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underpowering causing the amp to blow?

Discussion in 'Amps and Cabs [BG]' started by thebassmuchacho, Oct 18, 2003.


  1. Im wondering if im doing a bad thing by playing a 500 watt cab at 8 ohms with a head that can only push 275 watts at 8. Any tips, anyone?
     
  2. Stephen Soto

    Stephen Soto

    Oct 12, 2003
    No man. Just try not to clip it. If the cab was like 800 watts probably say yes, but it's fine.
     
  3. You will do no damage to the head if you are not PUSHING it.

    Where the trouble starts, is you KNOWING that the cab can handle all the head hato give, and overdriving it.

    Oce you start pushing the head too far, you can damage the head. Additionally, if you start clipping the amp, you can damage your speakers too.

    Just keep off the full throttle, and all shall be fine.
     
  4. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    Rule of thumb is that you can feed a cab twice the nominal power it's rated at. So you're safe.
     
  5. ok cool, ive head rumors that you can blow your speaker by underpowering it, so i was making sure. Thanks alot
     
  6. From what I understand of bass electronics, when you push your amp into hard clipping ( eg: overblown distortion ) your speakers are no longer receiving an alternating signal ( AC ) but something more like a Direct current ( DC ) signal, which is what fries your speakers. The output stage of your amp won't like it either, neither can take the heat it generates. The trick is, if you hear distortion, then turn down and all will be well. Speakers can take occasional spikes of DC without damage but not full on DC.
     
  7. notanaggie

    notanaggie Guest

    Sep 30, 2003
    NAW....

    What is happening is that you are feeding it square waves instead of the kinda-ugly-sinewave looking waveform of bass.

    Square waves have double the power for the same voltage, so that means your amp is trying to put out double-rating.
    The extra power is all harmonics, so if you have a tweeter in teh cab, that's where a lot of the power is going...

    Did I mention it probably sounds like crap? A little clipping makes bass thick and can cut through better, but a lot sounds like garbage....your mileage may differ, of course....

    If you do it enough to have a chance to hurt the speakers, AND you have ears, you will be knowing about it.........
     
  8. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    No you're not giving the speaker DC current when you start clipping it, unless something is really wrong with the amp. That's a myth. Nor are you giving it square waves, unless you are clipping it as much as it can be clipped on a continuous basis, and that's pretty rare. But notanaggie isn right on the money that you are increasing the power your amp puts out when you run it into clipping, and that much of the power goes into higher harmonics, thus putting your tweeter (if you have one) at greater risk.

    Don't worry about your cab being rated higher than your amp. That, *in itself*, is never a problem. The real question you have to ask is, is your amp big enough to get the volume you need. If it's not, you're going to overwork it and possibly sound like crap, and you need to get a bigger one.

    Thought to remember: if you're going to be overdriving your amp at any time to get more volume, you're actually safer with a higher rated cab than with a lower rated one, all else being equal, because it will be harder to blow. (Note I said "safeR" rather than "safe," because this is a relative judgment rather than an absolute one.) The real answer, however, is not to put yourself in that position and to try to get the gear you need so you don't have to work either the cab or the amp too hard.
     
  9. Stephen Soto

    Stephen Soto

    Oct 12, 2003
    Well, you really can... But just don't clip it to blow...
     
  10. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    worst that can happen is you clip the poweramp when pushing too hard - won't hurt the amp but it will deliver twice the wattage to the speaker
    so in your case you'll give the speaker 550 wattsif you do that long enough you can destroy the speaker by overexcursion
    it'll sound horrible too:D
     
  11. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    worst that can happen is you clip the poweramp when pushing too hard - won't hurt the amp but it will deliver twice the wattage to the speaker
    so in your case you'll give the speaker 550 wattsif you do that long enough you can destroy the speaker by overexcursion
    it'll sound horrible too:D
     
  12. ok cool, so is a 275 watt head gonna be ok for a 500 watt cab? And also Ive heard rumors of it being really easy to rewire an 8 ohm cab so that itll run at 4 ohms, but of course that sounds iffy and i dont want to do it, but can that be done and not hurt the cab?
     
  13. You can run a 1000w head into his 500wcab? What if you turned up the master to the max? I've never heard of this before.
     
  14. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Argh, so much misinformation.

    Richard Lindsey is right on the money. Read his post carefully, he speaks the truth.

    "Rule of thumb is that you can feed a cab twice the nominal power it's rated at"...

    IMHO that's a horrible rule of thumb, and chances are you'll blow a few cabs if you try it. While it is true that bass amps tend to deliver power in "bursts" (when you play the notes), there are many factors that can destroy speakers, including heat, excess excursion (like when you hit a loud low B and the speakers try to jump out of their sockets), impedance mismatch, and physical problems like coil sag/deformation.

    The truth about power "rating" is: it depends entirely on what kind of speakers are in the cab. Different manufacturers use different methods to rate their speakers. Bag End's or EV's might be able to take a little extra juice, but I wouldn't try pushing an Eminence past its rating (personally seen more than one Emmy cone fly all the way across the stage :):):) )

    "Ive heard rumors of it being really easy to rewire an 8 ohm cab so that itll run at 4 ohms"

    No, that's just a rumor, and it's not true. Don't try it, and don't hook up a cab with an unknown configuration to your amp. If the impedance is too low, you can easily kill a solid state amp that way. Tube amps may be "a little" more forgiving in this regard, and a too-high-impedance speaker is generally worse for a tube amp than a too-low (that's opposite from a solid state amp, where too-high just means that it'll deliver less than the rated power).

    My rule of thumb is, always use a cab with an impedance that matches what your power amp wants to see. For solid state amps, never use a lower impedance than the "minimum rated impedance", which should be written in big bold letters right on the amp somewhere. For tube amps, try to get an exact match, but never go outside of the range of 50% to 200% of the rated impedance.

    When it comes to the power ratings, you have to be "reasonable". For instance I use an SWR 750 with a pair of BagEnd's that are rated at 200 watts RMS each, but I never turn the amp up more than halfway, and I can definitely hear the sound start to change when I exceed the safety zone.

    Going the other way, if using a high power cab destroys your amp, then it's a crappy amp to begin with and you should get something different next time. "Good" amps (read: well designed) have protection mechanisms that will either disconnect the load or shut the amp down when it gets into the danger zone. Sometimes the "protection mechanism" is just a little red light that you have to watch, but a better design will actually save the amp "automatically" without requiring human intervention (or constant surveillance).
     
  15. Good thread. Another issue set straight.

    It's really amazing how persistent rumours can be.....

    You would think that with the arrival of the internet, there would be more good tech info available. Turns out myths and rumours spread harder than truth, probably because it is so overwhelmingly abundant, as opposed to true technical knowledge.

    The moral of this: don't believe anything you read, unless backed up by real technicians. How to tell? That's another story.
     
  16. notanaggie

    notanaggie Guest

    Sep 30, 2003
    Say there....Richard Linsey.....

    Not square waves?
    Well, as you point out that depends on level....the less the clipping the less ideal the square wave....Guitar amps on max gain boost do make pretty decent square waves on occasion...



    BTW....the harmonics produced by clipping are mostly odd harmonics...what that does sonically is REDUCE apparent sound level and projection from what it could be, like a muted trumpet.

    A LITTLE clipping probably does produce a reasonable amount of even harmonics, which tend to make sound louder and more projecting.
    Most amps are not perfectly symmetrical, and so they don't clip symettrically, which produces some even harmonics.

    But very much and it sounds terrible, distorted, with the fundamental tone muted and the harmonics much louder.
    The harmonics go way up into very non-musically-related tones, i.e. nasty buzzing, like a bad guitar amp......

    Better to get a bigger amp, I'd say.
     
  17. I'm amazed at the number of people who ask "How can I rewire my 8 ohm cab to 4 ohm" or vice versa "How can I rewire my 4 ohm cab to 8 ohm".

    I think we get a thread on this subject at least once a week. Some people even claim they know "all about Ohm's law" yet they can't figure out two speakers in series or parallel. So it shouldn't be surprising when lots of misinformation gets spread around.

    P.S. original answer in this thread is correct. You can't "rewire a cab" to make it 4 ohms instead of 8 or the other way around.
     
  18. degroove

    degroove

    Jun 5, 2002
    Wilmington, DE
    YEAH!

    Here is is:

    Changing from my current AMpeg B2 R (350 Watts SS) to a V-4B (100 Watts Tube). The B 2 R is stable to 4 Ohms. The V-4B can go from 8 - 2 Ohms.

    The cab I have is an SVT 410 HLF rated at 400 Watts RMS and is a 4 OHM cab.

    In a thread, someone stated that if I did this switch, I would be in danger of UNDERPOWERING my cab. Is this the case?

    Keep in mind that I know NEVER to crank any amplifier 100%. With my B2-R, I usually had it 80 - 85% at most. It also had a clipping LED!

    I am expecting to have enough headroom. My band plays the following:

    Guitar 1: Fender SuperChamp (18 W tube) or a 63 VibroVerb Reissue (35 W tube)
    Guitar 2: Crate stack (100 W SS)
    Drums:
    Me: V4-B (100 Watts Tube)

    Classic Rock Music Covers is the music. No slapping or popping!