BLowing a speaker

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by chetsmith, Jan 19, 2008.

  1. chetsmith

    chetsmith Member

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    I recently blew a 25 Watt Celestion Greenback with a 18 watt amp and I started researching why Speakers blow.... and I discovered that guitar speakers blow becouse they are not getting enough watts (Under Powered) as I read further the information basically said that your amp shoud be twice the wattage of the speaker... Is there anyone here that can go into more detail about this process???


    Thanks
     
  2. hamfist

    hamfist Member

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    Hey dude, somebody's giving you spurious info ! If anything, the speaker wattage should be approaching twice the amp wattage, for real safety.
    As for the "guitar speakers blow because they are not getting enough watts", that is complete BS, I'm afraid.
    Do a search, there's loads of info on this sort of stuff on the forum.
     
  3. chetsmith

    chetsmith Member

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    Thanks... Thats what I thought but maybe I mis read the info... it was not real clear... So I should install a 30 watt or greater for a 18 watt amp...

    What about the concept of speakers blowing becouse they are underpowered do you know anything about that?

    Thanks for your time
     
  4. John Phillips

    John Phillips Member

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    This is absolutely wrong. Speakers always, and only, blow because they are given too much power, sometimes in the wrong frequency range - period. You cannot blow a speaker with too little power.

    The reason you can sometimes blow a speaker with an under-powered amp is that by making the amp distort, the maximum power produced is a lot higher (up to double the clean rating of the amp), and will usually be in frequency ranges (very low and very high) that are inherently more stressful for the speaker.

    This is especially true of hi-fi and PA speaker where there is a crossover. If the amp goes into clipping distortion, the sound will be split (crudely, this is to illustrate roughly what happens) into very sharp transients (very high power in the very high frequencies), which because there is a crossover will be sent only to the tweeter, and sections of DC (very high power in the very low frequencies), which will be sent only to the bass driver - so instead of being equally distributed, the amp's whole power - up to double the clean rating - will alternately be sent to each driver, and will probably blow them since it could be more than double what they will be designed to take if the amp and speaker ratings are similar.

    The best way of avoiding this is by preventing the amp from distorting, so using a higher-powered amp than the speakers will help. In a typical music program, the peaks will be at least 3dB higher than the average level, and 3dB is equivalent to a doubling of power, so if you make the amp up to twice as powerful as the speakers, when it first audibly clips the power will still be below the maximum the speakers will take, and you won't blow them - provided you pay attention and back off a bit if it distorts. This is where that information you read comes from.

    But in a guitar (and usually bass) amp, distortion is expected and intentional, so you can't rely on this rule, and you have to go the other way - the speaker must be capable of taking everything the amp can throw at it, even fully distorted. So ideally, you need the speaker to have double the power rating of the amp, not the other way round.

    Your 18W amp could easily peak at 30 to 35W fully distorted, and although Celestion do rate their speakers conservatively by the standards of speaker makers, that's only a 25W speaker and you might have just exceeded what it would really take.


    So there are two rules:

    For PA, monitors or hi-fi, you want the amp to be 50% to 100% more powerful than the speaker rating, but you must not allow the amp to distort.

    For guitar and bass, you want the speakers to be 50% to 100% higher rated than the amp - or more if you like, you can't do any harm with even more safety margin.


    A 30W speaker would probably be enough for that amp, and a 50W one should be completely safe.
     
  5. chetsmith

    chetsmith Member

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    Perfect... Now that makes more sense. I really appreciate the help. I'm still learning the technical aspects of Amps and speakers, mainly becouse I do my own repairs etc.

    Regards
     
  6. Groovey Records

    Groovey Records Member

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    "For guitar and bass, you want the speakers to be 50% to 100% higher rated than the amp - or more if you like, you can't do any harm with even more safety margin.
    "


    Is this each speaker? or shared total power rating in a 2x12 or 4x12.

    i.e. 16 watt el84 into 2x12 celestion blues rated 15 watts = 30 watts in the right neighborhood.?

    Thanks
    joe tee


    "The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science. He to whom this emotion is a stranger, who can no longer pause to wonder and stand rapt in awe, is as good as dead: his eyes are closed." - Albert Einstein
     
  7. rockon1

    rockon1 Member

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    Total combined power rating. Bob
     
  8. DADGADammit

    DADGADammit Member

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    yep i blew a 120 watt cab with a 100 watt. i learned a lesson that day
     
  9. NitroLiq

    NitroLiq Member

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    Also remember that in a mismatched cab of say a blue (15w) and G12H (30w), you double the speaker with the lowest power rating so in effect the rating for the cab would be 30w not 45w.
     

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