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Tech question: multiple cabs, defeat switches, and output

Discussion in 'Amps and Cabs [BG]' started by cap'n crunk, Apr 17, 2010.

  1. cap'n crunk

    cap'n crunk

    Apr 6, 2010
    Here's a hypothetical that I need a reading on from you amp tech gurus (apologies in advance if this has been answered at some point in another thread, but searches didn't turn anything up):

    I have a 350w RMS @ 4 ohms, 450w @ 2 ohms, combo amp which has 2x10 drivers rated at 4 ohms. I plug in an extension cab rated at 4 ohms, so the combo's head is now running at 2 ohms. Then I hit the "internal speaker defeat" switch, silencing the 2x10s while keeping power to the extension. Here are the questions:

    1. Is the head still running at 2 ohms or 4 ohms? and
    2. Is the extension drawing 225 watts, 350, or 450?

    I can actually disconnect the internal 2x10s at the speaker jack, and if I do then presumably the head is running at 4 ohms and the extension is drawing 350 watts. I'm just wondering if by killing the speakers with the switch but leaving them plugged in, it's possible to "cheat" the system, running the head at 2 ohms and directing the full 450 watts to the extension. My guess is that it's not and that it's still only getting 225w, but the signal sounds much cleaner at higher volume with the 10s defeated, so I thought I'd ask. Thanks!
  2. The defeat is the same as unplugging - no sound, no power draw.

    You are only running @ 4 ohms
  3. JTE

    JTE Supporting Member

    Mar 12, 2008
    Central Illinois, USA
    Yep, the amp only sees the speakers that are in the circuit, so the load is whatever they are. Whether you unplug the internal speakers or hit the switch, the amp only sees the external speakers and is only capable of delivering the 350 WRMS for the 4-ohm rating.

    The speakers don't draw any power at all. None, nada, zilch. They're passive. They take what's given them. So, whether you're using the internal speakers by themselves or the external speakers by themselves, the amp is able to produce up to 250 WRMS depending on local current conditions, how you've got the amp EQ'd, and how hard you're playing. Speaker power ratings are one of the most useless bits of data in the world.

    The amp produces power (wattage is a measurement of power) which the speakers convert to physical energy, but the speaker doesn't do anything to draw the power.

  4. experimental bassist

    experimental bassist

    Mar 15, 2009
    The above comments are true, unplugged speakers are not loading the circuit therefore when the 10s are defeated the total load is 4 ohms.

    But I'm wondering about your comment that it sounds cleaner running just the extension cab. What kind of cab/speaker configuration is that cab?

    It could be that when you try to run all the speakers you're getting phase cancellation, or maybe the extension is just plain more efficient and is making better use of the full 350 watts.

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