[OnyxOcelot]

Hey Folks,
 
Just thought I'd check back in with my latest ES headphone driver build.  No grand new ideas here, but I was able to get access to a CNC machine capable of cutting rather accurately, so am pleased with the build quality.  I am still having some difficulty finding the best way of stretching the Mylar, but was able to get a taught set of diaphragms on the third or forth attempt.  My general impression is that they sound rather good: not very bass-heavy, compared to my last pair -- probably because the diaphragm is stretched tighter -- with clarity throughout the midrange.  
 
I still have yet to build a case, and am listening to them strapped on the front of the hollowed plastic shells of some cans from the 1970s.  An open backed case would give me a better sense of how they should sound, I believe.
 
 
I would like to understand more fully how each of the variables we can alter in making these headphones affects the output level and quality.  See below the variables I know and do not know for this current build.  IT is quite likely that some of these variables do not affect sound quality (stator thickness and hole shape seem unlikely to make much difference), but I would eventually like to know exactly which variables affect sound in what ways.  If anyone can suggest reading on the subject, or has thoughts, please let me know.
 
For the time being, I'll keep chugging away on these designs if you all keep chugging away on your designs.
 
Happy Building,
OnyxOcelot
   
 
 
SPECS:
 
KNOWN VARIABLES:
DIAPHRAGM AREA.......................................................3.5in x 2.5in
DIAPHRAGM MATERIAL...................................1.4 Micron Mylar film
DIAPHRAGM COATING.........................Static Guard anti-static spray
DIAPHRAGM-STATOR SPACING..........................................0.021in
DIAPHRAGM SHAPE................................................................Oval
 
STATOR THICKNESS..........................................................0.021 in
STATOR HOLE SHAPE.................................................vertical slots
 
POWER SUPPLY...........................Stax SRD-7 (Self-Biasing) Adaptor
 
UNKNOWN VARIABLES:
STATOR OPEN AREA 

 

[n3rdling]

You might want to use thicker PCB for your stators on your next attempt and also change the long slots to holes.  With such thin stators and those long slots, I imagine you're getting some stator resonating while music plays, which increases the distortion.  I think I remember somebody trying the slots before and saying they weren't getting much bass also. 
 
Output level depends mainly on spacer thickness/bias voltage/driving voltage, with some smaller influence from open area % of the stator design.

 

[chinsettawong]

Nice looking stators.
 
How do you stretch your Mylar now?  To get good bass, you need to tension your diaphragm just right - good stability and not too tight or loose.  It's not easy.
 
For me, stretching the Mylar to the exact right tension is still a problem.  I have to do and redo it many times until I'm satisfied.  Maybe it's time that we focus on experimenting the best diaphragm stretching method.  If you guys have any thoughts, please share them.  
 
 
Wachara C.

 

[chinsettawong]

I made this design a long time ago, but never tried to make it.
 

 
However, my CNC machine works better with drilling than routing.  So, I stick with drilling holes on the stators.
 
Wachara C.

 

[marsupialx]

Perhaps a non-workable idea, but the way a screen is tensioned on a screen door or a screen is tensioned on a silk screen, if the stator (or perhaps one of the spacers) had a groove around its perimeter, with an o-ring (rubber or metal) placed over the mylar (over the groove), when the assembly is bolted together, it would pull the mylar tight. Experimentaion would be required. Just a thought.

 

[OnyxOcelot]

  You might want to use thicker PCB for your stators on your next attempt and also change the long slots to holes.  With such thin stators and those long slots, I imagine you're getting some stator resonating while music plays, which increases the distortion.  I think I remember somebody trying the slots before and saying they weren't getting much bass also. 

Yes, I made slots instead of drilling to reduce machining time (I am working on borrowed machine time), but there have been some issues of stator stability -- no resonance yet, but a certain amount of bending as they are fit into the (makeshift) case.  
 
Wachara, on this pair of drivers, I tried a couple methods of stretching the diaphragm:
 
                  1. I built a rectangular frame with a bicycle inner tube stretched around the outside.  I would use gaffer's tape to attach the mylar around this frame, then inflate the inner tube.  I never quite achieved a high enough tension before the Mylar ripped using this method.
 
                  2. My most successful stretching methods have been the simplest.  I stretched the Mylar using many pieces of gaffer's tape on a flat, clean table.  This allowed me to progressively stretch the diaphragm by moving each piece of tape outward a bit at a time.  This method is in no way repeatable, but without a proper stretching jig, this has been quite successful.
 
I have noticed that I occasionally get a very high pitched whine in one or both drivers.  It does not sound like a noise issue in the signal, but something else: perhaps a repeated partial discharging of the diaphragm's static charge to the stator.  Perhaps something else.  It may or may not be humidity dependent.  Do any of you have any experience with this particular issue?
 
Best,
OO

 

[davidsh]

I have read that a whine might come from corona discharge. Especially forms at sharp edges on the stator

 

[chinsettawong]

Hi OO,

I'm not sure why you can't get enough tension on your diaphragm using inner tire method. As for me, I really have a hard time not to put too much tension on the diaphragm. It's so easy to pump too much air into the tire and that the diaphragm is too tight.

About the noise in the drivers, it's very common. Most of the time it's dust in the driver. You have to clean your stators and diaphragm very well before you put everything together. It takes a lot of patience to clean. Even a very tiny piece of dust can cause this annoying noise.

Wachara C.

 

[davidsh]

I plan to use dishwashing liquid for coating. How much is needed, give or take?

 

[Tachikoma]

With the antistatic fluid I have, I just splatter it on, and rub it till its even-ish.
 
I have another question about the coating, using the method I mentioned above, my pro bias drivers are now unstable at 580V bias, but its very hard to use less coating, keep it stable at 580V bias and still achieve perfect channel balance. Any ideas on how I might do that?

 

[chinsettawong]

If your diaphragm is unstable, it's because that you haven't put enough tension on your diaphragm.  It's indeed very difficult to tension your diaphragm just right.  Lowering bias voltage certainly helps the stability, but you lose some efficiency.
 
Wachara C.

 

[davidsh]

I'll be using 10 or 13 um food film for my physics speaker project. Sorrily, mylar was too expensive. Hopefully the film isn't too weak, but it's low density polyethylene. 
 
I'll try using rinse aid for dishwashers, couldn't find any dishwashing liquid. I'll probably just dissolve it in ordinary tap water.
 
Wish me luck... Or something. And feel free to shoot me down, but my options are limited.

 

[chinsettawong]

Food film can sound very nice.  Go for it.
 
However, the liquid soap, in my experience, is not very good.  If you really can't find the computer screen cleanser, you might want to try some easy to find glass window cleansers. I think some of them might just work.  
 
Wachara C.

 

[davidsh]

I just thought to myself that rinse aid is supposed to be anti static and it seems to spread out nicely on the film. I even succeeded in measuring a 20 Mohm resistance after applying it a bit excessively (2 copper plates separated by a couple of millimetre)
I'll see what I come up with, might find some screen cleansing gel.

 

[chinsettawong]

That sounds like a winner.  Go for it.