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How can 2 8 ohm speakers make an 8 ohm cab?

Discussion in 'Amps [BG]' started by Shinbone, Jan 17, 2014.

  1. Shinbone

    Shinbone Supporting Member

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    I have an Ampeg PF-115HE. This is an 8 ohm cab. It has a 15" speaker + a tweeter. Each of these is labelled as "8 ohms". Given that resistances in series add and resistances in parallel divide, how do they get the cab to come out as 8 ohms? I suspect the crossover has something to do with that but I'm not sure. Can anyone enlighten me?

    Also, if I were to remove the tweeter, how would that affect the resistance? Would it be the same as just turning the tweeter off?
  2. BurningSkies

    BurningSkies

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    The tweeter may be listed as 8ohms, but really most if not all common 'tweeters' in bass cabs are nowhere near that. Up in the range that tweeters operate, the load they create is negligible.

    If its a dedicated mid with a full crossover, its because of how your signal is being crossed over and how it feeds to the component parts.
  3. Grissle

    Grissle Supporting Member

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    Ya I believe the crossover divides the impedance one way or another. Someone who knows more will soon chime in.
  4. Mr. Foxen

    Mr. Foxen

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    Amp tinkerer at Ampstack
    Chances are the tweeter is just high passed. Since it isn't operating over the place where the power to the cab is, the impedance of the woofer is the one to pay attention to.
  5. Jools4001

    Jools4001

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    A crossover is basically applying two filters to the input signal - a low pass filter and a high pass filter. The load that the amp sees is the crossover, not the individual drivers connected to it.

    At frequencies lower than the crossover point the tweeter side of the crossover presents a very high impedance, so it doesn't really affect the sytems impedance if connected in parallel with an 8 ohm woofer. At frequencies higher than the crossover point, the woofer side presents a higher impedance so doesn't really affect the systems impedance and at the crossover frequency the impedance of both drivers is usually higher than 8 ohms anyway...impedance isn't simple resistance, it is frequency dependant and is all over the place, but a nominal 8 ohm impedance means that the system will never be lower than that.

    And yes, removing the tweeter will be like switching it off, but don't do that. Remove the cable to the tweeter if you must, but removing it will leave a hole in the baffle and screw up the cabs tuning...at worst making a blown driver more likely
  6. Shinbone

    Shinbone Supporting Member

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    Thanks. This is great info. Actually, the reason I was thinking about removing the tweeter (besides the fact that I don't use it much) is that I would like to create a port in the cab. This is a sealed cab but I've discovered that I like the tone a lot better if I push the flip-top lid back an inch or two creating a port on top. I saw another thread where somebody added ports to the back of the cab but I'm not sure I want to cut holes in it.
  7. beans-on-toast

    beans-on-toast

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    If you want additional details, it is explained here under impedance.

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