Fostex T50RP Incremental Mods and Measurements

[giorbe]

Hello,
I'm new and I can't open new threads so I'm using this one.
 
I read about a lot of mods but none addresses distortion on the mid highs, that is prominent on the stock t50rp.
 
All the measurements that I found are much more oriented towards the frequency response (that IMO is a secondary problem)
 
I found the dampening and securing the driver's area where the cables are soldered, with blue tac, greatly beneficial.
That part is surprisingly mobile, like a clarinet baffle, if left free it just resonates to those mid-high frequencies, creating distortion.
I wasn't able to find anything abut this problem and all the putty application mods seem to neglect this specific problem.
 
I didn't try to dampen the rest of the front baffle, where the driver is mounted, but it would reduce the reflex volume, and I'm not sure it would work well.
 
Any thoughts?

 

[ZGLISZCZ]

  Hello,
I'm new and I can't open new threads so I'm using this one.
 
I read about a lot of mods but none addresses distortion on the mid highs, that is prominent on the stock t50rp.
 
All the measurements that I found are much more oriented towards the frequency response (that IMO is a secondary problem)
 
I found the dampening and securing the driver's area where the cables are soldered, with blue tac, greatly beneficial.
That part is surprisingly mobile, like a clarinet baffle, if left free it just resonates to those mid-high frequencies, creating distortion.
I wasn't able to find anything abut this problem and all the putty application mods seem to neglect this specific problem.
 
I didn't try to dampen the rest of the front baffle, where the driver is mounted, but it would reduce the reflex volume, and I'm not sure it would work well.
 
Any thoughts?

Hi Giorbe,
It might be that part of diaphragm with soldering pads is resonating, its tin piece and its hold only from one side, It might be a good idea to use some tape to stick it to the baffle (and first secure both soldering joints with small pieces of paper) I wrap everything with bee wax so didn’t had any issue with that. But you might also mean plastic resonation which is one of main issue with stock tXXrp’s.

 

[ZGLISZCZ]

   
Thanks for this information.
 
I wonder if there is another variable that might be responsible for the impression that the bass quality/quantity is changed when uisng the Fazor? Some additional variables to consider include a thinner diaphragm, different rear side damping materials and their configuration, higher impedance, different magnets (strength and layout), and diaphragm tensioning.
 
It will be interesting to test for this effect with T50RP.

 
I will try to make two variants for fostex, one with 7 guides like you was thinking and another with each hole rounded separately. But that will be work for next weekend.
 
By the way I just received your diaphragm thanks again for that. Time for sound rebel came back

 

[giorbe]

  Hi Giorbe,
It might be that part of diaphragm with soldering pads is resonating, its tin piece and its hold only from one side, It might be a good idea to use some tape to stick it to the baffle (and first secure both soldering joints with small pieces of paper) I wrap everything with bee wax so didn’t had any issue with that. But you might also mean plastic resonation which is one of main issue with stock tXXrp’s.

 
Hi ZGLISZCZ,
Thanks for the reply.
Yes, I did fix the area with the soldered cables immobilizing it with blue tac. And it really worked well.
 
I reduced the reflex port sticking some tape with a small openings to increase bass, and that worked fine as well.
 
I first tried to dampen the bottom of the cups with blue tac (to reduce the possible plastic cups vibrations, that you mentioned) but that reduced the internal volume of the cup, reducing the bass response and putting the upper part of the spectrum in evidence, resulting in more evident distortion. At least this is what I think happened. I removed it.
 
What I'm surprised of is that all the mods I found don't explicitly mention this problem, and go in the opposite direction sometimes reducing the internal volume of the cups with various materials that are not transparent to bass frequencies - cotton is, wood and plasticine are not.
 
How do you use bee wax? A thin melt layer? That would be interesting because it wouldn't reduce much the volume of the cup.

 

[ZGLISZCZ]

Hi ZGLISZCZ,
Thanks for the reply.
Yes, I did fix the area with the soldered cables immobilizing it with blue tac. And it really worked well.

I reduced the reflex port sticking some tape with a small openings to increase bass, and that worked fine as well.

I first tried to dampen the bottom of the cups with blue tac (to reduce the possible plastic cups vibrations, that you mentioned) but that reduced the internal volume of the cup, reducing the bass response and putting the upper part of the spectrum in evidence, resulting in more evident distortion. At least this is what I think happened. I removed it.

What I'm surprised of is that all the mods I found don't explicitly mention this problem, and go in the opposite direction sometimes reducing the internal volume of the cups with various materials that are not transparent to bass frequencies - cotton is, wood and plasticine are not.

How do you use bee wax? A thin melt layer? That would be interesting because it wouldn't reduce much the volume of the cup.

Hi. You can use other materials just like most people. I didn't try much only bee wax, blutac and some less-blutac thing. I didn't like blutac but hard to say why it might be that it just didn't work for my mod. You can warm bee wax with hand and apply piece by piece or warm it in small pot and apply when its liquid. I made custom enclosure so I don't have experience with stock setup. But there are many guys there and you can be sure that someone will reply in few hours also blutac won't damp your fostexboys. Try also "just listened to some fostex t50rp WOW" thread. Fight with those bastards they can sing like nothing else.

 

[c61746961]

Hi ZGLISZCZ,
Thanks for the reply.
Yes, I did fix the area with the soldered cables immobilizing it with blue tac. And it really worked well.

I reduced the reflex port sticking some tape with a small openings to increase bass, and that worked fine as well.

I first tried to dampen the bottom of the cups with blue tac (to reduce the possible plastic cups vibrations, that you mentioned) but that reduced the internal volume of the cup, reducing the bass response and putting the upper part of the spectrum in evidence, resulting in more evident distortion. At least this is what I think happened. I removed it.

What I'm surprised of is that all the mods I found don't explicitly mention this problem, and go in the opposite direction sometimes reducing the internal volume of the cups with various materials that are not transparent to bass frequencies - cotton is, wood and plasticine are not.

Yep, I found the same issue when using closed-cell foams (Paxmate, etc). I don't think the cup vibrating is as big of an issue as the backwave bouncing around and re-combining with the frontwave, to fix that you have to absorb as much midrange energy as possible. In the end I settled with a combination of dense open-cell foam with latex foam (from makeup pads), no akasa on the cups, plasticine on the back of the baffle. This flattened the mids nicely.

 

[giorbe]

Yep, I found the same issue when using closed-cell foams (Paxmate, etc). I don't think the cup vibrating is as big of an issue as the backwave bouncing around and re-combining with the frontwave, to fix that you have to absorb as much midrange energy as possible. In the end I settled with a combination of dense open-cell foam with latex foam (from makeup pads), no akasa on the cups, plasticine on the back of the baffle. This flattened the mids nicely.

 
I'm under that very same impression, that the baffle and driver support, even if it's cheap plastic, are not the main problem. The baffle has a lot braces that should break any mechanical resonance, so does the driver encasing.
The cups are not that flimsy either, and they don't offer big resonating surfaces, structurally. They do offer a resonating chamber that is poorly dampened, as you said. I'll try your remedy, I'm assuming a sandwich loosely shaped on the cups,  behind the driver, not compressed.
Thank you for your suggestions

 

[c61746961]

At the moment I have two ~8 mm thick stripes of the dense polyurethane foam fit onto the cup 'wells' (the deepest sections of the cup), over that a makeup pad on each cup (no measurements at hand), they are a bit compressed, I will provide pictures if you're interested. I do have 1mm creatology felt lining all the inside walls of the cups, probably not necessary, though. I found the latex foam to be very good at absorbing treble energy, and the dense polyurethane foam a little better at absorbing the midrange. I tested this using a liberated driver and using pieces of equal thickness of the different materials between the driver and a microphone with level meter / my ears, at specific frequencies and sweeps, open-cell foams tend to absorb rather than reflect the energy.

 

[giorbe]

At the moment I have two ~8 mm thick stripes of the dense polyurethane foam fit onto the cup 'wells' (the deepest sections of the cup), over that a makeup pad on each cup (no measurements at hand), they are a bit compressed, I will provide pictures if you're interested. I do have 1mm creatology felt lining all the inside walls of the cups, probably not necessary, though. I found the latex foam to be very good at absorbing treble energy, and the dense polyurethane foam a little better at absorbing the midrange. I tested this using a liberated driver and using pieces of equal thickness of the different materials between the driver and a microphone with level meter / my ears, at specific frequencies and sweeps, open-cell foams tend to absorb rather than reflect the energy.

I think I understand your use of materials. If the makeup pad is not covering the driver completely the felt lining could take care of some escapee in the cup. Your measurements are very interesting and will help me selecting the materials. 
The kind of distortion that I hear is in the mid-high range, and I suspect that stuff like cotton or loose wool won't be enough, too high of a cut. Need to cut lower in frequency with more dense stuff, but not too low as with heavy foams.

 

[c61746961]

Yeah, don't forget to immobilize whatever you have put inside the cup along with the mobile parts of the driver assembly. I didn't perform measurements using REW or Arta, so I can't provide FR plots, I just performed quick meter checks to ensure that the attenuation I was hearing was really there.

 

[bluemonkeyflyer]

Random Thoughts On Modification of T50RP and Other Planar Magnetic Headphones

 

Reducing or closing the baffle port will increase bass at the risk of increasing bass bloat into the mids and veiling treble frequencies.

 

Sealing all the cup vents will reduce bass too much. Bass will sound even more rolled off and thinner than stock configuration. Treble may become more pronounced. Mids may sound more upfront.

 

Significantly reducing the cup vents to a single opening of just one of four cup vent slots ~ 1mm x 3mm (maybe more depending on the rest of the config) and closing off the remaining vents will tighten bass, reduce bass bloat into the mids, and reveal hidden treble.

 

T50RP uses a bipolar driver so it produces identical sound waves/pressure into the cup as out to the ear side. The "back wave" is 180 degrees out of phase with the "front wave." The back wave is one of the biggest problems that must be solved and controlled when using a closed or semi-closed cup like T50RP. Rear damping is much simpler if the design is fully open.

 

In stock form, the T50RP's back wave reflects and bounces all around inside the cup, producing standing waves, nulls, and peaks. Any of the back wave not absorbed by additional damping escapes through the damped cup vents. If escaped back wave "finds" an entryway into the pad-to-head chamber, you risk bass cancellation via nulls. Any back wave not dispersed, absorbed, and vented is reflected back to the ear side causing smearing and distortion of the front wave which we do not want to happen.

 

With regard to acoustic damping and mechanical damping, mechanical damping is much less important, IMO/IME. I think Fostex tried to control and limit mechanical distortion by strengthening the baffle with partitions and isolating the baffle from the cup with shock absorbers. In addition, the driver is solidly attached to the baffle with 3 screws, 2 large side snap tabs, and a foam gasket between the driver and the baffle. The 4 shock absorber rubber caps make direct contact with the driver at all four corners. This limits sympathetic resonance by decoupling the cups from baffles. 

 

Mechanical damping using many types of materials in the baffle compartments, as well as on the ear side of the baffle, such as plasticine, Newplast, tungsten putty, Dynamat Extreme, Acoustipack Lite and various combinations of these materials have generally failed to provide sonic benefits in my experience. I have not heard detectable sonic differences when blinded during A/B tests. I've also seen no significant differences in measurements whether these materials were used, or not. Granted, I've not measured distortion but what I hear is either limited distortion or beneficial distortion.  

 

Acoustic damping, on the other hand, results in significant, clearly audible, and measurable results. Acoustic damping reduces the internal cup volume (dimension) but this is not necessarily a bad thing. Filling the cup internal volume is not a bad approach at all. It's also not the only approach that can work but the backwave must be dealt with and controlled.

 

To handle the acoustic threats to good sound, damping materials like foam and cotton (just to name a few materials that work) may be added to the cup and baffle to disperse, absorb, and to provide controlled venting of the backwave.  Ear side damping may also be needed to manage ear side reflections. Materials of various types and in many different configurations result in very similar results. This I've personally experienced  when comparing various modified T50RPs. Others have posted similar opinions.

 

Using acoustically non-transparent materials as part of a mod configuration is fine but, when used in isolation, may not work. 

 

Pad Rolling is yet another variable to consider because of their inherent effects on sound quality are significant. The type of foam and density, composites of different foams, permeability of foam, vented vs perforated pads, and leather/pleather vs velour all have very real effects on the sound quality.

 

If anyone can provide information about how to detect and measure distortion, I'm interested in learning from you.

 

[bluemonkeyflyer]

   
I will try to make two variants for fostex, one with 7 guides like you was thinking and another with each hole rounded separately. But that will be work for next weekend.
 
By the way I just received your diaphragm thanks again for that. Time for sound rebel came back

 
Good to hear the T50RP diaphragm finally arrived.
 
Here are the other LCD2 v1 dimensions. 
 
Thoughts?
 

 
Thanks so much for all your help with CADs for T50RP and LCD2 v2 Wave Guides!

 

[giorbe]

 

Random Thoughts On Modification of T50RP and Other Planar Magnetic Headphones

 

(...)

 

If anyone can provide information about how to detect and measure distortion, I'm interested in learning from you.

 
Hi bluemonkeyflyer,
It is very nice to know that some of your conclusions are pretty similar to my much less founded first impressions.
I will assume a good mechanical structure, and go ahead reducing the chamber resonance.
So as c61746961 did I will try to reduce mid highs bouncing around the cup with appropriate materials, in order to avoid the reduction of the volume of the cup where the reflex is tuned for, that is bass. That is the reflex volume for the bass will stay the same if I fill the cup with mid high absorbing materials. Just like speakers are often filled or lined with rock wool or felt, same principle.
Different materials have different acoustic qualities at different frequencies
 
Regarding evaluating distortion, I don't have any other tool but my ears, that are not very reliable per se, but it is really easy to spot distortion in choirs, piano music, and opera human voice in general. Easier with classical music, kind of jumps at you, really.

 

[ZGLISZCZ]

Good to hear the T50RP diaphragm finally arrived.

Here are the other LCD2 v1 dimensions. 

Thoughts?




Thanks so much for all your help with CADs for T50RP and LCD2 v2 Wave Guides!

and today I received two t20rp's. I forget how "nothing special" they sound, and stock earpads comfort... But they burnin on my head quite fast. I have question about lcd wave guides. Is there anything that will cause issue if I will make whole element in circular shape with radius 91.5mm and 2mm or 1,5mm thick? Like too short screws or enclosure fitting etc.

 

[MuZo2]

T20 old or new?