[purk]

Leading the way again K P Wachara!!  Man...you really outdone yourself again this time.

 

[jgazal]

I have been reading the SR-009 imbalance thread and I think it is better to post my questions here.
 
Humidity, diaphragm tension and diaphragm coating resistivity were already mentioned in this thread. But humidity is usually mentioned as a factor when using the drivers and I have not found any mentions of the relationship between humidity and the coating process.
 
This article describes the effects of humidity in very sensitive production processes: 
 

By far, the most pressing need for humidity control in a semiconductor cleanroom comes from photoresist sensitivities. It's photoresist that demands the tightest (most expensive) control limits because its properties are so sensitive to relative humidity.

Actually, both relative humidity and temperature are critical for resist stability and precise dimensional control. Even at constant temperature, photoresist viscosity decreases rapidly with increasing relative humidity. Changing viscosity, of course, changes the thickness of a resist film spun-on by a fixed coating recipe. Reference 2 cites an experimental demonstration in which a three percent variation in relative humidity produced a thickness variation of 59.2 A (sic) in resist thickness.

In addition, resist swelling following a bake cycle can be aggravated by water absorption at high relative humidity. Resist adhesion can also be adversely affected by high relative humidity; low humidity (~30 percent) facilitates resist adhesion even without polymeric modifiers, such as hexamethyldisilazane (HMDS).

Relative humidity control in a semiconductor cleanroom is not optional. But, from time to time, it's good to review the reasons and bases for common, universally accepted practices.

"Why control humidity in a cleanroom?", Robert P. Donovan, Solid State Technology

 
Do you feel from your experience that coating the diaphragm in very humid days derives more drivers with imbalance? 
 
I see that DIY drivers use a FR4 ring to hold the diaphragm. What material the SR-009's ring is made of? Is it metal like the stators?
 
I wonder if, contrary to the use of a FR4 ring to hold the diaphragm, a metal ring worsens the dimensional control of the ring/diaphragm structure.
 
Anyway, if it is such a precise process, a 95% reject rate is not bad at all. And congratulations to all DIYers of this thread that achieve such stability!

 

[n3rdling]

I don't think humidity while coating will matter much for arcing, but it might have a slight effect on the resistance of the coating.  Humidity plays a factor in arcing because the voltage breakdown of air gets decreased with more humidity thus an arc in the small gap between stator and vibrating diaphragm.  Think of it like making the gap smaller.  Humidity while coating wouldn't really make the coating thicker (ie lessening the gap).  If it lowers the resistance this can lead to more current on the film and possibly more damaging arcs but it's probably not a big issue.  I believe Wachara lives in a very humid climate so maybe he's noticed a difference in summer/winter.
 
Whatever material Stax uses for spacers, it isn't something as conductive as metal.  That would assure constant arcing.  The reason FR4 is used is to isolate the voltage on the diaphragm from the voltages on the stators since FR4 has a high resistance.  There was one member here who had ants in his SR-009 a while back and he ended up making a thread with a bunch of detailed pictures of the drivers.  That's the only place I can think of that might show the spacers used.

 

[dude_500]

Humidity affects the coating material while in use, not while coating (at least as far as I've experienced and can conceive of). It also seems to accelerate the wear on DIY coatings. The pros have it figured out well enough that the coating isn't going to just go bad... at least not for most, maybe the SR-009 does.
 
I'd say it's not so much about arcing as it is about stability. Well designed headphones will simply never arc. But diaphragm stability is much much harder to design for. 
 
In my building experience, it is very possible to make a diaphragm in November that will last the entire winter, and the first humid hot spring day it's shot forever, eternally unstable.

 

[chinsettawong]

I too don't think humudity during the coating process has any bad effect on the coating material - my coating material is actually water base.
 
From time to time, my headphones do have some imbalance issue.  Most of the times when that happens, I find some small arc spots on the diaphragm.  The arc isn't severe, it just leaves spot marks on the diaphragm.  Recoating usually fixes it.
 
Wachara C.

 

[chinsettawong]

  I see that DIY drivers use a FR4 ring to hold the diaphragm. What material the SR-009's ring is made of? Is it metal like the stators?
 
I wonder if, contrary to the use of a FR4 ring to hold the diaphragm, a metal ring worsens the dimensional control of the ring/diaphragm structure.
 

 
Do you know that Stax also uses PCB as stators and spacers is their SR007?
 
In Stax Lambda series, the spacers are made of brass rings.

 

[jgazal]

Thanks n3rdling, dude_500 and chinsettwong. 

 
I knew spacers need to insulate stators and diaphragm, but I did not know the Lamda series had brass rings. So do they use electrical insulating coating at the rings or stators?
 
I have been a fool trying to figure out what is happening, because there are too many variables…

 

[chinsettawong]

  Thanks n3rdling, dude_500 and chinsettwong. 

 
I knew spacers need to insulate stators and diaphragm, but I did not know the Lamda series had brass rings. So do they use electrical insulating coating at the rings or stators?
 
I have been a fool trying to figure out what is happening, because there are too many variables…

 
It's easier to just show you the pictures than to try to explain it.
 

 

 
The stator is glued to a piece of plastic.  The stator is perhaps powder coated.  So, it's insulated.
 
When put together, the different thicknesses of spacer and stator makes the air gap of around 0.4 mm.  There is about 1 mm of spacing between the outer diameter of the stator and inner diameter of the spacer.

 

[urlgr-A]

Dear Chinsettawong. Yet another headfier  you have inspired. I have been reading and re-reading this magnificent thread for some weeks ( and I am still not finished) and decided to join the forum because I think it is time to contribute and say thanks to you and many others that have so kindly shared your experiences. I have been thinking a lot about making my own set of electrostatic headphones for some years and it was only when I was poorly with a bad stomach some weeks back that I came across this thread that seems to contain exactly the accumulation of info that I think I need to start having a go myself. So... post number 1 here goes...
I have taken on a big project, as I have to build myself an amp as well. I am looking at the exstata possibly. Yours or anyone else's advice here would be appreciated.  I don't have a CNC router but I can design using CAD and frequently use a 3D printing house for making parts for my other little projects. 
I want to concentrate on the design of the driver parts and experiment a bit and this thread has that well covered. 
So please tell me what you think if you can.
1) I am considering using the perforated metal sheet found in microwave oven doors as my stator material (scrapped doors I mean!). It is already beautifully flat (usually) and free from burrs and sharp edges. The holes are round and on some ovens are about 1 mm diameter, spaced 'very close' (not sure how close as it varies from door to door). I think the doors are made of steel sheet and insulated with some special paint. I can cut the shapes of the stators out of this I thought.
2) Is there a way you can specify the tension you use on your diaphragms that can be easily reproduced by me? For example, the size of the masses used to stretch the film, or perhaps you can specify the natural resonant frequency of the stretched film (such as what one might hear if it is gently tapped with a finger).
3) Have you considered stretching the film in one direction only? I mean rather than trying to stretch it in two or more directions. Don't some ESLs use film only unidirectionally tensioned?
4) Do you think it would be useful to have a mechanism built-in to the driver in which the film tension can be adjusted in-situ so that it would be possible to tune a driver to find its 'sweet spot' so-to-speak?
Thanks in advance!

 

[n3rdling]

1.  That sounds like a good material to use on the cheap, but it'll depend a lot on the open area %.  40-50% is preferable.  You can get a sheet of perf metal for pretty cheap online, might not be worth a trip to the scrap yard.  Be careful when cutting out the shape of your stator so you don't bend the metal at the cut line (metal snips will do this).
2.  chinsettawong has some pictures in this thread showing his stretching rig.  IIRC he uses something like 6 water bottles suspended off a small table.  I'm not sure how large the piece of Mylar is though.
3.  If you stretch only one way you might get some long wrinkles in the film which wouldn't be good.  I don't really see a reason to do this, especially with the tight tolerances for a HP driver.
4.  That's a pretty interesting idea but I think it would be really hard to implement.  The film is glued down to a spacer so really you're trying to have an adjustable spacer.  One way this might be able to be done is to use some kind of elastic spacer that is compressed between the stators.  You then try to figure out a way to cut a notch into that spacer and a notch into one of the stators, both all the way around.  Align them face to face and as you tighten the bolts holding everything together, the notches will 'slide' the elastic outward.  This is hard to describe but it doesn't seem like fun to try.  An easier thing to do is to adjust the tension by heat shrinking the film.  It wouldn't be reversible, but you could at least see what tension (measure the resonant frequency) gives you the best sound.  Normal PET film has about 1.7% shrinkage under heat, but there is heat shrinkable kind that has something like 25% shrinkage...only problem with that stuff is that it's pretty thick (~25 um I think).

 

[khbaur330162]

I like the idea of point #4. I agree, it seems complicated. Best I could come up with is you have oversized, overly thick spacers on a diaphragm set at the lowest "desired" tension within the range. The stators would be threaded inside the inner diameter of the oversized spacers (variable stator spacing). Attached to the stators, however, would be "secondary" spacers. These secondary spacers would actually determine your diaphragm's active area and stator spacing. As you screw the first stator into place its secondary spacer applies the desired tension to the diaphragm by pushing on the diaphragm creating a "plateau" shape in it. Then the second stator is tightened to clamp the diaphragm down. I'd imagine the threads would have to be pretty fine in order to achieve high adjustment resolution and I think it would be better if the secondary spacers were more like washers (not attached to the stators) so that they would move in/out freely and apply less rotational force to the diaphragm when the second stator is tightened.

 

[arnaud]

  I like the idea of point #4. I agree, it seems complicated. Best I could come up with is you have oversized, overly thick spacers on a diaphragm set at the lowest "desired" tension within the range. The stators would be threaded inside the inner diameter of the oversized spacers (variable stator spacing). Attached to the stators, however, would be "secondary" spacers. These secondary spacers would actually determine your diaphragm's active area and stator spacing. As you screw the first stator into place its secondary spacer applies the desired tension to the diaphragm by pushing on the diaphragm creating a "plateau" shape in it. Then the second stator is tightened to clamp the diaphragm down. I'd imagine the threads would have to be pretty fine in order to achieve high adjustment resolution and I think it would be better if the secondary spacers were more like washers (not attached to the stators) so that they would move in/out freely and apply less rotational force to the diaphragm when the second stator is tightened.

 
Or you could use a hair-dryer and heat shrink until both diaphragms resonate at the same frequency . Wachara does this by ear, any cheap microphone hold close to the electrode could easily measure the open air resonance. BTW, it's variable with stax phones but I recall in the 120-150Hz range for my omega 2 and SR009 phones.

 

[urlgr-A]

Hello khbaur330162
Thanks for you comments and ideas about adustable diaphragms. What you described is very similar to what I had in mind, if I understood you correctly. Anyways my thinking was as follows:
The diaphragm is held at a 'lower' tension in a large stiff outer frame (the frame  is a sandwich of two 'hoops') and the stators are inserted into each side of the frame with spacers already attached to them. Regarding the adusting screws, I was thinking about them being inserted radially in the large outer frame. The stators would  have their own frames with slots to accept the ends of the adusting screws. The end of each screw is conically bevelled at say, 45 dergrees. As the screw is turned clockwise, its conical end engages the slots on the stator frames such that it works like a wedge pushing the stator assembly up and stretching the diaphragm on a "plateau" as you said. One would have to adjust each screw in turn a little at a time.
I think your idea is better here with threaded stator halves, although this would mean the drivers would have to circular. I don't mind... Fine thread pitches yes indeed.
I have been sketching out ideas for these parts to be 3D printed in Shapeways black (nylon?) plastic.
 
Hmm... does look a bit of a challenge... still there's always that old hair drier

 

 

[chinsettawong]

You can also do it my way.  I always cut more spacers than I need.  I glue a few more diaphragms to play with different tensions.
 
If you want to do the adjustable diaphragm, please do not forget that you're playing with lethally dangerous high voltage too.  Be very careful.
 
Wachara C.

 

[chinsettawong]

By the way, if you tension your diaphragms using a tensioner, the diaphragms are already very tight.  The hot air from a hot air gun can't help to tension the diaphragm that much anymore.
 
Wachara C.