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View Full Version : Why do 16ohm-drivers measure 7ohms!????



Dr.db
04-08-2014, 03:21 PM
Hey,


this might be a silly question, but I simply have no explanation...
Most 2440 and 2441 alnico-drivers which have the original 16ohm diaphragms installed, measure something between 7-8ohms... far away from factory-specs.
And 2405H ferrit slot-drivers which have the original 8ohm diaphragms installed, measure something between 6-7ohm.... so pretty close to factory-specs.


Any help.... :confused:

Thanks a lot,
Olaf

audiomagnate
04-08-2014, 04:30 PM
It's called "nominal" (in name only) impedance because it's pretty much meaningless, plus, what you are measuring the is DC resistance of the voice coil, which isn't the same as impedance anyway. Just be happy knowing your drivers are measuring correctly.

ratitifb
04-08-2014, 10:43 PM
And 2405H ferrit slot-drivers which have the original 8ohm diaphragms installed, measure something between 6-7ohm.... so pretty close to factory-specs.8=16 from JBL...

please take a look at ;)
http://www.audioheritage.org/vbulletin/showthread.php?23869-075-2402-076-2403-2404-077-2405

martin_wu99
04-08-2014, 11:16 PM
Hey,


this might be a silly question, but I simply have no explanation...
Most 2440 and 2441 alnico-drivers which have the original 16ohm diaphragms installed, measure something between 7-8ohms... far away from factory-specs.
And 2405H ferrit slot-drivers which have the original 8ohm diaphragms installed, measure something between 6-7ohm.... so pretty close to factory-specs.


Any help.... :confused:

Thanks a lot,
Olaf
Please check out your universal meter:D

Mr. Widget
04-08-2014, 11:52 PM
8=16 from JBL... No!

Impedance is a complex number as represented in the plot below. This is the plot of a JBL LE14H-3 which is nominally an 8 ohm woofer. If you were to put an ohm meter on it you would measure a DCR of 5.94 ohms.

If that isn't confusing enough, once you mount the woofer in a cabinet the impedance will change rather significantly. :D In the case of compression drivers, changing horns has the same effect.

The long and the short of it... if you can't measure actual impedance plots and are designing passive networks... then don't sweat the impedance number. To the typical user the number is of virtually no significance.


Widget

ratitifb
04-09-2014, 07:07 AM
No!Si! :p

Actually "8=16 from JBL" means that foilcals are stamped indifferently by JBL 8ohms or 16 ohms for the same voicecoil (and the same acoustical load... eg 2405 horn/phase plug + rear load).

And yes, i agree that 8 or 16 are the impedance numbers with no significance when designing Xover...


Impedance is a complex number :yes:

(and i teach the complex impedance with DC resistance plus coil self effect vs frequency plus additional mechanical and acoustical (radiation) terms from driver itself and its load (eg added stiffness due to the closed box volume (like a spring))

http://www.audioheritage.org/vbulletin/showthread.php?19972-Does-a-horn-affect-a-drivers-impedance&p=200233&viewfull=1#post200233

and my own (old) impedance measurement performed on E145 drivers showing the coil contribution in impedance value at "high" frequencies
61855

Mr. Widget
04-09-2014, 07:45 AM
I realize you understand and yes it is true that in the past on certain models JBL was inconsistent with their foilcals, but most people don't get the significance of impedance at all. In the scope of this thread 8 ohms does not = 16. :)

I bet many here will take a cursory look at that impedance plot and think, "geez, that speaker has a terrible frequency response"... I've gone to the trouble to post the plot hoping that at least one person out there will now understand that DCR is the resistance of the coil at 0 Hz and not it's impedance.


Widget

tomee
04-09-2014, 08:36 AM
Are these both 8 ohms but labelled as 8 or 16? Is there some date when they really did put diaphragms with different DCR values into the drivers for 8 and 16 ohm versions?


I understand that in the 1960s JBL called the LE14A a 16ohm woofer at first, and then later it was 8ohm woofer - I have a few of each, and they all have about the same DCR, and the measured impedance is the same (after replacing the foam)

Mr. Widget
04-09-2014, 10:00 AM
Modern JBLs use G,H, or J as the last letter to signify the impedance. G=4 ohms, H=8 ohms, J=16 ohms.

In the past they used A and B and others but they were inconsistent.


Widget

Dr.db
04-09-2014, 05:28 PM
Thanks a lot for all your replies!!
You guys brought some light in my confusion.... :)

The intention of my question was the following;
I got a pair 2405 ( labeled 16ohm , Alnico ) and a pair 2440 ( labeled 16ohm ) mounted to a 2390 horn/lense....
JBL claims the 2405 to be 105db/1w and the 2440 mounted to the 2390 to be 107db/1w. Due to the fact that I personally hate the sound of resistors in a passive crossover, I was wondering if these two drivers would match in terms of loudness when connected to the same amp-channel....
I apprehend the midrange will be a little too loud....
So my idea was to swap the 16ohm Alnico-2405 against an 8ohm Ferrit-2405.... Due to the lower impedance the 2405-8ohm should receive more power from the amp than the 2440-16ohm and compensate the lower efficiency....

But I am on the wrong track, ain`t I ?

Mr. Widget
04-09-2014, 06:59 PM
But I am on the wrong track, ain`t I ?Yep.:D

Resistors are your friend.


Widget

ratitifb
04-10-2014, 03:05 AM
So my idea was to swap the 16ohm Alnico-2405 against an 8ohm Ferrit-2405...my understanding again is there will be no major difference (alnico/ferrite debate out) when the two drivers have the same voice coil, same diaphragm ...

Have you any DCR measurement on the two to confirm ?

Dr.db
04-10-2014, 03:27 AM
Sorry guys, I believe i missed a post of yous, respectively (http://www.dict.cc/englisch-deutsch/respectively.html) I didn`t get it right.

So 2405-alnico and 2405h-ferrit have the identical voicecoils, but different labels on the back. Sorry for that extra turn :o:


I have no current measurements of MY DRIVER`s, cause theyr stored somewhere at my parents place...

tomee
04-10-2014, 05:51 AM
Modern JBLs use G,H, or J as the last letter to signify the impedance. G=4 ohms, H=8 ohms, J=16 ohms.

In the past they used A and B and others but they were inconsistent.


Widget

OK, so these actually ARE different then, and not just a change in label? I'm asking because I'm wondering if 2426Js would be just as good to use as 2426Hs (in for example, DIY 4425s or 4430s). (so the H and J versions would have different voice coils, if they do have different impedances?)

ratitifb
04-10-2014, 07:00 AM
OK, so these actually ARE different then, and not just a change in label? I'm asking because I'm wondering if 2426Js would be just as good to use as 2426Hs (in for example, DIY 4425s or 4430s). (so the H and J versions would have different voice coils, if they do have different impedances?):yes: regarding these animals and one connecting pin of the voice coil is red marked for 16 ohms (J) while green marked for 8 ohms (H)

ivica
04-15-2014, 05:43 AM
Thanks a lot for all your replies!!
You guys brought some light in my confusion.... :)

The intention of my question was the following;
I got a pair 2405 ( labeled 16ohm , Alnico ) and a pair 2440 ( labeled 16ohm ) mounted to a 2390 horn/lense....
JBL claims the 2405 to be 105db/1w and the 2440 mounted to the 2390 to be 107db/1w. Due to the fact that I personally hate the sound of resistors in a passive crossover, I was wondering if these two drivers would match in terms of loudness when connected to the same amp-channel....
I apprehend the midrange will be a little too loud....
So my idea was to swap the 16ohm Alnico-2405 against an 8ohm Ferrit-2405.... Due to the lower impedance the 2405-8ohm should receive more power from the amp than the 2440-16ohm and compensate the lower efficiency....

But I am on the wrong track, ain`t I ?

Hi Dr.db,

I will NOT suggest You to do any change 2405A vs 2405H. Their efficiency has to be the same (neglecting may be demagnetized Alnico).
So the best would be to do some measurements 2440J with the horn at the distance 1 to 2m from the horn mouth, and 2405 under IDENTICAL conditions, and then to see what would be the difference in the sound levels, and then to decide how to adjust their levels.
Driver efficiency can vary (from the JBL technical spec) even from piece to piece.

Regards
Ivica

Dr.db
04-15-2014, 06:07 AM
I agree, a real-life test is always the best way to determine the facts. Especially with these old drivers....

I was at my parents place last weekend and pulled them out for a test.
I did put both drivers on the same amp-channel and I couldn`t notice the 2440 to be much louder. This was a very subjective test, no objective test-equipment was used.
But I didn`t miss highs and the level of the uhf seemed to be right.

audiomagnate
04-15-2014, 04:52 PM
...Due to the fact that I personally hate the sound of resistors in a passive crossover...

What do they sound like? The resistors I mean.

Dr.db
04-15-2014, 11:14 PM
What do they sound like? The resistors I mean.


In my opinion they lessen the dynamics a little....

4343
04-16-2014, 09:12 PM
In my opinion they lessen the dynamics a little....

Is that with non-inductive resistors?

BTW F is used to denote 2 ohm drivers as well as the G, H, & J designators.

Dr.db
04-16-2014, 10:59 PM
To be honest, I can`t tell anymore.... The last time we used resistors is quit a while ago. But in that time we`ve tried several types/brands, I remember that.
But ever since I tried to avoid them when possible.

shoshaw
04-17-2014, 04:47 AM
Thanks a lot for all your replies!!
You guys brought some light in my confusion.... :)

The intention of my question was the following;
I got a pair 2405 ( labeled 16ohm , Alnico ) and a pair 2440 ( labeled 16ohm ) mounted to a 2390 horn/lense....
JBL claims the 2405 to be 105db/1w and the 2440 mounted to the 2390 to be 107db/1w. Due to the fact that I personally hate the sound of resistors in a passive crossover, I was wondering if these two drivers would match in terms of loudness when connected to the same amp-channel....
I apprehend the midrange will be a little too loud....
So my idea was to swap the 16ohm Alnico-2405 against an 8ohm Ferrit-2405.... Due to the lower impedance the 2405-8ohm should receive more power from the amp than the 2440-16ohm and compensate the lower efficiency....

But I am on the wrong track, ain`t I ?


Are you using an active crossover? If you change from a "16 ohm" to an "8 ohm" driver, the crossover frequency of a passive crossover will change.

Dr.db
04-22-2014, 10:47 AM
Hi,


Im using passive crossovers...

Thanks for the hint!