Newbie questions about amps/cabs and power

Discussion in 'Amps and Cabs [BG]' started by beaupral, Apr 9, 2012.

  1. beaupral

    beaupral Guest

    Apr 9, 2012
    Hi all,

    I'm sure this kind of questions has been answered several time, but I can't find the answer.

    So I have a SVT4 Pro and noticed that I can either :

    - Plug one cab monobridged (with speakon wire)
    - Plug two cabs with speakon wire
    - Plug four cabs with phone jacks

    Also, I see that cabs, in general, can be chained.

    So basically, I have the following questions (and lets say all the cabs offer the same resistance):

    Does running 2 cabs with half the power is equivalent to running 1 cab with full power ?
    (ie.: does 2x200w is equivalent to 1x400w ?)

    If cabs are chained, will this affect the power they output ? (I'm guessing the resistance will be increased ?)

    If I plug 4 phone jacks, will each output as much power as 2 speakon wires or will the power be split in half in those phone jacks ?

    Thank you very much, this has been bugging for a long time!
  2. Check out ohm Loads in the FAQ.
  3. ClayBones

    ClayBones Guest

    Mar 8, 2012
    Northern Illinois
    A few basic 'rules...' An amp develops power into a load. The load is the speaker cab. A 4-ohm cab is twice the load of an 8-ohm cab. This being said...a properly designed amplifier should develop 2Xthe power into a 4-ohm load as opposed to an 8-ohm load. The answer to your first question depends on the impedance of the cabs and their power-handling capability. Connecting cabs together will affect the impedance the amp sees, depending on if the cabs are connected in series, or parallel. Series connections double the impedance, cutting the power in-half at a given setting. Connecting cabs in parallel Halves the impedance the amp sees (provided the cabs are of equal impedance.) Your third question cannot be answered accurately or safely without knowing the impedance of the 4-cabs, as well as the output impedance the amp will operate comfortably at. Feel free to contact me directly if you have any more questions.
  4. makohund


    Dec 12, 2002
    Your amp:

    RMS Power Output: 300-Watts x 2/900 (mono-bridged @ 8 Ohms)
    RMS Power Output: 490-Watts x 2/1200 (mono-bridged @ 4 Ohms)
    RMS Power Output (2 Ohms): 600-Watts x 2

    That's not the most crystal clear layout to follow, so here's a translation:

    RMS Power Output, 2 channels:
    300W x2 @8ohms
    490W x2 @4ohms
    600W x2 @2ohms

    RMS Power Output, bridged ("mono-bridged"):
    900W @8ohms
    1200W @4ohms

    Sorta yes, sorta no. Take your amp with an 8 ohm cab, for example. Your amp will feed it 300W on one channel, or 900W bridged. If you add a second 8 ohm cab, the power will get split evenly between them. BUT... notice that with the lower load (4ohms), it'll also be feeding them quite a bit more power. 490W on one channel, or 1200W bridged. So each speaker will be getting 245W single or 450W bridged. That's a lot more than half. Plus you get a "magic" 3db bonus from the speakers coupling together.

    Yes, it'll split the power in half between them.
    No, sending more power to the pair plus coupling benefits means it'll be quite a bit louder than running just the one cab on that power.

    Actually, decreased. The scenario I just gave works the same whether they are daisy chained (in parallel, which is typical unless you go out of your way to do it differently) or both connected to your outputs directly.

    Note that you have two power amps in that unit. I'm talking about behavior on a single channel, or in bridged mono. Learn how this stuff works with a single channel first, then you'll see how having two available introduces more flexibility (to go with the complexity).

    If you took it apart and looked inside, you'd probably see that the speakons and phone jacks for each channel are all wired together. These multiple taps of different types are no different from each other electrically, they are there simply for your convenience in connnecting whatever you have.

    In reality you have three outputs... one for channel 1, one for channel two, and one that conveniently combines a +1 and -2 (or equivalent, methods vary) from each side into a single more powerful "mono-bridged" output. All of the output plugs/jacks on the back just tap into one of those three, and can provide all of whatever power is there to be had.

    The "splitting" of power between the amp and the speakers is a function of the speaker loads themselves, not of the various parallel taps available on the amp or cabinet. Two 8 ohm loads on a channel will give you a single 4 ohm load, with half of the available power going to each. Two 4 ohm loads will give a 2 ohm total, again split equally. Three 8 ohm loads will give you a 2.66 ohm load, with power split evenly 3 ways. And so on.