Ohm meter fun....

Discussion in 'Archived Threads 2001-2004' started by Aaron E. Smith, Jul 24, 2001.

  1. Aaron E. Smith

    Aaron E. Smith Stunt Coordinator

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    Okeedokee, here is my question. A while back I had a little incident where a woofer on my front right speaker was blown. Thinking that this was the only problem, I removed it, sent it back to Vandersteen for repairs, and replaced it back into the enclosure. Now I think that I might have damaged some other drivers and I want to test the drivers individually. For this purpose, I have bought a handy-dandy little Ohm Meter from Radio Shack. What I need to know is, what information should I be looking for, a match in the Ohm rating? Will it tell me if the driver is not functioning?
    Thanks in advance!
    ------------------
    - Aaron
    "Some mornings, it's just not worth chewing through the leather straps"
    -Emo Phillips
     
  2. Kevin P

    Kevin P Screenwriter

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    Connect the ohmmeter across the terminals of the driver. You should get a reading, probably about 3-5 ohms for a "8 ohm" driver. Problems would be indicated by no/infinite reading (meaning the driver is open or blown), or a zero ohm reading (meaning it's shorted out).
    The meter will only tell you if the voice coil is conducting or not, so testing with the meter will only tell you if the driver is blown completely or shorted. You'll have to listen to the driver to judge if it's working properly--an overheated driver could have a warped VC that still "conducts" properly but will sound awful.
    KJP
     
  3. Aaron E. Smith

    Aaron E. Smith Stunt Coordinator

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    Kevin,
    Thanks so much for the information! In two paragraphs you were able to be more helpful and informative than Richard Vandersteen was in three days of talking to him, so I really appreciate that.
    (And for anyone interested, I will have a letter on my Vandersteen experiences when this situation is cleared up in a few weeks).
    Thanks again Kevin!
    ------------------
    - Aaron
    "Some mornings, it's just not worth chewing through the leather straps"
    -Emo Phillips
     
  4. Saurav

    Saurav Cinematographer

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    Does your ohm-meter use a DC or AC signal? I wonder what the DC impedance of a speaker driver would be... much less than 8 ohms, I would imagine.
    Edit: Make that DC resistance [​IMG]
    [Edited last by Saurav on July 24, 2001 at 07:33 PM]
     
  5. Aaron E. Smith

    Aaron E. Smith Stunt Coordinator

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    Quote:
    -----------
    Does your ohm-meter use a DC or AC signal? I wonder what the DC impedance of a speaker driver would be... much less than 8 ohms, I would imagine.
    Edit: Make that DC resistance
    -----------
    Howdy, I hadn't thought what type of signal it uses, I'll have to check the manual and get back on that (it is powered by two AA batteries, if that helps). The meter can test both AC and DC however.
    ------------------
    - Aaron
    "Some mornings, it's just not worth chewing through the leather straps"
    -Emo Phillips
     
  6. Saurav

    Saurav Cinematographer

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  7. Gary Kellerman

    Gary Kellerman Stunt Coordinator

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    IMO, the DC resistance which is the measurement of the wire wrapped around the speaker voice coil could be significantly important as to how your audio system plays.
    Thirteen years ago, I came up with a 5 channel passive system based on Dynaquad. As I experimented with different speakers that affected the sound field, I noticed that the DC resistance had to be within a certain range otherwise the soundfield was affected. My left and right stereo speakers read 6.9 ohms; the rears read about 6.2 ohms. I had a specifically designed center that read in the 6 ohm range. The soundfield was thrown off. Luckily, I had a raw speaker that read 7.1 ohms. This worked fine. Later experiments told me that the center DC resistance must be equal too or higher in DC resistance than the left and right stereo main speakers.
    If possible, when you have bought a pair of speakers for stereo and/or surround to measure the DC resistance. It is a "sort" of way of knowing that you have a "matched pair".
    I measured the DC resistance of some Canton's my brother owns. Both were dead on 4 ohms as specified by the speaker.
     
  8. Adam

    Adam Stunt Coordinator

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    Ohms is Ohms! It does not matter to the meter when measuring resistance. Resistance is measured without any current going though the circuit. It is the measure of resistance to electrical current, either AC or DC. If the circuit were live you would not get an accurate reading, the resistance in a circuit varies with frequency. Also, measuring a live high power circuit would likely damage the meter. If you were using a multimeter to measure voltage or amps, then AC or DC would matter.
     
  9. Brian Fellmeth

    Brian Fellmeth Supporting Actor

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    Adam's post above is not entirely accurate. Resistance is measured with current flowing through the component- DC current provided by the meter itself (which is why ohm meters need batterys to work). It is also untrue that the resistance is the same to AC or DC if the component (or circuit of components) has inductance or capacitance. Speaker leads have inductance because they lead to a voice coil. All simple multimeters and ohm meters measure DC resistance. The AC resistance is more difficult to measure requiring specialized equipment and is not a particular ohm level but a curve depending on the frequency of the AC.
     
  10. Saurav

    Saurav Cinematographer

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  11. Claude M

    Claude M Stunt Coordinator

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    Keep in mind the dc current from the meter's battery can damage a tweeters voice coil (if it has one).
     
  12. Saurav

    Saurav Cinematographer

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    That's an excellent point which hadn't occurred to me at all.
     
  13. Bob McElfresh

    Bob McElfresh Producer

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    Uhhh guys - dont forget that you need to disconnect one of the wires on a driver before you try to take a reading. If not, you are measuring the crossover circuit and the driver.
    Do you guys remember this old grade-school experiment:
    You take some copper wire and wind it around a large nail or railroad spike. Then you connect the ends of the wire to a battery. Now the nail becomes an electro-magnet and can be used to pick up paper clips.
    Here is another one:
    You take some copper wire and wind it around the cardboard center of a roll of toilet paper. Then you put a small compass in the tube and connect the ends of the wires to a battery. When you make the connection, the needle on the compass will move to a new position. Disconnect one wire and it moves back.
    This second example is EXACTLY how a speaker driver works. It is a coil of wire wrapped around a magnet suspended in the middle. The magnet is attached to a paper cone.
    When you put some electrical power on the ends of the wire, the magnet will move and pull the paper cone with it. If you connect-disconnect the battery fast enough, the movement of the cone will create some sound.
    Your audio signal make the magnet move in and out many times per second. This makes music.
    DC Resistance: On a speaker driver, the two wires are simply the ends of one long piece of wire. And if it is made of copper, the DC resistance is very, very low. Much lower than even 1 ohm. (Something like 0.018 ohms per foot - tiny)
    For a DC (fixed voltage and current) this is a dead-short or zero ohms.
    So why did Gary read ~6 ohms of resistance? My guess is that the driver-manufacturer put a resistor inside the driver. This would help prevent equipment damage.
    AC Resistance (Impedence): When you wrapped that wire around the toilet-paper tube and first connected the battery, a electro-magnetic field built up.
    It took some energy to build that field, but once it was built, no more energy was consumed. (Think of blowing up a balloon).
    But if you change the voltage on the battery, the field has to grow or shrink. When it grows, it takes some energy for a short ammount of time. When it shrinks, it puts some energy back into the wire.
    What if you tried to change the voltage... 10 thousand times per second (a 10 Khz tone)? A bunch of energy would be used up to change that magnetic field ten-thousand times per second. (Think of blowing up the balloon, letting some air out, blowing it up larger, letting air out, etc.)
    This energy lost can be measured as a voltage-drop (measured from the begining of the coil to the end).
    And in electronics, a device that eats energy causing a voltage drop is called a RESISTANCE and it is measured in Ohms.
    This resistance is called IMPEDENCE only occurs with a changing signal. And the ammount of resistance depends on how fast you are changing the signal.
    Real World:
    The DC Resistance of your speaker driver should be zero ohms (unless they installed a fixed resistor in the driver). But if you put your ohm meter on your driver to measure AC voltage drop, and started playing a slow frequency sweep from 50 hz to 15,000 hz, you would see the the voltage changes.
    If you did the math to calculate the resistance from the voltage drop, you would find your 8 ohm driver measures something like:
    ~2 ohms resistance at 50 hz
    ~3 ohms resistance at 100 hz
    ~4 ohms resistance at 500 hz
    ~8 ohms resistance at 1000 hz
    ...
    ~ 30 ohms resistance at 20,000 hz
    Yes, your speaker impedence CHANGES depending upon the sound.
    You have to remember this: They call a speaker/driver a "8 Ohm speaker". But they have dropped an important word:
    8 ohm nominal impedence
    The word "nominal" means that it is 8 ohms at a "nominal" frequency.
    Does this help?
     
  14. Adam

    Adam Stunt Coordinator

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    To defend myself. My statement "Resistance is measured without any current going through the circuit." meant that the circuit is dead. The power to the circuit is removed, whether it be the amplifiers signal or AC from an outlet. The only current used to measure resistance is that from the meter itself. Resisters must also, be isolated from the circuit it is in. Any reactive or inductive loads will give you a reading of the combined resistance of the circuit. But, thanks Saurav for pointing out how stupid I am.
    Regards,
    Adam
     
  15. Saurav

    Saurav Cinematographer

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  16. Aaron E. Smith

    Aaron E. Smith Stunt Coordinator

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    Wow, I never expected this topic to generate so much interest!
    Anyway, to reply to some points:
    1. I can't tell in the manual if the ohm-meter uses a DC or AC signal. I do know that is uses two AA batts., but I'm not sure if that indicates the type of electrical signal used.
    2. The meter can measure the following: DC volts, AC volts, DC current, AC current, and resistance. It is a Radio Shack 20-Range Auto Ranging Digital Meter (Cat. No. 22-806).
    3. I know the article wasn't intended for me, but thanks Saurav for the ohm meter article. I'll take a look at it so I understand this process better. (btw, I understood what Adam meant by his original post, although it could have been worded better).
    4. About damaging the tweeter's voice coilc, this is something I would absolutely want to avoid! However, the Vandersteen guide to testing their speakers with the ohm meter suggests leaving the leads in place for at least 10 seconds to stabilize the reading. Is this a bad thing? Could you elaborate on how not to use the ohm meter and how it can be damaging?
    5. About removing at wire. Wouldn't that depend on which thing it is that you're hoping to test. For instance, on the Vandy's, getting to the tweeter is quite a chore and the Vandersteen trouble-shooting guide recommends first testing the bi-wired terminals at the back of the speaker (of course, Mr. Vandersteen later did an about face on the usefulness of this test, leading to increasing frustration on my part with this process).
    6. Bob, I think your post helps, but I'll have to let it sink in. [​IMG] Really tho, I'm still sorting thru this whole ohm meter thing, but I think your post will come in handy.
    Anyway, as it stands, I think I've diagnosed the problem with a few of my Vandersteens and have sent them to California for repair. In the meantime, I'd love to learn more about this nifty meter that I've bought.
    Thanks again everyone!
    ------------------
    - Aaron
    "Some mornings, it's just not worth chewing through the leather straps"
    -Emo Phillips
     
  17. Bob McElfresh

    Bob McElfresh Producer

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    Aaron: I think Radio shack has a small paper-back book on basic electronics or "How to use your Ohm Meter". Either one would be a cheap $6 investment.
    Ohm meters are fun. To learn more, you might want to find a basic electronics book. And the first chapter is almost always on "Ohms law". (I think the Radio Shack books will include this as well).
    In general: your ohm meter cannot hurt any device. But you should NEVER try to use the ohm meter on any device plugged into a AC power outlet. You could hurt yourself badly while poking around.
    My suggestion: go to Radio Shack and buy this selection of electronic parts:
    - A small battery holder (C-size is fine)
    - Some small wires with aligator-clips (Sold in a pack)
    - Several small volume-controls (variable resistors)
    - A package of assorted fixed-resistors
    - Several of their small lamp-sockets that take a screw-in DC bulb and several bulbs.
    - A small 12v DC motor (toy size)
    - A small solar-panel
    For about $15 this will give you lots of stuff to play with and hook up to the ohm meter to get the concepts.
    I took almost the exact same parts list and attached everything to a piece of plywood. I pounded some nails next to each device and wired the leads to the nails.
    Then my daughters could use the wires with aligator-clips to create a bunch of different circuits.
    Good Luck.
     
  18. Aaron E. Smith

    Aaron E. Smith Stunt Coordinator

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    Thanks Bob, that's good info. Looks like I've got some reading to do! [​IMG]
    ------------------
    - Aaron
    "Some mornings, it's just not worth chewing through the leather straps"
    -Emo Phillips
     

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