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Which should be more powerful, head or cab?

Discussion in 'Amps and Cabs [BG]' started by daglar, Dec 14, 2005.

  1. I've read a lot of opinions and I'm quite confused... Some say that if you have a more powerful head than cab, you can burn the cab and others say that if you push the head to the cab's limits, the head will fry.
    So, should one of them be more powerful or should they both be able to handdle the same amount of watts??
    There is one more question for you, is there a device able to count the amount of power (watts) that is being sent to the cabinets??
  2. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    I always prefer to have a power amp or head that is rated much more powerful than the cabs. That way, you will have a ton of clean headroom. Speakers will put out alot if you have enough watts in reserve.
  3. Monomer


    Jul 22, 2005
    You want the power amp (head) to be more powerful than the Cab.

    more headroom, the better.
  4. Clean headroom has nothing directly to do with the ratio of your amp's power to your cab's power handling. Nothing at all. It has to do with the ratio between the max power your amp has available and the amount of that power that you are actually using in your situation.

    Consider a thought experiment: you are doing a folkie coffeehouse gig and you have a 2000 W amp. You can't play very loud at this gig, so obviously you have a ton of headroom--your amp isn't even breathing hard, much less sweating.

    Now imagine you have three cabs *of equal sensitivity and frequency response*, but with different power handling--say, 500 W, 800 W, and 1200 W. So with which cab do you have the most headroom in your playing situation? Answer: none, your headroom is the same with each cab. Because the cabs have the same output and differ only in power handling, the same output from the amp will yield the same acoustic output from the cabs. Therefore, you're using the same percentage of your amp's total power with each cab. Therefore, your headroom--the ratio of the power you're using to the max power available--stays the same. Ergo, cab power handling has nothing directly to do with headroom.

    You get headroom by having a bigger amp than you need. That's basically it. Once you have a way-too-big amp, your cab's power handling is basically immaterial, provided it's at least enough to handle the percentage of the amp's power that you're consistently using.
    Jah Wobble Fan likes this.
  5. Copy that post and paste it up the next time someone asks this. It is a good explanation.
    Jah Wobble Fan likes this.
  6. My suggestion is this: get an amp that has at least twice the power you think you'll need, then buy a cab that sounds the way you want it to sound and can handle at least the amount of power you expect to be using (which is unlikely to be the amp's full power on any consistent basis if you've chosen your amp well). As an extremely rough rule of thumb, I'd say start with a cab rating of around half your amp's max (e.g., 400 W for an 800 W amp), then go up from there (if an 800 W amp does the job for you, using it into a 1200 W cab is no issue.) Having a cab rated higher than your amp, in itself, causes absolutely no danger. Yeah, you could under some circumstances (explained at length in many other threads here) damage a 500 W cab with a 350 W amp, but if you used that same amp with an otherwise similar *200 W* cab in the same situation, you'd likely damage the lower-rated cab sooner and more seriously (because you'd be pushing the amp just as hard, but the lower-rated cab would be even less able to take the punishment). This shows why merely having cabs rated at less than your amp's max output, *in itself*, does nothing to give you headroom.
  7. That is a great explanation of what appears to be a widely misunderstood concept.
    Jah Wobble Fan likes this.
  8. Thanks for the info guys, it's just that I have a 310W amp connected to two 75W speakers (not for bass, just to listen to music) and I almost fried them twice by sending too much power to them... I don't want the same thing to happen to my cabs, so is there a way to check what the total amount of watts being send is?
  9. seamonkey


    Aug 6, 2004
    How do you know you "almost fried them" ?
  10. whitedk57


    May 5, 2005
    Franklin, NC
    The earlier poster's long description was basically correct, but what I found missing was a warning.

    If you have a cab rated at 100W and your head is pushing more than that - blown speaker eventually. Granted, the head is rarely at it's peak output for long.

    If you have a cab rated @ 1000W and you're pushing 300W continuously from the head - no problem.

    The headroom comes into play where you have more power (head & cab) than you need. If the guitar player is being especially noisy or the arena you're in is bigger than usual, you might have the extra power to account for that - headroom.

    It's like having a Porsche in a school zone. And then you get on the freeway. Conversely, try a Geo Metro in a similar case - not much headroom.
  11. whitedk57


    May 5, 2005
    Franklin, NC
    Also, it depends on the impedance of the speakers in your cab.

    If you have two 75W speakers @ 8ohms in series - 150W @ 16ohms means less power draw from the head.

    If you have two 75W speakers @ 8ohms in parallel - 150W @ 4ohms means more power draw from the head.

    You also need to check the minimum total impedance(ohms) from your cab. Most heads can't handle 2 ohms and will eventually fry.
  12. I had the amp sending around 120W to each of the speakers and after a few minutes I smelled that something was burning and noticed some smoke around the speakers... Thank God they weren't totally destroyed :rolleyes:
  13. But how do you know how much power you were *actually* sending to the speakers?
  14. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    Richard, good explanation. I just hate seeing people buying big cabs and then complaining that they are distorting or "can't handle my amp" when in fact the speakers aren't even being worked yet. A 250 hartke head is not going to really push an ampeg 8x10! I have seen this so many times.
  15. My amp (I'm saying again that it's not a bass amp) has an indication that shows how many watts are being sent (it says on the amp that it's a "FL peak power level meter" if that helps).
  16. seamonkey


    Aug 6, 2004

    Hmmmm - interesting piece of technology there - I've never heard of it. Must be real expensive or cheap snake oil. :) If so, maybe it's showing some "peak" wattage?

    I've burned out crossovers - they smoke, but never really burned out a speaker. I must be lucky.

    Sounds like you like your music loud so maybe you need speakers that can handle more power.
  17. danis


    Jun 7, 2005
    I have a CAV home theater amp that has v.u.meters,maybe yours too from what your are saying,and though it's not that accurate it can give you a "general picture" of the amount of watts that sends to the speakers.
    But then again you can never be sure...And that's because the watts that sends the amp are never "stable".For example you said that your amp was sending 120W to your speakers,but there could be peaks to 200W even if it was still saying 120W to the meter...And right now the smoke appears :)

    A 300W amp to 75W speakers is too risky and wrong combination.You must be very carefull and experienced and still you may make a serious damage to your speakers.

    Generally there are three ways to make damage to a speaker:

    1)Too many watts(the coil or the crossover are melting)
    2)Excessive cone movement(the cone unsticks- mostly in home speakers-non pro)
    3)Excessive distortion because the amp is clipping-is at it's limits(again the coil or the crossover are melting)

    I always prefer cab or speakers that can handle more power than my amp's wattage.(but not too much ehh,cause then we have other problems in terms of sound quality)With this way i feel safe and the risk for doing a damage is minimized almost to zero!

    Be safe and i hope i helped! :bassist:

    p.s. o protos ellinas poy vrisko edo mesa :)
  18. Sako


    Nov 4, 2004
    Charlotte, NC
    Okay, sorry to hijack, but this is related. This thread and the FAQ thread have been helpful, but I still have questions. I have always been a combo user so I've never had to worry with this.

    I'm looking to get a amp head and a 210 cab and a 212 -- one cab now, one later. The amp is 400 watts at 4 ohm, 250 at 8 ohm. It has a 4 ohm minimum, with a "burst power" cycle of 725 watts.

    Both cabs are rated at over 700 watts, and are available in 4 ohm or 8.

    1) If I get 8 ohm cabs, together they would be 4 ohms with the amp. So if I only use one, will the amp only pump out 250 watts?

    2) Am I crazy for thinking about two cabs with this amp? I mean, I see people with 500 watt heads using 1000 watt cabs.
  19. jonly


    Oct 3, 2005
    Springfield, il
    run a voltmeter across the speaker terminals. Volts^2 / ohms = watts.
  20. Chasarms

    Chasarms Casual Observer

    May 24, 2001
    Bettendorf, IA USA
    Yes. You'll have 250 with one cab.