR cable) or bypass the internal amplifier and feed the sub with an external power amp. Personally, I think you'd be better off procuring a modestly-priced used crossover for added flexibility.

    Riis
     
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  6. xring

    xring

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  7. Zooberwerx

    Zooberwerx Gold Supporting Member

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    Don't. A bridged mono config will run two daisy-chained 8 ohm enclosures but not four. Sidenote: I really don't care for this type of system as it incorporates a low pass filter in the sub but still allows the mains to "see" the full range signal. Seeing you don't have a crossover, we'll have to make a few adjustments to achieve the same goal. Oh, BTW...the "sub" ouput on your mixer is for subgroups, not subwoofers.

    *Configure your power amp in bridged mono with a high pass filter set at ~100hz. Connect the (L) main output on your mixer to the Drivecore's channel 1 input with an XLR cable.

    *Connect the Drivecore's channel 1 speaker output to one of your two main speakers with a bridge cable (amp: +1, +2 speaker +1, -1). Daisy-chain to the second main speaker with a conventional speaker cable. This should take care of providing a >100 hz content to the mains....which is what you want. Now, the fun part....

    *Snag yourself a couple of 25' (or whatever length you need) balanced TRS 1/4" signal cables. You cannot use speaker cables!

    *Plug one into the Drivecore's channel 1 "link / out" input and run to the input on the Yorkie subwoofer. Daisy-chain to the second sub with the spare TRS cable. Be sure to configure both subwoofer enclosures for line level as we'll be utilizing the internal 200 watt amps.

    Here's why this should work: you don't have enough in terms of external amplification (Drivecore) to run all 4 enclosures...IME. We're re-assigning the available power and programmable high pass filter to make the most effiicient use of the mains. The TRS > subwoofers is providing a "copycat" of the mixer's full range signal. The subwoofer's internal low pass filter takes care of the rest.

    You do own a fire extinguisher, don't you?

    Riis
     
  8. two fingers

    two fingers Loud Mouth Know It All Blowhard Gold Supporting Member

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    Zoob is on top of it, but (in my view) that is only a temporary fix. I would still get a crossover asap.
     
  9. Zooberwerx

    Zooberwerx Gold Supporting Member

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    A big hello to Pitt Co.! Agreed...as stated, my first choice would be the addition of a crossover. I'm just trying to economize by using the features already on hand (amp DSP, subwoofer internal power amps, etc). To tell the truth, I'm kinda curious as how the finished product sounds once cabled up / powered up.

    Riis
     
  10. walterw

    walterw Supportive Fender Gold Supporting Member

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    don't bother bridging anyway; you guys have two 1x10 200w powered sub cabs! bridging the tops would likely have your tops way over-powering the subs.

    might as well leave this amp stereo, and use one side to power both top boxes; that way, you can use the other side to power something else, like a monitor.

    also, while the subs are made to take a speaker-level signal and convert it (so it wouldn't affect the load the amp sees or anything) it might be nice to run a line level signal to them instead, maybe even one you could control separately from the tops.
     
  11. Zooberwerx

    Zooberwerx Gold Supporting Member

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    ^ The mains cabs would be seeing 250-300 each when run in stereo or daisy-chained from a single channel...may be enough given the circumstances. Overall volume would be adjusted via mixer. The balance between the tops & subs would be manipulated via the Drivecore's attenuators (mains) and the Yorkville subs' onboard attenuators. Not ideal but passable. The idea behind this system is to supplement a full-range main not act as a faux bi-amped FOH rig as I spelled out earlier.

    BTW, I mis-read part of the manual. As I understand it, the subs utilize the internal 200 watt amp irrespective of whether they're being fed line-level or speaker-level signal. If I'm wrong on this, please let me know.

    Riis
     
  12. xring

    xring

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    That is correct that the subs will utilize the internal amp whether feeding from a line or speaker level signal. I did not originally know that until I got the sub model that we use. The mains are rated at 400w rms ea. Stereo is pushing 300w to each side, thus the desire to bridge in order to get more headroom. I do know we need to watch the fader with 775w available each side. The only little worry I have is that I'm using a short binding post to 1/4 speaker cable, in turn running into a female 1/4 adapter to run bridged. I guess they've done it for decades with more power, but I plan to get a properly wired speakon on the amp side to use. Thanks for the replies.
     
  13. Zooberwerx

    Zooberwerx Gold Supporting Member

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    A speakon on the enclosure side is not a bad idea, either...provided the cab will accomodate a speakon! I buy and use the good banana plugs (heavy duty, strain relief) for various applications using the amp's binding posts.

    Riis
     
  14. xring

    xring

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    Oh yeah. The mains do have speakon inputs. I'd utilize them of course. Thanks again.
     
  15. jimfist

    jimfist Supporting Member

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    I agree with WalterW on this one. And, FWIW, the OP is not necessarily incorrect about the "Sub" out. Some Mackie boards utilize a dedicated subwoofer out crossed over around 75hz or something, with its purpose being exactly what the OP would be looking for, or so it would seem. Thus, connect the SUB out connector to the input of one of the subs, daisy chain the line output "jumper" to the second subwoofer input, and leave the Crown amp out of the equation entirely in terms the subs.

    for example: http://www.mackie.com/products/cfx16mkii/

    xring, what make/model is the mixing board?
     
  16. walterw

    walterw Supportive Fender Gold Supporting Member

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    that's just it, your overall headroom will probably be way limited by the subs; center-coupling them will help with that, but they'll likely still get quickly overrun by the tops if you bridge the tops amp.
     
  17. Zooberwerx

    Zooberwerx Gold Supporting Member

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    Thanks for the link. The 75 hz appears a little odd at first but, as with other designs, it's probably intended for the addition of a subwoofer to supplement / complement a full-range mains system which tend to demo a more pronounced roll-off in this region.

    Riis
     

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