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500w amp into 400w speaker?

Discussion in 'Amps and Cabs [BG]' started by kurosawa, Aug 21, 2012.

  1. If I plug a 500w head into a 400w speaker, will I fry the speaker?
  2. MoeTown1986

    MoeTown1986 Supporting Member

    Sep 14, 2010
    SoMD (Mechanicsville)
    its just a rating! use your ears. you can run a 1000 watt amp into a 100 watt speaker if you do it right.
  3. gjbassist

    gjbassist Supporting Member

    Sep 7, 2005
    Kansas City, MO
    Don't do it!


    Just kidding!
    As long as you don't crank the amp anywhere close to full volume it will be fine.
  4. 1958Bassman


    Oct 20, 2007
    "Use your ears"- that would be great if more people knew what to listen for.
  5. Tankdave


    Dec 2, 2009
    Wilts, UK
    I use a 900W amp into a 300W cab now and again and it's fine. Of course if I was to remove my brain and crank the amp to full power it will fry my very costly drivers.
  6. MoeTown1986

    MoeTown1986 Supporting Member

    Sep 14, 2010
    SoMD (Mechanicsville)
    True, but I would hope any musician would understand what "too much" sounds like.
  7. BurningSkies

    BurningSkies CRAZY BALDHEAD Supporting Member

    Feb 20, 2005
    Seweracuse, NY
    I run a 3000w amp and don't seem to have problems with speakers rated below 3000w.

    Use your ears. Listen for your speakers to distort. When they do, back off your volume, or your low end. Or both.

    Speakers tend to be rated for their thermal handling rather than their excursion capacity. That's usually done because the thermal number tends to be a bigger, more impressive number. The excursion limit is how far a speaker will move until it distorts (xmax) and then how far it will move before it shreds itself (xmech or xlim).
  8. How many ohms is the speaker cabinet (usually 4 or 8ohms)?
    If your amp works at a min of 4 ohms (eg. 500 W @ 4 ohms), and you run it through an 8 ohm cabinet the amp does not work at the full 500 W, probably closer to about 70% of the 500 W (eg. 350 W @ 8 ohms).
    That said, you can damage a speaker cabinet running 350 W into a 400 W cabinet as said before listen with your ears. If the speakers in the cabinet are straining, turn down.
  9. Interested in buying a solid state backup amp to drive a 4 ohm 15PR400. I don't plan on powering it into the ugly zone, but I *was* afraid someone might say "it's a thermal limit," 'cause I don't know what the sound of heat might be (I don't figure if the coil were to overheat, I'd know it 'til it was too late). There's been a lot about not putting a bigger amp with a lower rated speaker on TB lately and I wanted to know if something with these newfangled lightweight amps or speakers had changed things recently. Didn't figure it would hurt to ask.
  10. derrico1

    derrico1 Supporting Member

    Apr 12, 2005
    Charlottesville, VA
    No. You will fry the speaker if you hit it with too much power for too long.

    You could do this with an amp rated at 500 watts (or, say, 2000 watts). However, you could also feed the speaker too much power with an amp rated at 400 watts or 200 watts. This is because the amp's shorthand rating (500 watts @ 4 ohms) is only part of the story. And even an amp's long-form power rating doesn't specify the max level of power it will put out.

    Consider, for example, that your amp might be rated at 500 watts RMS driven continuously into 4 ohms w/ less than 0.1% THD from 20Hz to 20 kHz.

    If you drive its input hard enough, it's capable of delivering well north of 500 watts RMS into 4 ohms, but at a level of total harmonic distortion well north of its rated 0.1% from freq X to frequency Y. It's also capable of producing peaks well north of 500 watts (yada yada) into 4 ohms.

    If you're worried you might be damaging your speakers by overtaxing them, attend to them. Are they starting to distort what should be a clean signal from your amp? Is your cab starting to compress (in essence: you can raise the master volume at your amp, but the cab doesn't get louder)? Is the speaker cone jumping back-and-forth in the basket?
    • Yes? Dial it back.
    • No, no, & no? Your speaker's probably okay at this volume, no matter what the rating of your power amp.
  11. dincz


    Sep 25, 2010
    Czech Republic
    If your amp actually delivers 500W continuously for several minutes you'll probably fry the voice coil. In reality you might be delivering peaks of 500W if you're cranking it but the average power will be way lower - probably around a quarter.

    Exceptions might be very heavy compression or a bowed double bass.
  12. BurningSkies

    BurningSkies CRAZY BALDHEAD Supporting Member

    Feb 20, 2005
    Seweracuse, NY
    Fortunately, bass playing doesn't put out a high level 'program' style signal that tends to burn out voice coils, unless you're compressing a whole bunch. In reality, I've seen way more people with drivers that have been torn apart by over excursion than I have fried voice coils.

    In my own infamous bass playing career, I've lost 2 sets of drivers in about 30 years. Both from over excursion.
  13. lburton2

    lburton2 Les Is More

    May 15, 2008
    Detroit, MI
    As stated above, over excursion is usually what happens first. It makes sense, as the physical limit of the speakers is usually quite lower than the heat limit of the voice coils.

    I may be slightly off, but it's been my understanding that the physical limits of speakers in terms of wattage is usually about half of the thermal limit..

    In summary: You can definitely run the over-powered head into the cab, but if you start pushing the head close enough to it's full power potential you are most likely going to blow the drivers. You should be able to hear when this is happening.

    So if you do go this route and find that "sweet" spot with your head and cab to where it gets quite loud and no distortion or break up and yet you still need a LITTLE more volume.. Don't do it. (I've done it)
  14. It HAS to be a thermal thingee. There'd be no other way to express it. So that would make it a straight-ahead thing. I bought a Fdeck first generation HPF. That should go some way toward keeping me from having to use the power in inaudible ways. If the power were wasted that way, then I'd have to crank it higher. Also, I'm not real big on cranking the bass knob. That reduces the proportion of mids that makes the P cut so well. However, I am developing an interest in compression (solely for old school tonal flavoring), and as pointed out, that could well keep the amp "on" a good bit more than usual, jacking the voice coil temp considerably. LOTS of variables here...
  15. BurningSkies

    BurningSkies CRAZY BALDHEAD Supporting Member

    Feb 20, 2005
    Seweracuse, NY
    I still think its molehills to mountains. Unless you're running one of the 'super-woofers' with a very high xmax/xlim, your speaker is going to run out of headroom before you hit your thermal rating. I'd contend that if you're getting close to burning your coils you've been enjoying the sound of your drivers distorting out for quite a while.
  16. SactoBass

    SactoBass I like all-tube amps! Supporting Member

    Jul 8, 2009
    Sacramento CA
    BurningSkies nailed it. Don't be fooled by plaid-jacket-wearing marketing dweebs who insist on publishing the thermal rating. The mass produced cabs (I won't name names but we know who they all are) cannot actually handle that much power. As BurningSkies stated, the physical (excursion) limits of mass produced bass cabs will fail long before the voice coil burns. As a very general rule of thumb, it is safe to assume that the mechanical rating of a bass cab is about half of what the published (thermal) rating says.

    Of course, that is a generalization. In my experience, high ends cabs (eg, Bergantino, etc) can handle more power than that, partly due to the fact that people like Jim B rate their cabs more appropriately.
  17. dincz


    Sep 25, 2010
    Czech Republic
    When manufacturers do give honest specs they can be revealing.

    Aguilar SL112:

    Power Handling: 250 watts RMS, 500 watts peak
    Sensitivity: 99.9 dB 1W 1M
    Max SPL: 120db

    A quick calculation shows that 100W should give you the cab's max SPL. After that you don't get more volume - just more distortion and heat.
  18. OK thanks, I feel a lot better about the driver now with xmax of 5.75mm, plus using the HPF. Couldn't find a demo vid of the fdeck, but the sfx one is reassuring for the concept:
  19. Using the HPF you have a much better chance to fry the speaker. You might get lucky and hear it get quieter at extreme loudness, then it's time to turn down, not up.

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