Wednesday, July 26, 2017

Super Tweeters

I've long been a believer in super tweeter magic.  I've been enthralled by all the systems featuring super tweeters, from the MQD with Decca Tweeters which I heard long ago, to the Hill Plasmatronics which I heard many times, to the Iverson corona massless speaker I've only heard about.

Once you step down this path, as I did around 2011 by getting a pair of Elac 4PI's in slightly used condition, you end up spending more and more time keeping it going at the same or higher levels.  And if you've got a critical sense, you also do begin to wonder, sometimes, if it's all a waste of time, effort, and money.

Hardly anyone in blogs I've looked at is even aware of the Elac 4PI super tweeters.  No doubt this is mainly because of rarity and high price.  But I do wonder if they are as good as the best of the others anymore.  Now that I've determined that one unit is not working good anymore (most likely due to an accident in 2011 which resulted in very high power being blasted into them by a 500W amplifier I was using just because it was the best spare amp I had on hand), I'm not sure if I'd spend $2000 for a new pair, or whatever it would cost to fix them (which might be impossible, but in any case not cheap).  I bought the used pair I have as an impulse, just seeing them on eBay for $995.

At the time, I knew of little other than the Sequerra T1, which also sells for thousands of dollars, even used and needing ribbon replacments (T1 foils are apparently relatively easily replaced).  But now I see a world of other options in the ribbon category (including Raven and Raal), and many other categories (AMT, plasma, high frequency domes and ring radiators of various kinds, even very HF electrostatics).

But it appears a 14kHz resonance is a very common problem among ribbon tweeters when (1) the aluminum looses tension and "flaps around", (2) the aluminum falls out of the magnetic gap and isn't being controlled by a continuous magnetic fields, (3) the aluminum was too thick or improperly corrugated.

That is one advantage of the "fake" ribbon tweeters that have aluminum bonded to a plastic membrane.  The plastic membrane (and adhesive) does provide some damping to the aluminum so you are unlikely to get the metallic aluminum resonant signature (which is very nasty sounding) even when everything isn't 100% perfect (as things with more than a few days heavy usage may not be anymore--and you might not be aware of it at first).  On the other hand, then you get a plastic resonant signature.  The general "consensus" if there is one is that pure aluminum is better.  It gets way more respect anyway.  But many of the popular ribbons like Founteks are the bonded type.  Generally the real aluminum ribbons are more expensive, like the Sequerra T1, and the Raven R1, R2, R3, Line Source and Point Source, and the various ribbons made by RAAL.  (I just discovered the RAAL's as I am updating this entry, and they look like they might be the best of all, they have 100kHz bandwidth like the Sequerra, and uniquely they used embossed rather than folded foils, thinner than many others also.  One poster at DIYAudio says that foils thicker than 5 microns sizzle to his ears, and below 5 microns there is enough material damping to resist that.  The RAAL's are 5 microns, as is the new Raven Point Source, but the earlier Ravens may have been 9 microns.  Household aluminum foil varies from 12.5 to 25 microns.)

I happen to have purchased but never used a pair of Raven R2's, and from what I read now (it was a spur of the moment ebay buy for me) there are indeed highly respected, certainly the build quality is top notch (reminding me of the far more expensive Sequerra).  The spectrum plots I've seen show no trace of a 14kHz resonance, and the response goes to 50 kHz.  The Sequerra apparently uses an even stronger magnet, by legend it was from a defunct particle accelerator, and goes to 100 kHz.

I'm thinking of using the Ravens real soon now, but I also like my Dynaudio D21AF tweeters.  They have response to an astounding 40khz according to published specs and graphs.  I've always thought this tweeter sounded effortlessly transparent, which is why I used it in my modified LS3/5A which has been unused since 2005.  I still think so, and I wonder if it isn't a competitor to the ribbons.

Sadly soon after I bought my pair, Dynaudio quit making the D21AF and stopped selling raw drivers as well, only making extremely pricey box speakers.  I am not sure if they ever sold their most legendary tweeter to speaker builders, the Esotar 330D, priced about tenfold higher than the D21AF.  But even the Esotar doesn't seem to have the fully flat and extended to 40kHz high frequency response of the D21AF.  I took a long look at tweeters today and it seemed only a few had response extended beyone 30kHz.  Typically the most expensive tweeters have the least extended response, showing how unimportant that is to most builders.  Many of the lesser expensive tweeters have the most extended response.  The cheaper Discovery tweeters by ScanSpeak have more extended range than the pricey Illuminators which have the ugly projection.  I found a number of other good looking high frequency extensions below $30.  Ring radiators look to have good high frequency extension generally, but there are other choices with similarly good high frequency extension.  Generally one looks for the smallest domes or ring radiators.  Metal, ceramic, and diamond drivers also have good extension, but at much higher prices, and wrt the metal drivers at least I worry about resonances.

The soft domes that have extended response, like my D21AF, must be flapping around under their own mass (NOT the air resistance primarily, I believe) at higher frequencies.  But it seems like it hardly matters that much, distortion continues to be low, etc, and there seems to be little HF resonance.  A "floppy" soft dome tweeter "works" well enough.  Trying to make a hard tweeter immediately creates difficulties.  The earliest domes were harder plastic materials, and those were all unsuccessful because of internal resonances.  This would amuse Lao Tsu.

Meanwhile I worry about when and where an aluminum ribbon actually bends or how "surround" materials give to allow the ribbon to move.  The ribbon story is not so clear cut when you think about the suspension and how that works.  Or diffraction.

In principle, the lower mass and higher velocity ribbons should be the best super tweeters.  But in practice, all the details count, and possibly the domes are still more refined in many ways.

I knew of the Hill Plasmatronics in 1978, and heard many stories about the Iverson Corona speakers, but I hadn't realized that plasma speakers have been made more recently.  The Acapella Plasma is one of the most well known, and was once available for $2500.  The plasma is generated electronically.

I imagine a significant problem with aluminum ribbons: when there is no current flowing, there is no restoring force on the speaker.  Conventional dynamic drivers have an elastic restoring force which always returns the speaker to the center position.   While Dick Sequerra said that ribbons are at least as reliable as dynamics...I wonder about that.  I've seen lots of ribbons bent out of shape, especially Sequerra ribbons.

It seems to me now that if there is *any* visible deformation on a ribbon speaker, that is a significant flaw which could impact performance.  At least for the deformed part, it is not properly within the magnetic field, and the material junction between the deformed and undeformed part could cause internal reflections and resonances.  The bottom line is, just like with any other speaker driver, any visible damage is serious damage, and there could be other damage that isn't visible.  And with super tweeters, since it must process sound which can't be directly heard, it's problematic to determine if there is damage by listening also.

So maybe ribbons are not the ultimate great idea after all.  There are some other possibilities I hadn't mentioned so far:

The Sopranino electrostatic super tweeter (8kHz - 40kHz) looks interesting, however very expensive, and despite the inherent dipole nature of electrostats, it doesn't seem to be intended as a dipole, since it has all the transformer and stuff behind the driver.

The "Walsh Tweeter" is arguably not a real Walsh and it has no termination and faces up rather than down.  It was first used by Infinity, and only later by Ohm.  They haven't been made for a long time, though there are a few people who have made similar units and/or claim to repair them.  Since it involves using a very thin rigid foil in order to work something along Walsh principles, but isn't terminated, it seems like an open invitation for resonances, just as with ribbons, if not more so.

Piezo's and polymer tweeters?  Piezo's have always sounded nasty to me and I wouldn't want one as a super tweeter either.  Chemical action isn't necessarily as fast as electronic, and the electro-polymer based "super tweeters" on the Pioneer HPM 150 and 1500 were not noted for the best sounding transients.

Mundorf now makes a whole line of AMT type supertweeters, small enough to be nailed onto the frames of my Acoustats.  AMT's have a huge advantage in not requiring transformers and naturally permitting dipolar operation.

Fans of ceramic and "diamond" drivers are very suspicious about ribbon resonances and distortions.  They also point out that the "speed" doesn't entirely depend on mass, it's F/M.  Dynamic driver motors can produce enough force for basically anything.  The problem getting HF response out of dynamic drivers first comes from breakup resonances--which can be fixed with high strength such as ceramic or diamond domes--and secondly from voice coil inductances, which can be reduced by using less coil and higher strength magnets.

While I believe in and of itself an omnidirectional radiating pattern would be best...or best yet omni but notched out where it would otherwise "fire into" the side of the main speakers...and I'd even say this with dipolar main speakers...dipolar dispersion might well be good enough, and eliminates a lot of technical issues as well as giving many more choices.  (I only know one maker of omni ribbons--Elac--and no other omni supertweeters are worth considering...the Infinity Walsh isn't a true walsh since it is not terminated.)

I remain impressed by the textile domed D21AF I have.  I remember when I bought them in 1979, IIRC I had planned to buy more than two because I thought the price was only $29, but actually that was for the D21, the D21AF were $49.  I was outraged that I had misunderstood the ad (the "response to 40kHz and $29 price were not the same unit) and they were that expensive.  $49!  I had driven all the way from San Diego to LA to buy them.  I pondered whether I could be happy with the D21's and decided I couldn't be happy without the 40kHz response.  So I got a pair of D21AF's, more angry about the higher price than anything.  It did dawn on me about a year later (or was it the next weekend, I can't be sure now)...I should have bought as many D21AF's as I could.  I drove back to LA, went back to the same store, but D21AF's were long gone.

Now I see that the D21/2 was about the same, supposedly an improved version, still having the essential 40kHz response (though, the specs only say 30kHz).

And, surprise surprise, there *is* a close to equivalent speaker still made and sold by Dynaudio today.  It is the Esotar 110.  Performance looks very similar to D21/2, and there's ferrofield cooling.  It's probably a more rugged speaker, not that I need that sort of thing (my D21AF's are, as far as I can tell, perfect, despite having been directly driven by power amps for 20 years in crude biamp setups) now for 19-40kHz duties.  I'd prefer not to have the ferrofluid, actually, but I'd get some of these except for the new price I see: $1399.99.  The Esotar 110, though it has extended hf performance is actually the larger 28 mm size like the old D28, with more setback, and I'd prefer a D21AF actually.

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