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Switchable impedance amps?

Discussion in 'Amps and Cabs [BG]' started by chetorch, Jan 24, 2012.


  1. chetorch

    chetorch Guest

    Apr 13, 2011
    Is it uncommon for power amps to have this feature? I was looking at the Orange Terror head, noticed this and figured I would ask here. I googled and all I found was a Traynor head...
     
  2. this is common with tube amps, on solid state amps the impedance is automatic.
     
  3. chetorch

    chetorch Guest

    Apr 13, 2011
    I see, that makes sense, so the claimed wattage output for an amp is usually when put on a 4 ohm load?

    I have an 8 ohm 1x15 cab and find it difficult to tell if the stated power output is for 4 ohms or not. I'm guessing it is for those who want to stack cabs?
     
  4. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    No it's not! On most SS amps this is indeed true but for some they cannot output their power into both say 2Ω, 4Ω or 8Ω. The carvin BX500 can drive 4Ω cabs or 2Ω cabs. There is a switch to choose. If you get it wrong the amp can fail. If you see say 500W into 4Ω and 500W into 2Ω this is probably the case.
     
  5. If you have a tube output amp, the output power is not dependent on speaker load. If SS then the power rating will always mention the load impedance (8 ohm, 4ohm, 2 ohm).
     
  6. Yes point taken, I have a Line6 on the bench that a guitarist didn't switch impedance and blew a tripath.
     
  7. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    Then you may need a quadpath! :D

    OK it's STILL my b'day be nice! :p

    (66 FWIW! - <sigh> I'm sooooooo old! :scowl:)
     
  8. chetorch

    chetorch Guest

    Apr 13, 2011
    Hmm, this can be quite tricky! so for instance if an amp says its capable of outputting 500w at 4 ohms then two 8 ohm cabs will work to get the max power correct?
    and if the same amp is switchable to 8 ohms, then 500w go out to a single 8 ohm cab?
     
  9. chetorch

    chetorch Guest

    Apr 13, 2011
    happy b-day sir! :bassist:
     
  10. HAPPY BIRTHDAY PAUL!

    I may need a sixpath ;) not sure about the other chip as the right side shorted. :) Trying to get a complete module before I rebuild the old one. Line 6 is not very supportive of independents. :rollno:

    Nine more years for me to get to where you are.
     
  11. chetorch

    chetorch Guest

    Apr 13, 2011
    Ok, so if an amp can give 500w at 4ohms then that means its the minimum, as in no 2 ohm load ( unless stated.)

    In other words, it is ideal for two 8 ohm cabs in parallel.
    So 500w at 4 ohms into an 8 ohm cab produces about half the power?
     
  12. This varies with different amp manufacturers but an average might be closer to 65%. Manufacturers will usually list power ratings for non minimum impedance's.
     
  13. 1) That is correct
    2) That will vary some with design but usually more along the lines of 60-70% power at 8 ohms
     
  14. Hi.

    Happy birthday Paul.

    Musicians tend to be like wine, the older they get, the better they get, so I for one wouldn't hold old age against anyone, especially not You.



    As for the OP's question, most transformer coupled amps need the impedance to be matched. That means the majority of the tube amps and a minuscule portion of SS amps. In order to work, some output devices like tubes have to "see" the correct impedance in order to work. The very early transistor amps were the same.

    OTOH there's also OTL tube amps out there and the design becomes a bit more complicated in order to make things click even though the OT is taken out of the equation.

    Regards
    Sam
     
  15. chetorch

    chetorch Guest

    Apr 13, 2011
    thanks, people lots of good simplified info for my poor uneducated soul :)

    JxBass: thanks for the link!
     
  16. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    The basic premise is that for a SS amp, if have a 4&#937; load you need a certain voltage and current to put out say 500W into that load. If you drop the load to 2&#937; the amp will try to output 1000W. Not many can actually do this. If you drop the voltage, 2&#937; switch, and keep the current the same you can get that 500W safely. If you leave the amp at 4&#937; and connect a 2&#937; load things are going to fail.

    Amps that switch voltage rails, class H IIRC, are essentially doing the same thing. It runs at low power until the demand for more and then switches to high power to supply that demand.

    This is very much simplified but I hope it explains the situation with these amps.

    Thanks for the well wishes - I had a great birthday!!! :)
     
  17. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    All I can say is the if Line 6 won't help, then perhaps a six PACK instead of a six path might be the way to go! :D
     
  18. RickenBoogie

    RickenBoogie

    Jul 22, 2007
    Dallas, TX
    HBD Paul, I always assumed we were the same age, as we seem "internet compatible" so often, but I turn 53 in about a week. Who knew? Have a great day, and apologies to the op for the derail.
     
  19. My Boogie D-180 had a speaker out for 8ohm and 4ohm, and recommended using two 8ohm cabs if you used two cabs. I never noticed any difference in power either way.
     

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