Over powering a bass cabinet

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers T' started by MK.II, Sep 3, 2005.

  1. MK.II

    MK.II Member

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    I was wondering what the rule is on bass amplifiers and cabs. My brother is looking to buy a new head and bottom and the store that he's dealing with has told him that you need to over power the cab in a bass rig. They're telling him that a 500 watt head at 4 ohms is a perfect match for a 400 watt cab at 4 ohms. They're saying that under powering a cab, such as a 400 watt head with a 500 watt bottom will blow the speakers faster than the other way around. They also said that you need to over power the cab in order to drive the low end and so that it's a clean low end. My brother looked at an owner's manual for a bass head and it cautioned that you should never do what the store is telling him he need's to do (at least with their product). He showed that to the salesman and he said he doesn't know why they put that in the manual because it's wrong! :eek: The store has my brother confused now because he's always thought that you need a bottom rated at equal or more watts. :confused: The salesman said that's true for PAs and guitar amps but not for bass amps. I told my brother that what the store is tellin him doesn't sound right to me. Am I wrong? Am I just adding to my brother's confusion? :confused: Thanks in advance for any advice on this situation. :)
     
  2. Pentode

    Pentode Supporting Member

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    That is really bad advice he is getting from the store. With bass it is particularly important to have a cab rated for at least the same wattage as the amp. Bass puts out a lot of transient spikes which will easily blow underrated drivers. I would suggest that he does business with another store.
     
  3. MK.II

    MK.II Member

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    Well, at least somebody agrees with me!
    :dude Actually it's one of the salesman telling my brother this info, not necessarily the whole store. I also sent my amp tech an e-mail regarding this. He may be gone for the holiday weekend though.

    Now that I think about it, maybe I should have put this thread in the Amp Technical Info area?
     
  4. aleclee

    aleclee TGP Tech Wranger Staff Member

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    Consider it done. :)
     
  5. MK.II

    MK.II Member

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    Thanks!! :dude
     
  6. John Phillips

    John Phillips Member

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    The store is wrong.

    It's always too much power that blows speakers. Always.

    The reasons for the too much power can be a bit complicated though... and you may have to take into account that the speaker's actual rating may be lowered by other factors (frequency, cabinet loading etc).


    The reason that the idea has grown up that too little power can blow speakers is because the onset of distortion in the amp (especially if you push the power stage too hard) drastically increases the output power and changes the frequency balance. This is especially true of solid-state output stages where the output waveform will make a pretty good square wave when it distorts, which is effectively 'DC' in the flat sections of the waveform, separated by extremely high frequency transients - so now both ends of the frequency spectrum contain far more energy than they would normally. It's especially critical in PA-type speakers, since they have crossovers which send different parts of the frequency range entirely to one driver or the other, which can then drastically exceed its power rating - it's still the too much power that actually blows the speaker.

    A distorted amp can put out up to twice the RMS power. So if you take for example a 400W PA amp, and use it to drive a 500W speaker, the speaker is serious at risk if the amp distorts. It will now be putting out well over 500W. You'll most likely immediately blow the tweeter since instead of about 1/4 of the energy going to the high end (ie about 125W at the normal rating of the cab), now about half will, ie 250W or more. You may even blow the bass driver if it's rated for more 'normal' music signals, ie not distorted, since now all the energy in the DC sections of the distorted wave will end up there.

    So for PA, you do want to have an amp with more power than you need, to make certain it doesn't distort.

    Most guitar and bass speakers are rated for the distorted output of an amp. They have to be, since the makers know that's how they'll be used. Celestion in particular are very conservative, and you can usually assume that one of their "25W" speakers will take a fully-distorted 25W amp, even though the actual measured power is far higher. Other makers seem a little less so, but you can still be sure a 25W speaker will take a clean 25W amp, which most bass players try to keep them ;).

    So the simplest way to get reliability is to make sure that the speaker will take at least the power of the amp, preferably a bit more. For a 500W amp, you want at least a 600 to 800W cabinet. That way even if the amp does accidentally go slightly into distortion it won't blow the speakers. Alternatively, you can deliberately mismatch the impedances in order to protect both the speakers and the amp. With a solid-state amp, the output power will be reduced by typically around 1/3 if you use double the impedance of speaker, so in the case you mentioned a 400W 8-ohm cab would be about right, and the amp will be under less stress since it will be only having to put out about 350W. If you do that, you could also then add another 8-ohm cab later if you want a bigger sound and more power.


    Trust me, I've learned all this by replacing a large number of blown speakers (guitar, bass and PA) over the years, and I know what causes speakers to blow, and how to put together a rig to make sure they won't.
     
  7. MK.II

    MK.II Member

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    Thanks John!! :dude

    Thanks for the detailed explanation! I never knew that about the aplication of PA heads and speakers. You learn something new everyday! Obviously the salesman is applying the PA head and cab power ratings rule of thumb to bass head and cab ratings. For some unknown reason he said that guitar and PA head and cab power ratings are different from bass head and cab power ratings when he should have been saying that PA applications are different from guitar and bass applications. Confusing! Again, thanks! :dude
     
  8. theelectic

    theelectic Guest

    That is the biggest crock I've ever heard. Is he going to tell Ampeg they're wrong for pairing their 300W SVT with an 800W cabinet? That particular tube amp could output up to double its 300W rating on peaks. Good luck to any 200W cab trying to handle that!!!
     
  9. MK.II

    MK.II Member

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    That's exactly how my brother and I were looking at it!! Now my brother has to diplomatically reject the salesman's advice! I'm sure they'll sell him whatever he wants to buy, even though the salesman will probably think he's making a mistake! :)
     
  10. aleclee

    aleclee TGP Tech Wranger Staff Member

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    Diplomatic, hell. Let him know that you think he's wrong and why. If that's a problem, tell the manager that the bozo's trying to sell you a rig that's liable to crap out when you need it most.

    I'm cool with ignorant sales people who can admit that they dont' know. When they start pushing BS or falsehoods and won't accept facts from a customer who knows more than they do, I have no prob letting his boss decide whether that's appropriate.
     
  11. theelectic

    theelectic Guest

    To hell with diplomacy, this guy is apparently so full of himself, he thinks he knows more than the company that actually makes the amp?? Please. His reasoning doesn't make sense any way you look at it. If too little power can blow a speaker, how can applying the SAME overload condition with even MORE power be good? i.e. 400W distorted may blow a 500W cab. Well now, 500W distorted isn't going to be good for that cab either, now is it?

    If you REALLY want to be a jerk about it, get *him* to hook a 500W amp to a 400W cab (or better yet, a 300W SVT to a 200W cab) and play a couple of slap licks with the amp cranked up. I bet he'll change his tune then :)
     
  12. leond

    leond Supporting Member

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    OK, so I'll go against the tide. I think for a bass rig, it is better to have more power than the speakers can handle. Based on John's PA explanation (which I agree with), I think the bass rig is closer to the PA end of the spectrum than the guitar rig end.

    Now being realistic, if you don't have enough bass amp / speakers to handle the situation, you'll blow anything up. But, if I'm playing a gig that a 4x10 bass cab should be able to handle (and assume it's rated for 400W), I rather have a 1000W head that's idling than a 300W head that I have to push to get the sound level I need. That 1000W head is going to be nowhere near distortion with plenty of headroom. The 300W may be at max and ready to distort.

    I play bass with an Acme B1 cabinet (it's a very inefficient speaker cabinet with 1x10, 1x5 and a tweeter). It's rated at 175W. Most people say you need 350W head to drive it comfortably. I use a 350W head and have no problem. It uses a light bulb as a fuse. In 99 times out of 100, if someone is blowing the bulb out regularly, the solution is to go up in power. The cab gets a cleaner signal for the same volume. I'm not saying that you can put endless power into the cabinet. There is a max volume this (or any) cabinet can produce. To go above that volume level would mean adding more speakers.

    So back to the original post, I don't agree with the salesman saying you need to overdrive the cab but I do agree with his saying a 500W head would be a good match for a 400W cabinet. The reason is not to overdrive the cab but to give it "clean" power.

    LeonD
     
  13. MK.II

    MK.II Member

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    I know what you guys are saying and if this salesman was anything like many of the salesman from a certain chain store I would expect to hear this type of advice from him. BUT, this salesman is a good guy, very well respected and has worked at this particular store for probably 20 to 25 years and I've probably known him just as long. He will bend over backwards to please a valued customer. Obviously he just has his facts a little mixed up in this situation. I'm sure that if the correct info is shown to him that he will admit his confusion and be thankful that it was brought to his attention. He was confusing my brother because he is NOT known to give advice like this, generally speaking. My brother really isn't thinking that the guy is a crappy BS salesman, he's just thinking that the guy is probably wrong in this (rare) circumstance and would rather be sure of the facts before he makes his decision. My brother has no problem buying from this salesman, or store, now or in the future. Again, thans everyone for your advice! :dude :dude
     
  14. Pentode

    Pentode Supporting Member

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    MK.II, the best bet is to follow the manufacturer's advice. In my 25+ years of bass playing and having owned countless high power bass rigs, I have never seen a manufacturer that recommends going with a cabinet rated for less power than the amplifier. So your brother should check the manufacturer's literature and/or website for their recommendation.
     
  15. MK.II

    MK.II Member

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    Yes, I agree. That's the plan he was doing in the first place and what he is now going to stay with. Thanks!
     
  16. MK.II

    MK.II Member

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    After doing some research today I found out that there are many guys that like to run heads that have more power than their cabs are rated for! So the salesman isn't alone in his opinion on doing that! Their theory is basically like what John Phillips described for PAs, which is to make certain that the head doesn't distort and damage the speakers. The problem that can arise is that if you turn up the overpowered head a little to much you can still damage the speakers just like if you were to run a conservatively rated head at a cranked volume for an extended period of time. You have to be careful of not turning up too much in both cases. Seems to me that the answer is to run two cabs that will handle the power of whatever head you use at whatever volume you may have to possibly use it at. Even better yet would be to use cabs with high speaker efficiency ratings so that you get more volume at lower amp volume settings. Any thoughts? Thanks!
     
  17. leond

    leond Supporting Member

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    Below is a copy of a posting I made to this thread yesterday that addresses this.

    A bass player definitely wants a higher power amp than speakers. More power equals more headroom.

    LeonD



     
  18. John Phillips

    John Phillips Member

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    Absolutely correct. Using a head with more power than the cab will handle does not protect the speakers in any way, in fact you can blow them before you even realise they're over the limit - they don't always sound distorted (which is determined by cone movement) even at over the power handling (which is determined by the voice coil).

    However, if you do use a cab (or two cabs) with more total power handling than the amp can put out under any circumstances (even distorted), you're safe. Since this is usually up to double the RMS power, that may be what you need to do.

    Anyone that really thinks that using an overpowered head is safer (not just that it sounds better, which it might very well do) must be running their rig fairly conservatively... or not work in the repair business.
     
  19. theelectic

    theelectic Guest

    It STILL doesn't make a lick of sense to me. Say you have a 500W head into a 400W cab, but you run that 500W head so it only puts out 200W consistently, with occasional 500W peaks. So why not get a 200W amp that can provide its full rated output? What's the goal here, undistorted peak power? Seems like a dangerous game to play, one tweak of the volume knob and there goes your cab.
     
  20. John Phillips

    John Phillips Member

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    Exactly. The goal is a better tone, because the amp is running well below its capacity so power supply sag is not an issue, which means it will sound fuller, cleaner and more solid - especially at the low end.

    And in fact, short-term overloads with a clean signal are less harmful to speakers (even those without crossovers and tweeters) than ones with a distorted signal, because they don't alter the way the cone moves like a clipped waveform does - clipping restricts the cone movement and reduces cooling relative to the input power. But keep it up continuously and you're still in trouble.

    One reason the over-power approach does work for PA is because the ratio of peak to RMS power in a mix is really high - at least double, if not a lot higher - so the peaks may even make the speakers audibly distort (and so the sound engineer turn it down a bit... hopefully ;)) well before the amp continuously exceeds the RMS rating of the speakers.

    I'm not sure it works so well for bass, because the signal contains a lot of energy on every sustained note. And it does depend on the bass player. If they're careful and listen for speaker overload, you're less likely to get a problem than if they just crank it. This is more likely in a jazz/folk/world-music/etc context than in rock... :)

    The one thing I can tell you from having worked in repairs in general rock equipment is that you do not want the speakers rated any lower than the amp, and preferably quite a lot higher, or you see a lot of blown ones.
     

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