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4ohm +8ohm cab =?

Discussion in 'Amps and Cabs [BG]' started by Randy W Lopez, Feb 23, 2004.

  1. Randy W Lopez

    Randy W Lopez Supporting Member

    Oct 19, 2003
    Dearborn Heights, MI
    Endorsing Artist-MESA BOOGIE
    Not exactly sure what type of ohm load I'd be putting on my amp using a 4ohm cab and a 8 ohm cab. I'm using a SWR 900 and from what I understand, 4 ohms is the minimal it will take. What do you all think? Thanks
  2. I would assume that running an 8 ohm and a 4 ohm would not be good for your head since you would be running at more than 4 ohms.
  3. ccbass71

    ccbass71 Supporting Member

    Using 4 ohm and 8 ohm cabinets together would load down the amp at 3.2 ohms. If your amp's minimum rating is 4 ohm you could potentially damage the amp. I have seen people use amps in this situation and there was no problem. BUT I recently saw a guy run two 4 ohm cabs with the amp bridged at 4 ohm and after the show the plastic rack that the amp was in was REALLY hot. If you want to use two cabs, I would suggest going with two 8 ohms or get another amp that can handle 2 ohms.

  4. Electricmayhem


    Dec 18, 2003
    I wish I understood it...

    Just figured out the basic points of resistance...this is too much!
  5. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    Impedance really isn't that hard.

    If the cabs are in series, you just add the impedances. Eg 8 + 8 =
    16. That's why most amps/cabs don't have the speakers in series,
    adding a second cab makes the setup quieter!

    So, most cabs are in parallel. The general case for parallel cabs of
    impedance A and B: (A x B) / (A + B)

    Eg (8 * 4) / (8 + 4) = 32 / 12 = 2.7

    However, for the normal case of two parallel cabs of equal impedance,
    just divide by two.

    A further note: No matter what you do, two cabs in parallel will
    always come out lower than the lowest cab. So if your amp can do a
    minimum of 4 ohms, and you have a 4 ohm cab, you cannot safely add any
    cabs in parallel.
  6. ChenNuts44


    Nov 18, 2001
    Davenport, IA
    Series: R1 + R2 + ... + Rn = Rtotal

    Parallel: 1 / ( 1/R1 + 1/R2 + ... + 1/Rn)
  7. embellisher

    embellisher Holy Ghost filled Bass Player Supporting Member

    Actually, 2.67 ohms, not 3.2
  8. Mattski


    Jan 6, 2003
    Cleveland, OH
    Yep...2.67 Ohms (but I just round to 2.7)
  9. Whappo Grande

    Whappo Grande

    Feb 9, 2002
    Santa Clara, CA.
    Manager: AccuGroove Speakers
    Your SWR 900 will do 400 watts @ 4 Ohms per side or 240 watts per side @ 8 ohms.

    This means that in stereo, you could run your 8 Ohm cab @ 240 watts & your other channel could run @ 4 Ohm with 400.

  10. If I'm right:

    Running in mono, you'll find the 4 ohm cabinet receiving twice the power delivered to the 8 ohm cabinet. This can be used to advantage, making one cabinet louder than another, affecting the final balance.
  11. ccbass71

    ccbass71 Supporting Member

    Oops! You are right it would be 2.67ohms not 3.2. My bad. Either way it still wouldn't work.
  12. Randy W Lopez

    Randy W Lopez Supporting Member

    Oct 19, 2003
    Dearborn Heights, MI
    Endorsing Artist-MESA BOOGIE
    Math :meh: . Not my strong subject.
    I thank everyone for their assistance.
    Let's add to this, if I'm running biamp mode, I assume I'm using 2 different side's of the power amp which each excepting what ohm load I put on it. 1 side 4, the other side 8.
    Problem is the 15 is 8ohm and the 10's are at 4. I assumption is you generally want more power to your lower end, which in actuality will be the opposite.
    But then again, the 10's do have a wider dynamic range then the single 15.
  13. I would say just balance the output of the two power amps until you have a good mix. Or add another eight ohm 15 to your lows. At least you have some options with your amp.

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