2420 voice coil DC resistance?

[dougzilla]

A quick one, I hope. What are the correct DC resistance ranges for the voice coils in 16 ohm vs 8 ohm 2420 or other 1'' compression drivers that accept the current 2421 replacements (which are available in either 16 or 8 ohm versions, D16R2421 or D8R2421).

Reason I ask is that I recently purchased a pair of nominally 16 ohm 2420's on eBay, not with original wax seals. Upon opening them I found the aluminum diaphragm in one was damaged. I took both to A Brown Soun, authorized JBL Pro repair facility in Bay Area. I asked for both to be checked out and the bad diaphragm to be replaced with an aluminum version.

I had measured the DCR of both prior to bringing them in. One was 5.2 ohms, the other 5.4 ohms. The repair shop called me a couple weeks later and quoted me the price to install a new JBL diaphragm (which was about what I had expected) and said the other one was fine.

Then the guy said he would be ordering the 8 ohm part. I said, aren't they 16 ohm drivers, it says so on the label? He said no and that he had measured them himself, that I should have the 8 ohm to match. I said OK, since I haven't built the crossover network for these yet and figured I would compensate.

Today I get them back, and the new diaphragm measures DCR of about 2.5 ohms. I haven't installed these to test acoustic performance, since I haven't even built the cabs yet. But I am really questioning their statement and choice of parts. Thought i would consult the experts here before I go back to them.

Thanks in advance for the advice.

[4313B]

Here it is again:

D16R2420 5.9 - 7.0 ohms

D8R2421 2.9 - 3.5 ohms
D16R2421 5.9 - 7.0 ohms

D8R2425 2.9 - 3.5 ohms
D16R2425 5.9 - 7.0 ohms

[dougzilla]

Thanks, Giskard. I searched the forum and couldn't find the threads. Know this topic has been kicked around quite a bit.

My take on this is that I now have two mismatched diaphagms, do you agree?

[johnaec]

It sounds to me like they first measured your existing ones and thought no way would those be considered 16 ohm. I'd show him those specs Giskard posted and demand the 16 ohm versions you originally asked for.

I've dealt with A Brown Soun a lot and generally they seem to be OK, but you really have to be specific with them.

BTW - where in the bay area are you located? I just moved to Rohnert Park from San Anselmo myself.

John

[4313B]

Impedance curves for the D8R2425 and D16R2421 in a 2425 core.

[4313B]

Impedance curves with a 2344A horn load.

[dougzilla]

Here's an update on my 2420 diaphragm replacement situation with A Brown Soun.

When I called them to discuss the impedance difference last Monday, they quickly offered to order a new 16 ohm diaphragm from JBL and replace the wrong one ASAP. I called yesterday to see if the new one had come in. First they said it hadn't, then they said it had, and I could come over and they would install it on the spot. That's what I did, they apologized for the mixup and I left, reasonably happy.

I tested DCR yesterday and it comes in at 6.0 ohms, on spec for a D16R2421, but not by much. This morning, I thought I would have a look inside. Upon removing the cover, the newly installed diaphragm seems to be aluminum, but it has diamond shaped stiffening bumps around the outside edge, not the spiral shaped ones I know JBL used to use. Also, there is some sort of coating around the edge, which I don't recall JBL using in the past. Pictures are below.

In the group's considered, non legally binding opinion, IS THIS THE JBL D16R2421 PART, OR HAVE THESE GUYS TRIED TO PULL A FAST ONE ON ME BY USING SOME AFTERMARKET P.O.S.? Note I paid over $280 for the part and installation the first time around.

I sure hope this is genuine JBL, or the ZILLA part of my persona may emerge up in San Rafael....

[dougzilla]

A second shot.

[subwoof]

That is a real part. Quell the 'zilla...:)

I have some of them here and have ( blown ) examples going back to the 60's to show audio guys what the materials used to be...

Remember that in ALL of JBL's literature, they expressly mention "subject to improvement"

The old "spiral" ones went bye bye in the mid-80's. The diamond surround is more stable and has less distortion.

What you have is NEW adhesives, NEW dampening, NEW formers, TIGHTER tolerances ( done by computer controlled machines rather that chain smoking gossipy ladys on a friday aftenoon ) that ALL add up to a product that will **surpass** the original.

If you were to look at the " goes into" chart, you will see that most of the time it is suggested they be replaced in pairs to maintain imaging / performance in a stereo field.

And WHACK that guy for screwing up the DCR - anybody who went to the JBL reconing school ( mandatory ) would know that.

The DCR ( Direct current resistance ) is 1/2 to 2/3 of the impedance ( Alternating current resistance ) for most pro audio components BUT they can be VERY different for the exotic types!

DCR is measured at 0 ohms with any cheapo radioshack ohmmeter, ACR ( impedance ) is with an EXPENSIVE impedance bridge set to 1000Hz ( or so )...

Of course JBL doesn't require *any* of their service stations to use scopes, bridges or analyzers....

...sigh....

sub

[dougzilla]

OK, I can feel the flamebreath cooling and the tail is shrinking visibly. I'm glad, because the neighborhood doesn't look so good after I've been stomping around.

I knew that JBL is using the diamond shape on the Titanium diaphragms, but thought maybe they had stayed with original designs on the Aluminum ones. Progress happens.

I'll have to let my ears be the judge as to the difference in sound between the two types of diaphragm. Checked the other driver and it does have the older style that I had remembered. It looks fine, but I have to set up a test rig before I can report.

A slavish consistency being the hobgoblin of little minds, I will probably spring for another new one before too long, but the piggybank is about empty right now.

[subwoof]

Cool....

I was afraid if you let loose with the 'zilla I might have to unleash the dragon.....

sub

[johnaec]

A slavish consistency being the hobgoblin of little minds, I will probably spring for another new one before too long, but the piggybank is about empty right now.Maybe you can get them do give you one at cost, seeing as they put you to so much hassle. It'd be a shame to see a new, "more improved" model if you wait too long...

John

[dougzilla]

Well, if it's Dragons we're talkin', watch out, this bad boy guards the dark crystal (tweeter). Courtesy of my daughter, who appeals to his sweet tooth with the ice cream cone...

And John, appreciate the suggestion, but I'm not sure I want to push it with A Brown. I owe you a PM...