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Speaker DI Qs

Discussion in 'Live Sound [BG]' started by SpamBot, Oct 21, 2009.

  1. SpamBot


    Dec 25, 2008
    St. Paul, MN
    I searched for this, and no dice.

    I have a nice little tube amp. I get an awesome fuzz tone when I push it (REALLY push it) and it sounds great for practice. I would like to get this tone live, but it wouldn't be practical at 5w.......

    I've looked at various speaker DIs, mostly at the Countryman 85 and the new Markbass DI. While they are both designed to be used as speaker DIs as well as instrument DIs, the input impedance on the MB is 600 ohms and the 85 is 10k ohms IIRC.

    My main question is this: Is it okay to run my amp into this as long as I have the correct load (8 ohms, in this case) attached to the through jack? At $70, the MB is very tempting.

    Also: I have a 20w thermal guitar speaker that sounds hideous lying around - If I cannibalize it for the voice coil, can I put it in a box and use it as an 8 ohm dummy load for this amp?

    Also: Is a speaker DI hard to DIY? I imagine it would be, with the complex transformer and all, but I figured I'd ask.
  2. Mark Reccord

    Mark Reccord Supporting Member

    The input impedance of the MB DI is 12k Ohms in speaker mode assuming a load impedance at the mic preamp of 600 Ohms. The input impedance of a passive DI is dependent on the input impedance of the mic preamp. Most mic preamps have higher than 600 Ohm input impedance so the 12k would be a minimum. Short answer: as long as you have a correct speaker load (or dummy load) attached to the through jack on either DI, you'll be fine.

    Yes, as long as you don't take it apart. The magnet assembly is critical for cooling. Without it, the 20W speaker would probably be a 1W one. Am I reading you right that you just want to use the VC without the cone and magnet?

    Not too hard. You can buy the Jensen DI box transformer (or a Lundahl) off the shelf and Jensen has schematics for DI boxes using their transformers on their website. It probably won't save you any cash to DIY, but it would be a fun little project.
  3. SpamBot


    Dec 25, 2008
    St. Paul, MN
    Okay, I'm clear on the other points but a little confused here. You're saying that the voice coil uses the magnet as a heat sink? When you say, "don't take it apart", do you mean keep the speaker as it is? It is a guitar speaker, and will not handle the bass of the output.

    EDIT: Nevermind, I just parted out a diy dummy load..... not much to it. I'll save this speaker as high-wattage resistors will be a lot smaller.
  4. Mark Reccord

    Mark Reccord Supporting Member

    Yes. Most of a speaker's cooling is based on transfer of heat across the air gap and into the magnet. That's part of the reason why overexcursion is bad, because if part of the coil is out of the gap, its ability to cool itself is drastically impaired. So in order to maintain the thermal rating of that driver you'd need to keep it as it is and if it won't handle the bass you're probably better off with the dummy load.

    Did you say the amp was 5W? Then I'd suggest at least 20W of power resistor. It will certainly put out more than 5W if you're overdriving it and you don't want to be right up on the power handling limits of your dummy load. Those resistors will get really hot.

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