Simple question about running a moderate size PA.

Discussion in 'Mixing & Song Critique' started by killersoundz, Apr 6, 2005.

  1. killersoundz

    killersoundz Guest

    I'm currently in the process of putting together a pretty good kickin PA to host local shows and what not, and I know this is the dumbest question ever, but it's the only thing i'm left to ask. Do you crank the level on your power amps to what you know your speakers can handle and just depend on the output level of your mixer?
  2. jbeutt

    jbeutt Active Member

    Mar 2, 2005
    Home Page:
    what you're looking for is a proper gain structure. You want every piece of equipment to be working at an optimum level, but not clipping. You don't want to drive your amps high and clip them out.
  3. Kswiss

    Kswiss Guest

    usually you double the wattage of your speakers for your amps. So if you have 500w per side speakers, you need 1000 watts per side on your amps. When you push speakers too hard with clean power from a non-clipping amp, they get pushed to their limit and that causes distortion, but thats not how you jack up speakers. If you have 500w per side speakers and 250w per side amps, and the amps are clipping like crazy, thats when the speakers get jacked. When i run live sound, i put a limiter at 0db at the output of the mixer, then i adjust my amps at full volume until they clip, and then back off a couple clicks. That way you know that even if your getting mix crazy, no matter what you aren't gonna jack your amps or your speakers. Then if the sound is getting squashed, you back off your master volume rather than opening up the limiter...

  4. killersoundz

    killersoundz Guest

    really? if your amps are under powered but you push them to clipping that is worst for your setup then pushing tooo much power through the speakers? thats crazy.
  5. JoeH

    JoeH Well-Known Member

    Jun 22, 2004
    Philadelphia, PA/ Greenville, DE
    Home Page:
    really? if your amps are under powered but you push them to clipping that is worst for your setup then pushing tooo much power through the speakers? thats crazy.

    No, it's not crazy at all.

    When an amp is pushed into clipping....surprise! it is then outputting SQUARE waves, almost DC voltage to the voice coils. NOT GOOD for them. They'll burn out much much quicker with a clipped square wave hitting them than the occasional high-wattage transient.

    Underpowered amps are MUCH more dangerous to your speakers than overpowered, in most cases.
  6. killersoundz

    killersoundz Guest

    Yeah, I didn't mean "thats crazy" for real, I just never thought of it that way. After I read that I figured that the amp must put out "bad power" if its clipping, so I pretty much understood the concept.
  7. Kev

    Kev Well-Known Member

    Nov 13, 2001
    Joe is right
    it's not crazy.

    It may only be crazy if you had some sort off hardware level maximiser like the L1/2 such that it was possible to run very hight RMS levels without any peaks.

    This would mean that it would be possible to thermally overload the drivers even though the clip lights never came on.

    I don't think that is the sort of PA system we have here. So Headroom is all about amps being rated well above drivers.

    My rig had beasts ... even on the top end and so I very rarely lost a driver.
    Clipping is your enemy and only the Bass drivers can tolerate regular clipping ... I am talking about a fully active system here.
    Mid range clipping will sound crap and the drivers will suffer eventually.
    Highs even less to none at all.

    Highs and Mids in full range speakers can't tolerate regular clipping.

    jbeutt is right
    learn about gain structure
  8. Kurt Foster

    Kurt Foster Distinguished Member

    Jul 2, 2002
    77 Sunset Lane.
    One other thing to keep in mind about a smaller pa is to be sure you use very herky robust speaker cables.

    12 guage is good ... 19 guage is better. This is because a big ol' fat speaker cable will preserve the damping factor of the amps better than small skinny ones will, keeping the speakers from overshooting at excursion and bottoming out upon return.
  9. Davedog

    Davedog Distinguished Member

    Dec 10, 2001
    Pacific NW
    Gain staging is everything in PA.Large amps with the ability to provide clean power in transients will be a desireable attribute.Underpowered amps will eat your speakers and drivers in a short time,especially if you are doing complex program material with a lot of peaks and dips. Even the most basic compressor/limiter can be your friend live.I've owned a couple of moderate sized rigs....4-way setups...David Hafler power as well as (back in the day) BGW and Crown, and I used Furman Alesis and Symetrix limiting.All worked well...all are still available and all are quite reasonable in price.
    Your cabling will become quite important if you do shows where people have access to the arrays(stacks)....wildly gyrating musical enjoyers around the speaks is disaster unless you use cable connectors that will survive a kick or three.I used industrial twist-loks on all my speaker cabs. as well as very large industrial cable.A hard-usage SJTO or similar is easy to store and can withstand lots of abuse.It also comes in 14,12,10 and even larger sizes.It has a lot of stands and carries frequencies well.As for cable sizing, the electrical industry has made it simple as to size.The SMALLER the number, the LARGER the cable.AWG 12 is much larger cable than 19.However, there are other measurements used by the Audio cableing world and these have to do with the flux values as well as the impedance measurements per foot.
    When you are staging a PA,even in small venues,and you're doing this 'For Hire',industrial is always going to reassure you of a night of unbroken parts and a fair value for your clients money.And it saves you ,in the long run, maintainence costs that are easily avoided by simply designing a system with no compromises at the start.And this can be done with a good amount of research and looking at the used market for particular pieces.Good luck.

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