Can somebody help me with cab impedance with 3 speakers

Discussion in 'Amps and Cabs' started by Acid Test, Mar 4, 2014.

  1. Acid Test

    Acid Test Supporting Member

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    I have a Randall cab with a 2x12 and a 1x15 in it. The 15" speaker is definitely a 16 ohm speaker and I believe the two 12" speakers are also 16 ohms.

    The cab is supposed to have an overall impedance of 8 ohms. Is there any wiring that will allow this? I calculate more like 6.4 ohms.
     
  2. kodou

    kodou Member

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    If they are all 16 ohms then you will not get 8 ohms out of it.

    I calculate 2 series 1 parallel at 10.67 ohms and 2 parallel 1 series as 24 ohms
     
  3. Acid Test

    Acid Test Supporting Member

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    I agree. No matter how it is wired by the sticker next to the jack reads 8 ohms.
     
  4. kodou

    kodou Member

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    Adding to this

    3 Series will be 48 ohms and 3 parallel will be 5.34 ohms
     
  5. kodou

    kodou Member

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    I mean you could get away with 5.34 ohms with an 8 ohm amp but it definitely isn't as the cab states.

    How positive are you they are all 16? If there are two 8 ohm speakers in there you could get 8 ohm.
     
  6. Acid Test

    Acid Test Supporting Member

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    The only configurations I get are:

    24 ohms

    48 ohms

    5.33 ohms

    10.67 ohms
     
  7. Acid Test

    Acid Test Supporting Member

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    I see. When I measure the resistance across the jack, any idea what I might measure? For a 8 ohm 4x12 or 2x12, I get a resistance reading in the ~6.4 ohms range. This cab gives a resistance in the 4's - this is why I am worried.
     
  8. kodou

    kodou Member

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    Measuring DC resistance across the jack will actually give slightly less than the speaker impedance. 6.4 ohms would be approximately correct for 8 ohms.

    EDIT:

    Sorry I read your last message wrong.

    4 seems too low of a reading for 8 ohms.

    Can anyone else chip in. I am only a hobbyist.
     
  9. axelfoley

    axelfoley Member

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    Is there a small circuit board where the input jack is? Might have some sort of built in ohm correction.
     
  10. lostpoet2

    lostpoet2 Supporting Member

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    Ok, algebra students, here is the answer:

    -the 12" speakers are 8 ohms each and the 15" is 16 ohms
    -the two 12s are wired in series, giving you 16 ohms for the pair (8+8 = 16)
    -the pair of 12s and the 15 are then wired in parallel giving you 1/(1/16 + 1/16)) = 8 ohms for the cabinet.
     
  11. Acid Test

    Acid Test Supporting Member

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    Don't need an algebra lesson - I have a degree in math and taught as a graduate student. However if you would like a reading lesson, please read my first post where it is mentioned that all speakers are 16 ohms. Also why would the measured output be so low?

    Thanks for all of the input and will look for a circuit near the input. I plan to call Randall as well to see what is going on.
     
  12. drbob1

    drbob1 Silver Supporting Member

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    ACtually, that DC reading is perfect for 3 16 ohm speakers in parallel=5.3 ohms. That's what you've got, no matter what the plate says…
     
  13. GMGM

    GMGM Member

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    Is it this one?

    http://www.musiciansfriend.com/amplifiers-effects/randall-rs125cx-2x12-1x15-speaker-cabinet

    Unfortunately, I don't see much of a spec at this link, or on Randall's website.

    But if the speakers are all verified to be 16" Ohms each, then maybe there's a dummy load inside the cab (a large resister acting as an "imaginary speaker").

    But really - since the OP didn't provide a model number, and said he assumed the 12" were 16 Ohms - I am going to further speculate that the 12" are actually 8 Ohms each. It would be much easier for a manufacturer to have (2) 12" 8 Ohm wired in "series", and then take the resulting 16 Ohm combination and wire it in "parallel" with the single 16 Ohm 15" speaker. This series/parallel combination would also provide an 8 Ohm load.


    EDIT: Sorry, it looks like Lostpoet2 beat me to it. I guess that's what I get for not refreshing the thread before posting a reply!

    EDIT 2: And Axelfoley with the load correction.
     
  14. Acid Test

    Acid Test Supporting Member

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    All three speakers are 16 ohms and the overall impedance of the cab is 5.33 ohms. Randall confirmed this and mentioned it was an early design of the cab. They have since changed it.

    I asked if they were concerned about posting an 8 ohm sticker on the back of a cab that is 5.33 ohms and they said no. The tech support guy said most Randall amps can go down to 4 ohms.

    I told the guy my concern was putting an Orange CR120H head (8 ohms but not lower) into the cab. He wasn't concerned. I told him I was since there is an obvious mismatch and mislead - he said that is why they changed the design. Geez.....

    The cab is an RS125XL
     
  15. cap217

    cap217 Supporting Member

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    No matter what Randall says, I would use a 4 ohm tap to that load. An 8 ohm will work but you are running it a bit too hot.
     

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