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OK, newb question...what difference do ohms make?

Discussion in 'Amps and Cabs [BG]' started by rob_d, Mar 22, 2004.

  1. rob_d


    Jun 14, 2001
    When looking at amps I see some rated for 4 ohms, some for 8 ohms, and even some at 6. What exactly do the ohms do for the amp and how is it related to the wattage?
  2. Ohms are the load that the amp sees. Generally, the less the load the morepower the amp can deliver. 8 and 4 ohm cabs are usually what you see, if your amp is rated at 4 ohms, you can hook up one cab to your amp. Or you can hook up 2 8 ohm cabs, doing it that way divides the ohms in two, that is two 8 ohm cabs will give the amp a 4 ohm load. Many amps these days can run down to 2 ohms, so you could have two 4 ohm cabs or 4! 8 ohm cabs.

    more info is here ;)

    how ohms work

    Hope this helps.

  3. rob_d


    Jun 14, 2001
    Yeah, that does help, thanks. My cab is a 3x10 and is rated at 6 ohms, and my head is rated at 4. So if I wanted to add a 4 ohm cab I could right? What about an 8 ohm cab with my 6 ohm? If 8+8=4 then what does 8+6 equal? Yikes, this gets confusing.
  4. rob_d


    Jun 14, 2001
    Let me complicate this further....here's a configuration..would this work...

    I have the 600 watt head @ 4ohms. Could I run..

    a 400w 8ohm cab, and a 900w 6ohm cab?

    Would there be any advantage to this other than more power, more speakers pushing sound etc???

    Thanks, I'm a total idiot when it comes to this stuff.
  5. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    1/8+1/6= 3/24+4/24=7/24

    -> 24/7 = 3.43 Ohm

    This is probably too low, unless your amp can handle 2 Ohm.
  6. Eric Moesle

    Eric Moesle

    Sep 21, 2001
    Columbus OH
    And again, to be correct and sound more professional, the correct question of this thread is more properly "What difference does IMPEDANCE make".

    We live, we learn, we grow. Isn't life great?
  7. Yes you could run those two cabs *if* your amp can handle 2 ohms.

    Now to be a little controversial here, you probably could run both of those cabs together even with a 4 ohm head because you rarely are running full power into your speakers. Also impedance varies with frequency. if you saw a chart of the speaker's impedance versus the frequency there would be a huge hump at the resonant frequency of the cabinet. Unfortunately I don't have time for a real explanation of what that means, but the manufacturers use what they call "nominal" impedance, which is kind of a minimum across the whole frequency spectrum of the cabinet. That's where "4 ohms" or "8 ohms" comes in. That combination of cabinets sounds like it would be pretty high on the efficiency chart too, which helps you because you need less power for a given volume than a less efficient speaker.

    confused? ?:p

    stay low, BT
  8. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    Just curious, what cabinet do you have at 6 ohms?

    Also is your head tube or solid state? Tube amps are generally more picky about impedence than solid state amps.
  9. Mikemike

    Mikemike Miscreant

    Jun 18, 2002
    Minneapolis, Minnesota
    Would I be able to run a 4ohm head through a 4ohm cab and an 8ohm cab? I'm assuming I couldn't, but even so, what kind of danger would I be getting into, and how dangerous would it be? (I've been blindly doing this for the last week until I found out the other cab was 8ohms)
  11. Mikemike

    Mikemike Miscreant

    Jun 18, 2002
    Minneapolis, Minnesota
    So, if I were to keep using my 4ohm cab (the good one), I wouldn't really have any options for a second? It's a fifteen, and I would like to buy an avatar 2x10, but am I just stuck with the 4ohm unless I get two 8ohms?
  12. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    Unless your head can do 2 ohms, you are stuck. You could run two 4 ohm cabs in series, but with reduced power (i.e. adding the second cab would make the rig quieter). Plus the cables would have to be special.

    You might be able to purchase a power amp and run it off your head. Most power amps handle 2 ohms no problems. Or replace your head. Or sell the cab and buy two new ones.
  13. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Yes, the 6 ohm cab is definitely a "non-standard" value. Standard values are power of 2 (4, 8, 16, etc). If you try to use that cab with any other cab, you'll probably run into some strange behavior. Be careful.

    Here are the rules:

    1. Speaker impedance should match amp impedance wherever possible.

    2. For tube amps, subject to rule #1, you can usually go anywhere between 50% and 200% of the rated impedance, and still be "safe". There are exceptions (like Marshall Major's, SVT's, Fender 300/400PS and the like, and some class A amps), but generally that's a good rule for most "ordinary" tube amps.

    3. Solid state amps should be used with an "equal or higher" value speaker impedance. They usually have a "minimum impedance" rating below which it's not safe to use. If your amp says "minimum impedance 4 ohms", you can go higher but not lower. Sometimes even a small mismatch (like the one that JMX calculated) can cause problems. The higher impedance you use, with a solid state amp, the lower power your amp will crank out. So, an 8 ohm speaker won't be as loud as a 4 ohm speaker with the same amp (all other things being equal). There are exceptions to this rule for solid state amps too, but "most" solid state amps will work along these lines.

    The 6 ohm cab "by itself" will work with solid state amps rated at 2 ohms or 4 ohms minimum impedance (but not with solid state amps rated at 8 ohms minimum impedance). Also it will work with tube amps rated at 4 ohms or 8 ohms (but not with those rated at 2 ohms or 16 ohms).

    Sounds like seanm's suggestion is a good one, get a separate power amp to power the other cab. Run it off the "line out" of your existing amp, that way you'll (hopefully) preserve your existing tone.

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