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PA question

Discussion in 'Band Management [BG]' started by Hobbes, Aug 14, 2005.

  1. Just wondered if anyone had any opinions (or facts) on what the ideal ratio between the power amp and speakers is. For example, if I had a speaker which is rated at 500W RMS, 1000W program and 2000W peak, what would be the ideal amount of power (RMS) to be putting into the speaker from your amp?

    Any comments are gratefully received.
  2. For general PA use, the JBL site recommends matching the RMS power rating of the speaker to the RMS power rating of the amp.

    The JBL site has a document that recognizes 3 situations:

    1) where the amp's RMS power rating can exceed the speaker. Some people exceed the speaker's RMS rating in carefully controlled situations (recording room monitors, for example--and some folks here like to have tons more amp headroom than their cabs are rated). Remember this can be hazardous to the speaker, so it's not for everyone.

    2) General PA use. Match the speakers to the amp's power.

    3) Overdriven instrument use, such as an electric guitar amp speaker that will frequently drive the amp into clipping. They recommend a speaker rated twice the amp's RMS power rating for this extreme application.

    When I run sound, I frequently have the system running at the upper end of its limits, and all power amps are hitting the clip light occassionally. I have some pretty good 3-ways rated exactly what the amp is rated, and so far no problems.
  3. RicPlaya


    Apr 22, 2003
    Whitmoretucky MI
    You want to power your speakers with 1.5 to 2.0 times the RMS.

    So if your speaker is rated for 500 RMS....750 to 1000 watts per speaker is where you want to be.
  4. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI
    How NICE to see a concise, SANE answer to this question.

    I'll tell'ya, Hobbes: I have been practically infuriated over this subject several times, because of the BOGUS answers I've seen to this question here on TB! I mean there is such a wealth of knowledge - good, good advice - avialable here, but on this subject I've seen WRONG advice given several times; being that this community is so respected, I fear that who-knows-how-many folks have just tootled-off and used this advice to their hurt! ..and now they're archived forever with search capability!

    This is a question that takes just a LITTLE wisdom and knowledge to properly answer, and I've seen so many 'dogmatic blanket statements' thrown around here - and THEN stuborn yet baseless defence of these superstitions.

    The WORST is when someone just throws-out "amp power should always be twice the speaker rating". If some newbie power-punk kid takes this advice, he will SURELY destroy one or more speakers - SURELY! Even a responsible and conservative operator must be warned about this situation; the warning being that you can't make a mistake - you can't bump the master knob or power-down devices in the wrong order, for example - without a very-real chance of damaging a speaker driver. I know I'm a little-more speaking of bass amps rather than PA stuff, but still...

    Hobbes: also there's a little confusion maybe built into your question that relates to some of the crazyness... You summed things up by asking "what would be the ideal amount of power (RMS) to be putting into the speaker from your amp?". Didn't you mean "what would be the ideal (RMS) power amp RATING to be putting into the speaker"? This is not 'just getting picky with words'; firstly, the 'ideal amount of power' is that amount that produces the right audio volume - simple as that; secondly, in cases where extreme fidelity and headroom is desired, you SHOULD go with "twice the amp power", but the whole idea of 'headroom' is that there's 'extra' you don't use. Indeed - with the hi-fi-headroom scenario here (amp rated twice speakers), you can't DARE to "be putting (it - the amp's full-rated power, that is) into the speaker"! See what I mean?

  5. Cheers for the comments everyone.

    Basically I have seen a few different answers to this question in the past, so thought I'd post to see if I could get as near to a definitive answer as I could. I've been in gigging bands for years, but am in the process of uprating our PA system. I've just got hold of a pair of Peavey Hisys 4's (rated at 700W RMS) and am now choosing a power amp for them, hence the original question.

    Joe P, I did mean the ideal power amp rating, just some lazy use of English on my part.

    Anyhow, I'll be going with the general PA use guideline (thanks for that info nashvillebill) by looking to match the power amp rating to the speaker rating (or near enough, the amp I'm looking at is rated 750W RMS).

    Thanks again people.
  6. theshadow2001


    Jun 17, 2004
    I'm going to have to agree with Joe P on this one. I think if the PA is around 1000 watts (500 a side) Simply match the the speakers RMS equal to the amps RMS or a little higher wouldn't hurt either.

    It's probably a bit late to tell you now but my band is looking at a new PA system as well and whilst I was buying my mesa M pulse recently me and the guitarist tried a fantastic dB PA out. It had active speakers(i.e. you plug them in) with built in crossovers seperating highs lows and mids with real punch and clarity.

    No need for any power amps crossovers etc. It's just plug and play. Plus we can keep our powered mixer as the desk and use its amp to run monitors. Absolutley perfect! Anyways active speakers seem to get rid of a lot of hassle.
  7. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    EAW recomments 1.5 to 2 times the cabiniet rating. I bridge a QSC PLX 2402 for 2400 watts into each of my EAW SBX 220s. Scott Jordan at EAW said I'd be just a hair underpowered at this level, since the subs are rated at 1,400 watts continuous.

    The key is to recognize that all the extra power is not to drive the speakers at that level, but to provide ample headroom for transient signals. Those subs can handle every watt of the amps for short periods of time. Running them at maximum will kill them ,though.

    Yes, you will always have the knucklehead who doesn't understand that consistently overpowering the amp is as bad or worse than underpowering it.

    It reminds me of a former friend of mine who, when we were in college, had a Jeep with a V8 in it, and when we'd go out 4-wheeling (I had a Jeep also), he'd point his at the hill, and put the gas to the wood. He never learned the sound of valves floating and, sure enough, one day his engine went KOFF KOFF KOFF after it swallowed a valve. Lucky I had a strap to tow him out of the boondocks. Just because an engine will run at 7,500 RPM doesn't mean it will do it for any length of time. Same thing with amps and speakers.
  8. mwm70


    Oct 27, 2004
    Great thread and info.

    Another question in the same realm, What power handling number for the speakers do you use for determining amp power? Continuous, program or peak? I know what the definition of peak is, what are the definitions of continous and program. I need to get a power amp and the speakers I am using are rated at 350 continuous, 700 program, 1400 peak. The band is going to mic everything and run through these speakers for small bar gigs. Thanks for any help.
  9. RicPlaya


    Apr 22, 2003
    Whitmoretucky MI
  10. Continuous is the number you want.

    Get power amp with RMS rating 1.5 to 2x the continuous rating of the speaker.

    The people who are recommending matching the amp to speaker rating may be using the program rating of the speaker, which usually works out to about 1.5 to 2x the continuous rating of the speaker. In which case they're agreeing with the 1.5 to 2x people. Or they could be referring to continuous rating instead, in which case they disagree with the 1.5 to 2x people.

  11. Yeah, I guess the JBL folks don't have a clue what to recommend for their speakers. :rolleyes:

    Go ahead, be my guest, it's your speakers...run 1000 watts RMS into a PA speaker rated 500 watts RMS. Then wonder why a horn fried when somebody unplugged their guitar when the system was hot.... :eek:

    P.S. Here's a link to the JBL document:

  12. theshadow2001


    Jun 17, 2004
    I dunno. Why cant you use an amplifier that will give you way more power than you need so you only turn up to maybe 1/4 to 1/2 of full volume but still have speakers that match the RMS of the full output of the amp?

    For example if your using an amp rated 3000 watts RMS and speakers that can take 3000 watts RMS (or there abouts) but only ever use the amp at 1/4 or 1/2 the full output.

    This way you have the head room for sending non distorted signals but you also have the option to push a bit more out of the amp if need be without damaging anything. I'm not talking about pushing the amp to the point that it's clipping though, that would be silly.

    In college I do a course about manufacturing machines and automation and what not. Safety is a big issue in this field and you have to design machines with the idea that people are stupid and will do stupid things which will endager the machine and themselves. The machine would ideally be made so no matter how stupid someone acts they wont hurt themselves or damge the machine.

    Maybe the ideal is to have the amp rated at 1.5 to 2 times the speakers. From what I gather the amp is never run at full capacity in this scenario. But people being people and people being stupid what are the chances that someone will run the amp too hot and end up damaging speakers and everyones hearing as well.

    I know that if I owned a venue with house PA where someone who might be unfamiliar with the system or worse know nothing about what their doing at all could easily be involved with running the rig I'd be a little happier knowing it was that bit more difficult to blow the rig.
  13. In my opinion, the problem is this:

    Amplifiers operate on gain, they don't regulate the power being output.

    Let's say you have a 1000 watt amp, you drive it with a signal that puts out exactly 500 watts (into your speakers rated 500 watts) with the volume knob turned to 1/2 of max (lets say the knob went to 10, you turned it on 5).

    In the case of a PA, the signal from the mixer's main outs would be the signal driving the power amp. All is well and good....for now!

    But suppose the signal from the PA suddenly got much stronger, say some idiot unplugged his guitar. POP-thump-BUZZ for a few seconds, very loud. Or the singer cups their hand over the mic and feedback galore strikes!!! Or somebody bumps the master fader up full. Now the amp's settings haven't been changed--nobody's touched the volume knob, it's still on 5--but the input signal to the amp has suddenly gotten much much stronger. The amp will merrily multiply this stronger signal by the same amount of gain, and instead of 500 watts, it's pumping out a full 1000 watts (this could still be clean, without clipping!). The volume knob is still on 5, but the amp is going all out. Since tweeters and horns don't cope well with weird noises like feedback or unplugged guitar cords at high power levels, this can be very bad. Clipping is not the issue here, it's overpowering the speaker.

    For monitor use in a studio, or driving massive subwoofers or bass cabs, this situation may be rare, there shouldn't be mic feedback in your bass cab. And studio engineers can keep a pretty good tab on things, so the 2-to-1 ratio (amp to speaker) may not be a problem.

    But for general PA use, where there's lots of opportunities for screwing up, I support the 1 to 1 theory (amp matches speaker).

    Ultimately, though, I think the issue really goes back one level deeper. Instead of randomly buying a speaker (whether bass or PA) then deciding how much power they can "overdrive" the speaker with, I think most people would be better off if they determine how much sound volume they really need first. Then buy an amp with enough headroom to supply that volume along with speakers that can handle the full power output of the amp. This gives headroom for the amp as well as headroom for the speakers.

    Other people obviously prefer a different philosophy, but I'm more conservative with my approach. For people that may not be used to running a PA, I believe conservative=good. For those people who like to run with much more power, that's their preference, I'm not preaching to them or insisting they're wrong, after all it's their equipment and their situation.

    However, since there isn't always a "one size fits all" approach when matching speaker and amp ratings, it's my opinion that just throwing out a statement like "buy twice the speaker's rating" could get a beginner into a bad situation.
  14. Passinwind

    Passinwind I Know Nothing Supporting Member Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Here's some food for thought: http://www.prosoundweb.com/studyhall/studyjump.php?pdf=watts

    Notice the discussion of the difference in power rating methods for speakers and amps? Interesting, no?
  15. The JBL folks test their speakers with 6db peaks, as noted in the article. That means they're pumping short bursts of 2000 watts into their 500 Watt RMS speakers, and actually certified to take that under similar conditions. But absolutely, if you're using your 1000W amp pushed to the limit so it averages > 500 watts all the time, you certainly will blow the speakers. Momentary peaks are not going to blow stuff however, within reason.

    If you're running into clipping consistently, even matching the power amp rating to the speaker isn't safe. Clipping results in additional high frequencies, plus higher avg power from the amp than its rated for. The combination can easily overwhelm the tweeters and blow them.

    The bottom line is you or your sound guy have to pay attention. There's no escape from trying to squeeze 25 pounds of sound out of 5 pound speakers. No defense exists for stupid.
    Sure, if you get more amp/speaker than you need, so you aren't pushing the speakers, you can get a 1000000 watt amp with your 3000 watt speakers, never worry about blowing anything if 1000W is enough power to get the volume you need. Outside of people accidentally unplugging stuff while the power amp is live....

  16. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    I use a PLX 3402 to drive four 150-watt monitor cabs. With two 8-ohm cabs per side, I'm getting about 1,100 watts per side into 300 watts worth of speakers. Never had any problem of any kind. Why? Because I'm not so dumb as to crank them up into distortion. This is why god gave us input attenuators.
  17. theshadow2001


    Jun 17, 2004
    The article passinwind has shown is indeed interesting however I'm finding it a little difficult to keep up with but im tryin my best to come to terms with it

    The amp rated twice the speaker will amplify peaks by 6dB above the RMS as opposed to 3dB which an amp half the power and therefore evenly matched with the speaker wil do.

    Is this correct? Or have I got it wrong? If I am right I don't see how making peaks larger is going to do anything good for the speaker.

    It says in the document that the speakers RMS is measured using pink noise which includes signals 6db above the speakers RMS value. But how do you know your introducing a signal thats 6dB above what you haven't yet measured i.e. the RMS value of the speaker?

    To me this test seems to be testing the speaker to it's extremes like any test should. So I don't know why you would want to repeat these extreme's under normal operating conditions.

    Maybe I have got the whole thing confused and if I have I would apreciate if someone would put me straight in understanding this document (preferably with a bit of logic as opposed to "It just is" sort of thing:D) as I'm very interested in the topic.

    Also this whole discussion is making the whole active speakers option seem more appealing. :smug:
  18. Passinwind

    Passinwind I Know Nothing Supporting Member Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    The former amp will do that extra 3dB without clipping if you ever ask it to. That's the headroom that doubling amplifier power affords, in a nutshell. :cool: The idea is to keep peaks unclipped, not necessarily to make them bigger. Dig?

    That article comes from the Study Hall at www.prosoundweb.com . There is a tremendous amount of reference material on that site, if you're in the market for a PA you might want to spend a few hours over there.

    So far everyone seems fixated on the speakers blowing up. Amps blow up too though, and uprating the amp means less strain at nominal operating levels. To me, never seeing the clipping light come on qualifies as being conservative too, no?

    Anyhow, I have no problem with Bill and Joe's takes, but they're maybe gonna have to live with the fact that the vast majority of the pro audio community prefer to uprate amp power, and it works for a great many of us without any drama at all. But then, I can drive a Porsche at or under the speed limit all day too. ;)
  19. Well, I'm not going to call Bill wrong by a long shot. The operative word in your statement above is PRO audio community.

    Pro's show up with 1000 watts worth of amp/speakers when 500 is enough, then run the rig under its capacity. Bar bands, especially on budget, try to get away with just enough PA to get by, so they're running their stuff to capacity night after night. IF you run your system balls to the wall, the 1 to 1 ratio is probably going to last a lot longer than having extra power available that the speaker can't handle CONTINUOUSLY. THat's the difference, the 2 to 1 ratio allows you to reproduce more PEAKS cleanly. If the sound guy is (the usual scenario) the burnout stepchild cousin of someone in the band who doesn't know what they're doing, the 2 to 1 ratio can fry the speakers up pretty quick if you use that extra power for "normal" operation intstead of reserving it for "clean peak" reproduction.

    As Spiderman learned, with great power comes great responsibility.

  20. Passinwind

    Passinwind I Know Nothing Supporting Member Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    I agree, and Bill and Joe and I pretty much agree on the whole subject too, judging from our many previous go-rounds. And I'm not into telling anyone what they should do either, just trying to provide some balance.

    The original poster asked for the ideal recommendation, which I read as what will sound the very best. Uprating the amp will likely sound the very best when operated correctly, but it's moot if you don't need to wring every ounce of performance out of your system. Spec'ing a system that'll do substantially more than you currently need is a fine idea IMHO. It's always good to have a little in the bank. :cool: