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  • Power amp question.

    Hello every one,


     I'm a drummer and usually hang out in the drum forum, but I've escaped from there, and I have a question for all the live sound guys on here.

    My question comes from a conversation I had with a friend of mine about using a power amp to drive speakers, where the power amp is to "big" for the speaker ratings.  I realize that this situation is going to damage the speakers, that's not the question. 

    My friend said that he runs the amp at about 50% so that will save his speakers.  To give an example of the situation I'm talking about for lets say that some one is using a 1000W/channel power amp into speakers that have a 500W (program) rating.  In this situation you only turn up the amp halfway, which would decrease the power sent to speakers. 

    Is this arrangement/assumption accurate?  It make sense on a logical level, but I'm not sure that it would be true in reality. 

    This is probably a stupid question, but being a drummer I don't generally deal with these sorts of things, and it peaked my interest to find out more. 

     

    Thanks in advance. 

     

     


  • #2

    You will find lots of threads here on this subject. In short, you cannot "turn down" an amp. The controls are actually to set the sensitivity of the amp. Regardless of that setting, a strong enough input signal will still deliver full power the amp is capable of. With proper restraint, pro audio folks regularly use larger amps than you would suspect, but they do it with skill, careful operation and a knowlege that sometimes things just happen. Unless your friend is either lucky or skilled, he will eventually have an "uh oh" moment with a large repair bill following. 

    Rule of thumb for amps (and this is subject to lots of discussion even here) is that for highly compressed sources like pre-recorded music, using an amp rated for the speakers RMS power is correct. With live music, and depending on skill of operator, an amp capable of 1.5-2.0 times the RMS rating is OK. That works better for subs than mains. I always power at twice RMS, but I have about 50 years of experience. With less experience, use less power.

    You can't convince people of these things often until it is too late...

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    • #3

      I usually match amps up with speakers, giving the amp about 25-33% extra wattage headroom.

      In reality, and in a perfect world, you can match them 1:1,  IF and ONLY IF you never intend sending a peak/clipped signal to them.

       

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      • dboomer
        dboomer commented
        Editing a comment

        dcastar wrote:

        I usually match amps up with speakers, giving the amp about 25-33% extra wattage headroom.

        In reality, and in a perfect world, you can match them 1:1,  IF and ONLY IF you never intend sending a peak/clipped signal to them.

         

        So 25-33% is only 1 dB of headroom which is generally insignificant.  You could lose that in cables.

        Also sending a clipped signal is only a problem if it exceeds the sppropriate power level.  The clipping itself is not a problem.

         

        If the topic is speaker destruction one has to understand what the specs for the gear really mean and how they relate to real world conditions.  According to info published by Celestion, a major speaker supplied for many manufacturers, Speakers rated by the EIA method (such as JBL) at 500W may handle only 60% the power of those rated at 500W by the AES method.  So who's speaker are we talking about?

         

        Here's some info about how speaker ratings stack up ...http://www.peavey.com/support/technotes/concepts/THE_LOUDSPEAKER_SPEC_SHEET_GAME_2005.pdf

         

        And here for a reasonable way to plan amp power for your speakers ...http://www.peavey.com/support/technotes/poweramps/HOW_MUCH_POWER.pdf


    • #4

       

      My friend said that he runs the amp at about 50% so that will save his speakers.  To give an example of the situation I'm talking about for lets say that some one is using a 1000W/channel power amp into speakers that have a 500W (program) rating.  In this situation you only turn up the amp halfway, which would decrease the power sent to speakers. 

       

       

       


      Why do you think that?

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      • SYMBOLIC
        SYMBOLIC commented
        Editing a comment

        tlbonehead wrote:

         

        My friend said that he runs the amp at about 50% so that will save his speakers.  To give an example of the situation I'm talking about for lets say that some one is using a 1000W/channel power amp into speakers that have a 500W (program) rating.  In this situation you only turn up the amp halfway, which would decrease the power sent to speakers. 

         

         

         


        Why do you think that?


        That was what my friend said to me.  I'm a drummer and don't generally work with amps.  Not knowing exactly who an amp works it makes sense if you think of it like a garden hose spigot, which it clearly isn't from what the posts have said.  I was just wondering about how it actually works. 


    • #5
      Not a stupid question at all. Axis's answer is spot on.

      Also, a clipped signal can contain way more power in un-intended frequency spectrums because of the harmonics that are generated while clipping. Additionally, a clipping amp can produce up to twice it's rated power, another in-intentional byproduct.
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