Rule of thumb for matching an amp to a speaker wattage?

Discussion in 'Amps and Cabs' started by Scooter, Dec 13, 2005.


  1. Scooter

    Scooter Supporting Member

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    I recently blew a Red Fang (30 watts) that I was using in my Mazerati combo (32 watts). I never really pushed the speaker that hard, so I was a little surprised that it blew.

    So perhaps an expensive lesson was learned. Is there a rule of thumb for matching a speaker wattage to an amp? I did realize that I was "close" using the 30 watt RF in my amp, but I figured I wouldn't approach the danger zone unless I really cranked it.
     
  2. bscepter

    bscepter Member

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    I'm surprised. I would think that most manufacturers would underrate their products. Were you running it wide-open with some sort of gain boost in front?
     
  3. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    Many tube amps exceed their rated wattage, especially distorted. Some folks have mentioned amps that actually can put out double their rated power.
     
  4. Scooter

    Scooter Supporting Member

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    Actually I never ran the amp past 11 oclock. I do use pedals with the amp but nothing that really slams it.
     
  5. waxnsteel

    waxnsteel Member

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    Have you called up eminence? Sounds like a load of crap to me. I know tube amps are capable of putting out more than rated power, but a company that is selling speakers they know are going to be used with tube amplifiers should be taking that into consideration. You may not get much love having exceeded their wattage by 2, but I'd say it's worth a shot.
     
  6. toddyjoe

    toddyjoe Member

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    Speaker wattage can be a tricky subject, especially when you do not know how a particular manufacturer comes up with the wattage rating. For instance, does the manufacturer rate for the heat/frying limits of the speaker or just the physical limits of cone movement? Some manufacturers are known to under-rate their speakers (Celestion and probably Ted Weber), whereas other manufacturers may try to hype the biggest wattage they can support on a good day. Speaker quality and construction also play a big role. While some quality speakers hold up well and sound pretty good when they pushed, others simply fizzle and fry. From what I have read and experienced, Eminence does not have a reputation of conservative ratings on their speakers or the ability to take a beating like Celestion and Weber.

    Another thing to consider is old speaker wattage ratings were typically given as continuous ratings, meaning how many continuous watts of stereo music could be piped through them in a hi-fi application without blowing or frying. However, guitar and instrument amps have peaks of wattage that go well beyond an amp's stated rating, sometimes double or more. Many guitar amp makers also list an amp's wattage as the maximum clean output of the amp, yet tubes are capable of putting more watts out as they go beyond clean into distortion. These reasons might explain why you can see a Marshall 50-watter that actually puts out 40 or 45 watts or so clean but over 50 watts distorted and maybe up to 90 watts on peaks.

    One more thing I just thought of: do not equate "11 o'clock" on the amp as being less than half power or less than 16 watts in your particular case. Many amps reach maximum rated power much earlier than the maximum on the volume knob. Just a guess, but your Maz might be hitting 25 watts or more at the 11 o'clock setting and going well over 30 watts on peaks if you hit a hard chord, turn on a boost pedal, or dig in on single notes.
     
  7. Bloozcat

    Bloozcat Member

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    I have no idea where I got this from, but I've always used a "50% Rule". I choose speakers that can handle a minimum of 50% more than the amp's rated power.

    So, for instance:

    For a 20 watt amp, my speaker(s) would be rated to handle 30 watts min.
    For a 30 watt amp........................................................45 watts min.
    For a 50 watt amp........................................................75 watts min.
    Etc, etc, etc.

    I don't even know if this is a valid "rule", but I've never blown a speaker using it.
    YMMV...and all of that.

    About the only time I've not stuck to this "rule" is with my '65 Bandmaster head. I have 2-12" Weber C12N's that are rated at 25 watts each that I use in one of my cabinets. The head is 40 watts, and according to my formula, the speakers should be able to handle 60 watts, not 50 watts. Never a problem though.
     
  8. el34power

    el34power Member

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    FWIW. I smoked a Private Jack with a JCM800/2203 running on 2 tubes. Pre amp and master set at 2:00:confused:
    The speaker is supposed to be 50watts/100 peak.:NUTS
     
  9. Guinness Lad

    Guinness Lad Silver Supporting Member

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    It depends on many factors, what's the wattage of the amp, how much bass is being generated and is the speaker in an open or closed back cabinet. I also think it makes a difference if the amp is played using a clean tone or if there is alot of distortion present. I'm not sure on this one so I won't elaborate.

    First off it's best not to be below the amps rated output. Most amplifiers are rated for more then they put out but be cautious and use at least the same rating for the speaker as the amp, more is better if you always crank it (and use a lot of bass).

    More bass more cone movement. If the cone extends beyond the safe area (speaker cone extention) it can travel as it returns to it's starting point it may catch the inside edge between the coil and ? (I can't remember the other item) this sound is what is called speaker rub or cone rub.

    Open back cabinets should have a speaker in it which is twice the rated power then tha amp. The reason for this logic is there is more cone movement in an open back cabinet because there isn't a back wall to provide dampening. This tends to make the cone move more then it normally would and with increased movement comes friction and from friction you will generate heat. This eventually leads to all sorts of problems.

    Sorry for the forgotten info but I'm sure someone wil know all the technical jargon I left out. Perhaps a speaker guy will fill in my missing info.
     
  10. neve1073

    neve1073 Member

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    This is true. A friend of mine took his AC50 to a very good amp tech who warned him that the amp was putting out closer to 100 watts. I'm not sure how he quantified that, but he's a very good tech and I trust him.
     
  11. StompBoxBlues

    StompBoxBlues Member

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    I'm no expert, but I have read, and understand the theory behind this and it seems to me to be true.

    The thing is, clean guitar sound is still (until overdriven) basically sinusoidal. It looks like mountain peaks and valleys. Can be spiky on high frequencies or gentle rolling hills on lower, but the peak is just that...a peak. This is directly translated to current in the speakers, so the peaks are the speaker at it's extension (in or out).

    But a highly overdriven signal, with lots of clipping, looks like practiacally a straight line, a roof, and fairly straight line to zero, continuing to negative.

    When this gets translated to the speaker, this means that the speaker is having to stay extended for greater lengths of time...mechanically the speakers cone wants to bring it back to rest, so it is vibrating WAY more than the peak in sine waves. This means that it is dissipating many more watts for the same general signal if it were sine wave.

    As far as I know, again I'm sure lots of folks here know way more about this, this means that you better tack on, like was mentioned, at least 50% more if you use a lot of distortion.


    One piece of advice...talk to the people where you bought it, be honest and explain what happened and ask if this is something they see as abuse of the equipment, or if they believe it should have handled it. Maybe they had a bad batch, you never know til you try!

    It's cool to ask here, epecially first before contacting them so you know generally what is considered abuse and what is within the limits, but really the sellers or ESPECIALLY (I think you'll have more luck with the...) the makers. Just ask them, you got nothing to lose, you already have a blown speaker. If they had a bad run they either know it now from others, or will when others have the same thing happen. If it is their opinion that you misused the product, ask them for guidelines, and how their ratings stack up against other manufacturers. Good luck!
     
  12. Neil

    Neil Member

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    I am no expert in speaker design and my apologies to any experts who might read this. Here is how I think of this stuff. A speaker fails because it overheats. The heat produced is proportional to the square of the current flowing in the voice coil. The current in the voice coil depends on the amps ability to supply current into the speakers impedance and the back emf generated by the speaker (this resists the flow of current to the speaker from the amp and effectively protects the speaker. It is produced by the velocity of the voice coil moving through the magnets field). A distorted sine wave takes on the appearance of a square wave and in trying to replicate a square wave the speaker is spending time toward the extremes of its travel at low or zero velocity reducing back emf to close to zero (and allowing larger currents into the speaker).

    Another way to look at this is a square wave requires the speaker to more violently transition from moving to stationary (or vice versa) and to create the force required to decellerate (or accellerate) the cone requires more power (ie more heat) .

    This implies that, at a given output power setting, feeding an amplifier (not in distortion mode) a square wave (eg from a distortion pedal) is as tough on the speaker as power amp distortion. I dont know if this is true in practice.

    There is also a point about the phase angle between the signals current and voltage that effectively can allow large currents (ie lots of heat) to flow and so overheat the speaker at relatively low effective power. This needs some complex math to understand and my math isnt up to it.

    I run a 200 watt Hiwatt head into a 2x12 with greenbacks in. It sounds great and the speakers are probably safe because it is so loud I dont turn it up. They'll be toast if I ever play a big gig!

    Neil
     

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