The old ohm question again

Discussion in 'Amps and Cabs [BG]' started by Mario123, Jun 5, 2020.

  1. Mario123


    Aug 6, 2018
    Ok I gotta tone hammer 500
    Swr 4ohm 2x10. Can I add a 1x15 8ohm to this? I’m guessing it will drive the head at 6 ohms if I plug both into the head. I know I’m wrong hence the question. Can I?
  2. 2.6 ohms, not happy for 4 ohm amp.

    1/r¹ + 1/r² + etc = 1/Rtotal. Ohms maths.
    RyanOh, Pocket4, Al Kraft and 2 others like this.
  3. Nope!
    That head is 4 ohms minimum, anything lower will kill it. :dead:

    Here is the quick and most commonly used answer sheet.

    8 Ohm + 8 Ohm = 4 Ohms

    8 Ohm + 4 Ohm = 2.67 Ohms

    4 Ohm + 4 Ohm = 2 Ohms
    Doner Designs, RyanOh and Al Kraft like this.
  4. Wasnex


    Dec 25, 2011
    If you connect two cabs in parallel with the same impedance just divide the impedance by two. For example if you have two 8 ohm cabs (8/2 = 4 ohms).

    If you connect two cabs with different impedance in parallel, use this formula where Z is impedance:
    for example if you have an 8 ohm cabinet and a 4 ohm cabinet.
  5. Mario123


    Aug 6, 2018
    Thank you all for your Help
    Element Zero likes this.
  6. BassmanPaul

    BassmanPaul Inactive

    This can be simplified for two impedances. Z1 X Z2 divided by Z1 + Z2. 4 X 8 =32 divided by 4 + 8 +12 2.67.

    Or even simpler in terms of 8Ω impedances: 4Ω is two 8Ω loads. So 2x 8Ω and 8Ω is 3 8Ω loads. 8 divided by 3 is 2.67Ω.
    Wasnex likes this.
  7. Wasnex


    Dec 25, 2011

    You can do it that way if you like. :thumbsup:

    I prefer this formula because the form works with an infinite number of impedances:

    For example, if you have three cabs just continue the sequence:


    If I had three cabs with the same impedance I would simply divide the impedance by the number of cabs. For example 8/3 = 2.67 But this works as well:


    Or if the impedances are dissimilar


    IMHO this formula maybe a bit more complicated, but I think is also a bit more universally applicable.

    One caveat...I use a calculator ;).
    mikewalker likes this.
  8. BassmanPaul

    BassmanPaul Inactive

    I just use my brain! LOL
    Wasnex likes this.
  9. Killing Floor

    Killing Floor Supporting Member

    Feb 7, 2020
    Austin, TX
    Rise up bassists! Resist Ohm's law!
    I have a resistance to properly using "impedance".
    mikewalker likes this.
  10. k-3t5v5wkvuTLe-xiYyEmrVXOwBMlHWAb44mtVR5VIY.png
    mikewalker, Thundar, RyanOh and 4 others like this.
  11. TheReceder


    Jul 12, 2010
    Mind if I use yours too... mine doesn't seem to be working lately...
    BassmanPaul and Wasnex like this.
  12. Killing Floor

    Killing Floor Supporting Member

    Feb 7, 2020
    Austin, TX
    Are you sure your SWR 210 is 4 Ohm?
  13. Lobster11

    Lobster11 Supporting Member Supporting Member

    Apr 22, 2006
    Williamsburg, VA
    Apart from the ohms issue, I'd suggest getting two 8-ohm 2x10s rather than a 2x10 + a 1x15. Stack'em vertically and you'll have a killer rig. (I have two such rigs, one of which is SWR's.)
    S-Bigbottom likes this.
  14. They did make them in both 4 and 8 ohm, but it wouldn't hurt to test it. It's pretty easy to blow up a 4 ohm 210 so could easily be reloaded at 8 ohm.

    Multimeter on Ohms low range. Should show about 3ohm if it's a 4 ohm cab and 5 to 6 ohm for an 8 ohm cab.
    Wasnex likes this.
  15. john m

    john m Supporting Member

    Jan 15, 2006
    4 ohms in parallel with anything is less than 4 ohms. If your amplifier can’t support a load smaller than 4 ohms, you cannot do it.
  16. Doner Designs

    Doner Designs Steve Doner Gold Supporting Member

    Jun 2, 2012
    Metro Chicago Area
    Doner Designs is an alias for Steve Doner
    If you open up the 210 you might be able to change the impedance. It may have two 8 ohm drivers in parallel to get to 4 ohms together. Wiring them in series would make the cab 16 ohms which you could then safely combine with the 8 ohm cab.

    Note however that two cabs with different impedance will not be equally loud.
  17. micguy


    May 17, 2011
    Speakers in parallel add conductance. You get conductance (which is in mhos) by taking the reciprocal of the impedance. An 8 ohm cabinet has 1/8th of a mho of conductance, a 4 ohm one has 1/4th of a mho. So, for a 4 and 8 ohm in parallel, you add the conductances - .125 plus .25 equals .375 (or 3/8ths). To convert it back to ohms, take the inverse of that sum - 3/8 mhos is the same as 8/3 ohms, or 2 and 2/3rds ohms, or 2.67 ohms.
    agedhorse likes this.
  18. Pocket4

    Pocket4 Supporting Member

    Dec 9, 2013
    New Hampshire
    Or just review the sticky on this forum so we don't have the to call up the math again to the old ohms "homage".
  19. Killing Floor

    Killing Floor Supporting Member

    Feb 7, 2020
    Austin, TX
    Yeah, some smarty pants thought it would be fun to spell Ohm’s name backwards for the inverse. Flammable - inflammable, sine - cosine, Ohms - Siemens, chocolate - vanilla, salt - pepper, bassist - drummer.