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speaker impedance question

Discussion in 'Amps and Cabs [BG]' started by Matty, Oct 11, 2000.

  1. Matty


    Mar 17, 2000
    Hey all,

    I have a simple (I hope) question for you all. I need to know how much my amplifier can handle in terms of speakers.
    So, for example if I have an amp that will do 200w into 4ohms, how do I figure how many cabs i can plug into it. Again, for example lets say I want to plug in a 1x15 and a 2x10, how do i figure out which speaker impedance will work correctly,and whether or not I have enough power. Is there a formula for figuring this all out?

    Sorry if this has been covered, I didn't find it in a cursory search of the site.

  2. jazzbo


    Aug 25, 2000
    San Francisco, CA
    This much I know:

    Add the impedance of all the speakers you'll be connecting to the amp, and divide by 4, that'll give you what ohmage (I like that word) you can hook up to it. Now, someone else needs to answer the wattage part.
  3. White_Knight


    Mar 19, 2000
    Ok, you've got a 4 ohm head? That will let you do the following common combinations:

    A single 4 ohm cabinent
    Two 8 ohm cabinents

    The connection for the 8 ohm cabinents is parallel (the standard on every modern bass amplifier that I've ever seen). Which means using both speaker jacks on the amp or sending out one cable from the amp to the first speaker and then using the speaker's second jack to run the signal over to the next cabinent. So: yes, you could hook a 15" and a 2x10" if each cabinent is 8 ohms.

    Now, to the second part of your question: will 200 watts be enough? It just depends on a lot of variables. Some speakers sound like crap when they aren't being driven to a certian level (commonly called underpowering the speaker), while others take less power to drive them effectively. Personally, I don't think that you'd have any problems with underpowering most speakers with 100 watts going to each, though just to be sure I'd check out any speaker combinations with your amp so that you know how it's going to sound.
  4. timv


    Jun 7, 2000
    Chandler, AZ
    To get the most out of the amp, run it at four ohms. To do that you would want an 8 ohm 15" cabinet and an 8ohm 2x10" cabinet. As far as power make sure they can at least handle about 100 watts rms each at 8 ohms cause they will both be seeing about 100 watts. The power will split between the two if they are the same impedance. The formula that jazzbo mentioned isn't correct. Here's a page w/ a decent description. It's for car stereo equipment but it works. http://www.eatel.net/~amptech/elecdisc/spkrmlti.htm
    Here's another that shows the formula too:
    It's car audio also but it's the same.
  5. Phat Ham

    Phat Ham

    Feb 13, 2000
    Here's the equation for resistance in parallel:

    (1/R1) + (1/R2) = (1/ Total Resistance)

    So with two 8 ohm cabs:

    (1/8) + (1/8) = (1/4)

    So the total resistance is 4 ohms.

    For the purposes of answering your question resistance and impedance are interchangeable.

    [Edited by Phat Ham on 10-11-2000 at 11:06 PM]
  6. Folksie


    Mar 14, 2000
    What you could do also, is wire two 4 ohm cabinets in what is called "series." By wiring them this way, you are able to keep the 4 ohm load to your amp. Some amps (though few I know of) have their speaker jack wired in series. (take the Nemesis combo series for example). However, any amp can be wired in series. With some bassic skills and a little help from Bass Player magazine, you can use that option if you'd like. Here's the link to Bass Player's article on Impedance and how to wire in series:


    I found it quite useful, hope it helps.

  7. I'm sorry I have to say this, but I have to disagree with you, Folksie. Two 4 ohms cabs wired in series give you 8 ohms total. And wiring speaker cabs in series is something I always advise against. Unless they are two exactly identical cabs. Otherwise the speakers will influence each other, and in the worst case, produce annoying resonances, making them sound floppy or making them roar really bad.
  8. Folksie


    Mar 14, 2000
    I just read the series thing in bass player, and I very well might have gotten the info wrong. My bad. Thanks for the info though, that's a very good thing to know. Keep playing all. Sorry I led anybody astray!

  9. mikemulcahy


    Jun 13, 2000
    The Abyss
    Well now that leads me to this question, my 118 cabs have one input and no way to link to another cab.If i use a "Y" adaptor and run it to 2 8 ohm cabs, what does that do to the impedence. In other words is that in series or parallel?

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