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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

    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

    5stringDNA

    Oct 10, 2002
    Englewood, CO
    Good info Icez- thanx!
     
  6. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    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
    DC
    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

    NJL

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

    bump
     
  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
    Ireland
    bump, more people need this, or a physics text book.....