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Confused about watts vs undepowering/overpowering

Discussion in 'Amps and Cabs [BG]' started by Lecollectif, Feb 28, 2005.

  1. Lecollectif


    Mar 2, 2004
    Ok, I know there's a few threads running about this stuff right now, but reading them has gotten me even more confused about a topic I thought I understood.

    Having more watts in your head than your cab can handle is a good thing? What if you turn it up? Does it make a difference if it's tube?

    Please dumb it down for me?
  2. xonebass


    Feb 17, 2005
    Orange, CA
    Well of course the answer to your question really depends on the cabinet and the amplifier you select to a degree.

    But in general, having more power than your cabinet is rated can be helpful in reducing clipping. Clipping is what typically destroys speakers.

    So how much more power? In general terms, I would personally suggest buying a cabinet that is rated to 1.5 to 2X as much power (RMS of course) as you think you will need. Yeah that's pretty subjective - you'll have to figure that out on your own.

    Next, when purchasing your amplifier make sure that you have 1.3 to 2X as much rated watts as your cabinet is rated for. Why? So that when you turn up you don't clip the amp and destroy your speakers.

    So here's a couple examples:

    1) You play mostly small clubs and if you ever play a large club you use the PA. You think the loudest you'll ever need to get is 300W. You purchase a cabinet capable of 500W and an amplifier capable of pushing 700W.

    2) You play with 2 very noise guitarists who want to turn up to 11 all the time on a full Marshall stack. You often don't have PA support at venues (or it's crummy PA support). You think the loudest you'll ever need to get is 800W. You purchase a cabinet capable of 1200W and an amplifier to push it at 1400W.

    Of course this isn't a hard and fast rule and as you add more power (like the second example) you can reduce your ratios.

    My caviat is that if you ask 10 other bassists you'll get ten other answers. In that case the only way to find out is to try out a bunch of cabs with various amplifiers and find what works for you.
  3. Ostinato

    Ostinato Guest

    Feb 7, 2005
    Toronto ON
    You'll hear people talk about "overpowering" their cabs, especially when they're using some of the more boutique stuff that takes a lot of juice to power them. One thing I've learned is that a speaker's RMS rating is a nominal or "average" rating, so some guys use this as a starting point when deciding how much power to use.

    This is just my opinion, but I feel that for almost every playing situation, 800 watts available power is all you'll ever need.
  4. Lecollectif


    Mar 2, 2004
  5. Brian Middleton

    Brian Middleton

    Feb 4, 2005
    Here's how I understand it--more expert folks please chime in and correct me.

    The amount of power you're actually going to use in a particular situation is a given: you're going to turn up the level until your cabinet puts out a certain SPL, and the amount of power needed to produce that SPL is a given for that cabinet. If your cab requires 200W to produce the level you're looking for, then you're going to turn up the amp until it's putting out 200W. An amp that's capable of putting out 400W is going to handle this task more smoothly, and with less stress on your drivers, than an amp that tops out at 200W and is frequently on the edge of clipping.

    As far as the danger of blowing drivers with too much power, look at it this way: you won't blow a 300-watt cabinet by using a 600-watt amp unless you turn the amp up till it's producing more than 300 watts, and why would you do that--unless you *need* more than 300 watts? And if that's the case, then the problem is not an over-powered amp but an under-rated cabinet.
  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The real question is: what is more injurious to speakers, distortion sourced in the amp or distortion sourced in the speakers? Both the amp and the speakers will distort if pushed beyond their limits. But the distortion is different. The speakers will distort if pushed with too much power primarily via overexcursion, and this sounds so bad that you 'll probably turn it down before you hurt them. Amp distortion is more insidious, as it can wipe out your speakers before it sounds all that bad. Therefore the simple answer is to have an amp powerful enough so that if distortion does occur it will be speaker sourced rather than amp sourced.
  7. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI
    Is this just some phenomenon with bass? Rock guitarists regularly put clipped waves into thier speakers - from lightly 'soft' clipped, all the way to even very-very-very clipped ('distorted') signals.

    One thing that we can be technically dogmatic on is that a square wave ("perfectly clipped"; "extremely clipped" - a worst-case) will have a higher "working voltage" (a more descriptive, old-fashioned term that is now called "RMS") than a sine wave (technically the 'least distorted' wave there can be) for the same peak voltage. 'Peak voltage' is what should be compared because an amp's power supply is a low-impedance 'voltage source', and offers the valves or transistors this "supply voltage" - and no more - to drive speakers with. Working voltage for a sine is 0.71 peak, and working voltage for a square is 1.0, so mathmatically we can say that a square wave of the same peak voltage offers 1.4X the work. This means that for a fixed, set peak voltage into a fixed load impedance, a clipped wave could produce at-most up to 1.4X the power in a speaker than the purest 'pure' signal could. It wouldn't likely approach this in real life; I'd guess it'd be more like 1.3 or less, because of power supply sag, and that in real life a clipped wave wouldn't be a perfect square, and non-distorted notes wouldnt' be a pure sine.

    But anyway: we can pretty simply - like to a reasonable approximation - say that if your cab is rated for 1.5X the maximum clean-before-clipping amp output, you could crank everything to ten let the amp distort, and not hurt your speakers (I wouldn't like the idea of running'em continuously right AT max, though!).
    That makes sense for sure. I agree with just that. One-point-five minimum; 2X conservative - precisely because: you wouldn't want to blow your speakers!
    An amp that's rated for twice the power of a cab should certainly, definately be capable of destroying your drivers perfectly well WITHOUT clipping. That's practically the definition of "power handling" - right? - if you drive it with more than such-amount of power you'll wreck'em, right?

    I've never heard of a "maximum distorted-ness of waveform" rating - never, but always and only a 'maximum power dissipation'. Somethings spun here; I'm almost sure-of-it.

    I guarantee you that you're way more likely to hurt a speaker that's rated for half the amp rating, than one that's rated twice the amp rating! ...Yup, I'm sure.

    I think I'm going to end up with three Avatar twelves for my stage rig. That'll be fifteen hundred Watts-worth of speakers (for $700 shipped to Milwaukee). the reason I want to run so many is because volume won't be so sensitive to distance, being that when I walk right up to adjust something or whatever, a third of the power will be way down there blowing past my ankles, one-third down at my.. hmm.... well - the point being that only one of'em will be in my face, but as I move away, all three start to work together as more of a 'point source', giving me more of 'all' the power. It'll also look very cool.

    At any rate: I haven't decided on what amp I'm going with yet, but it'll be 300W or 400W or 500W or 600W or 700W or something like that, and whatever it is, I'm going to just go ahead and flip whatever knob I feel like anywhere I want - and HOW'S that going to be bad for my speakers??

  8. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI
    Whoah - Here's a great technical post from another thread:


    Now there's a reasonable reason that a bigger amp could be easier on speakers. Damping factor! Remember though that this angle only has to do with the power section of the amp. This would imply that clipping in another part of the circuit - or from a distortion pedal on the floor ould not likely cause this 'lack of damping' induced over-excursion.

    BUT! - I still say that if you DID turn the oversized amp up too far - OR maybe even left it alone, but increased the input enough - it'd be MORE likely to hurt your speakers than a signal from an amp that's within the cab rating, but had bad damping.


  9. Yup,

    except a pure square wave has twice as much energy as a pure sine wave and will produce twice as much power into the same load, not 1.41x. Power is proportional to the square of the voltage, remember ;).... So the theoretical maximum for an amp is twice rated power. Of course, in the real world you can't get near that because the portion of the output wave below the rail voltage is still sinusoidal, plus yer power supply sag and whatnot....

    We've had to drop the OTs from my amp project. :(
    There seems to be a demand for a more cost-effective amplifier system, so cap-coupled they will be...:D
  10. SirPoonga


    Jan 18, 2005
    This doesn't make logical sense. 200W is 200W, no matter if a 200W amp is outputting 200W or a 400W it outputting 200W. What causes the clipping?

    I'm new to bass guitar but I am familiar with power amp/cabinet relationship. I've never heard clipping on my home theater subwoofer that I built. The driver is rated 250W and I only have a 80W amp. It sounds awesome and I've had this sub for years.
  11. Your cab's power rating in itself has nothing to do with whether you will clip or not. If you have to turn up too high to get the volume or tone you want, you will clip regardless of whether your cab is rated at half your amp's power, the same, or twice, all else being equal. If you don't have to turn up too high, you won't clip regardless of the cab's rating.

    If you are clipping, that's not necessarily in itself dangerous unless you are giving some component of your system more than it can handle (remember that tweeters are typically rated for only a fraction of what a woofer is rated for, because under normal circumstances highs make up only a fraction of the signal).

    If you're pushing your amp too hard, you're actually safer with a higher rated cab. If you have more than enough amp to do your job easily, you derive no real benefit from a cab rated at half the amp's power--you'd do just as well, with no greater likelihood of clipping, with an equally effiicient cab that handles more power. You certainly can use a lower rated cab in this scenario, but it gives you no inherent advantage.
  12. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI
    Oo... Let's see: ahyeee-squarrred arrrr...(makes me feel like a pirate to say it that way); E^2/R; EI... Yup - you're right (looking at the in-phase components, anyway). I'm gettin' rusty (AND crusty - Arrrrr!).

    Good point.
    OTs are so cool to use. It bums me that they're so expensive.

    There's this idea that transformer design and manufacture is such a mystical black art or something - I've been in the transformer and reactor buisness for over a decade now (unfortunately my company has moved to all industrial reactor production, and farms-out transformers now), and I can tell you that it's not that tough! The trickiest kind of OT to build, I guess, are the ones for valves, because of the many turns of very fine wire required for the primary. This can run into design problems due to the inter-and intra-winding capacitance, turn-to-turn and layer-to-layer potentials, and production problems having to do with how delicate the wire gages can be to physically wind.

    BUT!! - I'm STILL waiting to see what transformers can do for modern SOLID STATE amps - here you don't have ANY of the aforementioned challenges to speak of! In fact, you'd probably have the option of going all current-heavy for the power transistors, and step the impedance UP for 8-Ohm speakers! That'd probably save you some dough on the power supply caps.

    Dang.. We sold-out our two multiple-winders ("stick-winders") to a small, kinda dumpy little transformer house on the other side of town. Those are just-the-ticket for valve OTs - I'm tellin' ya, Man: you could wind'em in-house. We used to use 40-gage wire on those things, and wind six transformers at a time, with thousands of turns per winding.

    Stacking the cores in is a no-brainer, quality test can be done with simple equipment, and you can send'em out to be vacuum impregnated if you want, or just varnish dip and bake them. I wonder how much he'd sell one of those old things for?.. Hm...

    Well anyway - I guess I've rambled about this kind of thing to you before, Mark.

    Wishing you the best -

  13. xonebass


    Feb 17, 2005
    Orange, CA
    Great replies to some of the issues, so let's recap some confusing issues.

    1) Why would a 200W amp cause clipping distortion when run at 200W as opposed to a 400W amp run at 200W?

    The primary reason for this is headroom. When you play your bass the input signal from your preamp will vary (based on how hard to you strike the strings etc.) So for you to able to play safely at a given level you need headroom (at least 3db preferably closer to 6db) to prevent clipping which causes distortion.

    2) Why can guitarists run their amplifiers distorted (or to 11) without causing problems with their cabinets?

    The real monster, is not just amplifier clipping, but the type of amplifer clipping that is being put out. As someone else mentioned "square-wave" clipping produced from solid-state amplifiers is highly dangerous to speakers, whereas tube clipping will not produce the same effect. Since most guitarists are running all tube (preamplifer and amplifier) they do not produce square-wave clipping. Most all bass amplification on the other hand is solid state or hybrid (where the amplifier is solid-state and the preamp is tube). Obviously problems of underpowering a cabinet will not be prevalent with an all tube bass amp (such as the Mesa 400+, some Aguilar models etc.)

    Also as stated earlier, distortion can happen to the preamplifier, amplifier or speakers. However, the most dangerous (most difficult to detect before damage is done) is amplifier distortion.

    3) But isn't it dangerous to the cabinet to run a 400W amplifier if my cabinet is only rated to 200W?

    Yes and no. It is dangerous if you run the 400W amplifier at 400W. However, it's pretty easy to dial in 200W by simply turning up the amp halfway. Also, if you have less power, you have less headroom. Don't make the mistake of believing that you can't hurt a cabinet if your amp is rated to less than your cabinet.

    4) But at home I have a home stereo system rated to 80W for a 250W cabinet and it never has any problems.

    Yes, but you would never run a hi-fi system at 80W (making the amplifier clip)! Your ears would fall off :D As long as you have headroom your good, that is the key.

    FYI, some hi-fi systems costing thousands of dollars are only 10W! Power is relative to efficiency. In the music cabinet world speakers surrender efficiency in order to maximize musical range. Hi-fi speakers have complicated cross-overs and 3 or 4 different speaker/tweeter combinations to simulate the full musical range.
  14. The point some of us have been making is that in this exact scenario, you are *even less* likely to harm a 600 watt cab of equal efficiency--the amp-cab ratio simply is not the point. The reason is that the benefit comes from having a bigger amp than your job requires, not from having a bigger amp than your cab rating.
  15. I have a little questions for ya amp nerds (... just kidding guys...).

    I like a little grit in my sound, so I've been running 350 watts from an old Peavey Mark VIII into my Avatar 2x10 Neo (handles over 500 watts, maybe 700ish) with my gain at about 7 (out of 10) and my master volume at about 2.3 (out of 10). This gives me a tone similiar to a very mildly pushed SWR Redhead combo. I get a slight bit of overdrive (pretty slight though, almost tube-like-ish).

    Is this bad? I'm in general not going at a volume that my cab should be able to handle, and I have my EQ settings pretty flat on my head, except for a bump in the 120-420-ish range. I run my flatwound strung passive Music Man copy with both volume knobs at full and no highs added or cut.

    Thanks a ton, I'm totally wondering if this is bad!
  16. At the levels you're talking about, you may well be OK, depending. Clipping, even SS clipping, isn't inherently immediately dangerous, despite the mythology, if you're not giving some component in your cab more than it can handle. I have clipped a 160 watt SS Peavey many times into a 400 W EV 15" w/o hurting it. Having said that, be careful if your cab has a tweeter. Tweeters are typically rated for much less power than woofers, for good reason. With heavier clipping, the percentage of highs in the signal can go up, meaning that your tweeter could get overwhelmed while you woofers are still coping.
  17. Not quite...;)
    The monster is too much power, no matter which way you get that power. Speakers are indifferent to actual waveform.
    If you have a 500W cab and an 800W amp putting an average of 600W into the cab you're just as likely to damage the cab as you would be with a 400W amp pushed far enough into clipping that it's putting 600W into the cab.....

    The differences between tube amp clipping and solid state clipping are actually quite miniscule. In a tube amp the HF harmonic series is attenuated (for various reasons) compared to a solid state amp. This translates into nicer sounding distortion but has little effect on the destructive power of that distortion. It may reduce the possibility of blowing a tweeter in a passively crossed over system, based on there being less HF energy in the signal.... In both types of amp, the tops of the waves are squared off when the signal reaches the voltage of the power supply. It should be noted that no amplifier will produce pure square-waves under any conditions. The part of the waveform below the clip point will be the same shape as the input waveform.
    A lot of guitarists use solid-state stomp boxes to get their distortion. All of those use clipper circuits that artificially clip the tops off the input waves to produce the distortion effect.

    The real reason guitarists aren't blowing speakers every 5 seconds is that manufacturers intentionally put speakers rated for more power than the rated clean amplifier power in guitar amps. They do this because they know that the amps are going to be overdriven and putting out more than their rated power. Check out any guitar amp. Any 50W combo will have a speaker rated for at least 100W.....
  18. You should be fine. :D
    If you were full-on overdriving the amp you might be on shaky ground.... Sounds like you're just overdriving your preamp a wee bit.
  19. +1;):bassist:

    Maybe not every single guitar amp, but not too far off! Case in point: a single Mesa Rectifier cab has a power handfling of 360 W, so a full stack would handle 720 W. That's because Mesa knows that Dual Recto users are going to turn those bad boys *up*. And a full Marshall stack with Celestion V30s handles 480 W (for a 100 W amp that could get a theoretical max of around 200 when dimed).