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2ax, 3a, LST TWEETERS


fedeleluigi

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>I would like to ask anyone who helped to design or worked

>with 2ax, 3a and LST tweeters these three questions:

>

>1)What material are the three red (or yellow) suspensions of

>those tweeters made of?

>

The material in the suspension of the tweeters is a urethane foam, with a clear butyl-rubber coating over the actual gap itself. On the top plate of the tweeter are three triangular slots. With jigs to position the voice coil and dome, a specified amount of two-part liquid foam was poured into each gap, then allowed to foam up to attach to the coil and the plate, "suspending" the dome in the gap. Once the foam cured, a liquid butyl-rubber was poured over the top of the gap between the dome edge and the top plate to seal the voice coil from dust and dirt, etc. Insofar as these drivers were hand-made to some extent, AR tested each and every tweeter very carefully, matching it to a lab "standard," and rejected any driver that varied more than a dB or so. They also tested drivers for distortion, another detail-production step which insured proper performance.

>2)What material is the stuffing behind the paper dome and

>3)what is its function?

In the AR-3 domes, i.e., the 2-inch midrange and the 1-3/8-inch tweeter, a specific amount of fiberglass wool was placed under the dome to dampen the driver, and the cavity under the dome. This material also caused pressure against the underside of the dome, and later a ring of fiberglass was placed under the protective screen to offset this, and to further improve damping. In the 3/4-inch dome, such as in the AR-3/LST/2a/5, etc., there was a small foam pad under the dome. This, too, was a type of urethane foam. This foam actually deteriorates or oxidizes, just like the surround materials, after many years. As the foam pad was used for damping under the dome, there is probably some very slight "measureable" difference in performance, but not likely to be any audible difference to amount to anything.

--Tom Tyson

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Thank you very much for all your answers.

I'm trying to fix one of these tweeters as closely as possible to original standards. I would like to know, if possible, more details about the stuffing under the paper dome. What material can I use? Must it touch all the paper dome? must it be joined by glue to paper and to the central magnetic pole?

What Kind of urethane foam have I got to use for the three suspensions?

Thank you in advance for every answers.

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>Thank you very much for all your answers.

>I'm trying to fix one of these tweeters as closely as

>possible to original standards. I would like to know, if

>possible, more details about the stuffing under the paper

>dome. What material can I use? Must it touch all the paper

>dome? must it be joined by glue to paper and to the

>central magnetic pole?

>What Kind of urethane foam have I got to use for the three

>suspensions?

>Thank you in advance for every answers.

Well, the material under the dome was a tiny little pad of urethane foam, and I think it was just large enough to stay in place without attachment; i.e., about the size of the dome itself. Unless you have a two-part urethane-foam dispensing device, you will be hard-pressed in replicating the original foam suspension. I have the specifications on several blue-line drawings, but I would have to search for it.

More importantly, it would be a gargantuan undertaking to try to duplicate the original production-foam techniques. Then, once you have put the foam in place, you have to apply the clear (non water-soluable) butyl-rubber compound around the gap. Then you would need to verify performance, etc., etc. You might spend a fortune trying to rebuild the dome tweeter; on the other hand, it might be quite a bit easier to try to locate some spare, workable, AR-3a tweeters on eBay (I saw some the other day) or from another source.

Repairing woofer surrounds is one thing, and really a great project for anyone, but repairing a dome tweeter is a much more difficult process.

--Tom Tyson

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Thank you very much again.

In these forum I also asked about the glue used by AR in AR 9, 10 Pi,11 etc. woofers to join the foam surround to the paper cone and to the basket. Unfortunately no one has answered. Do you Know what glue is it? I think that it is a poliacryilic sealant but I'm not sure.

Luigi Fedele.

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>Thank you very much again.

>In these forum I also asked about the glue used by AR in AR

>9, 10 Pi,11 etc. woofers to join the foam surround to the

>paper cone and to the basket. Unfortunately no one has

>answered. Do you Know what glue is it? I think that it is a

>poliacryilic sealant but I'm not sure.

>

> Luigi Fedele.

>

Luigi,

By the time of the Advanced Development Division speakers, there were several different forumlations and different specifications of glue dispensed from automated, air-powered, high-temp glue guns. The glues were somewhat like the hot-melt glue used to mount the grill frames to the speaker cabinets in the pre-ADD AR speakers. The glue went on in a whitish tent, and cured quickly to a clear color. The glue was compliant, yet strong. I have the formulas somewhere in some AR blue-line drawings, but I can't put my hands on it now; besides, it's academic anyway unless you are going into production.

As for attaching new surrounds, you can use one of the water-based, but water-resistant white glues that dry in 20-to-30 minutes. This gives you opportunity to position the cone properly on the cone sides and the metal speaker basket. Probably Carpenters' glue or something of that sort would work, but something more plyable once set would be better. Better yet, Madisound and others sell glue that works fairly well for this purpose.

--Tom Tyson

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