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Need to find out the Ohm of this cab...

Discussion in 'Amps and Cabs [BG]' started by django, Jul 14, 2005.


  1. django

    django

    Dec 20, 2004
    Stockholm, Sweden
    I read the ohm-faq, but i'm not that good at math, so I kinda hope one of you guys could solve this one for me...

    My friend gave me these two DJ-cabs, that a friend of him built. One of them doesn't work, though...

    It has two 15" 300W 6ohm speakers, plus two 6" 160W 6ohmsand a tweeter. All speakers are connected to some splitting-filter, the red cables from the 15" soldered together, and the black ones soldered together, same with the 6". The tweeter seems separeted, except the black cord that is together with the red ones from the 15":s.
    I was thinking of using them with my poweramp (phonic max2500, 2x750W/4Ohm (1x1500W/8Ohm), but I´m afraid to blow it. Can I use it in bridge mode safely?

    About the one that doesn't work, I've checked all connections, nothing is loose. What could be wrong? How can I troubleshoot it?

    Thanks in advance!
     
  2. jazzblade

    jazzblade

    Jun 21, 2005
    Northern Cal
    Hard to say from the description you gave. I'm sure it's wired series - parallel somehow.

    The "splitter thing" is the crossover and it is also rated for a max wattage input usually. Sometimes the connections to the speakers are labled showing what "resistive loads" should be attached.

    Easy way to find out is borrow/rent/get a ohm meter and measure the total resistance between the ground and hot lead at the input ...OR.. ask the friend that built it if he remembers what the total resistance is.
     
  3. django

    django

    Dec 20, 2004
    Stockholm, Sweden
    thanks for the fast reply!

    i will borrow an ohm meter I guess... any idea about the broken cab??
     
  4. Is the wiring/soldering all ok on the "broken cab"? Check it with the ohm meter. If you get no resistance there could be a blown speaker or a loose connection somewhere.
    Do you get any sound at all out of it?
     
  5. django

    django

    Dec 20, 2004
    Stockholm, Sweden
    i got some low farting through the 6" nearly audible...
     
  6. bassontherun

    bassontherun

    Jul 9, 2005
    I'm sure there are several of you out there that have this nifty little tool, so I'm posting for those who don't:

    I had been looking for an easy way to verify the resistance of bass (and PA) cabs quickly, without getting inside the box with an ohmmeter. A fella at a local music shop showed me how to make the world's simplest cab tool.

    You'll need a mono 1/4" plug, 2 short wires and two alligator clips. Simply solder the wires to the + and - poles in the 1/4" plug, then solder the gator clips to the other ends of the wires (I made one that's about 8" overall). You can now plug this into any cabinet jack, attach the alligator clips to your ohmmeter leads and voila!, you are now reading the resistance of the entire cab! Best damn tool I ever owned. Whole thing cost me about $2 using parts from Radio Shack.
     
  7. Unfortunately the resistance of the cab has little to do with the actual cab impedance. Any more than trying to measure the AC volts of an AC outlet with a DC voltmeter.

    Don't rely on what the ohmmeter reads.

    Find out what kind of speakers they are, they may have ohms on the back of the driver. Or the model# might allow you to look it up, contact the manufacturer of the driver to tell.


    Randy
     
  8. jazzblade

    jazzblade

    Jun 21, 2005
    Northern Cal
    On the broken cab:

    Ok them the only way to know is to get into the cab and measure the resistance of each speaker. (you might have a open wire or just a bad connection or bad crossover)

    NOTE: You will have to disconnect both wires from each speaker before you can measure it. Do each speaker one at a time. (Draw a diagram of each speaker wire before removing them so you can put them back where they belong)
    Leaving the wires connected when tracing resistance measurements will give you inaccurate measurements
     
  9. jazzblade

    jazzblade

    Jun 21, 2005
    Northern Cal
    Quite true Resistance will change with freq. but it should show the inital load the amp head sees. However it will not tell the user if ONE or more drivers are open or shorted.

    If the speakers ARE marked. at least thats a starting point that will at least tell you what the meter should read.
     
  10. Not sure what "the initial load the amp sees" means. The ohmmeter shows you the resistance, which is the impedance at frequency of 0, i.e. DC.

    Cab's impedance is rated around 1kHz, which is liable to be a totally different number.

    Can't check AC ohms (impedance) with a DC ohmmeter (resistance) any more than you can check AC voltage with a DC voltmeter. Just cause they're both called "volts" and "ohms" doesn't mean they're interchangeable.

    The voice coil is just a length of wire to an ohmmeter, a dead short except for the wire's resistance. The coil has impedance to AC that increases with frequency. Capacitors in the crossover are open circuits to DC, but conduct AC, impedance increases with frequency.

    So anything connected through a capacitor is invisible to the ohmmeter, but visible to audio signals. So the crossover is opaque to ohmmeter, but clear to AC signals.

    All that renders the DC ohms value you can measure with a multimeter worthless in determining the ohms of the cabinet.

    If you're just suggesting to use it to see if the voice coil is open, that should work like you suggested, testing each speaker individually and disconnected from other things.

    Randy
     
  11. jazzblade

    jazzblade

    Jun 21, 2005
    Northern Cal
    TOTALLY right Steve...DUH.. Forgot about the caps. Blonde moment??? I stand corrected.

    Since it's got some kind of LC crossover network, You sure can't measure the overall resistance of the good cab from the input jack.
     
  12. django

    django

    Dec 20, 2004
    Stockholm, Sweden
    i'm not really following... but if i gave you the complete schematics, i.e. how the crossover is wired etc, it would be possible to tell the "ohm", right?
     
  13. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Steve's right, the Ohm meter will give DC resistance. You can still work out the nominal impedance though. The DC resistance is typically a bit lower than the nominal impedance.

    A Typical 8 ohm cab will read DC resistance of somewhere between 5 and 6 Ohms of DC resistance. A typical 4 ohm speaker will read at something like 3 ohms DC resistance.

    Django are you sure they're 6 ohm speakers an not 8 ohm? I only ask because 6 ohm speakers are not common.
     
  14. Tim__x

    Tim__x

    Aug 13, 2002
    Alberta, Canada
    They are in car audio. If I haven't misunderstood your description, I'd say it's a 3 ohm cab.
     
  15. django

    django

    Dec 20, 2004
    Stockholm, Sweden
    ...and it said 4.0 ohm at the output.

    Would it be safe to plug it into the poweramp, which needs 4 ohm stereo and 8 ohm bridged, and use it in stereo (or parallell, perhaps) mode? Is it harmful for the amp to use just one of its channels? I figure I cannot use it bridged since it needs 8 ohms th

    about the cab that is broken. Neither of the 15" gave any ohm-readings at all, the 6" did though... so i guess they're blown, right?
     
  16. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Then it's a 6 ohm speaker (nominal impedance). Which is very odd considering it's got 2x15's in it. That means each 15" is either 12 Ohms (wired in parallel), or 3 ohms (wired in series). I can dig that 6 ohm speakers aren't uncommon in car audio, but what about 12 and 3 ohm speakers?

    Can I just confirm your testing method. I'm assuming you plugged a speaker cable into the cab and then measured the DC resistance at the other end of the speaker cable? In other words, you measured the CD resistance of the "entire" speaker cab?

    Or did you measure each speaker individually?
     
  17. django

    django

    Dec 20, 2004
    Stockholm, Sweden
    actually it is car/pa/disco speakers... i got them from a dj. Actually I found the pdf:s for the speakers here:

    www.ljudia.se/pdf/703129.pdf - the 6"

    www.ljudia.se/pdf/703125.pdf - the 15"

    where they clearly say the the impedence is 6 ohm.

    The cabs dont have speakon or a standard 1/4" outputjack, but these "binding posts" where you connect bare speaker wire. So I just measured the resistance between these two... and got 4.0 ohm.

    So is it safe to treat it as an 4ohm cab?

    another question: can I take a regular instrument cable and cut off one end and use the two inside wires to connect to the cab?
     
  18. DO NOT use instrument cable to hook up speakers!!

    Instrument cable isn't heavy enough to carry the current. Use a speaker cable, either buy one or make it yourself. Plain 16 gauge or heavier (smaller numbers are heavier, when it comes to wire gauge) 2-conductor cable from Lowe's or Home Depot and a 1/4" plug on the end. Don't fall for the claims of some of the cable vendors, you don't need to spend more than a few bucks on a speaker cable.
     
  19. ESP-LTD

    ESP-LTD

    Sep 9, 2001
    Idaho
    I wouldn't spend a lot of money on them. They are home audio type speakers (good low end, low sensitivity) but they won't take much power (300 'Max' so figure maybe 100w RMS) so overall they just won't get very loud for you. A 50w bass combo would probably be louder. There are a lot of specs missing in that PDF which leads me to believe they are not very 'good' speakers.


    NOT- (editted)
    Yes, you can treat them as 4 ohm speakers for load calculation purposes and be safe (with solid state amps).


    See the following; I thought you had a single 15 in each box.
     
  20. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    I'm not so sure yet. We've established each speaker is 6 ohms, but I'm under the impression each cab has 2x15's. If they're wired in parallel, that's a 3 Ohm load, loo low for a 4 ohm amp. If they're wired in series, it's a 12 ohm cab which raises wattage delivery issues.