Ohms and wattage question.

Discussion in 'Amps and Cabs [BG]' started by Regurgitator, Aug 7, 2000.

  1. Regurgitator


    Apr 27, 2000
    Which would be a better setup for me: I have a Peavey firebass head and a Peavey 410txf cab, I plan to buy either a Peavey 15 BX-BW cab or a SWR Big Ben 18" cab. Which do you guys think would be better? The Big Ben is 8 ohms and the 15 is 4. Would running the 8 ohm 18" sap all my wattage from the Firebass?
  2. White_Knight


    Mar 19, 2000
    It all depends: what's the Firebass rated at? If it's full wattage is put into 4 ohms, then you'll want to use either two 8 ohm cabs or one four ohm cab. If it's full wattage is 2 ohms (which I doubt), then you'll want to use two 4 ohm cabs to get the most out of it.

    Recap: 2 8 ohm cabs for a 4 ohm head
    2 4 ohm cabs for a 2 ohm head

    Personally, I like the Big Ben, so I would probably go for it but that's just my preference. Hope this makes sense to you.
  3. Regurgitator


    Apr 27, 2000
    Actually here is the spec on the Firebass:

    Rated Power & Load:
    700 W RMS into 2 ohms
    475 W RMS into 4 ohms
    275 W RMS into 8 ohms

    Which I guess explains why both the Peavey 410 and 15 cabs are 4 ohms loads. So if I run a 4 ohms cab and a 8 ohms cab, what is the load? 4 ohms? I think I will go with the 15, keep it more powerfull.
  4. White_Knight


    Mar 19, 2000
    I would go with the 15" in that case. I can't remember how to find out the ohms when using two different impedances (I think it would be around 3 ohms but I'm not quite sure) but I do remember that it isn't exactly the best thing to do. If Joris or Psycho is around here they could probably go into more detail about it. It wouldn't hurt your amp or anything, it just wouldn't give you the desired sound (sound would be very unbalanced between cabs as I understand it).

    Generally, 8 ohm cabs are meant to be run in parallel with another 8 ohm cab (making a 4 ohm load). Since you amp can handle a 2 ohm load, then running two four ohm cabs in parallel is just fine.

    When cabs are run in parallel (of the same impedance), the impedance is halved. When they are run in series, the impedance is added together (which I guess would be doubled).

    Also, you probably already know this, but running two cabs in parallel is what most amps are capable of. Very few amps that I've seen (again Joris or Psycho may know more about this) have a way to link cabs in series. The Crate BX-100, in fact, is the only amp I know of that can run in series.

    At any rate, that was quite a long answer to your question, so yes, run the two 4 ohm cabs.
  5. rcrimm

    rcrimm Commercial User

    Jun 20, 2000
    Meridian, MS USA
    Customer Service, Peavey Electronics
    The formula I use to figure a mismatched impedance is:
    X x Y divided by X + Y. In this case 8 x 4 / 8 + 4 = 2.67 ohms.

    Doing this is ok as long as the total is equal to or above the rated impedance of the amplifier, so it's no problem for the Firebass.

    It does reduce the power a little, and the lower impedance cabinet gets about twice the power that the higher impedance cab gets. Sometimes this is barely noticable, and sometimes it sounds really unbalanced. This would depend upon the particular cabinets used (efficiency, number of drivers etc).
  6. White_Knight


    Mar 19, 2000
    Thanks for the formula rcrimm. I wasn't sure how you work with mismatched impedances. Learn something new every day.
  7. Here's the formula for equivalent impedance when you have more than 2 speakers:

    1/R1 + 1/R2 + 1/R3 + ... 1/Rn = 1/Rtotal


    Hook (3) 16-Ohm cabinets in parallel

    1/16 + 1/16 + 1/16 = 3/16, then you have to make the numerator equal to 1, so divide 16 by 3 = 5.333 Ohms

    Hook a 4-ohm, a 8-ohm, and another 8 ohm in parallel
    1/4 + 1/8 + 1/8 = 2/8 + 1/8 + 1/8 = 4/8 = 1/2, so 2 ohms total.

    This formula is equal to the one given in the posts above when there are only 2 speakers involved.

  8. could someone explain to me series, parallel and daisy chaining? please do it like i'm stupid, because i am. i always understood parallel was running a cord from amp to cabinet and then cord from cabinet to cabient? i have a peavey mark iv and it has two outputs. where does this come into play? if i run a four and an eight ohm cabinet, the ohms will be something like 2 and two thirds, and the four ohm cabinet will get more power?