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Power Amp + Cabs Question

Discussion in 'Amps and Cabs [BG]' started by christliche, Nov 19, 2018.


  1. christliche

    christliche

    Jan 10, 2007
    Was doing a little research for a potential new rig I'd like to throw together. Came across a review on the Orange OBC15 1x15 cab, since I already have one and want to get a second one, so this review was helpful. The cabs are 400 watt 8 ohm. The reviewer seems to be saying he runs the full wattage out of the power amp @ 4 ohms, I am just wondering if there is potential to damage the speakers by doing this. Looked up the specs for the QSC GX7, and it is 1200w @ 4 ohm with a single channel driven, and 1000w @ 4 ohms with both channels driven. Not sure if the reviewer was just mentioning the amp can do 1200w @ 4 ohms, or if he ran out from one channel with the cabs in series, or if he is utilizing both channels at the 1000w each @ 4 ohms to each 1x15.

    "I have a SansAmp RPM into a QSC GX7 (1200w @ 4ohms) to power 2 OBC115 cabs. I'm loud enough for 2 half stacks and a heavy drummer. These cabs are rated at 400w each, but they are efficient enough that my power amp doesn't have to push them very hard to produce high volume."

    This is pretty much the exact set up I was imaging, just want to avoid blowing anything up.
     
    Last edited: Nov 19, 2018
    basscapes and BadExample like this.
  2. BadExample

    BadExample

    Jan 21, 2016
    Injiana
    You are wise to ask first.

    The first thing to do is boil the "watts" down to the same type. Are the 15's rated at 400 Watts RMS?
     
  3. mikezimmerman

    mikezimmerman Supporting Member

    Apr 29, 2001
    Omaha, Nebraska
    The QSC is a two-channel amp that doesn't appear to support bridging the two channels together, so either he's

    1) running each of the 8-ohm cabinets on its own channel, with the amp supplying a maximum of 725 watts to each cab (the "8 ohms both channels driven" spec), or

    2) powering both cabs from one channel, which results in the channel seeing 4 ohms from the combined cams and giving a maximum of 1200 watts, which is 600 watts per cab (the "4 ohms single channel driven" spec).



     
  4. christliche

    christliche

    Jan 10, 2007

    My thoughts exactly. Would going either of these routes cause damage? Or would it be safer/wiser to find a different power amp that will deliver less wattage per channel/per cab?
     
  5. christliche

    christliche

    Jan 10, 2007
    I believe it is RMS, from Musician's Friend and Amazon tell me.
     
  6. BadExample

    BadExample

    Jan 21, 2016
    Injiana
    You first need to get the watts we're discussing boiled down to the same unit of measure. Are the cabs 400 Watts RMS, Program or Peak? Without knowing that, we are guessing.
     
  7. BadExample

    BadExample

    Jan 21, 2016
    Injiana
    Haha, we replied at the same time.
     
  8. BadExample

    BadExample

    Jan 21, 2016
    Injiana
    Screenshot_2018-11-19_18-35-45.
    Divide the above figures by 2 and you should be very close to the PA's output in Watts RMS.

    If the cabs are indeed 400 Watts RMS, IMO one per channel would be your best bet. I used to be a firm believer in having an amp that can put out a little more power than the cabs can handle. I'm a bit slow, so after just a few years of reading @agedhorse 's comments on these types of threads, I've changed camps and now want to see the amp have a bit less power than the cab can "take."

    WARNING: The cab's wattage rating is the the voice coil (usually) and the amount of power it can take before the copper melts. It does not (usually) consider the mechanical limitations of the driver itself. I believe our @agedhorse has some familiarity with QSC amps, and if he chimes in, you would be well served to take his advice.
     
  9. ficelles

    ficelles

    Feb 28, 2010
    Devon, England
    The "no frills" answer:

    Dime that amp and you'll fry those cabs. Not enough power handling, and there is no way you can be sure you've pushed the cabs too far until they start making dalek noises, possibly accompanied by magic smoke.
     
  10. BadExample

    BadExample

    Jan 21, 2016
    Injiana
    Good point ;)
     
  11. Marko 1

    Marko 1

    Mar 9, 2009
    N.E. Ohio
    I'd put both cabs on one channel, 600watts per cab which imo is scary. One cab per channel = 725 per, which is scarier yet. My GX5 was great for PA or driving two 500watt 410s.
     
    Downunderwonder and BadExample like this.
  12. Nowhere in that other guys post does he say that he is running the power amp wide open, or implying that the cabs can handle more then their wattage ratings.

    There is no reason to believe that he is running that equipment at full tilt.
     
  13. BadExample

    BadExample

    Jan 21, 2016
    Injiana
    I didn't take the time to find it on their website, but I believe the power ratings of that amp will be in "Peak" Watts. So you can divide by two to get that to Watts RMS. If the cab is rated at 400 Watts RMS, I'd feel better with an amp rated somewhat below the 400 WRMS. Who knows what the 400 WRMS cab can mechanically handle? The only way to find out costs one (presumably expensive) 1 x 15 :D
     
    Last edited: Nov 19, 2018
  14. BadExample

    BadExample

    Jan 21, 2016
    Injiana
    Good points, friend.
     
  15. Marko 1

    Marko 1

    Mar 9, 2009
    N.E. Ohio
    I have one (GX5), pretty sure it's rated RMS/Continuous, not Peak. He'll cook or destroy those speakers imo. :)
     
    BadExample likes this.
  16. BadExample

    BadExample

    Jan 21, 2016
    Injiana
    I think walking 12 ft away from the preamp while using a 10 ft. instrument cable will likely provide the pop of death to the cab :D

    You would certainly know better than I on how the amp is rated. I'll take your word for it :thumbsup:
     
    agedhorse and Marko 1 like this.
  17. ctmullins

    ctmullins fueled by beer and coconut Gold Supporting Member

    Apr 18, 2008
    MS Gulf Coast
    I'm highly opinionated and extremely self-assured
    Remember that amplifier power ratings are like automobile horsepower ratings. Just because you have a Hemi with a 450hp engine doesn’t mean you can’t mosey on down to the grocery store. It only puts out 450hp when it’s being run flat out. And no sane person would run it flat out while doddling around town.

    Likewise, the amp, your preamp, your bass, and your fingers can and should control what portion of the 600 watts is actually going to the cabinet(s).
     
  18. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Not running the power amp's controls wide open doesn't reduce its ability to deliver rated power.

    It is rated are continuous average power (same as "RMS")

    If this were always the case, I wouldn't see so many speakers damaged from too much power, right?
     
  19. fourstringburn

    fourstringburn Supporting Member

    Jun 30, 2009
    New Mexico
    It wouldn't be a bad idea to use a small format mixer or compressor between the power amp and preamp. I use the Behringer RX1602 for visual level control between 2 of my preamps/compressor signals. This also has an effects send with level control which could be used to a FOH direct out.


    my rack.



    A rackmount compressor/limiter would be good because you have meters to see how much signal is going to the power amp plus the limiting feature can control signal overloads. A mixer or compressor has input and output level adjusting capabilities to set levels to the power amp without over loads which you don't want when running high power.

    What Aged Horse said about " Not running the power amp's controls wide open doesn't reduce its ability to deliver rated power." is correct. The level controls on power amps are attenuation controls, not power reduction.

    Depending on the amplifiers input sensitivity voltage level which typically is selectable from .775 volts (0dB) to 1.23 volts ( +4dBu), full power is achieved when the input drive level voltage from a preamp/mixer/or compressor output is reached at the power amp's input sensitivity voltage level.

    You can set the mixer/compressor final output level below 0 dB output on the meters and that will reduce the drive level to the amp thus reducing the amps power output.
     
  20. True but why ask if you aren't looking for maximum MOAR.
     

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