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Adding a cab w/different ohm rating

Discussion in 'Amps, Mics & Pickups [DB]' started by buffalobillh, Jul 31, 2005.

  1. buffalobillh


    Jul 20, 2005
    Endorsing Artist: Samuel Shen Basses, NS Design, D'Addario Strings
    I looked in newbie section, but this was not answered directly. I have a 15" cab that is rated 4 ohms. I am thinking about purchasing a 410 cab that is 8 ohms. I'm not worried about using either of them with either of my amps individually, but if I wanted to use them in a "stack", is there anything I should be aware of regarding difference in ohms, piggybacking (plugging into one cab, then connecting that cab to the other cab), etc.? If piggybacking is bad with different ratings, my amp has a crossover, and a low out and high out. Is ohm rating a concern there as well?

    Thanks for the help,

  2. ctxbass

    ctxbass Supporting Member

    Nov 6, 2003
    Central Texas
    Piggybacking them will create a 2.7 ohm load. If your amp is designed to run at 2 ohms it should be fine. If the amp says 4 ohm minimum, the amp will probably shutdown and may be damaged.

    Are your crossover outputs line outs or speaker outs? If they are line outs, you will need to send either the high or low signal to another power amp to power the other cabinet.
  3. Because it's half the impedance, the 4 Ohm cab would get most of the amp's power - double the watts of the 8 Ohm one, so you wouldn't have much power going to the 410.

    I suggest you sell the 15 and buy a good 4 Ohm 410. If you buy one with a good efficiency spec (eg Eden 410XLT - 106dB) it should be plenty loud. Many good 410's have more extended lows than 15's.

    What amp are you using? What sort of sound do you want?
  4. buffalobillh


    Jul 20, 2005
    Endorsing Artist: Samuel Shen Basses, NS Design, D'Addario Strings

    I have a Peavey Firebass 700 and an SWR SM-400 in the stable. I'm looking for the Bob Moore/Nashville sound. I'll be going gut soon, and I will be using the 410 cab alone w/UB, but I have some EB gigs where I could use a "stack". The Peavey's ohm card says 700w @ 2 ohms. I'm not sure on the SWR. I'll have to look again.

    What is the significance between 4 ohm and 8 ohm ratings on cabs? Is it purely an efficiency thing?

  5. Bill,

    The safe answer is don't connect a load that is less than your amp's rated minimum impedance. In reality, speaker impedance ratings tend to be very nominal to say the least. The actual impedance of most speakers changes dramatically with frequency.

    Although I would not recommend doing this, I have connected a 4-ohm 2x10 cabinet and an 8-ohm 1x15 cabinet to a GK 400RB amp several times without any problems. (The 400RB is rated for use with a 4 ohm minimum load.) In this particular case, I was very familiar with the real impedance characteristics of the speakers in use. You have to be very careful if you decide to do this however, as it can be a dangerous game. If your amp runs hotter than usual, it's telling you that it's overloaded.

    Mixing speakers with different impedance ratings is not necessarily a bad thing as long as you don't end up with a load impedance that is too low for your amplifier to drive safely.

    Finally, impedance and efficiency are not really related, and are 2 separate issues. In the old days, when amplifier power was very expensive and a 50-watt amp was considered very powerful, many speakers of the day that (by necessity) were very efficient were rated at 8 or 16 ohms or even higher.

    BTW your Peavey amp will probably drive anything that's not a dead short.