ESL diaphragm coating

[I_Forgot]

I needed to replace a diaphragm in my Quad ESL 63s during the recent rebuild. I have been experimenting with some weakly conductive coatings and found one that seems to work quite well (though it remains to beseen if it works for several years).

I was recently cleaning up a woodburning stove so I coiuld sell it so I picked up some stove-black at the local hardware store. The stuff is a wax + black pigment (carbon?) paste. I figured the pigment might be carbon and so the stuff may be conductive so...

I stretched a diaphragm and applied some of the goop to it with a paper towel. It goes on smoothly and it's easy to control where it goes. It can be softened further by putting a few drops of water on the paper towel. When the coating dries (about 30 seconds after application) it looks very much like the original Quad coating.

After applying it to the film I put a couple coins on it and measured the conductivity using my DMM. Depending on where I put the coins, it read between .1nS and 0.5 nS (that's equivalent to 10 Gohms - 2 Gohms. I let is sit for a few days to make sure it was fully dried and checked again and the results were the same.

I have installed that diaphragm in my ESL63 and it is working perfectly.

Stove-black costs about $2 for enough to coat about 1000 speakers.

I_F

[SY]

Nice bit of improv! Stability might be questionable, but if you don't mid getting in there and getting your hands dirty, you can always reapply more coating.

[Brian Beck]

It's good to find relatively common household products for use as diaphragm coatings, but one is never sure of stability, the effects of humidity, temperature and added mass, etc. I live in coastal Florida where it can be pretty humid (despite AC) in the summer, but we also have some pretty dry air days in the winter. So the effects of humidity are important to me. My ESL-63s seem to perform consistently however.

Perhaps this will interest some folks: I came across U.S. patent #5,590,3212 granted to Sony engineers for an ESL coating they developed. Take a look at it at www.uspto.gov. The inventors give several formulas, but they all have in common a water solvent solution containing a conductive high-molecular weight monomer, a chemical oxidizer and a dopant. Choices for these various compounds include some commonly available chemicals, some even household chemicals. The diaphragm is dipped in the coating (for varying times depending on desired conductivity) and then rinsed and dried. In other words, no painting or rubbing required.

So what? The coating is said to be extremely uniform on the surface, adds little weight, but most importantly, the resultant conductivity varies very little due to humidity, aging and temperature (curves are presented in the patent for these variables). The conductivities shown in the patent are higher than I’d typically want, at around 10^5 ohms/square, but one should be able to vary the solution concentration and dipping times to control the conductivity. While one can’t use the patented formula for commercial projects without Sony’s permission, this might be worth trying for home projects. I haven’t mixed one of these formulas yet, but may try it soon.

[Zvon]

Brian,

The nominated coating resistivity appears low for all but ESL tweeters.

Regards,
Zvon

The conductivities shown in the patent are higher than I’d typically want, at around 10^5 ohms/square, but one should be able to vary the solution concentration and dipping times to control the conductivity.

[meffi]

The correct Pat. Num. : 5,590,212

[Brian Beck]

Yes, you are right; I typed an extra number digit in my posting. Just checking to see who is paying attention Thanks

By the way, I find that patent searching at the US patent site is a great, and underused, way to learn about anything technical. I frequently search for audio-related patents. I have tried to print out every patent related to electrostatic speakers, but now my stack of patents exceeds 3 inches (ok, make that 76 mm). We often must relearn what the inventors were trying to teach us decades ago. In particular, one lesson that needs to be taught over and over again has to do with not over-insulating stators. If the insulation has too much resistance, it will consume all of the electric field, depriving it from the air gap. This is why that when wire stators are used, PVC insulation is preferred over, say, Teflon. The speaker will simply play louder with PVC! I’ll bet Martin Logan’s black stator “paint” is somewhat conductive. If course, it can’t be too conductive if it is to limit potential arcing currents.

[bear]

Many of the chemicals mentioned in the Sony patent are reasonably dangerous... I didn't read it all yet, there may be a unique combo that is only moderately dangerous and perhaps suitable for DIYers...

But if you wouldn't set up a plating line or anodize line in your work area, don't even think about this bit...

The Martin Logan "paint" is obviously "powder coating" - being black it contains a fair amount of what? Carbon.

The story from Jim Strickland is that he tried teflon(R) wire for the insulators and this permitted higher voltages, and better sensitivity, but that the speaker ceased to work in extremely dry climates! He teaches that leakage is required, and without leakage due to the moisture in the air, the field is decreased. Thus the use of PVC which is nicely leaky...

Which should tell us electronic builders something of interest too...


_-_-bear

[SY]

The problem with those coatings is adhesion- without treating the film surface, getting them to stick permanently will be nearly impossible. In order to get really good adhesion to Mylar or Saran, you need to do something like corona or plasma treatment to get the surface energy up to 45 dyne or higher. It can also be done chemically, but it involves VERY nasty stuff like chromic acid.

[I_Forgot]

Originally posted by SY
Nice bit of improv! Stability might be questionable, but if you don't mid getting in there and getting your hands dirty, you can always reapply more coating.

Yeah, stability is always questionable until someone tries it. I also used double stick tape to secure the diaphragm to the stator in that one, another experiment, but based on some experience- my stretcher table has tape that was put on it 15 years ago and it still works fine!

Quad originally cemented the diaphragm to the stator using a super-glue type adhesive that doesn't really bond with the film. I am amazed they were able to ship the things without all of the diaphragms busting loose.

My rebuilt speakers (rebuilt ESL-63 ) allow easy access to the driver panels, so it won't be any problem to replace a diaphragm if needed. All I have to do is bust open the dust cover, unsolder the connections, and remove the driver from the frame. In the original frames it would take a couple hours just to get to the darned things! Now I can have a driver out, re-diaphragmed, reinstalled, and a new dust cover in place in about an hour with most of the time used to install the new dust cover.

I_F

[I_Forgot]

Originally posted by Brian Beck
It's good to find relatively common household products for use as diaphragm coatings, but one is never sure of stability, the effects of humidity, temperature and added mass, etc. I live in coastal Florida where it can be pretty humid (despite AC) in the summer, but we also have some pretty dry air days in the winter. So the effects of humidity are important to me. My ESL-63s seem to perform consistently however.

I am not surprised that they perform consistently. The dust covers seal them up pretty tightly. I don't know how permeable polyester film is but probably not very- they use the stuff for helium balloons that manage to hold helium for weeks or even months at a time. If polyester can contain helium, it will act as a barrier to almost anything.

I_F