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Ohm question.

Discussion in 'Amps and Cabs [BG]' started by Digity, Dec 22, 2016.


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  1. Digity

    Digity

    Apr 7, 2014
    I have a 320watt combo at 4 ohms. I just acquired a cabinet that is 540watts at 8 ohms. My combo has an extension speaker output. Will I have any problem here ohm wise?

    Thanks.
     
  2. TomB

    TomB Supporting Member

    Aug 24, 2007
    Vermont
    You need to figure out the rating of the built in speaker. If it's 8 ohms, you're good. If it's 4 ohms, you're taking a chance.
     
    Old Garage-Bander likes this.
  3. The post above is spot on. Verify speaker impedances before running a second cabinet.

    Usually when a combo provides a 4 ohm power rating as well as a jack for an extension cab, it's more or less implicit that the internal speaker is 8 ohms and it's fine to add an 8 ohm extension. But it would definitely be prudent to verify everything. It would suck to let the smoke out of a perfectly good amp!
     
    Munjibunga and Old Garage-Bander like this.
  4. Al Kraft

    Al Kraft Supporting Member

    May 2, 2016
    Northern Virginia
    Excellent advice...always check! As an example, my Mesa Walkabout Scout combo has a 4 Ohm speaker. Some combos do come loaded with a speaker(s) that allow the combo itself to utilize the full rated power of the amp. If your combo has an 8 Ohm internal impedance driver, then hooking up a second 8 Ohm load will be fine.
     
    Old Garage-Bander likes this.
  5. If you look at the top of the amps and cabs opening page you will find a Stickey for these questions. It starts with AMPS FAQ's OHMS ???
     
  6. Often the ratings on a combo with an extension speaker Jack will include power into 4 Ohms and 8 Ohms.
    For instance, my Rumble 200 combo is rated for 140 watts at 8, and 200 watts at 4 when using an 8 Ohm extension cab.
     
    Al Kraft likes this.
  7. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    What is your combo and have you read the manual?
     
  8. Mandelbaum, Mandelbaum.
     
  9. Digity

    Digity

    Apr 7, 2014
    Eden Nemesis NC212, 320 watts.
     
  10. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    That is VERY valuable and relevant info. The cabs that are built into those old Nemesis combos are always 4 ohms as they sit, which means you are getting full wattage without an extension cab. The ext cab jack is wired in series instead of the usual parallel, which means you can use a cab of any impedance with it safely. The tradeoff, however, is that you will raise the total impedance the amp sees (I forget what 4 and 8 would equal...one of the ohm experts on here can tell you, though), and you will cut the wattage you're getting out of the amp. So let's say you use a second 4 ohm cab...that gives you 8 ohms, and your wattage will be cut anywhere by 40-50% or thereabouts. Using an 8 ohm cab with it will cut the wattage even more. So for that reason, it may not make sense to use an ext cab with it, as the gains you get from the extra cab will be offset by the wattage reduction. On the bright side, those old Nemmy combos sound pretty darn good as they sit and aren't quiet, so I'd just enjoy it for what it is and not worry about an ext cab.
     
    Last edited: Dec 23, 2016
    dbase and Al Kraft like this.
  11. Munjibunga

    Munjibunga Retired Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    According to this, you can attach any impedance external speaker to the amp because the external speaker jack is wired in series. (See page 6 and 7.)

    <Ooops. A second late and a nickel short.> But I did provide the manual.
     
    Al Kraft likes this.
  12. Al Kraft

    Al Kraft Supporting Member

    May 2, 2016
    Northern Virginia
    +1 to Jimmy's post...this series connection (vice the more typical parallel) is a bit of an oddity. From the manual:
    EXTERNAL SPEAKER OUTPUT
    This jack allows you to use an external extension cabinet. The jack is
    configured as a series connection point. You can use any impedance
    speaker in this jack without causing any problems for the amplifier. Best
    recommendations would be for a 2 Ohm or 4 Ohm extension speaker.
    Leave the internal speaker switch in the on position when using the
    extension cabinets.
     
  13. Munjibunga

    Munjibunga Retired Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    In series it's just additive, so a 4-ohm extension would result in a 8-ohm load, and an 8-ohm extension would give 12 ohms.
     
  14. Al Kraft

    Al Kraft Supporting Member

    May 2, 2016
    Northern Virginia
    At least we all came up with the same answer...which should provide the OP confidence that it's the right answer. :thumbsup:
     
  15. Digity

    Digity

    Apr 7, 2014
    I don't plan on setting it up like this permanently, I just don't have a power amp at the moment for the 410.
     
  16. rufus.K

    rufus.K

    Oct 18, 2015
    SoCal
    Even though you ay be pumping out less wattage in series, you will gain dispacement (at the expense of headroom).
     
  17. Digity

    Digity

    Apr 7, 2014
    So since the cab is 8 ohms when I get a power amp it should be 8 ohms as well?
     
  18. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Power amps will run at whatever impedance that the cab(s) present. All you have to do is make sure you don't go under the minimum impedance it demands.
     
  19. Munjibunga

    Munjibunga Retired Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    No. Four-ohm or even two-ohm.
     
  20. One thing to remember with series connected cabinets of differing impedances is that the higher impedance cabinet will get more power than the lower impedance one. This is opposite to what happens in parallel.
     
    TomB likes this.

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