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Head, Cabs and Ohm Assumptions...

Discussion in 'Amps and Cabs [BG]' started by Quadzilla, Mar 4, 2002.


  1. Quadzilla

    Quadzilla Supporting Member

    Please excuse my ignorance but if a head says (4 ohm minimum) under the 2 output jacks I assume that means that you can run these configurations:

    1 - 4 ohm cab
    1 - 8 ohm cab
    2 - 8 ohm cabs (one from each jack)

    I also assume that you CANNOT run the following:
    2 - 4 ohm cabs
    2 - cabs one at 4 ohm and one at 8 ohm

    Am I right on this? :confused:

    Thanks
     
  2. Johnny BoomBoom

    Johnny BoomBoom Supporting Member

    Jun 8, 2001
    Glasgow, Scotland
    The formula for working out resistance in parallel is:-

    1/Rt = 1/R1 +1/R2 ........

    This can go on however for your examples:-

    1. 1/Rt = 1/R1 therefore

    1/Rt = 1/4 therefore

    Rt = 4 ohms (Rt is the total resistance)

    -------------------------------------------------

    2. 1/Rt = 1/R1 therefore

    1/Rt = 1/8 therefore

    Rt = 8 ohms (Rt is the total resistance)

    -------------------------------------------------

    3. 1/Rt = 1/R1 + 1/R2 therefore

    1/Rt = 1/8 + 1/8 therefore

    1/Rt = 2/8 = 1/4 therefore

    Rt = 4 ohms (Rt is the total resistance)

    I run my two 8 ohms cabs daisy chained into each other - ie head to cab1, cab1 into cab2. This resul;ts in a 4 ohm load as they are in parallel

    ------------------------------------------------


    For your cannots :-

    1. 1/Rt = 1/R1 + 1/R2 therefore

    1/Rt = 1/4 + 1/4 therefore

    1/Rt = 2/4 = 1/2 therefore

    Rt = 2 ohms (Rt is the total resistance)

    ------------------------------------------------

    2. 1/Rt = 1/R1 + 1/R2 therefore

    1/Rt = 1/4 + 1/8 therefore

    1/Rt = 1/8 + 2/8 = 3/8 therefore

    Rt = 2.67 ohms (Rt is the total resistance)


    Hope this helps!!


    Edit - to prevent confusion (I hope)
     
  3. Let me simplify Johnny Boom's post:

    The answer to your question is yes - you are corect :)

    YOU CAN:

    1 - 4 ohm cab (total 4 ohm load)
    1 - 8 ohm cab (total 8 ohm load)
    2 - 8 ohm cabs (total 4 ohm load)

    YOU CANNOT:

    2 - 4 ohm cabs (total 2 ohm load)
    1 - 4 ohm cab + 1 - 8 ohm cab (total 2.67 ohm load)
     
  4. Quadzilla

    Quadzilla Supporting Member

    Thanks for explaining that in ENGLISH!!!
    :p ;) :D :)