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A quick guide to calculating ohms!

Discussion in 'Amps and Cabs [BG]' started by Ívar Þórólfsson, Mar 19, 2003.

  1. Ívar Þórólfsson

    Ívar Þórólfsson Mmmmmm... Supporting Member

    Apr 9, 2001
    Kopavogur, Iceland
    Hi all!

    I apolagize if this has done before, but I could not find this in the excellent amp faq that Joris compiled.

    There has been an abundance of ohm related questions here, so here I give you the quick answer to calculating ohms.

    R = ohm
    R1 = ohm of cabinet 1
    R2 = ohm of cabinet 2 etc.

    R1 * R2 * R3 (etc.)
    ----------------------- = R
    R1 + R2 + R3 (etc.)

    For example, if you have two cabs, a 8ohm cab and a 4ohm cab and you want to know how many ohms that will make, you simply use the formula here above.

    8 * 4
    -------- = R
    8 + 4

    So, 8 * 4 = 32
    and 8 + 4 = 12
    Finally 32 / 12 = 2.66666....ohms.

    If the total ohms number is lower than your amplifier specifications recommend, it is not safe to use that particiliuar combination of cabinets as it would most likely result in a fried amp.

    On the other hand, if the number is higher than your amplifier specifications recommend it is safe, but the amp won´t be pumping out as many watts as it could have.

    As a sidenote, if your amp is capable of 2ohms, and you have a 8ohm cab and a 4ohm cab, which is safe in this instance. The 4ohm cab will be much louder then the 8ohm one.

    Hope this helps!
  2. wushuguy


    Jan 9, 2002
    Flagstaff, AZ
    Thanks! I never really knew the formula.
  3. Ívar Þórólfsson

    Ívar Þórólfsson Mmmmmm... Supporting Member

    Apr 9, 2001
    Kopavogur, Iceland
    ->Bump in case anyone else is interested<-
  4. Does it matter if your cabs are running in series or parallel?
  5. 5stringDNA


    Oct 10, 2002
    Englewood, CO
    Good info Icez- thanx!
  6. Yes. That formula is for parallel, which is the most common way for two cabs to be connected.
  7. Phat Ham

    Phat Ham

    Feb 13, 2000
    That formula only works for 2 resistances. You'll get the wrong answer if you use it for 3 or more resistances. The proper equation to use is:

    1/R = (1/R1) + (1/R2) + ... + (1/Rn)
  8. NJL


    Apr 12, 2002
    San Antonio
    you da man!! very cool!

  9. Agreed. It's better to use the correct formula, then derive the formula for your application.

    1/R = 1/R1 + 1/R2 + 1/R3 + ... + 1/Rn

    In our (two cab setup), R3 thru Rn do not apply. They equal a resistance of near infinity.

    So, if R1 = 8 and R2=4

    1/R = 1/8 + 1/4 + 1/oo

    Note, 1/oo approaches 0...and will get there eventually :D So we just let it go to 0.

    1/R = 1/8 + 1/4 + 0

    1/R = 1/8 + 2/8

    1/R = 3/8

    Algebra Time!

    1*8 = 3*R

    R = 8/3

    R = 2.667

    Icez is right, but this is for you 2 2x10 / 1x15 (etc) users. Or for anyone that really loves equations.
  10. miccheck1516

    miccheck1516 Guest

    Feb 15, 2003
    bump, more people need this, or a physics text book.....