This thread is for describing, discussing and further developing mods to the Fostex TH900 headphones. So far I developed these with @playitloud.
 
History
 
It all started with my similar mods on Stax SR-007 and then SR-009, summarized here:
 
http://www.head-fi.org/t/754839/stax-sr-009-and-sr-007-mk1-earpad-diy-mods
 
Then I got the TH900 for the office, and tweaking started right away:
http://www.head-fi.org/t/595683/fostex-th900-impressions-discussion-thread/6930#post_10537146
 
Then @Playitloud joined: 
http://www.head-fi.org/t/595683/fostex-th900-impressions-discussion-thread/7815#post_10850139
http://www.head-fi.org/t/595683/fostex-th900-impressions-discussion-thread/7815#post_10850940
 
After countless of emails, messages, experiments with various materials, thicknesses, densities etc, looks like we have settled down enough to say this is good.
 
The results have been shortly described here:
http://www.head-fi.org/t/595683/fostex-th900-impressions-discussion-thread/9960#post_11537304
 
For convenience, I summarize the currently known best setup here, using some words and images from my earlier posts, too.
The following steps are all needed, with the exactly the same materials, dimensions, etc. Small details make big difference.
 
List of materials (for sourcing kudos to @Playitloud):
- active carbon foam, at least 12 mm thick sheets
e.g. http://www.amazon.co.uk/Finest-Filters-Activated-Impregnated-Aquarium-Filters/dp/B005AK9FQ0 (this is 25 mm thick)
- 5 mm thick wool felt from meervilt.nl, the white (important!) color; we have tried natural, black, grey etc; the white (washed natural) one sounded the best; has to do with density
  you need 2 pcs 73 mm diameter disks
- 2 mm thick white wool felt from meervilt.nl; you need 2 pcs 73mm diameter disks
- as above, but 3 mm thick white wool felt to try an alternative cup damping
 
List of tools:
- circular cutter like this:
http://www.amazon.co.uk/Olfa-Compass-Cutter-cutting-circles/dp/B000BK7NWC
- a good Phillips screwdriver
- a good thin blade sewing scissor for opening the earpads
- a good thin blade scissor (or equivalent) for trimming foam.
 
 
A. Modding the ear pads
 
This is the step requiring most skills.
Based on the previous experience in tuning the SR-007 ear pads, finding the optimum for the TH900 took quite short time.
Disclaimer: the modified pads can be restored to the likeness of their original state, but involves un-sewing and (careful manual) sewing again, which is not the same as before. So this is a simple, but destructive mod. However, if you need new pads, you can always get replacement pads from your Fostex dealer.
 

1. Unmount the earpads
by pulling the leather pad out, then around, until it comes off. Then, remove the white disk by rotating it with hand or a tool. Then try to mount them again in and out; after a few clicks-in clicks-out the movement becomes quite smooth. You can mount and unmount the pads more easily then, by putting the white disks into the leather pads and rotating the white disk together with the earpads with a short, firm movement. For experimenting I recommend not turning it until it clicks in, just halfway, it's enough for being in place.
 
For curiosity, listen to your TH900 without the earpads. It's more open sounding, right? Now we will try to make that more comfortable, and retain some of the cavern as well.
 
2. Open the ear pads
Undo the sewing on the edge of the white plastic base of the ear pads:
 

 
When done, the ear grill cloth comes off, and remove the foam filling:
 
  
 
Note that the foam filling is marked where it was bent because it's too high for the pads. We don't need the foam any more, archive it somewhere.
 
3. Make new earpad filling
Get this material:
http://www.amazon.co.uk/Finest-Filters-Activated-Impregnated-Aquarium-Filters/dp/B005AK9FQ0
Cut a 105 mm diameter disk.
Cut an inner 73 mm diameter disk.
Cut the resulting ring in two thinner, 12.5 mm thick rings (EDIT 22.Apr.2017: 14-20 mm high at the back, mine is 18 mm now)
Then, trim/round off a little bit the outer and inner top edges.
Then, trim the angle into it: in the front it should be 7-8mm high, in the inner side of front should be ~10-14 mm high, and gradually growing to 12-18 mm in the back.
 
The result is something like this, and you can mount it then over the plastic base of earpads:
 
  
 
Then fold the leather part back, and put the white mounting disk in its place (on the left):
 
 [ on the right: new dampers made of felt ]
 
Now you can mount and unmount the ear pads by matching to the guiding holes and slightly rotating them.
 
 
B.  Dampers
 
This step is completely reversible.
The foam dampers are the grey foam rings around the drivers in the ear-facing side.
The new ring dampers are made from 5mm thick white (washed) wool felt from meervilt.nl, of 73 mm outer, and 48 mm inner diameter, as shown in the picture above.
The dampers make the biggest change in the sound, because they control the driver and cups loading. This makes the bass tiny or huge, and the midrange shouting or recessed.
Try 4mm dampers and it will be very open sounding, but a bit shouting. Try bigger outer diameter, and will be more muffled. Try bigger opening (inner diameter) and you get bigger stage, but shouting mids.
Now you can put the rings around the drivers, it will be held there by the earcups. No adhesive is needed.
 
Note. Playitloud tipped to trim the edges of the felt dampers so that they become very slightly rounded with a radius of ~1mm (upper outer and upper inner, optionally also the lower outer and inner edges). This actually makes a well audible difference, consider it fine-tuning to your taste, on how open should it sound. I "voiced" mine to the SR-009 tonality. It is possible to make it even more open, but then there are other problems.
 
 
EDIT 22.Apr.2017. I am still using the felt felt dampers sometimes (I have about 10 pairs or various make and size), but mostly I use the original Fostex dampers nowadays. That is good news for modders, as this is the most tricky mod to get right. Funnily they sound better than the newer but identical dampers from the TX00 variants.
 

C. Cups
 
This step is completely reversible.
1. Unmount the earpads.
2. Take out the dampers.
3. Unscrew the 4 small screws, and take one of them completely out.
4. Take off the cup.
5. Remover the polyfill stuffing.
6. Cut a 73mm diameter disk from the 2 (or 3) mm thick white wool felt from meervilt.nl.
7. Cut a 48mm diameter disk, so that you get a 73/48 ring and a 48mm diameter disk.
8. Put that into the cup as shown below. Don't glue it, it will stay there without moving. There is little space between the back of the driver and the wood.
 

 
9. Before assembling the cups back, align the cups using the hole from which you removed one of the screws, and a spot light. This is safe and sure. Use very gentle force with the wood screws.
10. Mount the earpads: insert the white plastic holder ring into the ear pads, align one of the dents with the sewing on the side of the ear pad, push the dents into the location, then  with a short, firm movement, turn to click in the pads. 
 
I should note that do not mix this with any Lawton mods. That caused me a 2 months dead end side experiment. The Lawton mod is good on the stock TH900, for taming the bass, and it does work as advertised, but nothing from there works in this mod. Do not mix them - but if you're a tinkerer you'll try it anyway .
 
Also, in our experience high pads (Audeze, Lawton etc) don't work well with this mod, and in general with the TH900. The ear pads are integral part of the mods. See also http://www.head-fi.org/t/595683/fostex-th900-impressions-discussion-thread/6270#post_10241637.
 
EDIT 22.Apr.2017. As the cups got older, they don't show any 3kHz ringing on the CSD any more when no damping is in the cups. So I have removed all damping from the cups (similarly to the TX00 variants). Prior to this, I've been using Twaron Angelhair (http://www.twaron-angelhair.com/), it's the best damping material if you need one (kudos to @playitloud).
 
Testing, listening
 
With these mods my TH900 is playing in about the same league as my SR-007 and SR-009, except fluid smoothness and ultimate accuracy/neutrality/instrument timbre/tonality, which is why the Stax are still the best IMO... but so far nothing  can match the bass of the modded TH900, despite not being overshot any more - which gives them a kind of realism which is more physical than tonality related, but it feels like realism anyway. The lower register of string instruments is so palpable it gives you goosebumps. Real cello and double bass of course sound better, but this is as good as it gets from headphones today. Piano attack and decay, saxophone, percussions, and of course anything electronica sounds world class - in the context of the current headphone industry of course.
Since this last mod I could clearly hear much more difference between my home rig and mobile rig, between recordings, and even between two 10 cm USB OTG cables (call me nuts). Before I didn't hear much difference with the TH900.
 
Competing headphones would be the HE6+EF6 (I think my TH900 is better than them but the HE6 is slightly more neutral and a tiny bit smoother). But the TH900 is far more usable (portability, drive'ability - hence total system price -, isolation, etc) and did I mention the bass and sound stage? 
 
These mods are relatively cheap, but they make a big difference. Since the minimum order quantities, and the little materials actually used it is worth joining forces with someone and share the materials. 
 
Enjoy.
 
Finally, I would like to thank again to @playitloud for co-developing these mods, for being an inspiring buddy, sourcing master, tester and fellow music lover.

Wow.

Very informative, thank you.

You are such a boss! Thanks for this!

Wow those are some bold claims about quality change! Iv heard the Stax Sr 009, so saying they are almost on that level is a big deal to me. I have a few questions if you dont mind....

1. If I didnt feel like tinkering with the pads, could I just use Lawton Pads with this mod? How would this effect the sound you are going for?

2. About how difficult on a scale of 1-10 would you say this mod is to do? Im really not good at modding but if anything I'm brave and willing to try.

3. In what ways is this superior to Lawton mods other than cost?

Thanks for sharing your work! If I decide to try it myself be prepared for more questions, im a big dummy when it comes to these things lol.

goropeza said:

Wow those are some bold claims about quality change! Iv heard the Stax Sr 009, so saying they are almost on that level is a big deal to me. I have a few questions if you dont mind....

Click to expand...

 
Read it in context: plays in the same league, but there are a lot of differences. Those may be fine for some, and deal breakers for others. The Stax are better, still, but the point is many people won't miss.
 

1. If I didnt feel like tinkering with the pads, could I just use Lawton Pads with this mod? How would this effect the sound you are going for?

Click to expand...

 

No, and I have updated the text with that (plus a link). Higher pads would sound muddy and boomy.
 

2. About how difficult on a scale of 1-10 would you say this mod is to do? Im really not good at modding but if anything I'm brave and willing to try.

Click to expand...

 
If the original TH900 is 7, the Lawton basic mod is 7.5, this is about 9-9.5, where the Stax 009 is 10.
I don't know the other Lawton mods. 

Wow Zolkis, thorough as always. Great job.!!
 
Some of you may question the white felt and opt for black as it better fits the headphone. You can do that of course, but is will not sound as good. And the difference is quite severe.  White felt is undyed felt, and bleached. By bleaching the remaining dirt inside the felt is removed. Hence, this is the most pure 100% Merino felt that you can get. It feels very soft, very pure. Dyed felt, and even undyed felt but not bleached (Ecru) feels more harsh, stiff and coarse. And that is also how it sounds compared to white felt.
 
About the skill level to do these mods. I had zero skills when I started this journey. I even had problems removing the earpads from the headphone the first time. After I removed them the first time, putting them back and off gain is easy. Removing the cups is not difficult if you have the right tool, use good light, and be very careful not loosing those tiny screws. For the rest of the necessary skills, you will find some info below. This is how I did it. Maybe, Zolkis does it differently.
 
Dampers:
if you use a Olfa Cutter (see post Zolkis), it is relatively easy to cut 5mm dampers. First cut the outer diameter. You set the right diameter on the Olfa cutter. But I always make some test with a piece of paper, and check the final diameter, just to be 100%. Be aware, we are talking millimeters here. Of course you can always trim it with scissors afterwards.
The Olfa cutter (Mine is slighly different than the one Zolkis uses):
 http://www.amazon.co.uk/Olfa-1057028-Circle-Cutter-CMP3/dp/B001CEAMCY/ref=pd_sim_kh_8?ie=UTF8&refRID=1VM7M0DZJSWY78F8E86Z)
 needs some getting used to. It is important that you keep the center pin straight up (perfectly vertical). If you deviate from this position you do not get a full perfect circle. Hence, you need to train yourself a bit. Best is to buy a piece of cheap felt to give it a few tries, before doing the ‘real’ job. Next it is also very important that you the exact same center point that you used to cut the outer diameter, for cutting the inner diameter of the damper. With the white felt you can clearly see the needle position, but I always tip the needle of the Olfa cutter with a yellow marker (any color will do of course) before cutting the outer diameter. Then it is very obvious where to stick the needle to cut the inner diameter of the damper, as it leaves its mark on the felt.  You cannot cut through 5mm felt with the Olfa Cutter. Hence you need to use  a additional knife or scissor. And trim it to make it look good. Again, a bit of training will help you.
 
After you have cut the damper in the right dimensions you can further trim it to fine tune the sound. You can trim it by decreasing the outer diameter. Decreasing the outer diameter will result in less bass, or increasing the inner diameter which will make the sound more open. Be aware there are limits here. 72-73 outer diameter is about minimum, otherwise you damper does not fit well. Increasing the inner diameter will give you a more open sound, but again 48-49mm is about maximum. We have done many many tests with different dimensions. In the end, we always end up with 73 outer diameter and 48 inner diameter. Of course you can increase or decrease these diameters with about  1mm. Hence outer diameter between 72-74 mm and inner diameter between 47-49 will be good for the most of you.
 
You can also trim the dampers by making it a bit thinner around the edges, reduce the thickness a bit (max. 1 mm) around the outer and inner diameter of the damper. This also gives you freedom the tune the sound. Reducing the thickness of the outer edges of the damper will reduce the bass, reducing the thickness of inner diameter will create a more open sound. It will also take some training to trim it smoothly. Furthermore, you have to be aware that not all 5mm felt is exactly 5mm thick. There is variation in that and sometimes quite a bit. Small variations in thickness have a profound effect on the sound. By being able to trim the dampers smoothly you can create a damper that completely fits with your ‘ideal sound’.
 
Carbon pads
If you buy the carbon foam from Amazon, you can either buy 12mm thickness or 25mm thickness. For most of you  I guess 12mm is just a bit too shallow. I have very small ears, and they do not stick out, and for me 12mm is just fine.  In the end it is important that you do not make your pads too thick.  We found out that a thickness between 12-15mm is fine, choose a height close as possible to 12mm, but feels comfortable to you as well (ears do not the headphone)
 
Hence, better is to buy the 25mm thickness carbon foam from amazon.. You have more degrees of freedom. Carbon foam is ‘dirty’ material, because it contains many sandlike pieces of activated carbon. Your hands and nails get dirty working with this material, and also the surface on which you work to cut it. 25mm is thick, and it is difficult to cut through it smoothly. What I do is this. I  use a plastic flower pot with a diameter of about 11cm, put that on the carbon foam, and take a knife to cut around the plastic flower pot. For the inner dimension I use a plastic cup of about 60 or 65mm. After I have cut out the major shapes, I do all the rest with scissors.  It takes quite some time but it is doable. Especially reducing the thickness is in the end precise work. Pay attention. Again, some training will help of course.
 
A short note about the dimensions. We use 105mm outer dimension,  and 75 inner dimension. We started with 65-70  as inner dimension of the carbon pads, because that is similar to the dimensions of the Fostex pads. Increasing the inner dimension to about 75 opens  up the sound. We found out inner dimensions between 70-75 can be fine. Overdoing it will create a more hollow sound.
 
Felt pads for Cups
This is very easy. The Olfa cutter cuts through 2mm felt like butter. For the rest see section on how to cut the dampers.  
 
I like to thank Zolkis for all the work he has done, and the initiative to start experimenting with the Fostex TH900. We have learned a lot in a year of experimenting. It is really nice to have a partner in this to discuss many ideas and many many results.  I think we have bought about 4 to 5 square meter of felt of different thickness and color. But it was well worth it.
 
For me the most important result is the expressiveness of the midrange.  You hear so much more music in the midrange. You can call it more detailed, more transparent, and that is true, but it is also more (micro) dynamic. You hear so much more of the phrasing of musicians. It is addictive. My former reference the Sennheiser HD800 was also very detailed and transparent, but it missed the dynamic part. It was more removed. It is the difference between observing or feeling the music. The Fostex TH-900 lets you feel the music, now even more than before.  

Wow, thanks for that guys. maybe in the fall when I have more time to practice etc...

This would be a great mod to do if I could get all the stuff for the mod in the US. But does not seem so!

gurus said:

  This would be a great mod to do if I could get all the stuff for the mod in the US. But does not seem so!

Click to expand...

 
This looks like the same carbon foam for the earpad mod:
 
http://www.amazon.com/Finest-Filters-Activated-Impregnated-Aquarium-Filters/dp/B005AK9FQ0/ref=sr_1_4?ie=UTF8&qid=1429941940&sr=8-4&keywords=Activated+Carbon+foam
 
It is kind of expensive with shipping though. If anyone wants to go splitsies with me, please PM me to let me know

stjj89 said:

gurus said:

 
This would be a great mod to do if I could get all the stuff for the mod in the US. But does not seem so!

Click to expand...

This looks like the same carbon foam for the earpad mod:

http://www.amazon.com/Finest-Filters-Activated-Impregnated-Aquarium-Filters/dp/B005AK9FQ0/ref=sr_1_4?ie=UTF8&qid=1429941940&sr=8-4&keywords=Activated+Carbon+foam

It is kind of expensive with shipping though. If anyone wants to go splitsies with me, please PM me to let me know

Click to expand...

I'm not ready yet and am in a different country. Side bar going to San Fran in June, if you like pm any recommendations for bars, music, or live jazz etc if you want. Lastly I think this represents a biz opportunity for the creators, either from a diy supply or full service. Just saying.

whazzzup said:

Lastly I think this represents a biz opportunity for the creators, either from a diy supply or full service. Just saying.

Click to expand...

 
Agreed. I'd be happy to have Zolkis (or someone equally experienced) take a look at my TH 900's, but I'm afraid I'll butcher the headphones if I took a crack at it myself.

whazzzup said:

 Lastly I think this represents a biz opportunity for the creators, either from a diy supply or full service. Just saying.

Click to expand...

 
I consider this as an "open source" development. Take it, enjoy it, develop it further, share it. I would not have done it for money . My time is too expensive.
 
Though @playitloud also has a say on this, from my part it is also fine if someone wants to make this mod for others, and it's fine to charge a fair price for the work and materials.
 
My reason for that is the more TH900 are updated, the more the industry is likely to pay attention to these kinds of details.
I've been just wondering, why could we improve 3 very good headphones (TH900, SR-007, SR-009 and I would have a few clues for Audeze and Hifiman as well) with just a side-hobby interest and some tinkering, and why wasn't that done by the manufacturers, especially when they have a few decades experience in making headphones. And actually the same goes for speakers. 
 
@Zojokkeli  once we live in the same country, you can come over and compare your TH900 with mine. If you like it, I can help you making your mod. PM me if interested, usually I don't have much spare time but we could figure out something.

Hey man, modders like you guys help keep this hobby alive! You guys represent the optimism that there is something better out there. Look at what Mark Lawton and Dan Clark did for the community. Modders are the pioneers man. I am personally a big fan of the Lawton mods, but since yours are entirely reversible I may try these first.

The Lawton basic mod is also completely reversible . It's for people who like their TH900 as it is, but want to tame the bass a little bit. This mod is for people who want better midrange - and will also get better bass and sound stage. I think it's well worth the price for anyone joining now - I have spent the price of a good headphone on all these experiments, but still I say it was totally worth it.
 
Also, you become less susceptible to upgradeitis; instead, you try to improve what is is possible .