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Aux PA Speaker Cab Ohms Question

Discussion in 'Live Sound [BG]' started by SurferJoe46, Apr 9, 2010.


  1. If a speaker is stated to have 8 Ohms - is this really measurable with a good Ohm meter?

    Is there a corollary to STATED Ohms verses ACTUAL Ohms readings?

    ie: 16 Ohms STATED = X Ohms via a meter?

    It may be a loaded question concerning working resistance or a peak capacity at X Hz and power settings, and allowing for hysteresis, induction and standing waves, etc., and I quite understand parallel and series Ohms values, but this is leaving me in the dark a little.

    Bottom line - I don't want to call down Armageddon upon my PA amp and fry it. So far I've gotten away with it but-there's-always-that-niggling-doubt in the back of my mind.

    And ya know - I've asked a LOT of questions on this and other TB arenas, and made numerous posts trying to fill in for my over 40 years of not playing and the aspect of new amps, guitars and effects and such, but I find that in 99.99% of the questions I ask, I have gotten really fine answers and am extremely thankful for the intelligent and understandable returns by the talented people here.

    Thanks a lot! It helps in ways you may never know.
     
  2. t bake

    t bake

    Dec 26, 2009
    I ck my speakers and cables often with dvom to make shure i have no huge probs my 8 ohm cabs measure aprox 7.2 to 9 at the the end of the cable 20 to 50 ft I have never had many cabs measure dead on rated ohms to many variables, make shure they are close. If im working at the low end 2ohm total load i will get a lot more pickey. be shure your amp is ok with the load your running and rock on hope I was of some help ,Im shure someone will chime in and really set us strait. God Bless Gig On!!!
     
  3. There is no fast set corollary. The ohm rating is a nominal rating, and varies according to manufacturer, and even from speaker to speaker with the same manufacturer. Seldom does an 8 ohm speaker actually exhibit 8 ohms of resistance. Even the two identical speakers running off the line consecutively do not exhibit the same resistance. This is partly why I consider running an amp below a rated 4 ohm load is abuse, even if the manufacturer says it is ok, because a 2 ohm load can be less than one, and that's too close for me. Yes, a good meter should give you the actual reading, but don't freak if 8 ohms turns out to be six.
     
  4. Ahhh! You opened up my next series of questions.

    A 50 foot speaker cable is rated at X Ohms - and again this is a generalization - but is that impedance or Ohms that is the real important value ?

    Somehow I had the gut feeling that Ohms in a meter are not the same things as impedance of the speaker or load - which I worry about at night - sleep being seriously overrated! lol

    I'm making a sound check this afternoon for that huge outdoor gig I have starting tomorrow, and just wanted to have a set of checks and balances with which to work.

    I'm bringing everything I own to it and I don't want to fry or destroy anything.

    Again, thanks for you insight. It is valuable to me.
     
  5. Your ohmmeter isn't measuring impedance, it is measuring resistance.

    You can usually expect the resistance of a speaker system to be in the neighborhood of two thirds of it's nominal impedance.

    Your ohmmeter is not broken, those are 'actual' ohms.

    What problem are you trying to solve and what are you worried about?
     
  6. My Peavey (it's old, and it's mine and I like it - besides I'm old too) is rated at 16 Ohms for any external speaker load. It was really intended to drive it's own sidecab with a 16 Ohm 12-incher + folded horn. which I had at one time - but it walked away somehow.

    I have two 8 Ohm Kustom KSC10s that are rated at 8 Ohms and they have series outputs alongside the normal IN jacks.

    I was wondering if running them in series is going to take the amp head into Smokeland or not. This is going to be a long day and even though I've been successful in the past, it was never for an extended time like this.

    The Peavey has thermal protection, but at it's age I wonder if it can see thermal distress accurately and shut it down.

    Next time I'll install a couple of muffin fans for it - but the time isn't available right now.

    and....................

    Which SIDE of the neighborhood? The high side or low side?
     
  7. JackANSI

    JackANSI

    Sep 12, 2006
    PA
    Here is an impedance chart for a 4ohm subwoofer (first on a google image search, so I used it)

    SW263WA03-Impedance-curve.

    You see how the impedance changes over frequency?

    thats why a standard ohm meter isn't the perfect way to measure a speaker..


    Here is more:

    http://en.wikipedia.org/wiki/Nominal_impedance#Loudspeakers


    Oh yeah, in short, you should be OK. Just don't go balls deep with the master volume..
     
  8. I'd bet any amount of money that that is not a series output.

    You are running the two boxes in parallel, making for a 4 ohm load.

    If the amp isn't supposed to be running below 16 I'd avoid doing this. Even a Peavey has it's limits.
     
  9. JackANSI

    JackANSI

    Sep 12, 2006
    PA
    You can check that with a common meter..

    measure each speaker on its own, hook them up how you would to the PA, but not TO the PA. Then measure that... if its roughly double what each speaker was, you're in series. If its roughly half, you're parallel. You want series.
     
  10. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    alpha-music.com
    ??
    series connections from box to box in pro audio are pretty much non-existant. it will pretty much always be parallel.
     
  11. I just Ohm'd them out - the KSC10s and they show at 13.5 Ohms.

    That's in series. That's also with a 9V battery firing the analog Ohmmeter too - so that's prolly about right but my DVM is in the back of my shed - er, somewhere. .

    They have two jack ports in the back distinctly labeled IN & OUT, and one speaker at a time measures 7.3 Ohms close to it's posted rating +/- a bit, whereas putting the jumper from the OUT of the first one to the IN of the second gets shows 13.5 Ohms.

    This is with very short (4-foot) speaker cables) but I intend to separate them by 20-feet or so at the site.

    Kinda nifty. I figger with a little fudge that I can get away with this if I don't go to 13 o'clock with the amp.

    That gives me 2 x 110+horn and the Peavey basic speaker which is a 15" mounted at a 45º angle in the box for size considerations. It works pretty good like that and I just left it there.
     
  12. Found out something -

    The KSCs have been modded by (?) someone with stereo jacks and circuit changes inside the rear access ports in the cabs themselves.

    It appears they were set up as 8 OHM each and adding a second one would drop to 4 OHM - but someone (who?) added stereo female jacks that when used change the parallel to series and that's how they get to 16 Ohms.

    It only happens only when stereo plugs are used and both speakers are on line, with the "A" unit in the first part of the daisy chain and the "B" unit on the end - and if the mono jacks are inserted it defaults to a parallel speaker system. I was going kinda crazy trying to figure it out until I obeyed the A/B labeling and it all started working right.

    I don't think the jacks are standard from Kustom - as there is considerable de-soldering and re-soldering on the little boards to make the change, and the new stereo jacks themselves are metal, whereas the originals - which were hot glued inside the cabs for future access - were plastic and the PC-soldered on type.

    That's pretty cool! This way they suited my needs exactly!
     
  13. The gig went great - the A.R.T. tube preamps worked very well and nothing smoked at all.

    I had plenty of volume, although I think next time I'll not fire up stereo mics and just go mono for my vocals.

    Ran the KSC10s in a series/16 Ohms with 4-mics and the combo Peavey running it's own 15-incher at it's legacy values for whatever it could do and it ran great too. Nothing overheat and we had a lot of direct sunshine on us during the day. We even got some sunburn.

    I watched the line voltage and it got down to 102VAC a few times so I was a little suspicious that I'd brown-out an amp - but they survived OK.

    That's the price you gotta pay when you are 300 feet from the power panel - but I ran 10-3 AWG 220VAC to the bandstand and split it there and d-boxed it out to two 120VAC legs, but made sure that all the instruments were on the same 'A' leg for hum and incidental 'lip-zap' possibilities. There were no problems when it was running at all thankfully. The 'B' leg ran the lights and some aux things that also ran off power supplies since I figured they were at least Low-Volt transformer'd off the line and couldn't create independent loop-grounds and noise.

    Figuring it was a good idea, I went with an auxiliary ground anode driven into the soil for extra protection, back to the Common rail in the d-box. There is a lot of lightning in that area and it is forecast for tonight - so an ounce of prevention ----- ahem!

    For the past few hours I've been winding wires and untangling rats nests on the ground. What a mess! It was a good idea to get all the stuff packed and out-hauled as they expect some snow tonight. I don't see that happening - but the weather-guessers have to hit it right once in a while.

    Thanks for all the info about 48V phantom and balanced vs unbalanced cables etc. It worked well and I had a great crash course for you. Again - thanks!
     
  14. realbadgroove

    realbadgroove

    Apr 12, 2008
    Kansas
    It is a good Idea to check your speakers every once in a while to make sure they are running between the right ohms. Sometimes when things get to hot they will run way high but I have yet to see a speaker that was rated at 8 ohms that was right on 8.
     
  15. Right! I've seen a supposedly 8 Ohm speaker that was really 7.7 Ohms get to 9.2 Ohms after the coil warms up.

    That looks like about a new Ohms value about 119.4805194805% from the original 7.7 Ohms if my mental calculations are correct. Huh?!?

    -That's a lot of fudge zoning there!
     

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