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ohms?

Discussion in 'Amps and Cabs [BG]' started by terms bassist, Nov 16, 2001.


  1. I know a little about what this is. but I have never really had it explained to me in english if you know what I mean.

    what's the deal with ohms? eg. why do amps say 400watt through 4 ohms and 200watt through 8 ohms etc?
     
  2. ItchyBass

    ItchyBass

    Oct 12, 2001
    Eugene, OR
    Ohms is a rating of a speakers resistance. The smaller the number, the less resistance the speaker has.

    Using your numbers:

    A 4 ohm speaker will resist less, and therefore allow your amp to push 400 watts into it. An 8 ohm speaker will resist more, and only allow your amp to push 200 (or so) watts into it.

    The best way I have had it explained to me is using the analogy of a hydro-electric dam. The bigger the number, the bigger the dam. The bigger the dam, the less water can get through. The less water can get through, the less power generated. Damn! :D

    Most amps will run at 4 ohms, some at 2, and still others at 8. Be careful not to let your speakers combined ohm-rating (or impedance) go below the lowest impedance your amp can handle, or you can blow it up!

    I am sure many others will post some far more scientific answers to this question (or tell you to do a search and read the answers given to the other 20 people a day who post this same question – don’t feel bad, 90% of the salespeople in music stores don’t even understand the theory/rule/law of ohms). My advice, keep it simple:

    If you only want to use one cabinet (again, using your numbers), get a 4 ohm cabinet. If you want to use two cabinets, get two 8 ohm cabinets (2 x 8 ohms = 4 ohms). Either way you will be running your amp at 4 ohms, and get the full 400 watts out of it.
     
  3. cool thanks mate, that is a huge help. you just saved me from blowing up the biggest investment of my life so far. I thought it was the other way around. cheers!!
     
  4. now i'm confused.....:(
    i posted something similar a while back (i think here).
    quick recall: Ampeg B-2R 200w @ 8ohms
    350w @ 4ohms
    Hartke 4x10xl cab 400w @ 8 ohms

    ....so my question was...adding a 1x15 cab 8ohms, would run my amp at 350w?
     
  5. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    Yes.

    Two 8 Ohm cabs in parallel will result in a total load of 4 Ohm.

    So if you use a mono poweramp with 350W @ 4 Ohm, you'll get the full 350W.
     
  6. td1368

    td1368

    Jan 9, 2001
    Philadelphia
    I've been searching all over for the formula to calculate the resistance for multiple cabinets? I thought it was the number of cabinets divided by the total resistance.

    For example if I have a head with a 4 ohm min and I have one speaker hooked up with and 8 ohm rating. Can I add a 4 ohm cabinet? Or will that go below my minimum.
     
  7. 1/Rtotal = 1/R1 + 1/R2 + ..... + 1/Rn

    In this case,

    1/Rtotal = 1/4 + 1/8 = 3/8 = 1/2.67, so 4 // 8 = 2.67 ohms.

    So that would go below the minimum of 4 ohms in the amp referenced above.

    It's easy if both cabs are equal, then the result is just half.

    8//8 = 4
    4//4 = 2

    You have to use the full formula above when the impedances are unequal.

    Chris
     
  8. td1368

    td1368

    Jan 9, 2001
    Philadelphia
    Ahh I see. Thank you kindly.
     
  9. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    In parallel, 8 ohms plus 4 ohms equals 2.67 ohms. If your amp is rated for a minimum of 4 ohms, you probably shouldn't try to run a 2.67 load. It *might* work, but do you wanna take the chance?

    Here's the formula for parallel:
    Total impedance = 1/(1/a + 1/b + 1/c ... + 1/n)

    where a = cab 1, b = cab 2, c = cab 3, up to n, which is whatever your last cab is.

    So, in your case:

    Impedance = 1/(1/4 + 1/8)
    = 1/(3/8)
    = 2.67
     
  10. basslax

    basslax

    Apr 20, 2000
    Washington, DC
    :eek: oh my thats a lot of numbers for a bunch of bassists
     
  11. No one said it's easy!

    If you're a tech geek, like some of the residents here, its a piece of cake, but we had to learn just like you!
     
  12. Churchbassist

    Churchbassist

    Nov 3, 2001
    Iowa
    So would it be safe to assume that 2 4-ohm speakers wired in series to an amp that already has one 8-ohm speaker input would be ok?
     
  13. It will be alright if the following is true:

    1. The amp is rated for 4 ohm minimum load.

    2. The (2) 4 ohm speakers are truly connected in series, which doesn't mean just daisy-chaining from one cabinet to the next with a regular speaker cable. If you don't know what I'm talking about, do a search for instructions on how to make the series cable.

    If all that is true, then you'll have an 8-ohm load in parallel with an 8-ohm load, giving you a 4-ohm load total, which should be cool.

    Chris
     
  14. Churchbassist

    Churchbassist

    Nov 3, 2001
    Iowa
    Thanks a lot Chris for the quick reply. Didn't know if I would get one since this was an older thread. The amp I use is a Hartke 7000.
     
  15. jamminji

    jamminji

    Mar 22, 2002
    The hartke 7000 is a stereo amp. Each separate amp (channel) will take a 4 ohm load. So in your amp you could, for example, put one 4 ohm cab on the left side and one 4 ohm cab on the right side. Or you could put two 8 ohm cabs on the left side and two 8 ohm cabs on the right side, etc. Just make sure neither side goes below 4ohm. You can also run just one side (one 4ohm cab), or even have a 4ohm on the left and an 8ohm on the right. This last configuration will give you an unbalanced sound, meaning the 4ohm cab will be louder (more power to it) but that is ok as you can balance the volume with the left and right volume controls on the front.

    The 7000 is a flexible amp but one thing it won't do it bridge the two power amps like some other stereo amps. Your amp is two 350watt amps and can not be bridged to make one 700 watt amp. So the max you will get out of any one cab is 350W. This would be using one 4 ohm cab.

    jam
     
  16. I had a lot of problems with this too. If you go to Carvin's website, their resident sound tech wrote a big article on Ohm's and watts and all sorts of other things. It's a really good article, but it's long. Also, if you have problems you can email him and he's pretty quick to write back good responses. His article basically saved me from destroying my cabinet. Cheers!
     
  17. Bass18

    Bass18 Guest

    Jul 21, 2001
    Australia
    a simple way to figure out the impedence of two equal loads (note i said equal) is to half the amount of one load and you have your answer.

    For eg. 8 ohm + 8 ohm = 4 ohm

    Half of one load (8 ohm) is 4 ohm

    Of course remember this only works for TWO EQUAL loads.