Recap tweeter info

I'm still enamored of the Dynaudio D21 AF above all, even the various Esotars.

These are very hard to find, and even used and 40 years old command prices like 330 Euros.  There has not been one on US Ebay in awhile.

Anyway, needing something at least for the back if not the front (either having D21AF or ribbon in the front) I investigated other tweeters, and wrote these comments to a friend:

I thought I would like "wood" but having the LS 3/5A on one side of the Acoustats is, I have decided, too "woody."  Also it blocks too much of the airspace, now that I understand that diffraction is unimportant.  The smaller the blockage the better.  Though I also just bought a $249 32 inch stand with 4 heavy sand fillable pedestals (that's cheap for something like that).  Now that I'm no longer thinking necessarily to have the elacs or the LS 3/5A's on it, a slender steel rod attached to a very heavy base would do, if something like that could be attained.
These Vifas are the best thing I've seen at Madisound as an inexpensive super tweeter.  Not perfect, just the most extended HF response in a cloth dome.  The usual D21AF replacement suggestion is the Eton 19SD-1
https://www.madisoundspeakerstore.com/eton-soft-dome-tweeters/eton-19sd-1-%C2%BE-soft-dome-tweeter/
That costs twice as much, is already rolling off at 20kHz, is only curved to 30kHz (though it looks as if it could extend to 40kHz with less than 10dB loss, which may be all the Vifa does, but I suspect it collapses right above 30kHz instead).
Nobody seems to want to build a supertweeter cloth dome, but it looks eminently doable, just as good as the Dynaudio D21AF.  What you need is a flat plate, no inward curvature, and a small dome with good damping.  The Eton above has the usual trick inward curvature, which captures some air mass and hence lowers the resonant frequency of the tweeter, making it more useful in 2 and 3 way box systems.
Almost everyone does that always.  When they don't have as much inward curvature, they apply lots more damping, including ferrofluid, like this scanspeak:
https://www.madisoundspeakerstore.com/scanspeak-soft-dome-tweeters/scanspeak-classic-d2010/8513-20-mm-dome-tweeter/
Both of these however do have smoother looking curves, they just start falling off too much for a super tweeter.
The Vifa does a trick I don't really like that much, but it works.  It does really have a kind of ring, a large surround bulge.  So what happens is the inner part vibrates at "lower" frequencies, up to (one can guess, looking at the response) about 25kHz.  That gives the tweeter excellent performance from 10-20kHz, excellent dispersion (even if you don't want it, most do).  Then what happens is the surround takes over at 25kHz and extends the response to 40kHz, because it has less mass, the resonant frequency is higher.  The dispersion collapses at that point like a rocket because there's a wider radiator...but it's still only 3/4 inch.  The 40khz response may be a kind of accident, they didn't care, they just wanted the excellent performance (in dispersion up to 30 degrees) up to 25kHz, which comes from that bi-radial kind of design.
Anyway, it seems to me now any of these highly capable manufacturers could build a real D21AF nearly flat 3-40kHz equivalent they just don't want to, nobody cares, the high end super tweeter market it dominated by ribbons and AMT's as cheap as $20...but have their own, usually hidden, issues of metallic resonance (hidden best when brand new).  Reading DIYAudio I hear one trick ribbon makers often use is to hide scales so you may not really know how flat they are.  And then they extremely rarely show waterfall plots (Raal does, and looks amazingly clean, but you can still see dots at the metallic resonant points).
I've discovered one essential problem is that ribbons can exceed their own centering force, and become increasingly bent, which I don't think is good.  When you get bends you start to get internal reflections, resonances, and so on.  The restoring force is lost beyond a certain point, which seems often to be exceeded.
Now it may be that some of the better ribbons, like the Raal, actually use a non-metal suspension on both ends.  It's not totally clear from reading their info that is exactly what they do but I think so.  Then you have the problem of low mass wiring, for that they famously use circuit board trace.
I'm still thinking about that possibility.  I may try to sell my Raven 2's before I damage them myself.  They'd be better in a certain kind of multiway system, since they do have pretty good response down to 2kHz or so which is amazing for ribbons.  There's one on ebay now for $680 (I bought mine for $250 IIRC).  But that's not what I want.

Actually, I was wrong about the Vifa's.  Vifa shows a very high resolution graph in which even 1dB response variation looks large.  They are not quite as flat 10-40k as the D21AF, but not all that unflat either, not much different (though indeed slighly less "flat" but far more extended than) the Eton and ScanSpeak.  The Vifa response is quite respectably flat compared to most finished speaker systems.

(In both the Vifa and D21AF I detect a double upper resonance in the frequency response, rather than the usual highly damped resonance.  In both cases you can see 1dB peaks spaced fairly close together, in the case of the Vifa it's 21 and 23 Hz, for the Dynaudio slighly lower.  I suspect this is the surround resonance in both cases playing the role of response extender.  The upper resonance probably also pulls the resonance below it upwards.  I would generally prefer no resonance, but the sad truth is that all tweeter materials have resonances, and typical metal resonances (I understand beryllium and diamond are fine...but extremely costly) are the worst sounding, and oiled cloth resonances the least offensive.

Which was an extension to an earlier message

After more pondering, I decided to get 2 Vifa 3/4 inch domes.  If I really like them, I might get to more for dipoles.  Otherwise, they might be fine firing backwards from something better, like ribbons.

https://www.madisoundspeakerstore.com/vifa-soft-dome-tweeters/vifa-ne19vts-04-3/4-silk-dome-tweeter-4-ohms/

Incredibly nice for the $33 price, response to 40k and decent dispersion at 20k.  About the closest I've seen to D21AF, and the only D21AF I can locate now is in Italy and the seller wants 330 Euros for a 40 yo used unit.

I've decided I don't like ring radiators at all.  The Mundorf dipoles have limited dispersion and I've always been suspicious of AMT (and they use metal also).

Linkwitz has studied diffraction and determined it's less important than many people think.

http://www.linkwitzlab.com/diffraction.htm

Since I'm crossing over at 20khz, where even this driver has very limited response at 60 degrees, 90 degrees is moot.  I don't even have to bother with a 3" square baffle, no baffle at all will do fine.  I think resonance (such as from a wood baffle) is a more important issue than diffraction, and if I use no wood, there will be no wood resonance.  (I might use heavy wood and bolts to attach the speakers to the stands though, then taped with hockey tape).

If I use a ribbon, I could tape the exterior box with hockey tape also, no need to build a fancy wood enclosure (which I'm basically incapable of doing anyway).

And that was a follow on to an earlier message, which was also somewhat repeated here before:

I've been studying super tweeters.  The unused Raven R2's I have are well respected, were used in some very pricey speakers too.

But they use a relatively thick foil, which means they could have metallic resonances.  9 microns.

Magnepan uses a 2 micron foil, maybe better sounding but also easy to break.

A newer generation of Ravens includes a "point source" tweeter with 5.6 micron foil, which is about the best compromise.  But it's still corrugated foil.  Price is $400 each.  Other than the foil, the build quality of Raven tweeters is monumental, all the exterior parts are 1/4 thick or thicker metal.  It would look beautiful on the outside of a pricey speaker (but probably should be inside).  Response to 50khz.

A newer company Raal uses embossed flat rather than corrugated foil, also 5.6 microns, that sounds best of all, and they have a point source model also, and an upgraded one with amorphous core transformers for $400 each.  Looks are very plain compared to the Raven, but if I were buying one now, that is what I would get (and I might).  Response to 100kHz, just like the old Sequerra T1.

It does worry me now about all aluminum ribbons that they may have metallic resonances.  When brand new and tight, perhaps not, but as they get older and looser the metal can flop around, as has happened to my used and mistakenly abused Elacs.  Audible resonances can be stimulated by ultrasonic higher frequencies, it seemed to me that was happening with my left Elac tweeter.

A plasma tweeter was being sold as recently as 2000 for $2500, was used in the Acapella $120,000 speaker.  It generates plasma electronically using some kind of RF circuit.  No helium, etc.  Still seems a bit risky to me, even if I had money to burn.

Few soft dome tweeters have the extended high frequency response of the Dynaudio D21AF's I bought around 1979, almost flat to 40khz.  I'm still considering using them instead of ribbons.  But in that case, I would get a second pair for the backside, or something else for the backside.  They have been almost unobtanium for decades, hifishark shows one for sale in east europe.  Dynaudio's most prestigious Esotar tweeters do not have extended ultrasonic response (and they're also unobtanium mostly, except there are now clones).

I do think it is essential to have extra super tweeter on the back side, or omnidirection, or dipolar.  I can hear the difference in current setup.

Other decent dynamic drivers are berylium domes and diamond domes, but extremely expensive!!!

Metal domes and we're back to dealing with resonances again.  Beryllium is special because of extremely high resonance, like 60khz.  Diamond tweeters go to 100kHz, but now we're talking prices like $10k.

Among the soft domes, the most expensive drivers, such as ScanSpeak Illuminator, have only so-so ultrasonic response, if any at all.  Most designers don't seem to care about the ultrasonic response.  The cheaper ScanSpeak Discovery have better ultrasonic response in the smallest sizes.

To get decent response, very small domes 3/4 inch and smaller, and/or ring radiator style, can get to 30-40khz with some rolloff.

So one idea might be to combine D21AF on the front and cheap ring radiator or ScanSpeak Discovery on the back.

Or Raal supertweeter on front, with D21AF on the back side.

Or Mundorf AMT (air motion transformer) which is dipolar, light, and could be attached to the Acoustats.

I might also enquire if Elac can fix my supertweeters.  That would probably be more expensive than getting brand new Raals, if it could be done at all.

The Krell was shipped back to factory this week.