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Easy JBL D-130F Questions (I hope)

Discussion in 'Amp Central Station' started by Mowgli, Apr 17, 2021.

  1. Mowgli

    Mowgli TDPRI Member

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    I was fortunate today. I acquired a vintage Standel Imperial XV bass cabinet containing a JBL D-130F for much less than I expected. :)

    I haven't played anything through it yet because I wanted to verify the impedance before hooking it up.

    The impedance on the label states 16 ohms.

    I know that the JBL D-140 labels state 16 ohms but the majority of D-140s have 8 ohm impedances. According to the reports I have read and the one D-140 I own, they are actually 8 Ohm speakers. In other words there was a widespread labeling error by JBL regarding the D-140 and that has been well documented.

    The DC resistance on this newly acquired JBL D-130F measures ~ 9.8 - 9.9 ohms. This value seems too low for 16 ohms and too high for 8 ohms.

    Questions:

    > Is the impedance label correct on this D-130F (16 ohms) OR is this an 8 ohm speaker that was mislabeled like the D-140?

    > OR... despite the 16 ohm label do all D-130F speakers have a nominal impedance of 8-16 ohms?

    Thanks.
     
  2. Mowgli

    Mowgli TDPRI Member

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    I think I found an answer (a little odd but it is from Harvey Gerst on the Lansing Heritage forum). FWIW, H.Gerst designed the JBL D-130, D-130F and D-140F among others.

    Here's the quote from Harvey:

    "Okay, the real truth is that back then, JBL 16 ohm speakers were really 8 ohms, and the 32 ohm speakers were really 16 ohms. Why did we do that? Because that made them louder."

    I am curious what others think about this comment by H. Gerst.
     
  3. Jay Jernigan

    Jay Jernigan Tele-Afflicted

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    My D-130 says "8-16 ohm" and it reads in between. (I forget what) I pretty much always treated it like an eight ohm speaker and never had a problem.
    I have a Celestion that's labeled "16" and it's 12.
     
  4. AlfaNovember

    AlfaNovember TDPRI Member

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    A multimeter is always going to read a number close to the nominal rating, usually a bit lower, in my experience. Formally, the rating is for Impedance, which has a complex number component (remember i ?), rather than a simple integer Resistance. I think of it as "AC signals have impedance, DC has resistance". So I'd have called that a 16, and hooked it up to an 8 ohm output transformer without a second thought; it will present an easier load than usual.

    Old JBLs also have unusual polarity markings; the usual 'AA battery positive to red terminal should move the cone outward' doesn't always apply, and then there's the question of recone history. It's worth testing.
     
  5. Dacious

    Dacious Poster Extraordinaire

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    Your multimeter is reading DC resistance. Your speaker is rated in AC impedance which you cannot realistically measure with a DMM.

    Typically if you measure an 8 ohm impedance speaker it will measure 5-7 ohms DC resistance. This varies with temperature and also with frequency.

    Making speakers of less than rated impedance could lead to people going below the minimum impedance rated for their power amp which results in pfffft and all the smoke escaping.

    So usually 4 ohm speakers measure <4, 8 ohm speakers <8 and 16 ohm speakers < 16.
     
    Last edited: Apr 18, 2021
  6. Musekatcher

    Musekatcher Friend of Leo's

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    This is correct, and why they are labeled as such. The impedance changes depending on the frequency, where an 8 ohm speaker may actually measure from 6 ohm to 180 ohm, depending. But the only thing we need to know, is the measured DC resistance at rest, from a really low input current, like from a VOM or DMM should be about 90% of the labeled resistance. UNLESS..

    Its been cooked. At which point, its going to measure lower still. A speaker measuring 9.9 ohm, under accurate measurement, suggests to me, a 16 ohm that's been cooked. It may sound and function fine, or not. That's the only reason I can think of, for such a reading on a labeled 16 ohm reputable product like from JBL.
     
  7. Dacious

    Dacious Poster Extraordinaire

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    The famous EV10s measure low resistance - one of the reasons Paul Rivera liked it in Superchamps and in his later 6V6 based amps. He said he thought that was a reason for efficiency. Below 5 ohms for a nominal 8 ohm speaker.

    I had a pair of E-series 15s, both nominal 8 ohm and both measured 5-6 DCr at room temperature. One was original, the other s fresh recone.
     
  8. Mowgli

    Mowgli TDPRI Member

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    Thanks for all of the input. It was all greatly appreciated.

    The comments about how some reputable speaker designers (e.g. EV and Gerst with JBL) and an amp builder like P. Riveria had their own reasons for using or providing ambiguous impedance ratings on speakers demonstrates the need we all have at times to hear from many different people because "our mileages DO vary." Some issues are jigsaw puzzles with different people providing different pieces before the accurate picture can be seen.

    Given the DC resistance of 9.8-ish ohms and both Gerst's & Jernigan's comments I will treat it as an 8 ohm spkr and see how it goes.

    Musekatcher - By "cooked" do you mean excess current has damaged the voice coil? What should one expect to see with a meter and why should we see it?

    If the current burnt a segment of wire and thinned that segment of the coil, that should increase the DC resistance shouldn't it? If it added a carbon compound to the coil DC resistance shouldn't that also increase DC r? And excess heat (metal expansion) with either or both alterations above could deform the coil altering magnetic flux, possibly inducing unintended eddy currents, new magnetic field inhomogenieties, internal shorting, etc.

    This is why I ask; I'm curious about what measurable impact a "current damaged" voice coil can be gleaned from a single DMM/VOM measurement (provided the coil isn't "open" of course)?

    I was already aware that impedance is frequency dependent (a voice "coil" is, in large part, an inductor composed of a conductive/small resistive element. As such an alt. current will "induce" reactance contributing to the overall impedance): this is why I wanted to avoid confusion by indicating it's "DC resistance." But there was no way any of you could have known that I understood this in advance so I thank you and agree with your explanations.

    I was just unsure as to why a 16 ohm spkr - if it is truly a 16 ohm spkr - would measure 9.9 ohms DC instead of something like 11 - 15 (which is what most 16 ohm spkrs measure)?
     
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