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Cab Ohms too low... what will happen

Discussion in 'Amps and Cabs [BG]' started by didier, Mar 15, 2006.

  1. didier


    Aug 4, 2005
    OK, I learned to be aware of speaker resistance/ohms from pretty much the first day I switched from using a combo to a head and seperate cabinet(s).

    But I have always wondered a couple of things, and a search didn't really answer these questions for me:

    Can you fudge "a little"? Like say I had an amp, solid state, rated down to 4 ohms, but I really want to run a 310 and 115 together, which would be a combined load of 3.2 ohms. Will this lead to regret, shame, death? Instantly or will it take minutes or hours?

    I guess it perhaps depends on the amp... And would it matter maybe also how hard it's pushing? I would imagine with gain and volume at 50% ish it would at least last a bit longer than all knobs to 11.

    And really, WHY is it bad? As in WHAT is happening? I understand it will "fry", "blow up", "kill" the amp... I've been told that and told people that, but what does that mean? Heat, voltage, surge of too spicy salsa?

    I'm thinking mainly of solid state amps. God bless the tube amps and the people that play them, but I no longer swing that way.
  2. Herman


    Dec 25, 2005
    Lynchburg, VA
    First of all, how did you get 3.2 ohms as the combined load of these two cabs? What is the impedance of each of them?
  3. didier


    Aug 4, 2005
    5.3 and 8... maybe I calculated wrong, but still, please just supppose that I have a 3ish Ohm load?
  4. ghindman


    Feb 10, 2006
    It really depends on the amp, and how well it's built. Best case is an amp that's built over tolerance, and you'll just slowly lose your transformer due to overheating. Worst case is a cheaply built amp running part right to the edge of their tolerances, doesn't have thermal breakers, and you could blow caps, burn up your transformer, etc., potentially smoke and flames.

    Assuming a ~3ohm load into a decent amp rated for 4ohms, it'll run too hot, you'll get thermal cut-out pretty quickly, and you'll eventually lose the transformer.
  5. Mark Reccord

    Mark Reccord Supporting Member

    Yes, you can fudge a little. An amp rated at 4 Ohms minimum won't automatically shut down with a load of 3.2 on it (Like Herman said, 3.2?) and would be fine in most circumstances.

    Basically what's going on is that all amps can provide a certain amount of electrical current to a load before the ratings of the output transistors are exceeded. The current ratings on transistors are based on temperature. The current at the device's maximum operating temperature is the maximum 'safe' current it can provide. Once you start exceeding that current the devices can be damaged by the heat. Fortunately, most amps have thermal breakers that shut them down before permanent damage occurs.

    Now: Say you have an amp rated 500W@4Ohms. That means the max output voltage is 45V and the current at that impedance would be 11.25A. For simplicity, let's say that 11.25A is the maximum amount of current the amp can provide before exceeding the ratings of its transistors. If you try to put out the full output voltage of the amp at 2 Ohms, the amp will try to provide 22.5A to the load, which greatly exceeds the maximum rated current. The amp will either thermal out or destroy itself. Real amps are a little more complicated but that's the jist of what's going on.
  6. Herman


    Dec 25, 2005
    Lynchburg, VA
    No, that's right for 5.3 and 8 - now I'm just curious where the 5.3 came from.

    I think there are several factors involved and there's no absolute answer to your question. I think it will depend on how hard you drive the amp with this lower than rated load and how conservatively (or liberally) rated your amp is.

    The reason it can be bad is that with a load of 3.2 ohms, you're asking the amp to put out more current (for a given voltage) than if it were driving a load of 4 ohms. The harder you drive the amp, the greater the voltage (and therefore the current) into the load will be.

    As the current goes up so does the power being dissipated by the output transistors. Those output transistors are rated for a certain power dissipation and, if it's exceeded for too long, you can end up frying them.

    There are probably others who can add to or correct what I've said, but I think that's the main reason you shouldn't load your amp with an impedance lower than it's rated for.
  7. phxlbrmpf


    Dec 27, 2002
    If you're lucky like I once was, your amp's fuse will break, rendering your amp silent until you replace it. My amp itself still works fine to this day so I guess it wasn't damaged. (I ain't overstraining it any more, though.)
  8. thrashermatt


    Jan 26, 2006
    western mass
    once i played a show and by mistake i ran a 410 at 4 ohms and a 2x10 and 8 ohms one time on my ampeg svt350h (4 ohm min)... thats 2.67 ohms which is below.. after our set i realized what i did and got very scared for a moment. so i fudged with it at home and everything was fine... thats my encounter with that, but i wouldn't reccomend doing it...
  9. bigbeefdog

    bigbeefdog Who let the dogs in?

    Jul 7, 2003
    Mandeville, LA
    5.3 isn't common, but it's not unheard of. Some old 6x10s have this cab impedance, such as the Traynor I own (shown here):


    If his 310 is three 16-ohm drivers in parallel, that's what he'd get....
  10. tadawson


    Aug 24, 2005
    Lewisville, TX
    Actually, there is a third possibility. It may just sound like hell. If the "weak link" is the power supply in the amp, the low load impedance may pull the power supply down, and give you a distorted sound, much like clipping. If the output devices are rated for more than the supply, this could happen without output stage damage - the power supply would fail instead.
    (I learned the hard way that a used cab I had was actually dual four ohm and not dual 8 when I got it, and had this exact scenario occur . . . perhaps it would have thermalled if I let it go on long enough, but it sounded so incredibly bad that I would have had to been deaf to let it run that way . . . . )

    - Tim
  11. mVC


    Jan 28, 2006
    how is this calculated?
  12. Herman


    Dec 25, 2005
    Lynchburg, VA
    Two 16s in parallel = 8 (16x16/16+16)

    These two in parallel with another 16 = 5.33 (16x8/16+8)
  13. ibz


    Apr 14, 2005
    Columbus, OH
    Aren't most all 310 cabs 5.3 ohms like the Epifani 310 or similar. I think Bertigiano among others made a 310 too.
  14. The Berg was 4 Ohms.
  15. Mark Reccord

    Mark Reccord Supporting Member

    Yes, there's that too, absolutely. I left it out because I was trying to keep ithe explanation fairly simple. :D

    It still boils down to the load requiring more current than the amp can provide without straining.... If the impedance causes the power supply to sag like you described, what you're hearing probably isclipping. If the rail voltage sags, the output voltage that the amp clips at does too...:ninja:


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