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weird impedance reading on new cabs

Discussion in 'Amps and Cabs [BG]' started by Gabu, Jan 29, 2002.


  1. Gabu

    Gabu

    Jan 2, 2001
    Lake Elsinore, CA
    Hi guys,

    I just assembled 4 new cabs for use as monitors. Each has a single woofer, wired in parallel with a single tweeter, and 2 input jacks that are also wired in parallel.

    The woofer is 8 ohms. The tweeter is also 8 ohms. Since they are wired in parallel I was assuming that the resulting impedance would be 4 ohms. However, this was not the case when I measured them with my ohmeter.

    I measured each cab at about 16 ohms +/- ... I thought that was really odd, so I changed the battery in my ohmeter and tried again with the same result.

    Now I am ready to ditch the ohmeter and get a new one, if need be. But what I am wondering is if there is anything different about these piezo tweeters that claim they need no crossover? Could something that I don't know about these explain my weird impedance reading?

    Thanks!
     
  2. Gabu

    Gabu

    Jan 2, 2001
    Lake Elsinore, CA
    I forgot to add, I am measuring DC impedance.

    Thanks again!
     
  3. Chasarms

    Chasarms Casual Observer

    May 24, 2001
    Bettendorf, IA USA
    First. Double check your circuit. Two 8s in series would be 16 ohms. Perhaps a co-wee-kee-dink, but the reading that you are getting is suspect to say the least. If you are dead sure about your circuit, test each speaker seperately and see where you are after that.

    Chas
     
  4. chucko58

    chucko58

    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    Yeah, something is wrong here. An 8 ohm speaker should have a DC resistance in the neighborhood of 5-6 ohms. You *did* zero your meter before making the reading, right? Do you have any low-ohm resistors you can use for a reference check?

    Also, a tweeter is not going to be happy without a crossover. The bare minimum should be a non-polar capacitor; the exact value will vary depending on the crossover frequency you need.
     
  5. Jeezus Gabu, your avatar is scarier than mine.

    Try measuring the individual drivers as a good check. Then do the math. But something's up. Maybe a crossover network for the tweeter will let you have better control of the total impedance.
     
  6. Gabu

    Gabu

    Jan 2, 2001
    Lake Elsinore, CA
    Thanks everyone for your replies so far.

    First, the speakers are definately 8 ohms. These are old speakers that I had laying around and are marked as 8 ohm, but also have previously been measured at 4 ohms when they were in two sepperate boxes in parallel pairs.

    The tweeters are the unknowns. On their packaging it says, "no crossover needed." My understanding is that they kick in at 5khz. They are Piezo Tweeters. They claim to be 8 ohms.

    I am sure of my wiring as I have done quite a few boxes in series, parallel or series/parallel schemes. The main thing I know nothing about is tweeters. It's my first time fooling with them Piezo or otehrwise.
     
  7. Nightbass

    Nightbass

    May 1, 2001
    Seattle, WA
    Holy cow, you've discovered a superconducting tweeter with a resistance of -9.6 ohms!
     
  8. Gabu

    Gabu

    Jan 2, 2001
    Lake Elsinore, CA
    Yay!
     
  9. Gabu

    Gabu

    Jan 2, 2001
    Lake Elsinore, CA
    Maybe more specific questions are in order.

    This piezo tweeter says no crossover required, so I assume it only works at 5khz and above.

    If that is the case, then does it become infinate resistance at frequencies below 5khz?

    If that it does, then does the cabinet impedance change to 8 ohms for frequencies below 5khz, and 4 ohms for freq above 5khz?

    If for whatever reason my cabinet frequency was truely 16 ohms all the time that would be fine with me, however if it's 8 ohms for some frequencies and 4 ohms for others, is that okay for my amplifier?

    Thanks again.
     
  10. chucko58

    chucko58

    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    Got me. I have no experience with piezos.

    No, because the impedance of the woofer is mostly inductive, and doubles with every octave above its minimum impedance point. By the time you care about the piezo's output, the woofer's Z is probably in the hundreds of ohms, snd the piezo's impedance will dominate.

    There is no such thing as a speaker that presents a constant impedance load!
     
  11. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Piezos have very high impedances at low frequencies. It could be this is why you are seeing 16 ohms (as someone mentioned, a 4 ohm load should look more like 2-3 ohms of DC resistance).

    Disconnect the piezo, see if the 8 ohm woofer read 5-6 ohms, if it does you're OK.
     
  12. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    All speakers vary in impedance vs. frequency. Go look at any spec sheet and you'll see this.
     
  13. CYoung

    CYoung

    Nov 30, 2000
    Gainesville, FL
    brianrost, your answer is spot on. A piezo tweeter uses a crystal that vibrates when an electrical current is passed through it - in this case, an electrical signal. Most piezo tweeters simply do not respond to signals below their cutoff frequency - about 3-5khz typically. Their electrical characteristics may give false DC ohm readings. Check the woofer alone, then hook up the tweeter making sure your wiring is parallel, and hook it up for a sound check.

    FOr the record, you can still use a crossover with a piezo, just don't cross it over at a frequency lower than its designed cutoff. In your case, a 5khz 2-way crossover would be nearly perfect.
     
  14. Gabu

    Gabu

    Jan 2, 2001
    Lake Elsinore, CA
    Thanks again guys.

    I will disconnect the speaker on one and double check the impedance. (One should be okay since they are all behaving the same way)

    If all is right, then perhaps adding a crossover would be the thing to do. If I add a 8 ohm crossover, then it would always be (+/-) 16 ohms right? 8 ohms going through the crossover and then 8 ohms going through the speaker or the tweeter, as appropriate.

    One thing I would really like about these things being 16 ohms is that it would simplify wiring. Each of them has 2 input jacks wired in parallel so I could connect the amp to the first one, then wire the first to the second, and so on making the over all load an acceptable 4 ohms, and in effect only 1 cable on the ground.
     
  15. Hey Gabu,

    A piezo tweeter's impedance can't be measured using DC. Even if you're measuring AC, you'll find an impedance of about 30 Ohms to 1 k Ohms. A piezo is a crystal in a capacitor-like sandwich, so it can't be 8 ohms like a coil speaker. It's DC resistance is close to infinite (try it!)

    The power rating of the tweeter is *matched* to 8 ohms. That's why they say it IS 8 ohms.

    If your total cabinet measures ~16 ohms, something's wrong with your test leads or your meter (is it good down in the "few ohms" range?). Of course, bad wiring or a bad speaker could be the cause.

    An 8 ohms woofer measures around 5-7 ohms at DC. A crossed over cabinet will have a total DC resistance of that of the woofer. Your piezo equipped cab should be 5-7 ohms.

    I hope this is of some help.
     
  16. Gabu

    Gabu

    Jan 2, 2001
    Lake Elsinore, CA
    Thanks very much! I think I am going to buy a new ohmeter. Mine is about 15 years old, and I just don't trust it now.

    The most recent news on my new cabs is I made a cable with 2 1/4" plugs on one end, and a single one on the other.

    I wired it so that the single leads + connected to the + of the left 1/4" and the single leads - connects to the - of the right 1/4" plug. Then I connected the - of the left to the + of the right lead.

    This enabled me to connect two cabinets in series.

    I connected two of my cabs this way, and then conencted the other two cabs in parallel by connecting them to the jacks left over in the first two cabs.

    It seemed to work pretty well. Now that they have tweeters (and no hole acting like a port!) the cabs sound a lot less bassy to me.

    Thanks very much to everyone for your help.
     
  17. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Hey Gabu, as Joris said, a piezo tweet is itself a capacitor, so at DC it'll be an open circuit, or at least a very, very high resistance. Sometimes they're used with a shunt resistance to get a more predictable filter curve from a passive crossover, but yours probably didn't come with one already. And if it did, it would've given you a lower resistance reading.

    If you just connected the tweeter in parallel with the woofer, you should get a measurement equal to the woofer's DC resistance, which as others have pointed out, would be about 5 to 7 ohms.

    So I'd say if the woofers actually measure about 16 ohms, then they're definitely not 8-ohm drivers! Maybe they're 20-ohm woofers. ;)
     
  18. Gabu

    Gabu

    Jan 2, 2001
    Lake Elsinore, CA
    Thanks for the info! I am almost certain that either my leads are coroded, or my ohmeter is on the fritz. Either way, it's pretty old so I am going to replace it. I have a serial cable that I made to connect the two cabs, and I will daisy chain them to two others. I predict my load should be +/- 8 ohms. Then I have 2 12" mains and am going to sit those on 2 15" subs. This has been a fun project, so I may make the subs too! :D