A different question on speaker ohm ratings

Discussion in 'Amps and Cabs [BG]' started by whodom, Dec 8, 2007.

  1. whodom


    Mar 3, 2006
    Salters, SC
    A few years back, I bought some (cheap) 12" speakers from Radio Shack to replace the woofers in my wife's stereo system. These speakers had a feature that would be really cool in a bass cab: each speaker could be wired for 4 ohms or 8 ohms. There were three lugs on each speaker and depending on how you connected to it, you either got 4 ohms or 8 ohms.

    This would be a nifty feature on a bass cab- your amp could always put out full power regardless of whether you were running one cab or two cabs.

    I figure there must be some major drawback to wiring a speaker coil this way, like a loss of efficiency or frequency response or else SOMEONE would have done it.

    Any ideas on this?
  2. Jazzman


    Nov 26, 2002
    Raleigh, NC
    I would venture to guess it was a dual voice coil speaker, which is common with car/home audio.
  3. will33


    May 22, 2006
    That makes sense, most MI speakers are one voice coil, can't change the rating other than with multiple speakers and different wiring schemes.
  4. A9X

    A9X Inactive

    Dec 27, 2003
    It's most likely a tapped voice coil. Dual coils of 4R each will give you 2R or 8R and only 4R if you use one coil.

    Cheap and RS in the same sentence makes me think it's a compromised design trying to sell 'general replacement' drivers to people who wouldn't know a T/S parameter if it bit them on the ass.
  5. whodom


    Mar 3, 2006
    Salters, SC
    All of the above I'm sure is true. What I'm really wondering is it practicable to build a high-quality musical instrument speaker with a tapped voice coil to allow easy selection of impedence, or is this undesirable for some reason?
  6. A9X

    A9X Inactive

    Dec 27, 2003
    Not that I can see, as it affects the characteristics of the motor and possibly Xmax depending upon the way the coil is wound. I don't see the point.
  7. whodom


    Mar 3, 2006
    Salters, SC
    OK, that makes sense. I'd assume different gauge wire is normally used to obtain different ohm ratings? (i.e.- approximately the same number of turns on the coil using a larger or smaller gauge wire to obtain the desired ohm rating). The "tapped coil" likely uses just one gauge of wire and you're either using ALL the turns on the coil or 1/2 of the turns on the coil depending on how you connect it. I can see where this would definitely compromise the coil performance.

    The point would be having a more versatile speaker, especially when two cabs are used. You could vary the ohm rating to maximize your amp output regardless of whether you were using one or two speakers. Or lets say you have a 4 ohm minimum rated amp and you later switch to a 2 ohm minimum rated amp (the MBII and GB Neopak come to mind). You wouldn't have to swap your two 8 ohm cabs for two 4 ohm cabs to be able to use your amps full wattage.

    Anyway, thanks for the comments. Sounds like it's not practical.
  8. billfitzmaurice

    billfitzmaurice Guest Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Far more trouble than it's actually worth. 'Getting all the watts out of my amp' is very high on the list of things you shouldn't be concerned with, unless you're trying to play large clubs with a 40 watt amp and no PA.