Impedence and stringing cabs together.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by michael.e, Dec 12, 2005.


  1. michael.e

    michael.e Supporting Member

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    Hey!
    So, if I have a cab [with a pair of 8 ohm speakers] that is wired for 8 ohms and I want to install an "out" on the cab, in order to plug in another 8 ohm cab [again, 2-8 ohm speakers], does the impedence drop to 4 ohms? If so, would it be possible to keep the draw at 8 ohms in this configuration?


    8 0hm signal from amp 1>>

    >>>>>>>>>>>>>>>>>>>>>>>>>>TO--> Headbone amp switcher--> IN-8 0hm cab 1-OUT--> IN-8 0hm cab 2=?

    8 0hm signal from amp 2>>



    My intention is to use the two cabinets for both of the amps, whichever one I choose, via the Headbone.
    Thanks for any help. M.E.
     
  2. VaughnC

    VaughnC Supporting Member

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    If you have a cab with a pair of 8 ohm speakers, it can't be wired for 8 ohms unless you use only one speaker. Two 8 ohm speakers can only be wired for 4 ohms (parallel) or 16 ohms (series).

    If you are certain the cabs are 8 ohms each, the internal speakers could either be two 4 ohm speakers in series or two 16 ohm speakers in parallel.

    Two 8 ohm cabs in parallel will be a 4 ohm load....and there's no way to make this configuration 8 ohms. However, you could wire two 8 ohm cabs in series for 16 ohms.
     
  3. michael.e

    michael.e Supporting Member

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    Ohhhhh....... Okay, so I am being a big stupid! 8 + 8 + = 4. Okay, now I've got that.


    So, I have 2 8 0hm speakers in my Tophat. So that =4 0hms. Okay, that is clear.

    I am going to be receiving a new cab that is wired for 8 0hms [2 16 0hm speakers, right?].

    I have 2 amps that both want to see either 8 or 4 0hm draw, how would I make these 2 speaker cabs work in conjunction with each other in order to not fry my amps?

    What about those Weber Impedence Matchers?


    Hmm, such a quandry! Thanks, M.E.
     
  4. michael.e

    michael.e Supporting Member

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    Here is the "enlightened" configuration....



    8 0hm signal from amp 1>>

    >>>>>>>>>>>>>TO--> Headbone amp switcher--> IN-8 0hm cab 1 [2-16]-OUT--> IN-4 0hm cab 2 [2-8]=?

    8 0hm signal from amp 2>>

    Now, what do I do? M.E.
     
  5. drbob1

    drbob1 Silver Supporting Member

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    The way to calculate impedance is 1/r+1/r+1/r...=1/R
    So for your case, mixing an 8 ohm and a 4 ohm speaker cabs, you'd get 2.7 ohms. That'd be a little low, but probably safe with the amp set at 4 ohms. You'd be better off to sell the 16 ohm speakers, get a similar set of 8 ohm speakers, make both cabs 4 ohms and wire them in series for 8 ohms when used together...
     
  6. Wakarusa

    Wakarusa Member

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    One of the other things to be mindful of is (for lack of a better term) load division.

    If you have a 4 ohm load in parallel with an 8 ohm load (for 2.7 ohm total) then the current flow through the two legs (and the voltage drop) will be divided proportionally with the loads. That is, the 4 ohm cab will get about twice the signal as the 16
    [*]

    If, however, all parallel loads present the same impedance, then the input signal is divided more equally and the cabs will (assuming similar speaker efficiency) be more balanced for overall loudness. The way to achieve that with these cabs is to wire the internal 8 ohm speakers in series and the external 16 ohm speakers in parallel, then put the two cabs in parallel. The series-parallel hookup results in 5-1/3 ohms overall nominal impedance.

    Regarding the Weber load matcher -- haven't seen one, but will probably pick one up to test. I'm guessing that it's a transformer with some capacitance in series with one or both windings for phase correction. Ought to be interesting to see if the output is phase correct and also to throw it on the distortion meter and see how it performs. Anyone have one they wanna lend me for a few days? :)


    [*] subject to some variability because of the different impedance response characteristics of the speakers, but a good rule of thumb anyway. Google Kirchoff's Laws for a more formal discussion.
     

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