Proper PA power?

Discussion in 'Live Sound [BG]' started by girhen, Jul 5, 2013.

  1. girhen


    Mar 20, 2012
    I've been doing PA reading, and it seems I have cleared up at least one thing that I had backwards. I would have personally thought it best to have speakers that can handle more power than my PA can output so I never blow them, but people say to drive them with more power than they're rated.

    The trouble is, I see different rules of thumb. Drive 1.5x the speaker RMS, Drive 2x. Drive 2.5x. Drive 2.5 for metal, 1.5 for acoustic. Never drive more than 1.5x. Drive 1x. Ten sources, fifteen opinions.

    What are your power recommendations? It's a metal band along the lines of an Iron Maiden, Type O Negative, and Avenged Sevenfold blend. I have a Mackie 808m, which puts out 450W @4 ohms. My old drummer had the speakers, hence the need now.

    * This would mostly be for practice and small venues or house parties. The local venues here tend to have their own systems, if we can get a gig.
  2. Floyd Eye

    Floyd Eye Inactive

    Feb 21, 2010
    St. Louis
    Drive them with more power than they're rated?

    Um, keep reading bro. ;)
  3. Floyd Eye

    Floyd Eye Inactive

    Feb 21, 2010
    St. Louis
    Are you just looking for mains? Do you have subs? Monitors?
  4. girhen


    Mar 20, 2012
    I'm looking at mains now, monitors in the near future. Step 1: Get something to use for now rather running through a bass cab with the highs reduced for safety (and get my cab back!).

    What I've been seeing is that if a speaker is rated at 100W RMS @x ohms, to get an amp that's rated 150W (per the 1.5x rule) @x ohms. And if there are two identical speakers in parallel 100W each @y ohms (y coming in as half their original impedences), have a head that is rated for 300W @y ohms.

    But how much power RMS should I be looking for with my head?
  5. Floyd Eye

    Floyd Eye Inactive

    Feb 21, 2010
    St. Louis
    There really is no hard fast rule. Most guys have enough power to blow their speakers apart, but you have to use your ears. Don't pump 300 watts into 150 watt speakers full on and expect them to survive.

    If I were you I would get a whole different PA set up, but the simplest answer I can give you to your question is to just buy a used pair of Yamaha club series. Using the 808 you ain't going to do much better than that IMO.
  6. JohnMCA72


    Feb 4, 2009
    If you want to play loud, get the speakers that can do it. Size the amps according to what the speakers can handle, using whichever multiplier "rule" you happen to believe in.

    What you don't want is anything to be working too hard. That's when stuff starts breaking.
  7. Hi.

    First of all, the idea behind having more power output on the amps than the speakers can handle, originates from the same fact as the almost always misunderstood/mis-quoted underpowering "myth".

    An amp, especially SS, can have a lot more distortion content in the upper region of the frequency spectrum than in the mids/lows when pushed to their limits. Badly/cheaply designed amps are worse than pro level amps.
    On high SPL systems, that high frequency distortion can go un-noticed because the mids do not distort yet, or the driver distortion is masked by intentionally generated distortion. The result is a blown high-frequency driver by OVER-POWERING the driver.
    Low cross-over point on 2 way systems increases that possibility drastically.

    In the past, the amplifiers cost a lot more clamshells/watt than the speakers did, and that obviously led to using small amps with a lot of speakers. That also led to using horn loaded cabs, and since the horn shape design reduces audible distortion, the problems were actually worse than with direct radiator designs.

    Pros obviously knew these things just as well (or even better ;)) than they do now, but the weekend warriors and clueless engineers were buying high frequency drivers and repair kits on a weekly basis.

    USE YOUR EARS, and use quality gear and You'll be golden no matter what combination you choose to use.

  8. The rule you’re referring to is "more power than speaker", so the amps don’t have to run cranked to push the speakers - reasoning that the amps last longer because they idle along (don’t have to work as hard).

    The volume will only be as loud as what the speakers are capable of delivering. Start with speakers that cover your needs and have enough power to damage the drivers - but, never push the speakers to failure.

    Overhead - You can never have to much extra power!!!
  9. uhdinator


    Apr 20, 2010
    There's 3 ratings on most speakers.

    Continuous RMS (1Khz sine wave at steady amplitude

    Program RMS (music/varying freq's and amplitudes)

    Peak (momentary transients)

    A JBL prx415m speaker specs
    300 cont (1/4 of peak)
    600 prog (1/2 of peak)
    1200 peak

    You'll notice that if you only have the peak rating of a speaker, you can figure the cont rating will be 1/4 of peak, prog rating is 1/2 of peak.

    1.5 x 300= 450
    2 x 300= 600
    4 x 300= 1200

    A 300w cont rated amp will work but have no headroom for peaks without going into unacceptable levels of harmonic distortion if amplifier input is overdriven.

    A 600w cont rated amp will allow 3 db of headroom before unacceptable harmonic distortion. (2x cont rating)

    A 1200w cont rated amp will allow 6 db headroom (4x cont rating). It gives the best headroom but would require using a limiter to protect the speaker(s).

    Live music has greater dynamic range than recorded music that's compressed and limited during production. A system used for a DJ can get away with less headroom. A band system you would want more headroom.
  10. Zooberwerx

    Zooberwerx Gold Supporting Member

    Dec 21, 2002
    Virginia Beach, VA
    ^^^ This is what I use as a guide. Case in point: we're using (2) daisy-chained Carvin 1503's (400 / 800 / 1600) powered by a single XLS2500 (2400 watts / 4 ohm bridged mono). Simple case of overkill, right? Not really. Each enclosure has the potential to "see" 1200 watts which equates to 1.5 X program RMS of 800 watts. Plenty of headroom and the amp doesn't even break a sweat. Add some judicious limiting and common sense and we're good to go.

    This blurb was thrown at me several years ago by a builder several years ago: "...stupid behavior results in damaged speakers." IOW, pushing any driver / enclosure configuration beyond its design specs & limitations may result in component failure.

  11. uhdinator


    Apr 20, 2010
    ^ The 1.5 to 2x guide is based on continuous rating not the program rating. The program rating IS 2x the continuous rating.
  12. bongomania

    bongomania Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Even just a couple of days ago we had a post from the Tech21 company representative claiming that most pro audio guys run double the power that their speakers can handle, and his own guy runs THREE times the power. He made this statement with total sincerity, intending to show how real professional guys do it.

    Who am I to say they are all mistaken--especially the 3x power guy--after all, I'm not a professional soundman, nor do I represent a major brand!
  13. uhdinator


    Apr 20, 2010
    The problem is that "3 times the power the speaker can handle" means nothing without stating if your basing the 3x on the continuous or peak.

    A speaker can handle 300 cont and 1200 peak. 900w would be fine.
    3x peak would not.
  14. Zooberwerx

    Zooberwerx Gold Supporting Member

    Dec 21, 2002
    Virginia Beach, VA
    ...but they don't get much work because they've blown up all their enclosures!

    Just kidding. The extra power does allow for expansion / adaptation to meet the needs of the venue. I found this to be handy in more than a couple of situations.

  15. thumbknuckle

    thumbknuckle In Memoriam

    May 23, 2012
    Westfield, MA
    You never want the amp to be the limiting factor.

    The amount of power a speaker can make use of is a complicated thing, depending on crest factor, total power, frequency distribution, thermal power compression, and a bunch of other stuff. You can blow up a speaker box with much less power than the number on the box indicates and you can safely run it with much more. You just have to know what you are doing.

    Amps are much more straightforward, once you hit the rail voltage or current capability of the power supply you are done. That's all you get and it sounds terrible. This is also a good way to pop all your compression drivers with almost no warning.

    If you are using an amplifier rated at or below the thermal power handling rating of a speaker box you are leaving a lot of available SPL on the table. and you still have all the power you need to damage the box.

    Power is cheap, get a bunch of it.
  16. derrico1

    derrico1 Supporting Member

    Apr 12, 2005
    Charlottesville, VA
    I'm w/ uhdinator and zooberwax in the general school of thought about system headroom. That's a different question, though, than the way the OP is thinking about system design (essentially: "how do I match components to get a foolproof loud with *no chance* of blowing speakers?").

    The only rule of thumb there is that you have to listen to your system until you begin to know its safe operating range. If your speakers are chuffing or distorting, you need to back off the volume or adjust the eq, no matter what the component ratings.

    Give the OP's genre preference, hearing system distortion can be harder, but cueing on the the elements of the band's mix that aren't intentionally distorted can help—as will listening to a variety of source material when setting up the system.
  17. Zooberwerx

    Zooberwerx Gold Supporting Member

    Dec 21, 2002
    Virginia Beach, VA
    Ha-ha.....$14.95 at Pep Boys. I'll be a zillionaire!

  18. girhen


    Mar 20, 2012
    Ha, all good stuff. Add 3x rule to my list. :cool:

    I've always been paranoid about my stuff. I once noticed how far my speakers appeared to be moving despite being on an amp weaker than its max and turned down. Guess I just need to keep my wits about me, trust myself, and not let bandmates do anything dumb.
  19. ggunn


    Aug 30, 2006
    Austin, TX
    It's complicated. Yes, you can blow up some speakers/crossovers by driving them with a relatively small amp pushed far into clipping. I have done that and it was expensive. You certainly can blow up speakers by overpowering them as well. Done that, too.

    A better answer than saying speakers should be 2X the amp rating or amp 2X the speaker rating IMO is that they should both be capable of producing 2X to 4X the volume you will ever need. If you never push speaker or amp even close to its limit, it doesn't really make all that much difference which is rated higher and your sound will be much cleaner. Headroom is where it's at.
  20. I've worked as a sound engineer for the past 3 years,

    From my experience its always good to have much more power than you actually need, I've almost always only ever run my amps at half power (gains say about 12 o clock)

    Get a decent set of speakers than can handle the power :)
    Also, watch for your ohm ratings, if they don't match your output will be more of less what you expect!