Here is a translation of a post on Russian forum regarding modding T50RP's. It's not my post, so I wouldn't comment much on it.
And so I, having done an immeasurable amount of mods to T50RPs and still getting the same result, came to a conclusion that they can't be helped only with damping and their sound cannot be improved without modding the driver itself. In my opinion, their driver construction is flawed, while the membrane is of very high quality, albeit small. The effect a grille puts on the sound has been discussed there, and well, what we have with that driver? Fostex has saved up on magnets, put only 3 instead of 5 and replaced the others with metal plates which have 42 holes 2mm diameter each to pass the sound. Total equivalent area of these holes is ~132 sq.mm, which equates to a single 13-mm hole, about 15% of the membrane's surface. Also, the holes are so small, they're smaller than the emitted sound wavelength. I kind of remember from school's physics that it results in wave being emitted from one point with a spherical wave front. It's called diffraction IIRC. Then these waves sum up with different phases in different locations, and that's called interference. And more than that, sound is trapped within two bare metal plates and reflects from them multiple times before getting out. No wonder Fostex's sound is so sloppy, weak and muddled and is so different from other orthodynamics.
So, I've lost my temper and being on vacation, I've decided to do a radical driver mod, which was planned long ago. The result was as expected. I've tried two versions of modding, easy and complex. Easy method uncovers 100% of driver's area, complex - about 50%. Easy method is inspired by construction of HE-4, the magnets from one side are just completely removed, and the driver is damped on the back till proper tonal balance. Sensitivity is reduced indeed, but just by about 30%. Sound becames just like the proper isodynamic sound, with deep and mighty bass and clean and clear highs. It would be perfect if not some boominess. Hard to say what it's due to, perhaps because
there is some space where there were magnets, which makes a chamber of sorts. Perhaps it's possible to correct by making membrane closer to ear, but it takes a complex and irreversible modding of driver's enclosure. I'm not yet ready for such an experiment. So, I've modded the front magnets, by removing metal between holes in the front metal plates. I've left only 4 "joints" - in the corners and underneath the plastic grid. It was a tedious job, using thin drill and needle file (may be wrong translation), but it took only one evening. Used cotton wool to remove remaining metal dust from drivers. Modded magnets should be glued back properly so that they won't jump out and damage the membrane during reassembly. Be very careful during assembly\disassembly and remove anything metallic away.
I have also covered the gaps between rear magnets completely with thin 1mm natural leather to reduce reflections.
I've left one hole in the center to further tune the damping. Damping is a theme in itself and depends on wether
you've removed stock damper, on materials etc. and is done by ear. One thing is sure - damping should be quite
heavy, or there would be too much of boomy bass and echo. Anoter mod is spacer for earpads made out of polyethylene
foam 3mm thick. Ear just barely touches the grid then and sond becames fuller to my taste. 2 layers is completely
comfortable, but there's too much bass that way. After the modding, the phones sound completely different, very
interesting. Very deep and mighty bass without distortion and midbass peak. Slight recession on low mids and a rise
on upper mids, so they sometimes "shout" on wind instruments and high vocals. Highs are very extended, clean and
sparky - a typical orthodynamic. Reading LCD-2 and He-5 reviews, I think I've got something inbetween them for much
less money. Photos follow.
Comments before photos.
The improved driver from the front. I've left "joints" in corner as membrane barely moves there and they don't play
much role in sound.
Rear magnets with plates covered by natural skin, view from membrane side. Only 2 holes are left for further
Rear magnets from rear. One of the left holes is taped over from that side, as pleather earpads required more
damping. Damping can be tuned further by covering remaining hole with duct tape. I've covered about half of the hole
to get tonal balance for me.
If the holes won't be covered, driver should be damped in a traditional vay, by covering it with dense felt. Like
that, or like as in the Thunderpants.
Here,a 2 mm hole is punched in the felt, and a piece of plastic with 1.8 mm hole is glued over. That hole in the
plastic can be further taped over. If it's not covered, the balance seems to be like LCD-2s, bassy and a little
boomy. The more damping, the more highs and less bass and boominess. With optimal (for me) damping, sound is very
detailed and natural. I can't call it very open, as the headphones are almost completely closed. But there's no
effect of in-head localisation, sound comes from the front. That, combined with deep bass, creates an illusion of
being in cinema when watching movies. If the stock paper damper is not removed, damping should be done differently,
covering driver's rear partially with felt, for example.
I must note that everything has been tuned with 3mm earpad riser. It changes the sound noticeably. If there's no riser or a different riser, damping should be also changed.
Also, cups are covered with thin skin on the inside and filled with synthetic fibers (sintepon). But this is a leftover from previous mods and barely changes the sound. Most of effect is from opening the drivers. Besides that, in an attempt to reduce bass and boominess, I've drilled 3 more baffle vents 2mm in diameter in addition to one stock. They are placed in 3 remaining corners. hard to say how they effect the sound, but it seems that compression on the bass has been reduced.