Great work sofar. If you hear ringing, you can do two things:
 
1. use the 2mm ECRU felt pads I send you for the cups. It is a bit thicker than the white felt and see how it works. 
2 Reduce the inner opening of the carbon pads to about 70mm. I strongly advice you to buy the amazon carbon foam. I know shipping costs are 'huge' (about 15 dollar) and seem unfair, but is is just better. Could be that the Chinese carbon foam does not damp enough, and that combined with the relatively large inner opening (> 75mm) could cause the 'ringing' problem'. 
 
If you find the mids too forward, you can slighty increase the height of the carbon pads to about 15mm at the back, and 10mm at the front, and see how this works for you. You can always decrease the height if you wish. Or you can try the 2mm felt rings I send you. That will improve damping, reduce ringing, but maybe the sound is a bit more dry. You have to experiment here a bit.
 
Good luck..
 
BY the way, the carbon foam I send you is NOT from amazon, it is Finnish I got it from Zolkis. The amazon foam is even better and more comfortable than the Finnish carbon foam I send you.....

sweetmeat said:

  zolkis, I've been reading this thread and found it very interesting, I recently performed the Lawton basic mod on my TH600. My impressions are here:
 
http://www.head-fi.org/t/748434/dampening-modifications-to-fostex-th600-headphones
 
You have compared your mod to the Lawton mod and deemed it superior, and I haven't A/B'ed the two so what do I know, but your mod omits one important upgrade that my Lawton mod covers very well: the Lawton mod stops the ringing that emanates from the concentric rings behind the magnet.
 
The next time you have you cups off, try tapping those rings and you will see what I mean. The result of eliminating this ringing effect is a cleaner upper midrange and treble, and a blacker background. I think that the problems some folks are having with the TH600/TH900 treble may originate from this ringing.
 
I'm surprised, given your propensity to mod, you haven't addressed the ringing, along with all that other stuff. I don't believe it would interfere with your other mods, but again what do I know. It seems like a low-risk no-brainer to me because that ringing is 100% distortion.

Click to expand...

 
Yes, you are right, I/we have not dealt with the treble at all yet. When I started removing the Lawton basic mod parts, I left the ring damper there for long time before I removed that too, in order to minimize variables. Didn't notice protruding ringing. But once you mentioned this, I am interested to try this next time, maybe this weekend.  I listen to my modded Stax most of the time when I am at home (they are incredible), but the TH900 leaves me little to want at the office. That little, however, does include the treble, which is not that great IMO when compared to Stax (higher in level, and more grainy, though the felt ring dampers improved graininess somewhat). It is also true that I don't use dust grills any more, which also smooth (smear?) the treble, driving the problem sideways. So any further improvements on the treble are very much welcome. But fundamentally IMO it is a driver issue: it needs some light coating.

playitloud said:

 
BY the way, the carbon foam I send you is NOT from amazon, it is Finnish I got it from Zolkis. The amazon foam is even better and more comfortable than the Finnish carbon foam I send you.....

Click to expand...

 
Indeed, the amazon carbon foam is somewhat denser and blacker than what I got locally. And indeed it sounds quite different (!), and better. 
The effect on the sound of the apparently minuscule structural differences between makes of carbon foam and especially wool felt are just amazing.
I also strongly advice to get exactly, repeat, exactly the materials we have tested: we have spent a year on trying different materials.
Of course later one can try further materials, but in order to have a reference, this is the best we have managed to come up with.

sweetmeat said:

 
The next time you have you cups off, try tapping those rings and you will see what I mean. The result of eliminating this ringing effect is a cleaner upper midrange and treble, and a blacker background. I think that the problems some folks are having with the TH600/TH900 treble may originate from this ringing.

Click to expand...

 
I have tried this, and works like you described, but the subjective effect is bigger with the felt back damper. That's probably why it sounded too dark in the stage of the mod (when I used thicker felt pads) in which I decided to clean away the Lawton mod. But now, using only the 2 rings is enough to tame the treble [ringing], and darkens the sound just a little bit. I will listen to this for a longer while and let's see if the good impression holds. 
 
Thanks for this tip, I planned to act on the treble issue, and now it's pretty well solved. It saved a lot of time .

On my way to do the mods. First step. 
 
 

Does anyone have an efficient way of attaching the ear pads back?
I've tried to put it back the way described in the main impression thread, but gosh it was a real pain in the a** process that took me 30mins? 
 
While removing by turning the ear pads took barely 10 seconds on both sides  

yuanathan said:

  Does anyone have an efficient way of attaching the ear pads back? I've tried to put it back the way described in the main impression thread, but gosh it was a real pain in the a** process that took me 30mins? While removing by turning the ear pads took barely 10 seconds on both sides 

 

Click to expand...

 
Assembling should take the same time, if the white plastic holder pads click in easily. I recommend "training" the holders by clicking on-off until they move smoothly. Then mount the holders inside the ear pads, rotate them to adjust the position of the legs, then hold together the leather and the holder inside it, put into the mounting holes, and with a short firm turning click in the holders. 
However, when I was testing, I never rotated them until they clicked in, only until they got tight enough to not fall off. It was pretty much the same. When I was done with testing, I clicked them in for the final position.

I'm finally going to order the recommended carbon foam just to make sure I'm not missing out on anything here. Do you think this 12mm sheet will work?

http://www.amazon.com/Finest-Filters-Activated-Impregnated-Aquarium-Filters/dp/B007NYQDMK
 
The reason I want a 12MM sheet is so I don't have to cut the 25mm sheet into half. I had problems slicing in uniformly in half the last time, and I was worried that might have affected the symmetry between the pads. Losing 0.5mm in height shouldn't make a huge difference right?

Yes, this should work. Trim the front part to about 8 mm thickness, as described earlier, and slightly round/trim the edge close to the ear around the circumference.

Finally ordered it! Now to wait about a month for the foam to arrive...
 
Meanwhile, I am enjoying my modded TH900 with my new HP-A8

The foam is in! Time to get to modding tonight

Also, zolkis, how do you like your TH900s after installing Lawton's ring dampers? If I remember correctly, you mentioned that installing them improves the treble. Would you recommend it, and can you install it in a reversible manner?

stjj89 said:

  The foam is in! Time to get to modding tonight

Also, zolkis, how do you like your TH900s after installing Lawton's ring dampers? If I remember correctly, you mentioned that installing them improves the treble. Would you recommend it, and can you install it in a reversible manner?

Click to expand...

 
Yes, it's reversible, and did calm down the treble. Happy modding!

zolkis said:

   
Yes, it's reversible, and did calm down the treble. Happy modding!

Click to expand...

 
Did it improve the quality of the treble, or just reduce sharpness/shrillness? I have no issue with the treble being shrill right now, but I do wish it was a little more realistic and textured like on my HE-560 (which makes string instruments sound BEAUTIFUL). Any comments on this?

Realism comes from less shrill ans hash among other things. No mods will make the treble much better, but can enable the existing capabilities. It won't become a Stax, but it will be better than it was.

zolkis said:

  Realism comes from less shrill ans hash among other things. No mods will make the treble much better, but can enable the existing capabilities. It won't become a Stax, but it will be better than it was.

Click to expand...

 
That makes sense. I'll contact Mark Lawton and see if he sells the rings separately.