4 ohm Head into 8 ohm cab

Discussion in 'Live Sound [BG]' started by Demon_Hunter, Jun 23, 2008.


  1. Demon_Hunter

    Demon_Hunter

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    I WANT OUTTA HERE!!!!
    Can you run a 4 ohm Head into an 8 ohm cabinet without inflicting any damage on either item?

    Thanks!!
     
  2. Earthday

    Earthday

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    Generally a head will have both an output at 4ohms and an output at 8ohms. Check your manual and see if this is the case. For example my head puts out 240 watts at 8 ohms and 350 at 4.
     
  3. anderbass

    anderbass

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    What brand/model amp are we talkin here?
     
  4. Deacon_Blues

    Deacon_Blues

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    The amp manufacturer states only the minimum impedance (resistance) it can handle. At this impedance, the amp pumps out the most watts. A lower impedance will force the amp to give out more watts than it is able to and you will blow at least your amp, whereas a higher impedance won't hurt anything in the system, only the output in watts will be lower.

    Then it won't hurt to know some basics about impedance when connecting more than one cabinet to the amp:

    Two 8 ohm cabinets in parallel gives a total impedance of 4 ohms.

    R(tot) = R1 x R2 / (R1 + R2) = 8*8/(8+8) = 4 ohms


    Two 8 ohm cabinets in series gives a total impedance of 16 ohms.

    R(tot) = R1 + R2 = 8+8=16


    It's is rather uncommon to connect cabinets in series, you virtually always connect them in parallel. Check the amp's manual for more info.
     
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  6. IanStephenson

    IanStephenson UnRegistered User

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    For solid state amps the rating is a minimum so a 4ohm head can drive anything higher than 4 ohms.

    Valve amps can be a bit more fussy, and usually have a switch on the back to set the speaker impedance.

    Ian
     
  7. Demon_Hunter

    Demon_Hunter

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    I have a Gallien-Krueger 200MB..and its speaker is blown :crying:
    I can run it with speaker on into a cabinet at 8 ohms(but the speaker is blown and sounds like poo) or turn the speaker off and it runs at 4 ohm...but the cabinet I want to run it into is 8 ohms
     
  8. anderbass

    anderbass

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    Sure, you can safely use your amp with an 8 ohm cab, but your combos amp section will be running at a reduced power level compared to using a 4 ohm.
     
  9. sailorofcheese

    sailorofcheese Supporting Member

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    hi, hopefully you can answer my question. i have a trace elliot ah1000-12 and i dont know if im reading this correctly in my manual.

    "There are two speaker output jack sockets provided, allowing two speaker cabinets to be connected to the amplifier. The combined impedance of these speaker cabinets must not be less than 4 ohms. Two 8ohm cabinets is the ideal load for these amplifiers.
    If you are using one 4ohm cabinet, then no further speakers can be connected to the amplifier or the combined impedance will fall below 4 ohms. If this occurs you will just be wasting power in heat generated by the output stage and no more volume will be available than with the single 4ohm cabinet."

    so what they're saying is. two 8ohms is better, one 4ohm cab alone is the limit.? thats what im getting out of it. i just dont want to make a mistake.....
     
  10. McCalister999

    McCalister999

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    That's the jist of it.
     
  11. MakiSupaStar

    MakiSupaStar The Lowdown Diggler

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    I have a power amp that outputs 1000W at 4 ohms. I run two 8 ohm cabs daisy chained together to make 4 ohms. However, there are many gigs were I just use my 4x10. It's a Carvin "red eye" 600W handing at 8 ohms. I've had this thing for 8 years. I've been running this set up for about 3 now and haven't had any problems. That cab works like a champ.
     
  12. Mr Grady

    Mr Grady

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    so...what would happen if you ran a 100w 4Ohm amp through 300w 8Ohm speakers?

    i understand that the speakers would be limited by the amps output, i just want to make sure no damage can happen

    as i understand it, as long as you didnt crank the amp up too high it wouldnt be a problem.......(each speaker is connected by a jack to seperate outputs) makes sense?

    cheers
     
  13. mrokern

    mrokern TB's resident Rush freak

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    To make sure I'm understanding you, you've got a head rated at 100 watts at a 4 ohm load, and you're looking to power two 8 ohm cabinets (rated for 300 watts each) with it.

    The amp will see a 4 ohm load, and you'll be putting 50 watts into each cabinet at max.

    That's a pretty low-power amp...

    -Mark
     
  14. lindseyp

    lindseyp

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    If those two channels are connected in parallel inside the head -- both of my heads are built that way -- your amp would see a 4 Ohm load and put out 100W, shared by both speakers. You'd have some headroom to work with. If your head has a dual power amp section with 100W at 4 Ohms for each channel, the head would deliver an 8 Ohm load to each speaker (because that's the speaker's rating), probably around 50-60 watts each.
     
  15. Marley's Ghost

    Marley's Ghost Supporting Member

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  16. liltommyg

    liltommyg

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    Here's a simple way of visualising the impedence problems;
    Think of a water pipe; we'll use inches instead of ohms ok,

    4 inch pipe joined to 4 inch pipe= no disruption in flow ( all power applied is delivered)
    4 inch pipe joined to 2 inch pipe = back pressure in pipe ( not all power applied is delivered)
    4 inch pipe joined to 8 inch pipe = low pressure in pipe ( not all power applied is delivered)


    In electrical terms these last two are not really spot on but you get the idea
     
  17. Growly Lytes

    Growly Lytes

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    Apparently you cant damage your speakers with to little power so your good to go with that set-up. 2 8ohm speakers into a min of 4ohm they will get equal power. The amp can handle anything above 4ohms.
     
  18. anonymous101511

    anonymous101511 Guest

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    You could run an 8 ohm cab, or a 16 ohm cab or a 32 ohm cab; ANYTHING 4 ohms or higher is fine. Doesn't have to be 4 or 8.
     
  19. JohnMCA72

    JohnMCA72

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    There's no "better". It's just like Deacon said, above:

    - 2 speaker jacks
    - Combined load must not be LESS THAN 4 ohms
    - 2 @ 8 ohm cabinets is the IDEAL load

    Above suggests strongly that the 2 jacks are wired in parallel. (8 * 8) / (8 + 8) = 64/16 = 4 ohms combined, which is the stated minimum limit for the amp.

    "If you are using one 4ohm cabinet, then no further speakers can be connected to the amplifier or the combined impedance will fall below 4 ohms." Should be pretty obvious, however (assuming the 2nd cabinet is also 4 ohms):

    (4 * 4) / (4 + 4) = 16/8 = 2 ohms - well below the minimum impedance, & very likely to release some magic smoke!

    How about a 4 ohm with an 8 ohm?

    (4 * 8) / (4 + 8) = 32/12 = 2.67 ohms - still not good (below the minimum).

    2 @ 16 ohm?

    (16 * 16) / (16 + 16) = 256/32 = 8 ohms - that'll work just fine, because it's at or above the stated minimum impedance.

    Etc. for various other speaker combinations. If you go beyond 2 speakers, you have to multiply all the impedances, and divide that by the sum of all the impedances. The math is easiest if you stick to 2 or fewer speakers.

    Again, though, there's no "better" or "worse" unless you get below the threshold where you can damage something.
     
  20. i_got_a_mohawk

    i_got_a_mohawk

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  21. blairquik

    blairquik

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    Simple answer: Yes.

    Ideally the speaker input impedance should be matched to the output impedance of the head as this means all the power will be transferred to your speakers. However, you can run higher impedance speakers although the output power will be less. NEVER have speakers with a lower impedance than your head. This will overload your head and will make you cry.

    Example:

    300W head with matched 4ohm to 4ohm = 300W output
    300W head with 4ohms running 8ohm cab = 260W output
    300W head with 4ohms running 2ohm cab = bang, smoke, sadness.
     

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