Diaphragm tensioning for a 34mm capsule?

[frobozz]

Hi everybody, I found this machine I got many years ago from a friend, and I started investigating how these 34mm capsule are made. First Thank you to this wonderful forum and people posting here for all great suggestion I read ! But many questions still need an answer after I tried to Reskin a broken K78 with a simple Mylar from my kitchen (I don't know its thickness... I only wanted to check I could make it )
In the pictures you can see what I did. First I tightened the Mylar to the tensioning ring with its 6 screws,
Next I put the ring on the plate over the loudspeaker. I played a sinusoidal wave of 440Hz and turned the tensioning ring (a threaded ring pushing the mylar out of the plate allows tension adjusting) until I could clearly listen to a resonance in the diaphragm.
Next I put the tensioned Mylar and its ring on the capsule leaving the silver spacer under the mylar and screwed. Final cut and it is done. See pictures for reference.
But I have some questions:
1. I know that the tension of the Mylar affects the bandpass of the capsule, but the question is: in this K87 or in a K47 which is the right resonance frequency?
2. Is there any place where I can buy the Mylar already sputtered with gold? I have many broken diaphragms K47, K67,K87 in my lab and I would like to try Reskin by myself. (The sputtered Mylar you can see in the pictures is broken too)
Thank you in advance!

 

[soliloqueen]

generally they're tuned about 980hzish I think. Really all around 1khz in every direction depending on the manufacturer, but that's after the size decrease post-clamping. if you want to do it the rode way (with a speaker tuning) you may need to do some drumhead math to figure out what tuning in the jig will get you 980 on the final capsule

 

[frobozz]

Thank you for your answer! I got the point. Some additional test are needed because the tension will be different after the new Mylar is fastened to the capsule and cut.

Any chance to buy the Mylar with sputtered gold for 34mm capsules?

Please help

 

[rmburrow]

Sputtered gold on Mylar is hard to get and expensive to make. I use 1 micron nickel. Somewhat harder to work with but stable once in place. Tolerates tension. Be careful, depending on the manufacturer and model, some of those capsules used 40 volt polarization.

 

[AGhostStory]

@rmburrow Where do you source your 1-micron nickel mylar sheet?

 

[bremusound]

I think it‘s pure nickel, not metallized Mylar.

 

[AGhostStory]

Oh, thanks. After looking around, that seems just as hard to source.

 

[Murdock]

I also asked @rmburrow some time ago about his source. But sadly never got a reply.

 

[rmburrow]

@rmburrow Where do you source your 1-micron nickel mylar sheet?

A site on ebay had some thin nickel sheet but I electrolytically "grew" some thin nickel, the electrolyte can be nickel sulfate and slightly acidified with a few drops of sulfuric acid, the donor nickel and the plating media approx 2 cm square, the plating can be calculated from Avogadro's number and the current flow controlled with a small rheostat in series with a battery DC supply. (Keep the current way down for a good smooth plate job!) The mass of the plated nickel is negligible and must be handled carefully!

 

[Екатерина]

Is there only nickel film? Without PET film as the medium, this may be a great solution, but I only have a thickness of 10 microns here

 

[AGhostStory]

@rmburrow Thanks so much. I'm going to have to experiment with this. Do you think a mirror-polished copper anode would be a good start? I'm guessing that your plate media is almost mirror-like in smoothness to make it easier to remove the grown nickel however I could be wrong.

Insulating the nickel from the backplate seems to be another challenge I will have to think of. I'm not sure how Schoeps originally did it in their capsules.

 

[rmburrow]

Is there only nickel film? Without PET film as the medium, this may be a great solution, but I only have a thickness of 10 microns here

Just nickel and the thickness can be on the order of 1 micron since the metal tensions better than any plastic...

 

[rmburrow]

@rmburrow Thanks so much. I'm going to have to experiment with this. Do you think a mirror-polished copper anode would be a good start? I'm guessing that your plate media is almost mirror-like in smoothness to make it easier to remove the grown nickel however I could be wrong.

Insulating the nickel from the backplate seems to be another challenge I will have to think of. I'm not sure how Schoeps originally did it in their capsules.

You will definitely need to experiment with this and there are conductive "jellies" out there that simplify the process. Yes, the plate media must be mirror like in smoothness. The backplate of the mic capsule should be recessed slightly (maybe 0.1 mm or so) and automatically insulates the membrane from the back plate.

 

[Екатерина]

That's really a good idea. Someone once said that the spacing between the backplates is about 25-35 um, the diameter of the holes on the backplate is about 1.2 mm, the material thickness is 1.2-1.5 mm, and the holes cover about one-third of the backplate area. This may be helpful