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using the correct amps

Discussion in 'Sound, Music, and Intercom' started by Les, May 4, 2005.

  1. Les

    Les Well-Known Member

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    I'm just considering buying some speakers but I was wondering, If I had a cabinet that was rated at 300w RMS, 600w PEAK @ 8ohms, should I buy an amp that delivers 300w at 8ohms?
     
  2. soundman1024

    soundman1024 Active Member

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    Acutally it is normal to get an amp that handles more power than your speakers. If you could provide more information it would help people out alot, such as how many cabinets you plan on getting, if you plan on bridging the amp, and other factors can change what amp people here would be able to advise you. You will get signifigantly more power out of an amp in mono mode with 4Ω load then you will get out of an amp in stereo with an 8Ω load.
     
  3. JasonH

    JasonH Active Member

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    Get an amp that is rated for 600 watts at 8 ohms. Thats your safest route.
     
  4. wolf825

    wolf825 Senior Team Emeritus Premium Member

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    NO. :)

    I could just give you the answer..but I want to try and explain this for fun and the amusement of my peers as this is a difficult subject to explain.. Amp and speaker ratings are tricky to read and understand--as there is no real universal "standard" for rating amps and speakers. Some manufacturers will rate their gear using a 1Khz tone (which is a very powerful frequency and will give them a higher response, higher SPL, and better numbers that makes their gear look GREAT when it may SUCK and do 80 or so watts LESS at full range or lower frequencies)...and other makers may rate it using Pink Noise, white noise, fixed tones & harmonics etc...and still others have their own unique way to rate their gear for RMS or peak (what "peak"--how was it measured??) or whatever method they use that probably involves voodoo dolls.. Either way a lot of "ratings" tend NOT to be disclosed in the spec's for method..and the ones that do--they hope you do not know what they are talking about but hey--it sounds technical so it MUST be good.. However....this is a rant all its own.. :)

    Typically if you have a speaker that is 300w max at 8 ohms, you will want an amp that is at least, IMO, 1.5 to 2 times or greater that speaker max 'peak whatever' rating....so a 450-600w max amp at 8 ohms works well, minimally. If you have a 600 watt PEAK speaker--think along the lines of 800-1200 watts of power or greater. Higher the better actually works in your favor in this instances...especially if you have to consider that you may connect MORE then one speaker to that amp--and while your ohm rating may drop, your wattage capacity increases--and is then divided by the number of speakers. I hate the math..and someone check me on it.. but for Example--Mr Speaker wants 300w at 8 ohms MAX, 450 at 4 ohms MAX...and you are hooking up four (4) speakers (2 per side) which will give you a rating of 4 ohms per side... So 450w speakers x2 = 900 watts at 4 ohms.. You want an amp that can push over 900w at 4ohms....cause at max output each speaker will then get its 450 watts max.. Not a problem--most amps give you the 8ohm and 4ohm ratings..and they are CLOSE. If you get an amp that does 450 watts at 4ohms--your max ability to each speaker then becomes only 225watts....and not the 450w it can handle...

    Why a stronger amp then the speaker? Well consider this...if you have a speaker that has a max rating of 300w, and an amp that has a max rating of 300w, due to heat and resistances that build up in an amp while being used, your amp may actually begin to clip or peak out and provide LESS wattage then the max rating it was tested and rated under. Or given the specs may be BS--it may not come close to giving you full range max watts that it claims were "max". It may clip and provide only 250 watts at FULL RANGE and 300 watts at 1khz....or even MUCH less wattage at full range. Point is, Underpowering a speaker is brutal abuse to a speaker as it causes the speaker to work harder and more heat is generated in the coil--especially if its is expected to be driven at its max performance. The speaker that can do more then the amp can most likely put out, is NOT safe driving. When an amp CLIPS (the little red or green light goes on suddenly)--it can send distortion to the speaker.. Distortion is that muddy garbled messy sound that is suddenly there and then gone and everyone hates it.. That distortion is in essence a square wave of electrical energy..and its asking the speaker to over exert and produce a square wave garbled mess cause the amp can't send out any more.. Distortion like that on a regular basis will KILL a speaker quick and you only get to use LESS then the speakers max capability. With a higher wattage amp, you can run the speaker to its maximum performance without concern for clipping at the amplifier.. All my amps are twice the power rating minimum for the speakers they drive....and my speakers tend to last for years of daily use at hard levels.

    Think of underpowering a speaker it in terms of cars and engines-- Its like using a 4cyl engine to drive a 4x4 truck...you will burn out the engine long before you get the max use from the truck.... So for your amp think about 1.5 to 2 times the max rating for your sepakers and you should do just dandy....

    -w
     
  5. Foxinabox10

    Foxinabox10 Active Member

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    But, can't you absolutely avoid blowing a speaker by having your amp power less than the speaker?
     
  6. Les

    Les Well-Known Member

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    Thanks wolf - suddenly JBL Eon powered speakers are sounding really good to me lol. I'm thinking I could get a pair of those for cheaper than a mediocre set of speakers and amp.

    Here's an off-the-topic question: Do the Eon 15's have pole mounts in the bottom?
     
  7. JasonH

    JasonH Active Member

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    Yes they do, AFAIK
     
  8. JahJahwarrior

    JahJahwarrior Active Member

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    and no, you cannot avoid blowing a speaker, no matter what amp you use. speakers can blow at way less than their rating for two main reasons:
    1) your speakers are crap, or lemons
    2) you are overpowering your amp, and you send a square wave for too long to either your woofer or your horn, but I know it can kill a horn fast

    theres some dude name....um.....shavano, that's it! He has a great article that says the same thing as wolf, except a little clearer I think, and it's longer too :)

    Simply put, if your amp is straining to pump, then 1) it's going to distort. Even a little distortion sounds bad--I think you can notice it at like 5% THD. Most amps have a raiting, like 2% THD at so and so db, or SPL, or something like that, if I remember right. Anyways, the harder you push the amp, the more likely you will get distortion that doesn't sound good. 2) you have no headroom. say all of a sudden you want it to be LOUD!!! Great, you have a 250 watt at 4 ohm amp giong into a 4 ohm load, say the total load is rated for 250 wattsat 4 ohms. perfect match, right? nope. You push the amp, which distorts and sound like crap, andmight hurt a speaker, or your amp. 3) you push the amp too hard and it's gonna die alot sooner than otherwise. If you are going up to it's limit, then it's gonna run hotter, when it runs hotter things die faster, and if it's in a rack, it heats up the whole thing more, which might lead to other stuff dying. Or you can buy a really good rack fan, which costs more $$, and might make noise. (though noise probably isn't an issue at a rock concert) 4) if you push your amp harder, and it clips, it really is likely to send a square wave out. If you don't know about waves...here's a little intro to them. Basically, AC electrical current switches back and forth 50-60 times a second, from positive to negative. (and I think it switches on two leads, so you always have voltage) I don't know what an ac wave looks like...anyways, there are sine waves, which are kina like curvy lines. That's not what you'll ahve going out of your amp. If you talk into a thing that I forgot the name of, then you will see the waves made by your voice, they will probably look jagged and funky and not regular. Speaers work by moving air back and forth--they move both forwards and backwards. If you hold the speaker forwards, you can hurt it, same if you hold it back. A normal, non clipped wave will move the spaker back and forth, but if you have a sine wave it will hold the speaker out, then hold it back, the length of the cycle depends upon, well, the length of the cycle...a square wave can be made by a function generator, it's literally like blocks. A sine wave that is clipping will have the tops of the roling peakers chopped off, forming a mix between a sine and a square. And that just kills a speaker! :)

    man I write way too much. I could condence that. But I'm gonna go listen to soundwaves.

    ^I mean music :)
     
  9. BNBSound

    BNBSound Active Member

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    That's exactly the opposite of the way it works. Speaks aren't blown by excess excursion caused by over powering them. They're blown by under powering them. When you drive the amp into clipping to get the volume you want, you're sending straight DC electricity to the voice coil which cooks it.

    Industry rule of thumb is to get an amp that's twice as powerful as you need it to be and drive it to the desired volume level without clipping it. You do have to be careful not to blow your horns out, but a little math can give you a safe level to set your input attenuation at the amp.
     
  10. JahJahwarrior

    JahJahwarrior Active Member

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    well, I just want to say, that I'm pretty sure you'd blow your speaker if your speaker is say, 400 watts at 4 ohms, and you are pushing say, just a little under clipping, on your 1200 watt at 4 ohm amp. It might not, but I would think that there would be a point where you blow it because you do over power it.
     
  11. BNBSound

    BNBSound Active Member

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    You're right, you can't just slap together any old combination and expect it to work. We're talking about planning out a system here. A 100w pole speaker on a 1000w amp wide open doesn't work. But if that's what you've got you throttle down the amp and off you go.

    The problem arises when said pole speaker isn't capable of filling the space when fully driven. On the other hand, with my four mains cabinets at 600w peak each, it wouldn't be out of the question to run them on amps that give them each 1000w wide open. The break point is that the cabs are well suited to the environment (generally) and I can achieve the SPL that I want without clipping the amp, or making confetti out of my drivers.

    It's also not uncommon to overdrive subs. To run say, 2000w into a cone rated for 1000w because it's not a test bench situation. You're looking at kick drum hits where the event lasts a few dozen milliseconds, not a sine wave at full rated power. The voice coils get a few hundred m/s to cool off between each hit.

    If any of you browse prosoundweb, the Lab Sub project is two 12" cones in a folded horn that aren't really happy untill they're seeing 2000w. But we're talking about purpose designed drivers, not off the shelf, low end pole speakers, etc.
     
  12. wolf825

    wolf825 Senior Team Emeritus Premium Member

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    In addition to BNB's comments, the key is you want to balance your system properly so you get the maximum output out of your speakers without neither the speaker or the amp being overly driven or stressed...and to drive it to levels without clipping.. If you have to drive a 1200w amp to clip on a 400w speaker to get the level or coverage you need, then you obviously need a larger system or more speakers to achieve the level you need.... Sure--driving a a speaker to the max with a high power amp can destroy the speakers too--but again if you have to overdrive to that level--you need more or different cabinets for the situation / venue or coverage... IMO...

    -w
     
  13. chslighttech

    chslighttech Member

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    Somebody who does sound for a University told me that instead of blowing the speaker you burn it out. He said that when you overdrive it you blow it and when you underpower it you actually burn it out.
     

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