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Determining Cab Impedence

Discussion in 'Recording' started by apstrong, Sep 16, 2009.

  1. apstrong

    apstrong Active Member

    I have access to a couple of older Gallien Krueger bass cabs, a 1x15 and a 4x10. The model number on the 1x15 is unreadable, and there's no labelling of any kind on the 4x10, so I gave up trying to find info on the web. Then I pulled one of the speakers from the 4x10. Nothing. They're wired in series, but no identifying marks on the speaker itself, so I can't even look up the speaker and do the math. Basically, I have no idea if either of these cabs is 8 ohm or 4 ohm or 27.254 ohm. How can I find out before I plug my Mesa Boogie 400+ tube amp into them at the same time and blow something up?

  2. dvdhawk

    dvdhawk Well-Known Member

    The great majority of G-K cabinets are 8-ohm.
    (including the 410RBS 4x10" and 115RBS single 15")


    This is another job for a multi-meter. They test voltages & ohms, among other things.

    **note - for reasons I can't explain alnico speakers can lie to a meter

    Are you sure all four speakers are in wired series?

    Jack+ to Speaker 1+
    Speaker 1- to Speaker 2+
    Speaker 2- to Speaker 3+
    Speaker 3- to Speaker 4+
    Speaker 4- to Jack-

    If you had four 8ohm speakers all in series, you'd have 32ohms - which seems unlikely

    Most 4x cabinets are a combination of series and parallel.
    Pair 1 in series (8ohm+8ohm =16ohm)
    Pair 2 in series (8ohm+8ohm =16ohm)
    Jack wired to the 2 Pairs in parallel (16ohm x 16ohm / 16ohm+16ohm=8ohm)

    I hope that helps.
  3. Boswell

    Boswell Moderator Distinguished Member

    As dvdhawk says, the cabinets are likely to be wired to present 8 ohms to the amplifier.

    Multimeters are all very well, but they measure resistance and not impedance, and in the world of power loudspeakers, these two can be quite different. What you can say is that if you get a reading of (say) 5 ohms from a speaker removed from the cabinet and that when it is replaced the reading from the total cabinet is 6 ohms, they are likely to be wired in series-parallel, and you should set your amp to expect an 8 ohm load.
  4. apstrong

    apstrong Active Member

    Thank you gentlemen. It's about time I got a multimeter anyway, so I'll pick one up, pull the speaker again and see what it says. My best guess is that the 1x15 is the RBS, but the 410 looks quite different from what's in the RBS manual (and thanks for the link!) so that's the biggest mystery now.
  5. dvdhawk

    dvdhawk Well-Known Member

    I'd just about bet the farm the cabinet is 8-ohm, unless a previous owner has screwed it up.

    Measuring the resistance on a normal speaker with an ohm-meter will usually get you in the ballpark.
    And FYI it's common for an 8-ohm speaker to read around 5.6 on a meter. (about 70% lower)
    Where a typical 4-ohm speaker will read somewhere between 2.5 - 3 on a meter.

    The impedance specification on a speaker cabinet is a nominal rating and not an exact measurement.
    It's s not a constant when in use, it's all relevant to the voltage/current coming from the amp - which would fluctuate with the music.

    If the G-K 410 cabinet has a piezo tweeter in the middle, like some do, don't worry about it's impedance.
    Piezos have extremely high impedance and won't be a factor one way or the other.

    A cabinet with a capacitor inline with the tweeter or a passive crossover will completely skew the readings.
  6. apstrong

    apstrong Active Member

    Alrighty, one analog multimeter later (with little AA battery for power)...please correct me if I'm using this thing wrong. I assume the red lead goes under my tongue? ;)

    Pulled a speaker on the 410, didn't disconnect the wires, attached red lead to positive terminal and black to negative. Set multimeter to show ohms. It said 16 ohms.

    Disconnected the wires from the speaker and repeated the test on the speaker itself: multimeter says 20 ohms.

    Then I tried to trace the pathway of the wires inside the cab without totally disassembling it, and here's what I think it looks like:

    speaker configuration:

    1 2

    3 4

    Two wires coming from the 1/4" jack on the back, red and black. Red goes to 1+, and then another red wire from 1+ to 3+. Black wire from 1/4" jack goes to 2-, white wire from 2- to 1-. Mystery-coloured wire from 2+ to 4+ but I am assuming it's red, white wire from 4- to 3-.

    That's a ridiculous way to explain it. Here's a diagram...


    The green lines are white wires. White on white didn't work so well. Kevin is not an issue at this time, but I will keep you posted if the situation changes.
  7. apstrong

    apstrong Active Member

    ps. There is no inline tweeter, just 4 10" speakers. And Kevin.
  8. apstrong

    apstrong Active Member

    I'm starting to doubt that I'm using this device correctly, because the 1x15 speaker says 12 ohms on the multimeter.
  9. apstrong

    apstrong Active Member

    Aha, I forgot to zero the ohmeter before using. New results: rating while all 4 speakers still wired together = 8.5 to 9 ohms; speaker all by itself: 12 ohms.
  10. dvdhawk

    dvdhawk Well-Known Member

    Your diagram doesn't look like all the drivers could be in phase wired that way.
    Let's not get so hung up on impedance we forget about wiring the speakers in phase. If you can't clearly see a red mark or (+) on the tabs you'll have to test the speakers to determine the right polarity.

    You can momentarily touch a half-dead 9v battery across the tabs and if the + and - of the battery match the + and - of the speaker, the speaker cone will push outward (which is correct). Don't hold it there, so you don't damage the voice-coil. It will suck back into the basket if it's backwards. Pretty high-tech. Get the polarity backwards and your speakers will suck - literally.

    Try this:


    Platypus not included
  11. apstrong

    apstrong Active Member

    Thanks, I will take a closer look inside the 410, I might have totally messed up what's actually going on in there, I couldn't see everything and was operating by feel for some of it.

    The good news is that the 1x15 read 5.7 ohms just as you predicted, so I'm confident it's a 8 ohm cab. One down, one to go.

    But if the other reading was right, 12 ohms for the 10" speaker, it's probably 16, which means it probably is wired in parellel series and it's also an 8 ohm cabinet. But I'll double check the wiring to be sure and give the 9v battery test a go just for fun.
  12. dvdhawk

    dvdhawk Well-Known Member

    Make sure when you're testing an individual speaker it is not connected in any way to any other speaker.

    Assuming they are all 16-ohm speakers:

    A) If each individual driver is 16-ohms and it's in series/paralell you end up with 16-ohms total for the whole cabinet.

    B) If you ran all 4 of the 16-ohm speakers in parallel the result would be 4-ohms total for the whole cabinet.
  13. Davedog

    Davedog Distinguished Member

    I have never heard of a Gallien_Krueger bass cabinet being anything other than 8 ohms except the large 8-10, the multi sized speaker cab's...ie: 2-10+1-15 etc.

    But you're on the right track.

    4 16ohm speakers in series/parallel is 16 and 4 16ohm speakers in parallel is 4.

    Your 400+ is a beast and will run at most impedances.....but I wouldnt ever run it lower than 4.
  14. apstrong

    apstrong Active Member

    Dammit Jim, I'm a bass player, not a mathematician. Ok, think I have it figured now. Opened up the 410 again. Each speaker, when disconnected, tests at 12 ohms, so they are all probably 16 ohm by themselves. So far so good.

    The + pole on each speaker is marked with a red dot, but I did the 9v battery test on each to be sure, and the speaker moves the right direction, so all's well there.

    Now the weird part. The wiring is almost like dvdhawk's diagram but not quite. Here's what it is:


    So the wire #1 comes from the jack to 1+, and the other wire from the jack goes to 2+. What the?? And 1- is connected directly to 2-. Otherwise it's the same as dvdhawk's diagram.

    Is this thing just wired wrong? Or is this a different kind of wiring altogether?
  15. apstrong

    apstrong Active Member

    And here's some more weirdness for you. I just rewired it so that it's exactly like dvdhawk's diagram and then checked the ohmeter (with all speakers connected): 8 ohms on the nose. Which it shouldn't be if it's now (4) 16 ohm speakers wired in series/parallel.

    [insert sound of hair being torn out]
  16. dvdhawk

    dvdhawk Well-Known Member

    Speaker #2 is out-of-phase in that drawing. I have a friend who used a 4x10 cabinet for guitar and preferred one speaker out-of-phase. He liked the resulting weirdness. He just had to be sure not to mic that 10". The speaker's excursion (distance the cone travels) wouldn't be very far using a guitar. But if this were a bass cabinet, I think one speaker sucking air while the other 3 were pushing air would be really bad.

    As far as your 8-ohm reading goes, don't worry about it. Hook it up to the Mesa and see if you like the way it sounds.
  17. apstrong

    apstrong Active Member

    I will, my real concern was hooking up mismatched cabinets, and which outs on the Mesa to use. So now that the 410 is rewired and they're both looking like 8 ohm cabinets, and the amp has two 8 ohm outs, I think that's going to work just fine. My amp has a history of blowing fuses at really inconvenient times, its usually fine in my studio when the volume is low, but in live situations with the gain up significantly things have gone bad, so I wanted to make sure the cabs were not going to make that any worse than it already is. But I'll try the two cabs one at a time first and see how they sound before I use both. The reliability of the amp is a separate issue I need to investigate. Time to take it to a tube amp specialist, I think.

    Thanks for all the info everyone!
  18. apstrong

    apstrong Active Member

    Doh, I mean I should use the two 4 ohm outs on the amp, right? Two 8 ohm cabs in parallel means a 4 ohm load in the end. I'll get this down eventually :)

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