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changing impedence of a cab?

Discussion in 'Amps and Cabs [BG]' started by Matt R Miller, Mar 22, 2004.


  1. Matt R Miller

    Matt R Miller

    Apr 27, 2003
    Wisconsin
    I have an Ampeg Portabass 2x12, which is rated at 4 ohms. I'm using an Ampeg B2R head (350w at 4, 200w at 8). I'd like to be able to use a second cab in the near future, and have 2 questions:

    1) How would (can i?) get the 2x12 changed to 8 ohms so I can use another 8 ohm cab with it to get 4 ohms?

    2) Would doing this and adding say a 1x15 increase the total output I get from the setup?

    Thanks for any help.

    -Matt
     
  2. Scott D

    Scott D

    Apr 21, 2003
    Minneapolis, MN
    1. No. - (But you could make it 16 ohms...)
    2. Yes.
     
  3. seanm

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

    Feb 19, 2004
    Ottawa, Canada
    1. You could either wire them in series to get 16 ohms as romeoarmada said. Or you have to replace the speakers.

    2. Depends on the speakers. Two loud 12s replaced with two louds 12s and a lower output 15 will actually be less loud. However the Ampegs are inefficient and a more efficient 15 would help.

    Really the PortaBass cabs are designed to be standalone.
     
  4. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    Also, you might mess with the crossover point of the speakers.
     
  5. That cab isn't very efficient at 93db. Even with the max power your amp can feed it you'll only ever reach a max SPL of 118.4db. That's not very loud for a 2x12. I think if you want to make things louder you might want a more powerful amp and/or some more efficient speakers. Anything with an efficiency of 100db or better would get pretty loud with your 350 watts. There's not a whole lot of arguing with physics, sorry!
     
  6. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Yeh that's a pathetic efficiency. Depending how they arrived at it. Usually it's at 1 meter but the frequencies vary. It's been a year since I've dinked with speakers and they were all 15's but an average rating on a quality speaker was 98 and toastie ratings were 100 - 104. Can't remember exactly but seems you have to double your amp wattage for every numeral increase in efficiency to get equal output - if that's not right, it's close. I was amazed when I read it. I couldn't tell that kind of difference with the stack of 15's I collected but it definitely makes a noticeable difference. And I'm talking 15's, smaller speakers should be even more efficient. BUT, more efficiency doesn't mean a better sounding speaker, just a louder one.