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My assumptions....well somewhat. (help?)

Discussion in 'Amps and Cabs [BG]' started by ohmanohjeeze, Jun 24, 2002.


  1. ohmanohjeeze

    ohmanohjeeze

    May 6, 2001
    tucson, az
    So yeah, I've got a 2ohm head, with 600 watts. Now I'm not too sure about all this, but if I were to buy two 4 ohm cabs (say a 2x12 and a 1x15), would that be able to take full advantage of the wattage? Another thing, if I toss a 4x10 up there, and 1x15, and plug one into each, ignoring the other input on the back of the 4x10, would this result in less wattage? Is a 8ohm 4x10 with 2 inputs run the head at 4 when both are utilized? I'm confused. Oh and please suggest some combonations that would help me with this. (cab wise):confused:
     
  2. I wouldn't run any amp at 2 ohms. Efficiency drops dramatically going from 4 to 2 ohms of output load, resulting in heating problems. This is why most amp manufacturers rate their amps at 4 ohms, not 2. Thermal shutdown ain't cool (literally).

    If you want VOLUME, get 8-ohm 2x12 and 1x15 cabs. That would be a killer rig.
     
  3. ohmanohjeeze

    ohmanohjeeze

    May 6, 2001
    tucson, az
    So if it were to run at 4 ohms would the amount of wattage be deminished? I don't need all 600, I'm just curious as to how all of this works out.
     
  4. You would get less output power, yes, but it'd probably be in the 450-watt range--still plenty.

    Speaker area is a lot more important at determining sheer volume than power output, anyway.
     
  5. ohmanohjeeze

    ohmanohjeeze

    May 6, 2001
    tucson, az
    Makes sense....thank you sir.
     
  6. ohmanohjeeze

    ohmanohjeeze

    May 6, 2001
    tucson, az
    anyother suggestions?
     
  7. geshel

    geshel

    Oct 2, 2001
    Seattle
    ?? If the amp says it's 600w at 2 ohms then I'd say that would be a better estimate (than 450). And as far as heating and inefficiency, again, if it's rated at 2 ohms, then it *should* be just fine. If not, hopefully it's under warranty. .

    (Peter's second point is pretty good though - doubling the speakers can be a larger volume increase than doubling the power)
     
  8. geshel

    geshel

    Oct 2, 2001
    Seattle
    The two inputs on the 4x10 are just two inputs. You only need to use one. The impedance of a cab doesn't change unless it has a specific impedance switch on it.
     
  9. ohmanohjeeze

    ohmanohjeeze

    May 6, 2001
    tucson, az
    So when i plug into a 410 I only need to use output from the head into one input into the cab, and it would make no difference to use both?
     
  10. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    i agree that if it is rated at 2 ohms it is capable of running at 2 ohms. what i would suggest is this: at 4 ohms, run your amp at your desired eq at maximum loudness, so the clip led just barely flashes, if you have one. turn everthing off, (if its tube, let it cool down a bit.) add your other 4 ohm cab and turn back on. you should have much more headroom. (it worked for me) you will be able to turn up somewhat from here, but it can be more damaging to clip at this level than at a lower level. these in conjunction with the right speakers will be more than enough amp. cab wise i agree that the 212+115 or 212+118 is great for larger venues. for smaller venues i'd go with a 212+210. needs less room to breathe
     
  11. If you take a look at many PA power amp manufacturers' specs, you'll notice that the power ratings for 4- and 8-ohm loads are at 0.01% or 0.05% THD, while 2-ohm loads might be at 0.5% or even 1% THD. I don't pretend to know enough about electric power engineering to say why that's the case, but I'd rather not be running my power amp into distortion by default.
     
  12. Turock

    Turock Supporting Member

    Apr 30, 2000
    Melnibone
    Yes, one output from the head to the speaker cab. The other output is for adding another speaker cab. The other input on the cabinet is another place you can add a speaker cabinet.
     
  13. ohmanohjeeze

    ohmanohjeeze

    May 6, 2001
    tucson, az
    Okay, it's just that i was told that only plugging into one of the inputs on the cabinet means you're only running half of it or something like that. Oh well.
     
  14. jobu3

    jobu3 Artist formerly known as Big Joe

    Feb 17, 2002
    Mountain Top, PA
    DON'T plug into both inputs of one cab unless it has a mono/stereo switch on it!!! that will send the signal back to your amp. not good! if your only running two cabs then use the 2 speaker outputs on the back of your amp (1 to each cabinet) instead of daisy-chaining (running them in parrallel). it's more efficient that way. i curse the guitarist who lead you in the wrong direction!!!! (spits in the eye of the wretched guitarist):D ;)
     
  15. Turock

    Turock Supporting Member

    Apr 30, 2000
    Melnibone
    Whoever told you that gave you some poor information. And I would be suspect of any other advice from this person, At least check it out here first, as you did this time. There is a wealth of knowledge on this forum.
     
  16. ohmanohjeeze

    ohmanohjeeze

    May 6, 2001
    tucson, az
    Yeah....I've notice there is a wealth of information on here (and hostility). But of course he was a guitarist.
     
  17. geshel

    geshel

    Oct 2, 2001
    Seattle
    Not really - if the amp is mono (or you're using the two outputs from one channel), it'll just be a parallel connection - effectively the two cables would become one cable. *But*, since there's no reason to do this (if you want thicker cables, buy them! :) ), don't.

    If you sent two different channels to the cab this way (effectively cross-connecting the two) I wouldn't want to be around to see what happens to the amp :(. (note: even on bridgeable amps this isn't bridging the two channels).
     
  18. ohmanohjeeze

    ohmanohjeeze

    May 6, 2001
    tucson, az
    Okay, one more quesion (I think) say I get a swr workingman's 210 and a workingman's 15, they are both rated at 8ohms and 200 watts. So i would have a total of 400 watts at 4ohms correct?
     
  19. geshel

    geshel

    Oct 2, 2001
    Seattle
    If you hooked them in parallel it would be 4 ohms, and a total power *handling* of 400 watts. The actual power you'd be using depends on the amp you hook up to them.
     
  20. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    You're not "running the amp into distortion" by running at 2 ohms per channel. An amp that produces significantly more THD at 2 ohms than at 4 or 8 is either poorly designed, or maybe a tube amp. ;)

    Power ratings are done at a specified % of THD; that's just to define the onset of clipping at some arbitrary but repeatable point.