Rebuilt L100 123A Woofers: Measuring 11.5 ohm

[analogkid]

Hi,

I'd love a little help confirming 11.5 ohm is not a good result on a 123A woofer.

I am in the process of completely restoring a pair of L100A's. While doing initial tests of the woofers, I measured both at 11.5 ohm on my digital multi-meter. This seems like it is too high - I was expecting ~8 ohms, but I'm looking for confirmation. In fact, both woofers are registering exactly 11.5.

Both woofers were rebuilt with new cones, surrounds and voicecoils by a reputable firm. I am rebuilding the N100 crossover from scratch as the original was a complete loss.

Many thanks in advance!
John

[Wagner]

Hi,

I'd love a little help confirming 11.5 ohm is not a good result on a 123A woofer.

I am in the process of completely restoring a pair of L100A's. While doing initial tests of the woofers, I measured both at 11.5 ohm on my digital multi-meter. This seems like it is too high - I was expecting ~8 ohms, but I'm looking for confirmation. In fact, both woofers are registering exactly 11.5.

Both woofers were rebuilt with new cones, surrounds and voicecoils by a reputable firm. I am rebuilding the N100 crossover from scratch as the original was a complete loss.

Many thanks in advance!
John

You are taking your measurements directly at the transducers' hookup terminals, correct?
And you are sure your meter is on the correct setting and is good working order?

If the answer to both of the above is "yes" then your reputable firm did a shit job.

That is approximately TWICE what the voice coil's DC resistance should be on that driver. Not a good result. Not really relevant, but that is fairly unusual for both of your woofers to need re-coning? Were they abused or have damaged cones?

The DC resistance in ohms for the 123A-1 and the 123A-3 should be:
4.5 Ohms - MINIMUM
5.5 Ohms - MAXIMUM
These are JBL's specs and coincide with all of the 123As which I own or have ever checked

http://manuals.harman.com/JBL/HOM/Technical%20Sheet/L100%20CENTURY%20ts.pdf

http://manuals.harman.com/JBL/HOM/Technical%20Sheet/L100A%20ts.pdf

http://manuals.harman.com/JBL/HOM/Technical%20Sheet/L100A%20LATE%20ts.pdf

[Mr. Widget]

While doing initial tests of the woofers, I measured both at 11.5 ohm on my digital multi-meter. This seems like it is too high - I was expecting ~8 ohms, but I'm looking for confirmation. In fact, both woofers are registering exactly 11.5.
It sounds like you are confusing AC impedance and DC resistance. A typical DMM will only measure DC resistance. You need a more specialized meter to measure AC impedance. The 8 ohm spec is for the system's AC impedance.

That aside, as Wagner points out, you need to measure the woofers independently of the system and the DCR figure should be significantly below 8 ohms.


Widget

[Wagner]

It sounds like you are confusing AC impedance and DC resistance. A typical DMM will only measure DC resistance. You need a more specialized meter to measure AC impedance. The 8 ohm spec is for the system's AC impedance.

That aside, as Wagner points out, you need to measure the woofers independently of the system and the DCR figure should be significantly below 8 ohms.


Widget
True; systems identified with a NOMINAL SYSTEM IMPEDANCE of 8 Ohms (what that number on the back panel of a speaker system indicates) will still generally measure "at" or less than 8 Ohms DCR (usually between 5.5 and 6 Ohms or less), occasionally spot on and rarely more; I have yet to measure a system (total) rated @ 8 Ohms IMPEDANCE (or whatever the given system's NOMINAL impedance rating may be) that exceeded that stated nominal value in DCR. Not saying it doesn't happen, but this is a rule of thumb for most complete systems when measured using the DCR scale on a DVOM.

A healthy L100A cabinet (system), rated at 8 Ohms SYSTEM IMPEDANCE as shown on the foilcal with good caps and clean pots should measure at or close to 4.5 Ohms DCR when measured at the binding posts.

If your 123As measure 11.5 Ohms DCR (DC Resistance) at the transducer's terminals then your reconer screwed up your job.

[analogkid]

True; systems identified with a NOMINAL SYSTEM IMPEDANCE of 8 Ohms (what that number on the back panel of a speaker system indicates) will still generally measure "at" or less than 8 Ohms DCR (usually between 5.5 and 6 Ohms or less), occasionally spot on and rarely more; I have yet to measure a system (total) rated @ 8 Ohms IMPEDANCE (or whatever the given system's NOMINAL impedance rating may be) that exceeded that stated nominal value in DCR. Not saying it doesn't happen, but this is a rule of thumb for most complete systems when measured using the DCR scale on a DVOM.

A healthy L100A cabinet (system), rated at 8 Ohms SYSTEM IMPEDANCE as shown on the foilcal with good caps and clean pots should measure at or close to 4.5 Ohms DCR when measured at the binding posts.

If your 123As measure 11.5 Ohms DCR (DC Resistance) at the transducer's terminals then your reconer screwed up your job.

Thanks all for the great responses and information/confirmations. In short, I was correctly measuring the woofers and indeed can confirm they each measure 11.5 ohms DC while only connected to the +/- leads on my multi-meter. I checked my dmm by measuring some other things including the LE5 and LE25 to make sure the dmm was not faulty.

I spoke with Orange County Speaker just a few minutes ago and they were helpful and just as surprised at this result. I am sending them back to California for them to double check and hopefully fix the issue (at their cost). The person I spoke with (Eric) was nice enough to check the impedance on a couple coils they had in inventory to make sure their inventory was ok.

Since, the reconing took place in 2014 (they sat on my shelf while I worked on the cabinets) - its nice that OCS is thus far saying they will likely cover the costs of repair and shipping.


Thanks again,
John

[Lee in Montreal]

Could the 123A simply have been reconed as a 16 ohm instead of 8 ohm? That would explain the 11,5 ohm resistance... :D

[Wagner]

Thanks all for the great responses and information/confirmations. In short, I was correctly measuring the woofers and indeed can confirm they each measure 11.5 ohms DC while only connected to the +/- leads on my multi-meter. I checked my dmm by measuring some other things including the LE5 and LE25 to make sure the dmm was not faulty.

I spoke with Orange County Speaker just a few minutes ago and they were helpful and just as surprised at this result. I am sending them back to California for them to double check and hopefully fix the issue (at their cost). The person I spoke with (Eric) was nice enough to check the impedance on a couple coils they had in inventory to make sure their inventory was ok.

Since, the reconing took place in 2014 (they sat on my shelf while I worked on the cabinets) - its nice that OCS is thus far saying they will likely cover the costs of repair and shipping.


Thanks again,
John
Sorry for your hassles; please post and let us know that/when they've made it right for you.
Hope it's more than "likely"; it shouldn't cost you another nickel.
I would be interested to know if they were able to rebuild your drivers with white cones and what was wrong with them initially?
Good luck with your project, hope they turn out well.
Post a picture when you're done.
Thomas