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4 ohm head into 8 ohm cabinet

Discussion in 'Amps and Cabs [BG]' started by Precision345, Nov 5, 2013.

  1. Precision345


    Feb 19, 2013
    Firsty, I apologize for this thread because I know speaker impedance has been threaded to death on these forums.

    My question is:

    Can I safely run my Gallien Krueger 400RB IV @ 280 watts (4 ohms) into my Ampeg 4x10 cabinet @ 500 watts (8 ohms)?

    From what I understand it should be fine, the head will just use less then the 280 watts of power? Is this correct?

    Also, if I wanted to, I could run two 8 ohm cabinets with the head and it would half the impedance of each cabinet to 4 ohms?
  2. P-oddz

    P-oddz Supporting Member

    Apr 7, 2009
    Milwaukee, WI
  3. Precision345


    Feb 19, 2013
    Fantastic. I was a little worried for a bit that I could damage my head. Thank you for your help.
  4. The amp doesn't "use" wattage, it outputs wattage. With SS amps how much is possible is determined by the load (in ohms impedance) the speaker cab(s) place on the amp head. 0 ohms being a dead short and possible fried amp head.
  5. DiabolusInMusic

    DiabolusInMusic Functionless Art is Merely Tolerated Vandalism Supporting Member

    In regards to your second question, yes, you could run two 8 ohm cabs together for a 4 ohm load.
  6. jumblemind

    jumblemind I also answer to Bryan Supporting Member

    Aug 27, 2011
    Speaker cabs don't actively "suck" power away from the amp; it's more like they set the size of the conduit that determines how much juice the amp can put out. The only way to really damage your head with speakers is to add too many such that it dips below its minimum safe rating (in this case 4ohms), and you seem to understand how the Ohm math works to keep that from happening.

    Pretty indepth FAQ on ohms is here:

    And check out the Over/Under Powering Cabs section of links in the general FAQ:
  7. Precision345


    Feb 19, 2013
    Pardon me, output is what I meant to say.
  8. Precision345


    Feb 19, 2013
    Turns out my Ampeg 4x10 is actually 4 Ohms. So say I were to link it together with an 8 ohm cabinet in series, it would decrease the impedance to 2.6 ohms? That's running with my 4 ohm head
  9. Just wanted to make sure you know that amps will output power depending on the load attached to them, their designed power and how much you ask them to amplify the incoming signal. Speaker cabs get rated by how heavy a load they will present to an amp's output (ohms impedance) and how much power they can accept or "survive" from an amp (watts). :)
  10. Precision345


    Feb 19, 2013
    Thanks for your knowledge and help. :)
  11. MDBass

    MDBass Supporting Member

    Nov 7, 2012
    Los Angeles, CA
    Endorsing Artist: Dingwall-Fender-Jule-Dunlop-Tech 21-Darkglass-Nordstrand
    Your head has a minimum load of 4ohms, meaning you can either run a single 4ohm/8ohm cabinet or two 8ohm cabinets safely.

    Running a 4ohm and 8ohm cab together will decrease the load below 4ohms, more than likely causing the power section to overheat/melt/catch fire.

    Don't do that ;)
  12. ^ what he said! :)
  13. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Series increases impedance, parallel is how things are usually connected though, and that decreases it. Even if you have a cable from one cab to another that is almost always parallel connection.
  14. Good catch Mr. Foxen I missed the word SERIES in the post!
  15. In case you need this, OP...

    Series resistance: Add all values together as a total.

    Parallel resistance: 1/(1/R1 + 1/R2) = Total resistance.
    So, if you have 4 and 8, it would be : 1/(1/4+1/8) = 2.666
    Hope that makes sense :D
  16. The experts recommend not series wiring odd cabs. Something to do with impedance not being a constant across the frequency range, combining them electrically in series does bad things.
  17. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    I've not heard that from experts, but have heard it said, so I asked Alex at Barefaced about it:
    The downside of series if you kill a speaker, it cuts off the other one, and for vlave amps, that is open circuit and damage, hence the SVT cab being all parallel and separate internal enclosures so a dud speaker has less effect on the whole function.

    Downside of mixing cabs is same regardless of series/parallel, phase fun etc.
  18. Precision345


    Feb 19, 2013
    Ok, so I had the equation right but I had parallel and series mixed up? In PARALLEL if I were to use an 4 ohm head with one 4 ohm cabinet and one 8 ohm cabinet, it would decrease the impedance to 2.6 ohms?

    But I don't want to do this because it will lower the impedance below the nominal impedance of a 4 ohm head so this could cause damage, i.e melting/fire/ending the world.

    Could I use an 4 ohm cabinet and an 8 ohm cabinet with an 8 ohm head? haha I think I might be a bit confused on what I thought I previously understood
  19. Precision345


    Feb 19, 2013
    This wouldn't work either then with the Parallel resistance 1/(1/R1 + 1/R2) equation because it would be the same? Right?
  20. gard0300

    gard0300 Supporting Member

    Jan 10, 2011
    Vandalia, Ohio
    You would have to have a head or amp stable at a minimum 2 Ohm load to run a 8 Ohm and a 4 Ohm in a bridged set up. I've always used the the formula: cab 1 ohms x cab 2 ohms divided by cab 1 ohms plus cab 2 ohms. So 32 divided by 12 = 2.66 ohms

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