[wualta]

Yes, it's '70s isodynamics time again. So far I've found only one 'phone of this type that in stock form was ready for the future Head-Fi market, and that was the 1978 North American (NA) market Fostex T50, but it was expensive, had limited distribution and was made in low numbers, at least for the NA market [long story-- we'll tell you in the big Ortho thread]. All the others I've come across so far have needed extra mothering to sound the way they should, which is, usually, just this side of a good electrostatic. What would happen if Matsush'ta, one of the world's biggest electronics/electrics combines, got into the act? Surely they'd bring something new to the marketplace.

Turns out they did. Thanks to HF member tyre, behold the EAH-830:

It was announced to the world impressively in an article in the May 1978 AES Journal: AES E-Library: Linear-Drive Headphones with Eardrum Response by Sakamoto, Naraji; Kusumoto, Syoichi; Matsumoto, Takashi; Hasegawa, Mitsuhiro
How does it differ from the Fostexes and the Yamahas, all of which arrived in '77 or the year before? First, inside it's square. Don't be fooled by the round damping pads.

This is the back of the driver assembly:

When that is unscrewed, you have, below left, a square serpentine style diaphragm between square magnets. Below right, the diaphragm of the dreaded 2002 Fostex T50RP. Eerie resemblance, huh?

Below left, a closeup of the aluminum "voice coil" tracks. Note the embossed circles to stiffen the center of the diaphragm, presumably so it doesn't "dome" as high in the center, thus allowing the magnets to be a little closer together. Note also the black felt buttons in the center of the magnets-- do these gently clamp the center of the diaphragm or just act as soft bumpers for excursions that are beyond respectability? Below right, a scan of a naked EAH-820 diaphragm with its dazzling streamline moderne appearance. Looks like part of a Radio City bas-relief. Thanks to new member Icsaszar for the scan.

[size=xx-small]DETAIL OF ABOVE PHOTO.[/size]

[size=xx-small]SCAN OF EAH-820 DIAPHRAGM SUBMITTED BY HF MEMBER ICSASZAR[/size]


Cool, huh? Looks just like one of Infinity's EMIT tweeters, miniaturized. The way it's all put together is unique. Tomorrow I'll add some more bla bla bla and we'll dig into it.

 

[tyre]

Interesting, I had no idea that Technics was such an innovator, I was expecting something more along the lines of a typical iso driver. I've previously searched through the USPTO website looking for any patents related to Technics' "linear drive" but came up empty handed.

 

[facelvega]

Wow, self-defrosting. Nice one. Hey, Wualta, I'd really appreciate it if you'd hazard some comments on the comparative sound quality on your many orthos. I realize it's a quicksand place to venture on such wonky-sounding old headphones, but it's damned interesting.

And yes, I'm working up some comments on my two pairs of orthos complete with detail photos, so stay tuned.

 

[Carl]

Quote:

Originally Posted by tyre Interesting, I had no idea that Technics was such an innovator, I was expecting something more along the lines of a typical iso driver. I've previously searched through the USPTO website looking for any patents related to Technics' "linear drive" but came up empty handed.

Matsu****a made a number of odd headphones back in the day. The EAH-80 electrets were another example, also with dribble inducing (?) square pads.

 

[wualta]

Some construction notes:
The Technics has the most elaborate internal design of any iso 'phone in The Lab's collection, which, thanks to Tyre, is edging up toward 20.

Note that no lead-in wires are attached to the diaphragm contacts. Instead, two little hourglass-shaped translucent elastomer springs pin contact buttons soldered to the lead-in wires against the two shiny aluminum antibuttons on the diaphragm frame, which is made of phenolic PCB material, only thinner. By making the diaphragm frame free of soldered connections, diaphragm replacement is a snap. I'd say it was field-replaceable, but getting all 8 screws out without losing the little elastomer springs or one of the screws... faugh.

Note too the nice smooth machining on the ferrite magnet faces. Much better-looking than the machining on the Yamaha magnets. In an iso driver the magnet faces repel. These magnets aren't very strong. Yet the 'phone is plenty loud (for an iso). Must be that big diaphragm area. Speaking of which, note the two nodal damping buttons on the two magnet faces that constrain the center of the diaphragm. Isodynamic speakers often have nodal dampers or clamps to keep certain rogue areas of the diaphragm from behaving badly, and I'm guessing that's what's going on here. Either that, or it's a way to keep the diaphragm from hitting the magnets during enthusiastic musical outings. This is beginning to look like a cross between the early Orthodynamic ring-radiators and the let-it-rip diaphragms from A-T and Fostex. Interesting. This means those magnets are very close together.

The squarish driver reminds me of the famous Wharfedale Isodynamic and eerily presages the disappointing teensy square driver in the current Fostex line.

The diaphragm material doesn't appear to be Kapton, which is usually yellowish. It's probably a Mylar/Melinex relative, which probably explains the heat-warpage visible in the photo.

Hey, dig the damping pads! Somebody at Technics knew this type of driver needed mechanical damping, so they used what looks like the right kind of material, nonwoven porous fabric. But it's not enough. The highs are weak, the bass is see-through, and the whole 'phone has that hollow, make-all-the-notes-sound-the-same effect you hear from an underdamped moving system whether it's a speaker or a phono cartridge or a headphone.

Can it be "fixed"? I think it can. There's certainly enough extension on both ends of the frequency spectrum to make me think that getting this nicely-embossed diaphragm under control would give us a nice open-back 'phone with some serious power behind it.

The enclosures that hold the drivers look like metal, but they're not; they're made of a glass-reinforced plastic that Polaroid made popular on its SX-70 camera a few years previous. The headband, however, is made of enriched lead and is covered with some of the most massive leather I've ever felt on my skull. So ironically, despite the high tech, the headband is a typical off the shelf '70s item and the 'phones end up weighing a ton. Big cloth-covered earpads are comfy, which is good, because the headband presses them tightly against your ears. The coiled cord, like all coiled cords, dreams of tying itself into a knot named Gordie. So even if the drivers can be brought up to potential, the EAH-830 is a little difficult to live with by today's standards.

Still, it's a magnificent try. All the ingredients are there for a better than average iso. They came so close to miss so completely. Unmodified, you wouldn't want to keep them.

Which brings us to the comparo FacelVega (which was not a glib Chevy Vega, despite the name) asked for. The truth is that none of the isodynamic 'phones pushed into the marketplace by the various manufacturers sounds good when compared today to the SR-X Mk 3, with the lone exception of the Fostex T50, which sounds like an SR-X with better bass. But why compare headphones like the Technics that can be had for $20 on eBay to one of the best headphones ever built? Well, for one, just about everybody on Head-Fi owns an SR-X these days (Carl has two of 'em), so anything eventually gets compared to them, and for two, with some cut and try, an iso with a decent driver enclosure can have its moving system damped to the point where it sounds very much like an SR-X with better bass (don't believe me? Try it-- it's easy and cheap).

The big picture in a snapshot: As is, compared to an excellent modern 'phone, the old isos, with certain Fostex exceptions, lack. They all sound more or less droopy at both ends and humped in the middle. The hump comes in different places on the frequency response graph, but there's always a hump. Modified (that is, damped), the hump goes away, and they're.. well, amazing.

Which one (again, aside from the T50) sounds best out of the box? If I were forced to pick one and couldn't modify it, I'd still go for a Yama YH-1. It's not as entertaining as the Fostex T30, but it's a heckuva lot more comfortable and has a flatter response to start from.

I'll be modifying the T20 and T30, since preliminary futzing has shown that they will respond very well to extra damping, so stay tuned for that on the Orthodynamic Roundup thread.

01-02-07: I appreciate your patience in staying tuned. A continuing severe illness in my family that had its onset 4 days after this post was written has put a severe damper on these projects, but I won't abandon them.


.

 

[Carl]

Quote:

Originally Posted by wualta (Carl has two of 'em)

Yes, both both of them have a less than 100% operational driver (alas, both lefts). Of course, on aggrigate, that still leaves me with one full headphone...

 

[facelvega]

Here's a little info on my new orthos, to show how the East Germans went about damping:





So, the driver goes on top of the felt, the floating positioning ring around both, and the clip/cover squeezes them down into place. There's a little blue foam lining the back of the housing, which as you'll see from the second pic, is only slightly open, with small holes out to more foam and a grill at the clip points and center. Low-tech, but very functional.

The reason I asked about sound quality above is that I've only heard my two German orthos, these and the PMB100's, and I've had an unusual experience with them. The PMB's had an interesting but muddy sound out of the box, an interesting experiment with no special results it seemed. However thanks to torridear's insistence on huge power, I persevered, and thank goodness I did. I don't know what it was-- the disassembly and cleaning, the better amplification, some burn-in (they seem almost never to have been used), or most likely just getting accustomed to a very different sound, but with time I've come to regard these as probably my best headphones. They are certainly the best for voices, pianos, acoustic guitars, anything midrange. They lack the clarity of my best dynamics and can't handle lower percussion well at all, but they make the dynamics seem somehow fake, empty. Perhaps it's the very fast, punchy response characteristic of isodynamic drivers. Note that I have not yet damped them, this is how they sound to me completely open.

Now, the East German headphones are a bit different; they totally lack the broad soundstage given to the PMB's by their unibody Jecklin frame, but their damping gives them a more even and perhaps extended frequency response than the big MB's.

A friend of mine, a medical doctor, is one of these people who love punchy response and couldn't care less about soundstage-- naturally he's a Grado man. I've had him try a half-dozen of my dynamics, and he's tried all the current top beyer, AKG, and Senn offerings. The only thing he found acceptable at all were my Koss A250. Then I let him try my DDR orthodynamics. He loves them. Wants to find a pair. He said they sound like SR-225's. Once again, I haven't modified them apart from putting on a modern plug.

Once again, though, I wouldn't say these have the detail or percussion of a good dynamic. So, either the good Doctor and I are just both suckers for the orthodynamic sound, or the Germans were particularly good at handling these kind of drivers. I'm guessing it's a little of both.

best, Erik

 

[wualta]

Quote:

Originally Posted by facelvega So, the driver goes on top of the felt, the floating positioning ring around both, and the clip/cover squeezes them down into place. There's a little blue foam lining the back of the housing, which as you'll see from the second pic, is only slightly open, with small holes out to more foam and a grill at the clip points and center.

Very interesting. The driver looks like it's already got some felt or fabric on it. Does it? Is it glued over the entire surface of the magnet? The blue foam must act as a spring to hold the extra felt disc against the driver.

Could you by any chance go in for an extreme closeup of the driver alone?
 
Quote:

The PMB's had an interesting but muddy sound out of the box, an interesting experiment with no special results it seemed. However thanks to torrid ear's insistence on huge power, I persevered, and thank goodness I did. I don't know what it was-- the disassembly and cleaning, the better amplification, some burn-in (they seem almost never to have been used), or most likely just getting accustomed to a very different sound, but with time I've come to regard these as probably my best headphones.

Whoa! and these are the ones with no damping at all! How did you give them Mo' Powa? Did you rig up a speaker-output to headphone jack adapter? What differences did this make in the sound?

Does the naked MB driver that we saw in Willi's posts just sit naked in the Jecklin housing? No foam, no felt, no fiberglass? Wow.

 
Quote:

They lack the clarity of my best dynamics...

Because the highs are weak, or some other reason?

 
Quote:

...and can't handle lower percussion well at all..

The "muddy" sound again? All the bass blobs together into one inchoate rubbery mass? Or is there actual breakup and bottoming?

 
Quote:

Now, the East German headphones totally lack the broad soundstage given to the PMB's by their unibody Jecklin frame..

Yeah, it's practically closed, with a resistive vent. [but see post #10, below] Not too different from the early series Orthos, which had the funny circumferential vents backed with strips of felt. Wotta way to treat a dipole.

 
Quote:

...but their damping gives them a more even and perhaps extended frequency response than the big MB's.

That sounds about right..

 
Quote:

Once again, though, I wouldn't say these have the detail or percussion of a good dynamic.

It's very possible they still don't have enough damping. [UPDATE: Well, not quite-- in their stock layout bass isn't well cared for.] Isos that need damping like it rough-- damp them a little and they beg for more. They have what you might call a deep need. One felt disc, even two, is often not enough, as we saw with the Technics. With the YH-100 I had to resort to a quasi-labyrinth that turned the backwave 90 degrees and sent it through the layer of felt edgewise rather than straight through it, and I think it could use even more damping than that. The payoff, as you might imagine, is very percussive bass.

Even the manufacturers who "got it" didn't always understand what it would take. Audio-Technica put a layer of stiff white nonwoven polyester (it looks a little like Tyvek) on the drivers that Radio Shack used in the Realistic PRO 30, and it still wasn't enough. Do the DDR drivers look anything like these, with both leads coming out the side?

[size=xx-small]REALISTIC PRO 30 DRIVERS, SHOWING DAMPING LAYER, ca.1983[/size]

[size=xx-small]FOSTEX FIRST-VERSION T20 DRIVER IN ITS CLAMPING FRAME, NON-INTEGRAL FOAM DAMPING PAD (WAD, REALLY) REMOVED[/size]

Or do they look like Willi's photos of the PMB 100?

And yeah, that's a sneak preview of the T20 project.

..

 

[facelvega]

Quote:

Originally Posted by wualta Very interesting. The driver looks like it's already got some felt or fabric on it. Does it? Is it glued over the entire surface of the magnet? The blue foam must act as a spring to hold the extra felt disc against the driver. Could you by any chance go in for an extreme closeup of the driver alone?

Yep, the driver has a thin woven fabric glued to its face, as do the PMB's. The blue foam is doing exactly that. Sorry, no closeups, I've put them back together and the clips on the faceplate are fragile. I've already had to re-attach one.


Quote:

Whoa! and these are the ones with no damping at all! How did you give them Mo' Powa? Did you rig up a speaker-output to headphone jack adapter? What differences did this make in the sound?

No speaker-output rig yet. I've just tried a bunch of different combinations. So far the best is sending the beefiest sound card signal I can into a go-vibe, while the second-best is running through a somewhat colored but very powerful headphone out on my old Sansui. Not ideal, but it's what I've mostly been doing since I realized the Sansui was up to it.


Quote:

Does the naked MB driver that we saw in Willi's posts just sit naked in the Jecklin housing? No foam, no felt, no fiberglass? Wow.

Yes, the housing actually seems contrived to suspend the thing in midair next to your ear. There's just a tiny bit of foam backing the outer grill, but that comes nowhere near the driver.

Quote:

Because the highs are weak, or some other reason?... The "muddy" sound again? All the bass blobs together into one inchoate rubbery mass? Or is there actual breakup and bottoming?

The highs are syrupy to my ear on both pair, pleasing but not exactly crisp, though the crispness itself now often sounds false in comparison. The bass on the PMB's is weak in the lowest registers, on the HOK80 it's a little better, but in both cases it handles something constant, like a tuba or bass guitar, better than something instantaneous like a bass drum hit.


Quote:

It's very possible they still don't have enough damping. Isos like it rough-- damp them a little and they beg for more. They have what you might call a deep need. One felt disc, even two, is often not enough, as we saw with the Technics. With the YH-100 I had to resort to a quasi-labyrinth that turned the backwave 90 degrees and sent it through the layer of felt edgewise rather than straight through it, and I think it could use even more damping than that. The payoff, as you might imagine, is very percussive bass.

This sounds convincing, but I'd hate to lose that soundstage of the PMB's headspeaker design-- at once this extremely fast, direct sound and the openness of soundstage at the same time. I'm torn. Also, because of the unibody frame design, it wouldn't be possible to get any damping material up close to the driver on the PMB's anyway.

Quote:

Do the DDR drivers look anything like these, with both leads coming out the side? Or do they look like Willi's photos of the PMB100?

One lead is off the side, and the other is out the center on a flat lead glued under the fabric on the inner side of the driver. More like the PMB than the Realistic.

 

[wualta]

Quote:

Originally Posted by facelvega The highs are syrupy to my ear on both pair, pleasing but not exactly crisp, though the crispness itself now often sounds false in comparison. The bass on the PMB's is weak in the lowest registers, on the HOK80 it's a little better, but in both cases it handles something constant, like a tuba or bass guitar, better than something instantaneous like a bass drum hit.

These descriptions are bang on for one of the Yamahas before owner intervention. The bass you describe is exactly the way people have described electrostatic or planar-magnetic speaker bass. Transients come through, but a sustained note eventually meets up with the backwave swimming its way through the room and the bass cancels itself. [UPDATE: Wrong-- the bass is limited on the Mk 1 HOK-80 and the PMB 100 because the driver was tuned (tensioned) high for open-dipole duty-- possibly the HOK was originally meant to emulate the sound and headstage of the PMB 100. Who knows.]

 
Quote:

I'd hate to lose that soundstage of the PMB's headspeaker design-- at once this extremely fast, direct sound and the openness of soundstage at the same time. I'm torn.

I understand. This is exactly the tradeoff made between the SR-X Mk 3 and any of the Stax Lambda series. The Fostex T50 shows that you can have some openness and still have proper damping, and it's a worthwhile trade, but anytime you damp any headphone mechanically you will lose some of the blissy open-air feeling of a headphone like the Lambda or the PMB 100. It's a choice, and the only simple way around it is to have both types of headphone: get yourself back on eBay.de and score some nice cheap Yamahas to experiment on. Or maybe a second-version Fostex T20, since they're a snap to disassemble. I wouldn't want you to change anything in the PMB-100; it's much too rare.

 
Quote:

One lead is off the side, and the other is out the center on a flat lead glued under the fabric on the inner side of the driver. More like the PMB than the Realistic.

Yep, that sounds like the PMB or early Yamaha driver. Since the Soviet isos used the PMB driver (or a copy), I'll bet that's what's in the HOK.

EDIT and UPDATE:
 
Not sure if it's a PMB design or not. It looks like one of the 46mm drivers found in the second-tier Yamahas (HP-2, YH-2): the magnets are a little thinner (and they turn out to be 45mm), but there are no clamps-- the sandwich is glued together-- and the holes are arranged in a grid, Yamaha style, rather than radially or circumferentially. It's a "pinch" type (center electrode clamps the center of the diaphragm) as well. Very much a Yamaha-style driver, just nowhere near as efficient. Interesting.

Unfortunately, at least in my limited tests, there appears to be a hard mechanical limit to the diaphragm's excursion, severely limiting the driver's bass (and to a certain extent, midrange) output.

Note too that the back inside of the earcup [below, left] appears to be a more or less flat blank wall but for those four tiny slots around the circumference. But wait-- it turns out to be completely removable! Perhaps it was an early reflex disc? Behind it is an open earcup, just what we wanted [outside view, below right]. The vents are the horizontal louvers, a little hard to see as vents in this photo. Behind them is metal screening covered by a thin layer of foam. Definitely the most complex, not to say baffling, earcup design after the Technics. Great workmanship but cheap materials. Great sound to about 200Hz, then WAAaaaaaaa; that is, it drops like a rock, and adding even a little bass boost makes it sound as if the diaphragm is bottoming out. Fooey. A puzzling headphone that, if it weren't such a PITA to work on, would be an ideal host for a Yamaha HP-2 driver.

....................................

Many thanks to FacelVega for supplying an HOK 80 (made by a Hermsdorf company with a long history) for this project.
UPDATE: There was an improved version of the HOK 80 that had better manners and more bass-- the HOK 80-2.

 

[lcsaszar]

Hey Wualta,

Thanks for processing my scan an republishing it in this thread. Actually my EAH-820 and your EAH-830 differ only in some minor details. The external face of the 820 is formed from the plastic case as a grille, painted with silver paint (unlike the 830 which looks for me woven fabric). There are small holes between the grille pits. The earpad was the cheapo leather-looking fabric, but the black layer peeled off with time, and now just a brown fabric earpad remained, which is more decent and comfortable IMHO. There was a thin plastic cover between the earpad and the driver (probably for protecting it from dust), which I removed.

I measured the foil thickness with a screw micrometer. The foil itself is 0.6 mil (15 um) and the aluminium layer is 0.8 mil (20 um), 1.4 mil (35 um) total. Allow 0.1 mil tolerance for my measurements. This seems a bit thick for such a small surface area.

More details to follow...

 

[wualta]

Quote:

Originally Posted by lcsaszar I measured the foil thickness with a screw micrometer. The foil itself is 0.6 mil (15 um) and the aluminium layer is 0.8 mil (20 um), 1.4 mil (35 um) total. Allow 0.1 mil tolerance for my measurements. This seems a bit thick for such a small surface area. More details to follow...

Thanks for getting out what Head-Fi member Carl would call your tiny, tiny ruler to measure the foil. Interestingly, your measurements are very close to the ones published by Yamaha for their YH-100-- 12 microns for the plastic film substrate and 9 microns for the foil track. Your next assigment, should you decide to accept it, will be to measure the thickness of the Stax Lambda Signature's diaphragm, but that's another thread for another day.

The 830 diaphragm is simply one turn of the snake (it's "serpentine" after all) more than the 820. A little harder to get good yields, but more surface area means more bass and of course a higher cost. It also means more mass, which means more damping will be required. Instructive to compare Matsu****a's grasp of the problem (two thin nonwoven fabric discs, the upper of which is only attached at its perimeter) to that of the East German designers (a coin of dense felt about an eighth of an inch thick).

Icsaszar, is there any possibility of your getting another EAH-820 anytime soon? What's the market like where you are?

 

[lcsaszar]

I bought my pair of EAH-820 long, long time ago. One may get another pair only by chance. I have seen one on eBay (it's a pity I missed it):

http://cgi.ebay.com/ws/eBayISAPI.dll...m=230060193781

However, I have seen a technical brochure on this line of product:

http://cgi.ebay.com/ws/eBayISAPI.dll...tem=5808933993

Delivery and payment costs to overseas don't make it worth buying it for me. Perhaps some of you technically inclined...

 

[wualta]

I was afraid you'd say that. In that case, the build-it-from-scratch alternative looks more and more attractive. All I can do is wish you good luck.

The later isodynamic headphones that used the smooth unconstrained diaphragm (ie, ones not clamped in the center and not corrugated) used thinner diaphragm material, as you might expect. If you can find a plastic film thinner than that used by Technics that will take reasonable heat and stand up to the photolithography process, more power to you.

Here's a question for those interested in the EAH-830's slightly bizarre construction: Technics claimed in the AES paper cited in the first post that the EAH-8x0 series achieved its engineering goals by using "acoustic circuits including double cavities for controlling the desired responses, which have twin peaks in the frequency response as measured with a dummy head."

I'd expect to see a labyrinth or a tuned cavity or something like that. And twin peaks? That's just scary. Any guesses, based on the photos of the innards, where these might be?

.

 

[lcsaszar]

There is definitely no double cavity. The diaphragm is about 4.5 mm (3/16") from the front surface, not counting the earpads. There is no leak from the back to the front. There is a 'cavity' behind the diaphragm, it is 17 mm deep (11/16"), undamped, apart from some thin dust-protecting fabric. It is vented to the outside through 9 x 9 small rectangular holes.