MOSFET Madness Strikes: Hitachi HMA-8500 Mk II

[wualta]

I love it! All this hidden Walsh-driver knowledge comes pouring out!

Mods: feel free to shove this over into the Speakers forum.

ARGH! Actually, it could have been saved, it was probably only the surrounds and centrator that were damaged, the rest is metal.

Not sure what a centrator is. Probably what we would term a spider, a circular folded treated-fabric suspension bit designed to keep the voice coil centered as it moves, but yes, there's a good chance Axel's Walsh driver could've been saved if the metal cone had come through unharmed.

But there was another 2-part Ohm cone:

This is the Ohm A, extremely rare. If you have one of these, you have something really special.

Ilimzn's right, though-- you're more likely to have a G driver.

The grayish is titanium, the silverish is aluminium. It's a Walsh-style almost full-range driver. It could probably also work in a sealed enclosure, but originals were passive radiator.

Yep. It sacrificed the coherence of the low end of the Walsh A and F and gained some efficiency and economy.

The angle of the cone is chosen such that the speed of the wave propagation times its cosine equals the speed of sound in air. This way, when the speaker is set to 'fire downwards', the horizontal projection of the wave is an exact analog of the acoustic wave in the air, creating a completely coherent cylindrical wavefront - in theory. Because the driver is relatively short, the wave looks more like a barrel, which is closer to what an ideal point source would create.

Excellent distillation of the operating principle. Hadn't heard about the barrel distortion of the wave. Assuming the wave can fit on the cone, wouldn't the wavefront tend to be cylindrical?

A pair of these speakers would, on a scale of 1 to 10 in the discipline of imaging, score about 12.

Interesting you should say that. Doubters maintain staunchly that Ohms create eight-foot-wide flutes and anamorphic Pavarottis. Could you give us some arguments pro?

It should be noted that the current 'walsh' drivers from Ohm are not really true Walsh transducers, but they are still bending-wave based.

Depends on how you define "true", but in a sense yes. Tweeters replace the titanium section (above 8 to 10KHz) and the vented box replaces the cardboard section. Their wild 360-degree dispersion has also been tamed and shaped to make them easier to place in domestic rooms.

AFAIK the only company currently making true Walsh drivers is German Physiks (at a very high price).

http://www.german-physiks.com/index1.html
True about the price, but perhaps they're not "true" Walsh drivers (or even Dick drivers) either, since a conventional cone woofer is used to produce the bass, from about 150 Hz on down. Beautiful workmanship, though.

You might notice that the driver has vertical slits and scorings on the membrane, as well as a few small pieces of stick-on kapton tape stuck on the inside. Do not remove them or disturb them, they are there to damp membrane imperfections. For the same reasons, carefully avoid any damage to the membrane, such as bends or kinks, as it destroys the ability of the driver to create a consistent bending wave on the cone surface.

Fascinating bit of information, and important for owners whose guests might be tempted to gefingerpoken. Thanks, Zeljko!
Now-- in the limited time remaining, tell us how you've come to be so intimate with the Walsh design, and am I too far off in thinking that bits of antenna theory have been applied to loudspeaker design in this case?

 

[ilimzn]

Impressive - mind-boggling I should say!

What happens if I plug only one? (since I only have one...)
Do I need two TA-N80ES bridged for phase + and phase - to feed it??

oh well, I guess I'll keep this G(ee) driver as... as... I don't now what!

A TA-N80ES should be able to drive one no problem. It is well known that to sound best, the Ohm F needed a truckload of power, unfortunately, it could not take as much power reliably
The G has slightly higher efficiency. If you want to try it out, check that the coil is ok. Not sure any more what the resistance is, I think it's about 4 ohms. Also, check CAREFULLY that the membrane moves freely (a few mm is enough, press on the outer edge of the surround, on two oposite sides, to create equal force). If all is well, you need some sort of a box, can or bucket, fill it with foam or something similar, if needed cut a hole for the down firing wave and set the driver on top. Hook up and play

 

[ilimzn]

Not sure what a centrator is. Probably what we would term a spider, a circular folded treated-fabric suspension bit designed to keep the voice coil centered as it moves

Yes, that is it, exactly.

But there was another 2-part Ohm cone... This is the Ohm A, extremely rare. If you have one of these, you have something really special. This example is owned by an AKer and the photo is his as well.

Compared to the Ohm A, spare VFETs for a Yamaha B1 are not rare at all
Last I saw, there are perhaps only a few pairs, not all complete, in existance. The Ohm A driver is quite huge, IIRC, even bigger than the F's.

Excellent distillation of the operating principle. Hadn't heard about the barrel distortion of the wave. Assuming the wave can fit on the cone, wouldn't the wavefront tend to be cylindrical?

Only if the cone had infinite height, this would be an ideal line source. That would be, however, speaking in terms of free space, whereas there is usually a floor and ceiling
A very small Walsh driver (shorter than the minimum wavelength produced) would make a good approximation of a spherical wavefront.
In pactice, however, due to limited length and a floor and ceiling, you get a lobed pattern, with an envelope resembling a barrel.

Interesting you should say that. Doubters maintain staunchly that Ohms create eight-foot-wide flutes and anamorphic Pavarottis. Could you give us some arguments pro?

Not in my experience. What they do create, is a sort of 'fisheye' distortion to the soundstage, it resembles a semi-circle. Unlike a regular speaker, the impression of a soundstage remains even in completely preposterous positions, such as inbetween the speakers, or off to one side, but the 'picture' is distorted, sort of like perspective distortion, on top of the 'fisheye' effect. Because of this, the positions may not be 100% accurate, but for a good recording, a small source, remains pinpoint small. Any pan work in the studio is IMMEDIATELY and rather blatantly revealed as spatial inconsistency, and this is how you get 8 foot wide flutes - probably separately miked and mixed in. In general, speakers that come close to line or point sources tend to reveal this. No such effect with live recordings at all. Also, these reveal imperfections in MP3s regardless of bit rate, so much so that a complete layman whio has never heared a comparison, can tell the difference.

Right. These are the ones with the tweeters replacing the titanium section (above 8 to 10KHz) and the vented box replacing the cardboard section, the ones that come in perforated coffee cans. Their wild 360-degree dispersion has also been tamed and shaped to make them easier to place in domestic rooms.

Actually, save for the supertweeter, you can ask Ohm to customize the dispersion somewhat.
The new 'walsh' drivers are paper membrbranes of a rather peculiar construction, the real science behind improved efficiency is in a different construction of the magnet. I've had the pleasure of investigating the innards of an Ohm G3 driver (shich is actually a similar one they currently use on the smaller Walsh series). It is not any sort of extreme high tech, but that can only be taken in it's favor, given the results it produces. It's easy to throw crazy money into something, not so easy to make it work with perfectly ordinary materials.

True about the price, but perhaps they're not "true" Walsh drivers (or even Dick drivers) either, since a conventional cone woofer is used to produce the bass, from about 150 Hz on down. Beautiful workmanship, though.

Actually, they are true Walsh drivers, the reason why they are not full range is the bending wave characterisitic, the speed changes. This is why Ohm A, F and G have different cone sections (A also has a different angle). GF basically optized their driver around titanium by limiting the frequency range it is used in. This is actually not a problem, since for low frequencies, for which he enclosure dimensions are small compared to wavelength, you automatically get omnidirectionality (in normal speakers, this is why something called 'baffle step compensation' has to be used). Ohm A, F, G and all the current ones, operate in 'Thielle-Small' mode under a certain frequency, so the walsh effect is not used full range, strictly speaking. For these lower frequencies, the orientation of the cone makes no difference (kind of like you can turn a subwoofer around in any way and get a relatively suimilar result). GF only uses a separate driver for these frequencies, because the DDD driver has semi-rigid cone termination, it does not have sufficient membrane excursion to work as a normal speaker for low frequencies.

Now-- in the limited time remaining, tell us how you've come to be so intimate with the Walsh design, and am I too far off in thinking that bits of antenna theory have been applied to loudspeaker design in this case?

Well, yes and no re antenna design but certainly the idea to use different paths for waves that travel at different speeds has been used before (like traveling wave tubes).
As to how I came to know all this, well, my very first, and epiphanical experience with audio was through an Ohm F - I was only in my early teens then but that one I still remember, and probably willa s long as I live.
I also had a pair of G3 for a while, and a few other lesser Ohm models pass through the house, spoke with the guys at Ohm many times too. Had the oportunity to see the GF driver in action, as well as the Manger. Speaking of unobtania, I also had, for a while, a single (GRRRR) Infinity Walsh tweeter... And... I also tried to make my own Walsh driver , a project which I will have to take up again some time in the future.

 

[wualta]

...for a good recording, a small source remains pinpoint small.

Interesting. Nice to have that confirmed. I have to say I've never noticed outsized instruments on the FRS-11s, just unfolded space around the instruments that are already there.

The new 'walsh' drivers are paper membranes of a rather peculiar construction...

Many of the neo-Walsh drivers use plastic cones. Do a Google image search.

Actually, [the German Physiks DDDs] are true Walsh drivers, the reason why they are not full range is the bending wave characterisitic, the speed changes. This is why Ohm A, F and G have different cone sections (A also has a different angle). GF basically optimized their driver around titanium by limiting the frequency range it is used in...

Calling anything "true" is usually semantic gymnastics. If we define a "true Walsh" to be one that holds to the original vision of a single full-range driver making waveform-accurate omnidirectional output over that range, people who call the new Walshes "not-true" would have to call the German Physiks DDD "not-true" as well. Having said that, whether there's an audible advantage to using the Walsh principle all the way into the high treble and all the way into the deep bass [foregoing the separate woofer of the GP design and the separate tweeter of the current Ohm line] is another question entirely.

Ohm A, F, G and all the current ones, operate in 'Thiele-Small' mode under a certain frequency, so the walsh effect is not used full range, strictly speaking. ... GP only uses a separate driver for these frequencies, because the DDD driver has semi-rigid cone termination, it does not have sufficient membrane excursion to work as a normal speaker for low frequencies.

Which makes sense. When I finally saw how flexibly the F's cone was mounted I confess I wondered why Ohm hadn't done somewhat the same thing, although with a passive radiator [like the G, which I discovered only later] than a separate woofer. The more rigid cone mounting would've eliminated the suspension sag problems and possibly given Ohm a better chance to properly terminate the energy at the cone edge. As it is, yes, the F goes into pistonic ("Thiele-Small") mode below some [low, but how low?] frequency and becomes a more or less conventional woofer. Which is fine, but very inefficient (due to the mass of the driver).

Well, yes and no re antenna design but certainly the idea to use different paths for waves that travel at different speeds has been used before (like traveling wave tubes).

I confess I haven't thought of TWTs since studying the first masers and lasers in the '60s. I think you've found the radar connection we've been looking for, though, since TWTs were and are used in radar work. For some reason, there's very little public information on Lincoln Walsh's other engineering work and nothing at all about his life except that it ended in 1969, so this is a piece of a very tough puzzle. Thanks again.

 

[caygees]

MOSFETs are the greatest for audio outputs, in my opinion much better than Bipolar transistors. Long term goal is to convert all output sections of all my amps to MOSFETs. The j49, k134 are no longer made, but there is a company that has bought the rights to manufacture fets with the same arcitecture. I think I still have a set or two of the 49/134 layin around here somewhere, new, unused. Durn things cost too much to use them now. Another addition to the growing collection of electronics stuff. I need to collect stamps, matchbox cars or something, all this stereo gear in my house, me and my dogs have to live outside cause that last set of Warfdales took up the only room left inside....