한 때 헤드파이 포럼에서 오쏘다이나믹에 대해 많은 연구(?)를 진행하고 프로젝트 위키도 만들던 시절이 있었다고 합니다. 그 운영자가 지금은 종적을 감췄지만, 당시에 많은 컬랙션들을 수집하기도 했고, 많은 유저들의 노력으로 많은 자료가 정리되어 있고 잠들어 있습니다. 현재는 위키가 닫혀서 추정만 할 수 있지만, 그래도 좋은 자료들이 많아서 몇개씩 정리해 보고자 합니다. 본문은 당시 위키 운영자가 작성한 오쏘다이나믹의 티어표입니다. 번역은 제가 개인사정이 있어 아직 못해 곧 할 상태고요.

다만, 첨언을 하자면, 이분들 성향이 우리나라와는 조금 괴리가 있어 몇몇 부분은 걸러서 들으셔도 무방합니다. 또한, PMB의 제품군이 너무 과소평가된 부분도 존재는 하네요. 재미로 보시면 될 듯합니다.

Top Tier: 

Fostex T50 (original North American version, 1978-80)-- Really stinkin' rare. Keep your eyes peeled but don't hold your breath. Sounds very much like the Stax SR-X Mk 3 but with bass. Not basshead bass, but reasonably clean bass. One of the hallmarks of the sound of this phone is emotional, not acoustic: when you wear the T50 you get a feeling of being placed into the hands of very competent people who've thought the whole design through and sweated all the details. My guess is that I like it so much because everything seems wonderfully balanced. The response is smooth, extended (but not too much) and flat. Damping is factory-tuned, but because there is damping, headstage is not Lambdalike as you might expect from one of the few open-back isodynamics. It's reasonably efficient (for good dynamic range) but not so much that you'll hear residual amp noise. There's no sense in which this headphone is either too much or too little. However, with all the different levels of Hell you'll have to go through to get one, the psychic bottom line looks better for one of the 2nd tier 'phones, which can be modded to give similar but not identical results. Did I say rare? Rare. They never made another headphone like it, and neither did anyone else. Not to be confused with the 2002 Fostex T50RP, an entirely different headphone and interesting in its own right but with a less-flat response and with early production sabotaged by earpads that robbed it of any and all bass. 

Yamaha YH-1000-- (we're only supposing this one belongs here) On paper, the only bonehead maneuver Yamaha pulled on this 'phone was putting the big earcup attachment/adjustment block right behind a vented earcup, but yes, Yamaha did vent the earcup on this rare early signature model and used mechanical damping too! Not too many people own this phone and fewer have described its sound, but the ones who own it or owned it do like it a lot. I've studied the service manual and it's obvious Yamaha was pulling out all the Orthodynamic stops. Rare upon rare. Neither Eric nor I have heard it and only one HFer currently owns one.

Close to top:

Fostex T50v2, aka "Mk 2" (1980--86?)-- (including NAD RP18, and Maior RPT-50) For the 1980 model year, Fostex changed the T50's headband from a pivoted double leather-covered band to a single padded plastic piece, and this is the headband we see on the OEM Maior and NAD models. Further, since the Maior's innards look the same as the T50's but don't sound the same (there's an elevated upper midrange and treble), and since the Maior sounds very much like the later pro-line Fostex 'phones in their unmodded state, we strongly suspect (but don't know for sure) that Fostex changed the sound of the 1980 Fostex-branded T50 as well, which is why we're provisionally calling the 1980 style and the OEM clones of it T50v2. To give you a basis for comparison, I like the modded YH-100 better than the Maior, which suffers because it induces a "so close to the T50, but.. aaarrrghh!" bitter aftertaste of betrayal with the associated emotional discombobulation resulting in angry sesquipedalia. Pfui! Now, having said that, it IS a T50 in every other respect-- it just sounds like there's a little guy inside with a graphic equalizer who's hellbent on futzing with the sound. Sigh. Could you live with it without modding? Yes, but it depends on your music, your gear, and your tastes. Again, not to be confused with the much later T50RP. UPDATE: Ortho stalwart Smeggy has modified a NAD RP18, and indeed it lacks the treble peak... but the price paid is a smoothly but heavily rolled off treble, a sound not unlike the T30's (see below). Work continues...

Fostex T30-- Great potential. Very similar to the T50 inside, but with a slightly less open magnet structure and a sound that's tilted basswise; thus I consider the T30 to require modification, but you could live with it, especially if you have a simple set of tone controls at hand. Shockingly uncomfortable to wear. Just about as rare as the T50.

Sansui SS-100-- Fostex-made, with a driver that's a cross between the one in the T50v1 and the T30 but slightly closer to the T50. Sonically similar to the Maior, but less bass. Rare again.

--Everything below this point requires modding. I've arranged the 'phones in order of sound quality aftermodding. My opinion, of course.--

2nd tier:

Yamaha YH-100-- This is a vented-closed 'phone, so it'll never sound exactly like a T50, but it can sound really freakin' good in a variety of ways, and swt61's custom open-back wood YH-100 transplant shows real promise (which I haven't yet heard for myself). Has bass and treble extension beyond that of the T50.

Fostex T20v2-- another good all-rounder, not rare, usually reasonably priced, and easy to work on. Needs very dense damping material, however, which may take some time to find.

Yamaha YHE-50S-- another very rare one but since it's based on the slightly more common YHD-3 (which I haven't heard, but which should be very similar), I thought I'd include it. Think of a miniaturized YH-100 with an open(ish) back.

The JadeEast Collection-- two rare 'phones owned, as far as I know, only by JadeEast. Based on what's inside these phones and JadeEast's reaction to other known quantities, I feel safe putting them here: the PWB Dyna-X, which uses a driver similar to the one in the Realistic Pro 30 (see below) and a strange vented baffle like the AKG K501's, and the Fostex-built Concept CE-h, an OEM model using the drivers from Fostex's T20v1. JadeEast's mod for the Dyna-X changed it from an also-ran to a champ, he says, and I have no reason to doubt him because it's exactly what I would've done.

3rd tier: I'd swap this whole group with the Pro 30's group below. I'd even give the Pro 30 a 2.5th tier of its own. 

Yama HP/YH-1-- A good place to start your career as an apprentice Orthodynamicist. The low bass of the YH-100 is missing, but everything else is there with the simplest of mods (the socalled Stage One-- a simple felt disc or two placed behind the driver). If you're not a basshead, get one but watch for a break in the insane online auction prices.

Yama YHD-1 and 2-- Similar sound signature to that of the HP/YH-1, but duller and bassier stock. Kicky minimalist design; still surprisingly heavy due to the magnets. A work of art that's also a headphone with the promise of goodness after modification. Some people like them unmodified. Among the most efficient of the Orthos or orthos.

Audio-Technica ATH-2 -- Not too common and slightly crippled by a thick, stingily-perforated baffle. One of the very few inexpensive isodynamics with an open back. Easy to work on. Driver virtually identical to the one in the Pro 30.

(maybe) Fostex T20v1-- Fostex's most famous headphone, used in recording studios worldwide, but not audiophile material without modding, and difficult to get sounding just right. Comfortable, not expensive, easy to work on, but not very efficient and has an upper midrange peak that's hard to tame. 

4th tier: (remember, I'd blend this into the 2nd tier group or just slightly lower.)

Realistic Pro 30-- Cheap to buy, easy to mod, can give astounding results for effort expended, good solid bass, compact on the head except for the "antlers". What's not to like? Well, when properly damped it tends toward brightness. Sentimental favorite, since it was my first wholly successful mod, circa 1987. The 'phone that should've spread isodynamicry to the masses, and may yet.

Yama HP-3/YH-3-- I added this one to this group based on reports indicating that some examples (but sadly not all) are a poor man's YH-100. Not the most common of the Orthodynamics, but not rare (yet). Variants abound: Yama HP-50(plus A and S versions)-- all designed to be sold with Yamaha electronic organs. The HP-50A is wired mono but can of course be converted to stereo. Unfortunately, it's a matter of luck to get one of the bass-heavy examples. The sound of this headphone varies more than any other-- everything from a stock HP-1 sound to a YH-100 sound.

Elektronika TDS-5M-- Visually, a Russian copy of the Yamaha HP-1 (!), but with a difference: inside is a copy of PMB's 55mm driver, but improved-- it has bass! Too bad it never shows up on US eBay.

5th tier: (I'd move the Fostex T40v1 out of this group. What remains are interesting 'phones but ones that respond very little to modification-- that's why they're here. Most use the 55mm driver made by Peerless MikrofonBau, PMB, now known as MB Quart.) 

Fostex T40v1-- I'd bump this one way up to 3rd tier or 2.5th tier. The lack of deep bass is a deficit but not a huge one and bass EQ works well. Very extended and smooth top end, like no other non-stat headphone I've heard (although the AKG K501 is close). One of the easiest mods gets you there.

MB Quart PMB 100-- Best of this group (aside from the T40v1). Early version used a modified and damped PMB 55mm driver. Wonderfully flat, though "dry" sounding (as opposed to the "liquid" sound of a good electrostat), from about 100 to 150 Hz on up. Below 150 Hz there be hippogryffes but not much bass, which will be found to have fallen off the edge of the map and lost in the shag. Strange Jecklin-design headset that promises much but delivers only a little. Some violently disagree with this, but I don't hear the spatial enhancements promised by the Jecklin open-frame architecture. And to me, the sacrifice of that much bass isn't worth it. Later version, which used a standard PMB 55mm driver and has a nonadjustable headband, is difficult, if not impossible, to improve, by which I mean add bass.

Telefunken TH 700-- Made by PMB in Germany (clone of the PMB 8) using PMB's big 55mm pinch-type driver. Not much bass or treble.. and it's factory damped! Maybe with a lot of work... or a new fundamental insight... Rare.

B&O U70-- I'm a big industrial design freak, but enough is enough. A work of art first and always, and only secondarily a working headphone. Unlike many orthos, though, the U70's driver doesn't improve significantly in a more sympathetic chassis. Uses the same PMB driver as the TH 700. Top octave sliced off. Not much bass. It's at least easy to work on since its aluminum-clad plastic body snaps together.

Burwen PMB 8-- from the look of the thing, likely to sound very much like the Telefunken, since it has what looks like the same driver in a similar enclosure. Can you say OEM? Rare.

Dual DK 830-- another OEM version of the Burwen and Telefunken.

RFT HOK80-2-- Made in East Germany, the old DDR. This phone and the next three all use what appears to be the same 44mm East German-made driver, believe it or not. Not much bass (very reminiscent of the PMB 100) and very inefficient but decent mids and top end. Horrible earpads with a lifetime of about 2 years max, but still serviceable even after the seams have split. A very brave attempt considering the political and economic conditions of its birth. Historically significant and shows rare sensitivity to the driver type. Convertible to open-back. Will accept incredible amount of bass boost and will deliver real bass if you have the amp and EQ for it. Snap-together construction using sometimes brittle materials let some bass backwave leakage occur. Difficult to work on and not for everybody, but hard to ignore. Not rare on German eBay.

Goodmans OHP-10-- Very nice PMB-made (West German) headset with an East German driver inside. With flatter earpads and some better baffle seals... As it is, it doesn't sound as flat as the gnarly little HOK80-2!

Burwen PMB 6-- from the photos, likely a variant of the Goodmans

Dual DK 330-- under the cosmetics, an OEM clone of the Goodmans. Dates back to 1975, making it one of the earliest isodynamics.

6th tier:

Yama HP/YH-2-- Sounds very much like the HP/YH-1, but unlike the HP/YH-1, used foams that haven't held up, so be prepared to vacuum/brush out sticky, crumbly residue. It's down here strictly because of this risk factor. Otherwise it would be up there with the HP/YH-1.

Technics EAH-830-- Very innovative technically but with weak magnets and an uncomfortable, unnecessarily heavy headset and blah sound.

RFT HOK80 (v1)-- The first version of the HOK80. Interesting, but NO bass and very inefficient. What's there is surprisingly good, but...

7th tier:

Current production Fostex? Yeah. They wouldn't be so bad if they weren't so entirely bass-free and entirely unmoddable, at least by me. It's possible that in a close-coupled, small-volume headset (like, say, the Pro 30's), this little (35mm square) driver could begin to make its low end audible without massive EQ, but this would take major surgery, since the clamp that holds the driver together in all Fostex RP 'phones since about 1987 is part of the baffle, which means that to remove the driver intact it must be jigsawed out. UPDATE:Frabjous day! Fostex has given us hope for the whole of their current production by giving it bass. How did they do this? They flattened the earpads on the T50RP. The new earpads are more dense as well. That's all they did. If I hadn't heard it with my own ears I wouldn't have believed it. The upshot is that with these new pads, this whole line of headphones suddenly becomes moddable. With the rise in price of all other desirable 'phones, this changes everything.

Wildcards: We don't know if these will respond to mods and be great, or lie there and razz us. Some are so rare they've only been spotted or owned by one known-to-HF person.

Wharfedale Isodynamic The original model, the first isodynamic headphone to reach market (1971) and owned and loved by a few HFers, usually many years ago. Hope to get my hands on the unique almost-true-ribbon driver inside this one someday. UPDATE: HF's own transplanted Brit Smeggy has reunited with a pair of Wharfedales and has pronounced them even better than he remembered, but he had to craft properly-sealing earpads for them, the originals having long since perished. Very inefficient but up for the game. They're second only to the Fostex T50v1 in out-of-the-box sound quality. 

Wharfedale ID-2 / Leak 3000-- I know nothing about these and have never heard or read of anyone daring enough to own them. Leak and Wharfedale were both bought up by the Rank Organisation, so it's not surprising to see them selling what's essentially the same headphone, which, a bit disappointingly, is a conventional isodynamic design

Amfiton TDS-15 and Echo TDS-16 (early version)-- These tantalizing Russian (Ukraine) 'phones manage to stay just out of our reach. The TDS-15 is basically a Russian Fostex T50, with bar magnets and a serpentine voice coil. The TDS-16 is more like a YH-1 inside (perforated disc magnets, spiral voice coil).

조만간 번역을 해야 하는데… 사간날때마다 해 보도록 하겠습니다.