measure ohms

Discussion in 'Amps and Cabs [BG]' started by bizzaro, Jul 10, 2001.

  1. bizzaro


    Aug 21, 2000
    I should know how do this but I don't. I have tried every way I know how to get a reading but can't seem to figure it out. I have a multi volt/amp meter and I can tune the ohm meter to zero but don't know how to hook it up to get an ohm's reading, say from speakers etc. I have looked all over the internet and can find tons of formulas-but nothing that shows or explians how to get a reading.
  2. Set the VOM to ohms. A scale of 1x or whatever is appropriate to measure 8 ohms.

    Turn on the meter, and it should read nothing or Over Limit when the leads are not connected to anything. Connect the two leads together, and you should get a very small reading, or zero. Analog meters let you zero the scale when the leads are connected together.

    Put on lead on each speaker terminal. If you are measuring a cabinet, you will have to use a speaker cabinet cable, unscrew the cover from the plug and put the leads on the tip + sleeve terminals.
  3. bizzaro


    Aug 21, 2000
    I have done that many times and it reads 0. Just like if I touch the leads together to 0 the meter. Does the speaker have to be on,(playing music), when you test it?
  4. Maybe your meter isn't precise enough to read near-zero values, like speaker impedance. An 8 ohms cabinet reads 5-7 ohms. Maybe you're reading the correct value, but you just can't make it out on the scale of the meter.
  5. Take it down to Radio Shack and have the guy compare it against another meter. You will also get a lesson in how to use it in the process..
  6. By the sounds of it you've got an analogue meter - one with a needle - rather than a digital.

    You will almost certainly not be able to accurately measure low value Ohms on a X1 resistance scale on such a meter. It will look like a short (zero ohms). The meter simply wil not resolve 8 Ohms to your eye.

    Select a more sensitive range to read these low resistances. Each meter manufacturer will have a different way of doing this but you should look for a 'divide by 10 ' or 'divide by 100' Ohms range and then you should be OK to measure low value resistances. If your meter doesn't have a more sensitive Ohms range there's nothing you can do other than buy / borrow one that has.

    If you're still not sure test the cab / speaker with a 1.5 volt battery: JUST ONE, 1.5 volt battery. Connect it with a bit of wire to the cab. You should hear a definite albeit small 'thump' from the cab as you connect and disconnect the battery and, becuse it's DC, you can easily see the cones move. This is a quick dab-on-dab-off type of test and is DEFINITELY NOT intended as a permanent connection.

    Rockin John
  7. notduane


    Nov 24, 2000
    Wouldn't he be better off with an impedance analyzer
    vs. a DC ohm meter ? I would think any ohm meter is
    going to see a "short" ( ~0 Ω ) because it's looking at
    a coil of wire.
  8. No.:D The coil of wire has a resistance. OK, it might not be very high but it has resistance nevertheless. The coils of speaker cones are wound with fine wire and such wire has a relatively high ohms / metre thus quite a short piece measures, well, a few ohms.

    There is also the point that a (digital) multimeter can be had for just a few $ or £ whilst an analyser might cost $$$$ or ££££ or more.:D

    Measuring the resistance of a speaker cab is no problem with a multimeter. But it is resistance, NOT impedance. In reality the two are very close but they're not one and the same.

  9. The general relationship between speaker resistance and speaker impedance usually works out so that you just round up the resistance to the next "normal" impedance value.

    Measure 3 ohms, probably 4 ohm speaker
    Measure 6 ohms, probably 8 ohm speaker

  10. bizzaro


    Aug 21, 2000
    I did previously use an analog meter that was not sensitive enough to register low ohms though I was using it correctly. All of this electronic stuff is totally new to me. Thanks to suggestions in this thread I made a trip to Radio Shack and purchased a more sesitive digital LCD multi meter that works great.
    SSSOOOOOOO now---The difference between resistance and impedance is???? :D
  11. Excerpt from "TalkBass FAQ" (being written as we speak):

    1.2. Resistance and Ohm's law

    Resistance is another basic aspect of electricity. It determines how much current will flow though a certain load at a certain DC (Direct Current) voltage level. To be more precise: 1 Volt is generated when 1 Ampere flows through a 1 Ohm load. In a formula it looks like this: V=I*R. Voltage equals current times resistance.

    Calculating resistances isn’t very hard. When you put resistances in series, you can just add up their values. With parallel connections it gets more complicated: say you have two resistors R1 and R2. Their parallel value Rs would be Rs=(R1*R2)/(R1+R2). The following is a more general formula, in which you can put as many resistors as you like in parallel: Rs=1/(1/R1+1/R2+1/R3+….+1/Rn))

    1.3. Impedance

    Impedance is a different phenomenon, and is used to describe AC (Alternating Current) loads. It adds the variable of frequency. Impedance can be a constant thoughout a frequency range. In that case, it can be treated just like resistance in that frequency range. However, impedance usually varies widely with frequency, as is the case with loudspeakers. The specified value for loudspeaker impedance is given at 1 kHz, and may range from half to 10 times that value at other frequencies. If you try to determine the impedance of a loudspeaker with an ohmmeter, you'll find a value lower than the specified value. An ohmmeter measures resistance, which could be seen as impedance at a frequency of 0 Hz.

    When calculating with loudspeaker impedances, the specified value at 1 kHz is used. Nowadays, loudspeaker impedances are standardised. The most common values are 4, 8 and 16 ohms.

    No more questions about ohms, m'kay?

    Just kidding.
  12. mikemulcahy


    Jun 13, 2000
    The Abyss
    <----- Is feeling REALLY stupid right now!!!

  13. notduane


    Nov 24, 2000
    Ohhh, Dr. Mike :( . Just throw out some o' them $10
    medical woids and call back in the mornin' :D.
  14. bizzaro


    Aug 21, 2000
    I promise "No more ohms questions" in this thread!! BUT HOW ABOUT the the series - parallel question??? Parallel is simple enough. I had to put together a cabinet for a gig the other night and I used two 8 ohm 15's. For the life of me I couldn't get the two hooked up in series. I tried + to - between speakers which only reversed the polarity (not good). And just about every combination I could think of but I couldn't get the meter to read what the output should be for series. It always split the single reading, never doubled it. I wanted to use the cab in parallel anyway (4ohms),so it really wasn't a problem. I just wanted to see if I could hook it up in series to try and educate myself in the process of putting together this cabinet. (OK so it is related to ohms but I worked really hard trying not to use the ohm word).
  15. jack tip to speaker 1 red,
    speaker 1 black to speaker 2 red,
    speaker 2 black to jack ring

    imagine a wire starting at the jack, running down and into one speaker, then out of that speaker and down to the next, then out of that speaker and back to the jack.

    that's series

  16. notduane


    Nov 24, 2000
    Ooooo, purty pitchahs :D

    • [​IMG]


  17. MikeyD


    Sep 9, 2000
    That they are! If you let Joris use them for his amp "FAQ", that would be great!
    - Mike
  18. notduane


    Nov 24, 2000
    No problemo Mr. Mike :D. I...uhhh..."found" `em at
    the Shavano Music site. I think the web-dude there
    mentioned that it IS o.k. to copy stuff :).
  19. Very nice job on the pix. A picture is truly worth a thousand words here.

    I also notice you are using the standard "+" forward notation, where the speaker cone moves forward (away from the magnet) when a battery is connected as shown in the drawings.