ReliaBill Engineer
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Posts posted by ReliaBill Engineer
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I clamped the dome down flush with the steel mounting plate. I used that steel washer with 3 legs soldered to it. The magnet clamps the washer down. A magnetic jig, if you will. Then I applied the red butyl at the 4 points. Let it cure for an hour. Then thinned the clear butyl so I could apply it with a brush. Now letting it cure overnight.
Once cured, when I release the jig, the preload from the batting should push the dome out, giving it clearance to move when a music signal is applied. Fingers crossed…..
Thinned butyl sealant:
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I got the gap cleaned and the voice coil cleaned. Pretty obvious that AR allowed the glue to get into the gap and on the voice coil, pretty much glueing the foam at the 4 points and on the coil windings. Very little clearance between the center magnet and inside edge of the coil.
Besides all of that, the stack of polyester batting inside the dome puts a VERY substantial preload on the dome. I put a HEAVY steel washer on the dome, and it isn’t enough to force the dome and voice coil down into the gap!I tried a 1.2 volt battery on the leads, to center the voice coil…. It expelled the dome with tremendous force! I won’t do that again.
The batting:
Using the heavy steel washer. I need a biggerweight!
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Anatomy of the early 2ax Dome Tweeter:
This dome tweeter has a unique build. The cloth-phenolic dome is light, although heavy by modern soft dome standards. It uses enameled aluminum wire on the voice coil, which is why you don’t see solder on the brass binding posts below the tweeter’s mount; the wires are twisted on the posts.
The suspension assembly is very unique. When in the voice coil gap, the stack of batting inside the dome supplies an upward (outward) spring pre-load to the dome. Of course, it also damps vibration and kills sonic energy inside the dome. The 4 liquid-foam deposits (yellow, applied as a foam liquid during assembly, then allowed to cure) anchor down the dome, against the pressure of the dome’s internal batting. The faceplate has 4 cutouts for anchoring the foam away from the voice coil. Then, a thin, sticky, clear elastomer sealant is applied on top of the foam, and on both the perimeter of the dome, as well as on the perimeter of the face plate. This elastomer sealant is allowed to cure. When all is cured, the dome is suspended by the batting that pushes the dome outward, and held in place by the 4 dollops of yellow foam and the clear elastomer membrane.
Below, the polyester batting that is compressed below the dome. It provides damping and spring force outward:
The four foam cutouts in the faceplate for anchoring the liquid foam:
Below, the four foam suspending anchors to “float” the dome:
Below, with the clear elastomer sealant applied, that forms a dust seal, and a resilient retaining membrane, to allow the dome to achieve pistonic motion when a music signal is applied.
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I spent last night formulating my plan for repair, the sequence and materials I’ll use. I tested using a silicone membrane for the sealant. Also the 4-point suspension. Hopefully I can get this done over the weekend.
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So now I feel I have no option except to remove the dome. So I did. I’ll have to carefully clean the windings and the motor gap. Then put the damping back in and replace the suspension and sealant.
Sealant removed:
The dome freed:
The damping batting inside the dome:
Dome completely freed:
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Pretty sure I figured out the problem. I don’t think my ears were lying to me. Output is greatly reduced. Looking closely at the tweeter dome, it was messed up at the factory 58 years ago.
In these pics, on the right side vs the left, the voice coil is wedged in the gap; halfway out on the right side and flush on the left, and glued that way. Gently pushing down, there is no movement.
Below, VC windings visible near leads:
Below, windings visible:
Below, other side of dome is flush with mounting plate:
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20 hours ago, RoyC said:
AR is known to have used butyl and (later) latex rubber compounds for this type of purpose...but the type of compound is irrelevant. These were manufactured in the mid-60's, and AR was the first to try various rubber and foam formulations.
If you scrape it, you will find it to be elastic and not "rock hard"...unless somebody applied some kind of additional material over it at some point. With that said, there is no doubt that it has stiffened with time, and the primary problem has as much to do with its initial degree of migration into the voice coil gap. If you are able to get it started in a "safe" area of the dome's perimeter, you can literally pull and peel it out. I have worked on many of these tweeters and have done this countless times.
Before doing anything, try listening to the tweeter with no cap in line vs with. You will likely be surprised at the subjective difference. The low DCR of this driver is as instrumental in its reticent response as the material around the dome. Unlike modern designs (like your Polks), these tweeters are only meant to enhance "sss" sounds, and little else. Your impressions are not unusual for folks new to early AR-dom.
I'm sure you can improve the response with your plan, but don't expect it to be a dramatic difference. Modern speakers simply have more upper mid and high frequency output.
Roy
Roy, you are correct in every aspect! The sample that I scraped from my tweeter was a drop of grill glue that managed to get dropped near the tweeter perimeter, just on the edge of the elastomer sealant. Boy! Do I feel embarrassed!! I was being *too* careful in my “sampling” of the sealant.
Yup! I am learning much about AR-dom here.
So….in your trials and travels, are these phenolic domes capable of more output by modifying their suspension, or renewing it? Or in other words, what is your experience in their change in output after *you’ve* changed out their suspension?
I haven’t yet molested the original tweeter. I’m being very cautious…
I’m going to attach a 1-minute clip of the speaker here. Only the caps replaced and pots cleaned. Tweeter at full output on its pot, midrange at 65% on its pot setting. I move my phone’s mic to within 1” of the tweeter. You can hear the tweeter’s output. Playing Bert Kaempfert’s “Wonderland By Night” from the 1960 Stereo LP. _________Everyone let me know what you think. Does this seem normal for this tweeter?
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2 hours ago, JKent said:
Hmmmmm. Roy said the tweeter crosses over at VERY high frequency. Bill said the tweeter output is woefully inadequate. I know my EARS have become woefully inadequate at very high frequencies as I age. Maybe it’s not the tweeter 😉
I tested the “ear” theory last night. Like you said, I needed to verify whether my ears were fooling me, or the tweeter output was low.
Using a frequency generator app on my phone, I injected signals from 1kHz to 15kHz into my AR speaker. At 8kHz the sound from the tweeter was barely audible. At 9kHz and above, the sound disappeared.
So I rotated the balance knob on my amp from right to left. Amp is in mono mode, to ensure L and R speakers see the same signal. AR speaker on the right channel, my Polk on the L channel. As I went up in frequency (1kHz steps) from 1kHz to 15kHz, my Polk was very audible up to 13kHz, the limit of my hearing.
On the AR, the XO does seem to be at 5kHz, as I had output from the tweeter. But its output dropped precipitously after 8kHz.
This should be an interesting project going forward. My wife loves the Polks, the 1989 RTA-8T, and is losing patience with my fiddling on the “old” AR speaker. What she doesn’t understand is that I hear greatness from this AR. But it can’t be realized with the tweeter not producing in the highest octaves. I know they’ll sound great once I get the tweeter working to its full potential. Without the upper octaves, the speaker will suffer from an apparent lack of “air”, detail, and balanced sound.
Edit: As an added tidbit, my father’s 1966 Wharfdale W70D speakers were his “cat’s meow.” He was very proud of them, bought new in ‘66. But they suffered from a beaming quality, and a sizzling paper cone tweeter with a felt surround. Already these little AR speakers have a promise of a more open, more spacious and detailed sound. My dad is still alive. In talking to him, he said he was sold on the “British” sound, and his Garrard turntable with Shure M44 cartridge. He considered the AR speakers, but got a “good deal” on the Garrard Lab 80 table and Wharfdale speakers….
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Thanks for the heads up!
Replacing the tweeter would be an absolute last resort. But if I should, I have several great candidates. But the impedance would require a new XO design, or at least a new value of capacitor. The tweeter output, as is, is woefully inadequate. It’s very restrained, even at max setting.I had already known the heavier dome requires a stiffer suspension, and at 5 kHz+ the displacement was small. But I doubt the clear sealant is butyl. It’s hard as a rock on my tweeters. Butyl rubber doesn’t usually fail in that mode. Butyl rubber is what I’m going to use for the 4-point suspension. But a softer, more compliant mix. I may try a compound which uses micro encapsulated silicone grease; it has compliance yet damps extremely well. I’m going to try a thinned clear silicone for sealant. Membrane thin. It’s stable and won’t harden. (I used to work for Goodyear Tire and Rubber as a technical engineer with the compounding engineer) Butyl rubber is used for the inner case of tires; it’s stable and doesn’t react to oxygen or nitrogen, it’s heat stable, and is stable under constant flexing.
I’ll do before and after vids of this track. This is before:
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My backup plan is to replace the tweeter. I’ve got many in inventory to choose from, including a couple of Dynaudio 28 mm soft domes.
But, I’ve done a lot of small work like this before. Fact is, my rebuild of a Shure VN35E stylus, and an (Micro Acoustics) MA 309, and Shure VN5MR and VN5HE styli was more demanding than this.
Of course, I’d rather keep these original tweeters. So here’s my plan:
Using a 10x loupe goggles and powerful LED headlight, use a razor to cut the sealant around the dome, except for where the yellow blobs are. Scrape away the sealant from the base of the dome, but not ON the dome. Then apply my own elastomer “blobs” next to the original yellow blobs (X 4). Let it cure. Then cut away the yellow blobs. Then apply a clear sealant I’ll make from thinned clear silicone. A very, very thin coat. Let it cure. This way I won’t come close to the VC windings, nor mess up the centering and spacing of the original dome or windings.
Im actually a little surprised by how non-precise these domes were originally assembled. The yellow blobs are all different sizes, and the sealant looks to be painted on by hand.
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Perhaps I’ll have to be the pioneer here…
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For an early model 2ax, I’m looking at these phenolic domes. The domes are relatively heavy, compared to a silk dome, treated silk dome, or thin polyamide dome, or even a titanium or beryllium dome. So I expect a stiffer suspension for this heavier dome. But these well-aged dome suspensions seem to have hardened and stiffened beyond their initial compliance.
In looking at the suspension, it appears the 4 yellow globs are both a suspension and damper. Then over the top of it is a clear flexible sealant, that has hardened. The voice coil is attached to the underside of the dome, in the magnetic gap.
Am I correct? Anyone in here willing to share their knowledge here?
Im pretty sure that my tweeters are being restricted in motion. While I’m getting output, I don’t think I’m getting the full output that was available when new.
I’ve rebuilt many vintage cartridge styli, from cantilevers, to tips and suspensions. I’ve also rebuilt tweeters using various parts I’ve found on eBay. I’ve formulated various room temperature curing elastomers that I use to form new cantilever suspensions for cartridge styli. I’m wondering if I should attempt to rebuild these tweeter suspensions.
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Yikes! No wonder I had it wrong!
The mid and tweeter are breaking in nicely. I’ll give them some time before making a decision. But it would be nice to know what the options are.
I have to say that from what I’ve heard so far, they sound better than my dad’s 1966 Wharfdale W70D speakers.
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Thank you for correcting me on both issues. I was under the impression the AR2 was from 64-76, the ax version from 69-76.
Any info on the tweeters?
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I’ve been working on these speakers for a couple of weeks. I see there are different sub-models from 1969-1976. Can anyone tell me where mine fit in?
Also the tweeters sound good, but can they be rebuilt?
Ive already cleaned the pots and replaced the caps.
A couple of sound vids. Just doing 1 speaker at a time. The other hasn’t been corrupted, grill has never been removed:
https://youtube.com/shorts/hPa7-tz-ujY?feature=share
AR2ax (Early) Tweeter Dome Suspension
in Acoustic Research
Posted
Looks a tad messy. But it sounds really good! Clear, focused, dynamic, well damped. Loud, too! Plenty of tweeter sound at 50% pot setting.
I decided to secure the leads with black RTV silicone.
