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Ohm Math

Discussion in 'Amps and Cabs [BG]' started by Bassman 100, Oct 22, 2010.

  1. Bassman 100

    Bassman 100

    Apr 4, 2008
    Can someone add (6 + 8 ohms) please

    I can add = numbers no problem, I know 8+8=4 ohms & so on, but I'm confused with this one unequal numbers
  2. Alex1984


    Jan 16, 2010
    48/14 = around 3.5?

    For a tube amp, I'd use the 4 ohm setting.
  3. mongo2


    Feb 17, 2008
    Da Shaw
    Series: 14

    Parallel: 3.4
  4. john m

    john m Supporting Member

    Jan 15, 2006
    6 ohms in series with 8 ohms = 6 + 8 = 14

    6 ohms in parallel with 8 ohms = (6 x 8)/(6 + 8)= 48/14 = 3.4
  5. Bassman 100

    Bassman 100

    Apr 4, 2008
    Fantastic thanks guys & know I know how to add.
    Follow on Question. I have a MarkBass MiniMark. 150 Watts. If I add an 8 Ohm Cab I get 250 Watts. I have a Phil Jones 6B Cab which is 300 Watts, but 6 Ohms, am I out of luck here? (mismatch 0.6 is this significant?) if I have my math correct
  6. FunkMetalBass


    Aug 5, 2005
    Phoenix, Arizona 85029
    Endorsing Artist: J.C. Basses
    Unless the amp is 2-ohm stable, yes, you're out of luck.

    Does the 0.6-ohm difference matter? Probably? I'm no expert on amp design, but I imagine there is a fairly small tolerance associated with an amplifier's stability at running with impedance outside of the scope of the amp's specs.
  7. Alex1984


    Jan 16, 2010
    With solid-state, the main concern is current draw (ie. heat). If you're not pushing it really hard, and you don't have any other alternatives, I think it'd survive, but not something I'd want to do repeatedly. On a side note, 6 ohms, really?
  8. Resistors in Series = R1+R2+Rn

    1/Resistors in Parallel = (1/R1) + (1/R2) + (1/Rn)
  9. I think a 4 Ohm amp should handle a 3.4 Ohm load since that's the average Ohm level (I'm NOT paying for your amp if it melts!). Just don't crank the amp to max, or at least periodically check the amp for too much heat if you use this load. A 2 Ohm amp will handle this just fine, one of the reasons I like my Yamaha BBT500H.
  10. mrkreuzschlitz


    Jun 30, 2008
    Dacula, GA
    I've seen people run 4 ohm amps at 2 ohms all night. Now, I'm not recommending this... at all, but its possible.
  11. By george, he's got it.

    Serial or parallel matter a whole lot as you can see.
  12. Folks, when the company who make the amp says don't use it below 4Ω it's a warning that you should listen to. I've repaired way too many amps that have blown outputs because of doing precisely this. It's simply not worth the risk. It's called abuse and a persons warranty will be void in these cases.
  13. greenboy


    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    Yeah, and some people are assuming that an 8-ohm cab or a 4-ohm cab doesn't actually dip lower yet, than its stated nominal loads. But that's not the case. Since these loads are all over the map, amplifier designers are already addressing some lower impedances than the stated nominals. No sense to throwing gas on the fire. Do it right.
  14. Another OHM MATH question.

    If two cabs have equal resistance (say, two 8-ohm cabs) but different wattage handling and you run them off the same channel in parallel (making the 4-ohms together, right?), how do figure out how much wattage they can handle?

    If one is 300watts max and the other is 800watt max, what's the max wattage you should pump into them to avoid damage?
  15. jmlee

    jmlee Catgut? Not funny. Supporting Member

    Jun 16, 2005
    Halifax, Nova Scotia
    If both have the same impedance and they're in parallel, the current produced by the amp will be equally split between the two speakers and the power "absorbed" by each will be the same. So...600 watts out of the head should translate into 300 watts to each. You want to protect the "weaker" of the two elements in the circuit.
  16. Thanks for the quick reply. Of course, I won't NEED that info til I get another cab, but I really, really want one so I want to be ready when I do! :D
  17. Exactly, double your weakest link and that is the "THERMAL" limit of the combined cabs (mechanical limit may be less).
  18. the formula to figure out resistance in parallel is:


    divided by


    So- to figure out two 4 ohm loads in parallel-



    2 ohm load.

    Got it?

    Series resistance- just add the numbers together- 4 ohm + 4 ohm = 8ohm.

    I agree with Paul- never EVER go below your amp's rating. While some MAY tolerate it most will not. Blowing your amp is not worth it.
  19. I have no intention of blowing any amp. Gear is spendy!
  20. Stick_Player

    Stick_Player Banned

    Nov 13, 2009
    Somewhere on the Alaska Panhandle (Juneau)
    Endorser: Plants vs. Zombies Pea Shooters
    Too much work!

    Use this: Impedance Calculator

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