New diaphragms and voice coils for vintage German mics?

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Curious. While it may not suit your project, it may suite mine! I enjoy repurposing materials - a frequent feature of many DIY solutions. :) So I press for the source you said you found.

In Message #29 you express a desire to source "a tweeter voice coil with the same coil and diaphragm dimensions (coil is about 19.5mm, entire diaphragm is about 32mm), and seeing if i can attach it."​
In Message #30 Mr. @Whoops said you could not source that. (referring to what you describe in Message #29)​
In Message $31 - you said your had already sourced that. (still referring to what you describe in Messag4 #29)​

Please indicate the source you mention in Message #31 as it may help me solve my issue. :) Thanks loads. James
Here you go. Like I said, I only found speaker drivers, so the diaphragm is probably too thick and the voice coil likely won't work well for a microphone. But you seem very very excited to try this out since you keep quoting me so go for it. Let me know what you decide to do with it.
 
I've modeled a mold for vacuum forming this diaphragm, but am not sure what thickness of material I should aim for. Does anyone know how thick dynamic microphone mylar diaphragms typically are?
 
Does anyone know how thick dynamic microphone mylar diaphragms typically are?

Pretty sure the center dome is thicker / stiffer, to aid in the voice coil maintaining its shape. The wide surround / suspension between that and the outside edge should of course be thinner and more flexible.

Think of a dome tweeter, but with a smaller, flatter dome, and wider surround.
 
I've modeled a mold for vacuum forming this diaphragm, but am not sure what thickness . . .

Just thinking aloud ... thinking of your RE-20, what "baffles" me is ...

a) I wonder how much of the the iconic sound of RE-20 is dependent on its cartridge, proper, and/or its unique Variable D sound chamber in its large, vented body. This makes me wonder how another large dynamic cartridge might sound in it. I think HEIL makes a large diaphragm dynamic cartridge microphone ... I wonder how its cartridge would sound in the EV body.

b) And I wonder whether you could cannibalize another large diaphragm dynamic cartridge, perhaps like what HEIL makes, to source a diaphragm and voice coil assembly ... um ... not that you have not thought of that! While Bob Heil passed away earlier this year, and policies may have changed under new management, he was known for servicing his products and selling replacement parts, so I wonder whether you could obtain a large diaphragm donor cartridge without breaking the bani or running over budget.

Again, just thinking aloud. Sometimes I get it right ... although my wife might disagree. Good luck. James
 
b) And I wonder whether you could cannibalize another large diaphragm dynamic cartridge, perhaps like what HEIL makes, to source a diaphragm and voice coil assembly ...

You could do for sure, and that's actually how people repair diaphragms in Dynamic capsules, they use donor microphones, normally cheaper microphones that use a diaphragm/coil with the same exact dimensions of the one you want to fix.

But to able to do that you need to find a donor diaphragms that has the same dimensions, similar corrugations, and the voice coil needs to be the same diameter and depth. And that's quite difficult to find. For the RE20 a donor diaphragm/coil with the same exact dimensions might not even exist.
 
Helpful perspectives here—I don't have an expectation of doing any sort of repair that sounds "good as new", at least with the equipment and techniques I have access to. This is half me trying to experiment and learn more about my microphones, and half me hoping that *maybe* I'll end up with something that works well enough for some purposes.

I'm going to have this mold 3D printed in resin, and then buy a cheap dental vacuum former and some varying thickness mylar sheets and see what, if anything, works best. I'm thinking the mylar will naturally thin out when it's soft and pliable, so maybe theres a bit of wiggle room between the thickness of the sheet, heat of the vacuum former, and vacuum pressure. I could be wrong—never vacuum formed before.

You could do for sure, and that's actually how people repair diaphragms in Dynamic capsules, they use donor microphones, normally cheaper microphones that use a diaphragm/coil with the same exact dimensions of the one you want to fix.

But to able to do that you need to find a donor diaphragms that has the same dimensions, similar corrugations, and the voice coil needs to be the same diameter and depth. And that's quite difficult to find. For the RE20 a donor diaphragm/coil with the same exact dimensions might not even exist.

This is almost definitely the best way to do it, but in my (limited) search, it seemed like all dynamic mics I could find, that were not rare, sought after, or generally expensive (even for nonworking units), used diaphragms much smaller than the RE-20.

In any case, nothing tangible to lose at this point, and a lot of perspective and knowledge to gain!
 
Just thinking aloud ... thinking of your RE-20, what "baffles" me is ...

a) I wonder how much of the the iconic sound of RE-20 is dependent on its cartridge, proper, and/or its unique Variable D sound chamber in its large, vented body. This makes me wonder how another large dynamic cartridge might sound in it. I think HEIL makes a large diaphragm dynamic cartridge microphone ... I wonder how its cartridge would sound in the EV body.

b) And I wonder whether you could cannibalize another large diaphragm dynamic cartridge, perhaps like what HEIL makes, to source a diaphragm and voice coil assembly ... um ... not that you have not thought of that! While Bob Heil passed away earlier this year, and policies may have changed under new management, he was known for servicing his products and selling replacement parts, so I wonder whether you could obtain a large diaphragm donor cartridge without breaking the bani or running over budget.

Again, just thinking aloud. Sometimes I get it right ... although my wife might disagree. Good luck. James
Thanks James, this is interesting advice. Will definitely look into this if my DIY approach doesn't pan out (which is absolutely likely).

Also, I want to apologize for any snark that may have come across in my previous messages. It's often hard to gauge communication, and I sometimes have trouble differentiating between condescension and just genuine curiosity — stressful week for me so I hope I haven't come across as insulting or rude.
 
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Sorry in advance for triple posting — but I'm doing a bit more thinking about the voice coil. I realized in an earlier message I mentioned 150 ohms, because that is apparently the output impedance of the RE-20. I'm not smart enough to know, though, how the transformer, magnet, and additional circuitry might affect that impedance — I can only assume it does affect the impedance, and the resistance of the voice coil alone is different.

Can anyone tell me what a typical voice coil resistance might be in a dynamic mic like this? I don't expect to be exact, but by my measurement, the original voice coil was about 40AWG (which is what I'm using to wind the new ones), and it would take a lot of wire to get to 150 ohms, much more than would fit on an RE-20-size coil.

Currently, I'm doing 2 layers of 40AWG copper enameled wire, 19.5mm inner diameter, 3mm coil height. Worth mentioning I didn't think to get a good measurement of the original coil's height, so I'm going off of the amount of magnet exposed in the capsule, as well as the thickness of the diaphragm's metal disc, to make something that fits.
 
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I want to apologize for any snark that may have come across in my previous messages. It's often hard to gauge communication,

NO WORRIES, old man. It IS difficult to gauge attitudes - just as it is sometimes difficult to convey humor - in this black and white two-dimensional medium. I appreciate direct, open replies. It's all good. Besides, a little salt and pepper makes a more interesting dish. James
 
The only person in the World that I know of that is able to perform quality repairs on Dynamic capsules and Diaphragms is Esa Tervala. He repairs many diferent capsules like AKG D12 (and family), Sen MD421, Sen MD409....

Maybe he is able to repair RE20 Diaphragms or voice coils, or can source replacements.
It's a mater of contacting him
 
Small update, for anyone who may be curious. I just purchased Mylar in 1 mil and 1/2 mil thicknesses from a model airplane shop. The 3D printed molds for the diaphragm are still in production, and I'm very curious to see how such a thin material holds up to vacuum forming. I have a feeling I may need to create a positive of the mold based on the 3D printed negative and press the heated mylar between the negative and positive molds, instead of vacuum forming, but we'll see. I'm also curious about how the surface finish of the mold will affect the result — I have a feeling I may need to figure out a way to polish the mold, although the print resolution should be quite high as it is resin.

I've also had some additional luck in my experiments winding and positioning the voice coil. By my own estimation, the voice coil should be the same thickness as the metal plate that the diaphragm adheres to (about 2.3mm for the RE-20), so that if the diaphragm is ever fully negative, the voice coil doesn't bottom out on the capsule housing (if anyone knows better, please correct me). I have been winding "practice" voice coils on a 19mm polished chrome bobbin in 2 layers, finishing with super glue. This is an appropriate size to fit in the tiny (maybe 1/4-1/2mm) space in the RE-20's capsule. I did test a coil in the old, damaged diaphragm, and I did get sound out of it, but it was very very low output, and had basically no low end at all. I think this is due to a combination of damage to the diaphragm itself, imprecision with attaching the voice coil to the diaphragm, having the wrong dimensions for the voice coil (this is before I realized the voice coil shouldn't be taller than the diaphragm plate, so I have a feeling the coil was getting trapped or bottoming out), and not using a flexible adhesive to glue the voice coil to the diaphragm (I used super glue for this test, which is brittle, and I plan to use contact cement in subsequent tests). In any case, it is a proof of concept that I'm happy to have found.

Also, I have figured out a very low tech way of positioning the diaphragm plate against the capsule housing, without the strong capsule magnet pulling the plate away from me and pinching or destroying the voice coil in the process. I stick 2 sewing needles through the magnet wire holes in the capsule, and then through the corresponding holes in the diaphragm plate. This allows me to get the diaphragm very close to the capsule, and then carefully pull the needles out so I can get precise positioning. If I had a lathe, or just some way to hold these parts with precision, that would certainly be better, but this is the best I've come up with in my 2 bedroom apartment.

In interest of sharing all the information I can — I also found this really helpful article while trying to figure out the appropriate thickness of Mylar to use. Maybe someone will find it helpful as well.
 
Keep going @christopantz !

I admire & envy you a little bit.
Admire, because having a broken AKG D12 & MD-409 (Sennheiser). I always wanted to try what you are doing now.

Envy a bit, because right now I have zero time & space for this type of DIY.

Keep going man, we’ll be Cheerleading you (minus ponpons - plus beards).

M
 
Ive repaired a few very ancient dynamic mics by replacing diaphragm/voice coil with a higher impedence headphone type drive unit .
the new transducer is mounted magnetically above the original polepiece/magnetic gap , helping focus a little extra magnetic force on the coil ,
All bets are off regarding original specs , frequency responce , impedence , but you can make unusable usable again .
 
Keep going @christopantz !

I admire & envy you a little bit.
Admire, because having a broken AKG D12 & MD-409 (Sennheiser). I always wanted to try what you are doing now.

Envy a bit, because right now I have zero time & space for this type of DIY.

Keep going man, we’ll be Cheerleading you (minus ponpons - plus beards).

M

Thanks so much for the encouragement — it truly brightens my day. Every piece of valuable information I know about sound recording, electronics, and repairs is from people who were generous enough to share their knowledge freely because it's the right thing to do. The least I can do is pay it forward by keeping no secrets on whatever techniques I discover, even from my super non-expert perspective — if I'm wrong, someone will correct me and we will all be better off for that.

Ive repaired a few very ancient dynamic mics by replacing diaphragm/voice coil with a higher impedence headphone type drive unit .
the new transducer is mounted magnetically above the original polepiece/magnetic gap , helping focus a little extra magnetic force on the coil ,
All bets are off regarding original specs , frequency responce , impedence , but you can make unusable usable again .

This is interesting — I hadn't thought of using a new driver assembly as a whole, the furthest I got with that was thinking about harvesting the diaphragm and voice coil from a headphone driver, but not using the magnet, but your technique is so much simpler and wouldn't require an exact match for the diaphragm and coil dimensions. How do the repaired mics sound?
 
Admire, because having a broken AKG D12 & MD-409 (Sennheiser). I always wanted to try what you are doing now.

Mate, of course everyone is free to do whatever they want but please listen to my advice.

Those 2 microphones are very valuable microphones with an high price in the market.
Repairing those capsules and diaphragms is really really hard, and needs someone highly experienced and knowledgeable.
If you ever try to repair those mics yourself there's an high risk that you might end up damaging them for good, probably not salvageable after that point.

Esa Tervala is able to repair those microphones and bring them to original specs, he repairs both the D12 and the MD409,
you pay for the repair but then you have a working mic that has an high resale value.
Esa is the only person in the world that I know of that is able to do this properly, send the mics to him and save them

Best regards
 
Small update, for anyone who may be curious. I just purchased Mylar in 1 mil and 1/2 mil thicknesses from a model airplane shop. The 3D printed molds for the diaphragm are still in production, and I'm very curious to see how such a thin material holds up to vacuum forming. I have a feeling I may need to create a positive of the mold based on the 3D printed negative and press the heated mylar between the negative and positive molds, instead of vacuum forming, but we'll see. I'm also curious about how the surface finish of the mold will affect the result — I have a feeling I may need to figure out a way to polish the mold, although the print resolution should be quite high as it is resin.

Mate I wish you the best of luck in your developments, keep on going.

If I find any information that might help I will share it here and let you know
 
Hi Christo,
The mic I repaired using this technique dated back to the 1930's ,a Grampian MCR
in this case Im certain the modern transducer was far superiour to what the original ancient one sounded like . But I cant say anymore as I didnt measure freq resp. or anything like that .

As far as construction goes a headphone driver is quite similar to what you find in a dynamic mic ,but with a smaller magnet assembly and no rear porting arrangements .

There is plenty of 30mm headphone drivers available , Try matching capsule diameter with the RE-20 and find a way of sealing up the rear of the capsule so it joins with the RE-20 rear chamber . A small section of plastic tube might work there .
 
Mate, of course everyone is free to do whatever they want but please listen to my advice.

Those 2 microphones are very valuable microphones with an high price in the market.
Repairing those capsules and diaphragms is really really hard, and needs someone highly experienced and knowledgeable.
If you ever try to repair those mics yourself there's an high risk that you might end up damaging them for good, probably not salvageable after that point.

Esa Tervala is able to repair those microphones and bring them to original specs, he repairs both the D12 and the MD409,
you pay for the repair but then you have a working mic that has an high resale value.
Esa is the only person in the world that I know of that is able to do this properly, send the mics to him and save them

Best regards
Ok thanks,

Would you mind sharing his details over here on this thread, for future reference/someone wants to get theirs fixed by him ?

M
 
Small update, for anyone who may be curious. I just purchased Mylar in 1 mil and 1/2 mil thicknesses from a model airplane shop. The 3D printed molds for the diaphragm are still in production, and I'm very curious to see how such a thin material holds up to vacuum forming. I have a feeling I may need to create a positive of the mold based on the 3D printed negative and press the heated mylar between the negative and positive molds, instead of vacuum forming, but we'll see.

For vacuum forming there's an intermediate thing between one-sided and two-sided molding, where you use a one-sided mold and push the plastic most of the way toward it, into the right general shape, and let vacuum take it the rest of the way.

So if you're using a negative mold, you'll have something roughly the positive shape, but smaller, to push the plastic into the mold.

For one-offs or a-few-offs of reasonably large objects, people often do that with their hands in work gloves, gently stretching the plastic toward the parts of the mold that it's hardest for the vacuum to stretch it into. Once the plastic has been stretched some, it's thinner and easier for the vacuum to suck it the rest of the way.

For larger volume things, people will often make a felt-covered thing to push the plastic into the mold in the right places. (Felt covered so that plastic can slide across it and not be cooled much by contact.

In general you do NOT want your molds to have a polished surface, because the plastic may stick, and it's harder to suck the air out everywhere. You want the mold to be rough at a scale significantly smaller than the thickness of the plastic---small enough that the plastic will not mold to it and become rough at that scale, but still large enough that air can work its way out through the tiny gaps the roughness creates between the plastic and the mold.

(Worse case, the plastic may stick to the mold. Normally you heat the plastic enough to be stretchy but not enough to be sticky, but if you misjudge, and have a smooth mold, that can be bad.)

For very very low volume stuff, like one to ten things, people often make molds of some kind of plaster. (Plaster of Paris or something denser and stronger like Hydrocal, or tougher like Water Putty.) That naturally gives a microscopically rough surface and considerable porosity---you can suck the last bits of air right through plaster.) Plaster molds tend to wear out quickly, so one option to make a silicone mold to cast several plaster molds from, if you're making a moderate number of things.

Molding very thin plastic is tricky, because it tends to cool quickly and give you less working time and other forms of slack. For example, if you have a felt-covered thing to press the plastic toward the mold, just contact with the felt may cool it too much. You may need your mold and that other thing (if you use one) to be kept warm so that they don't cool the plastic too much before it conforms to the mold.

(Vacuum forming is easiest with plastics between about one millimeter and and four millimeters thick, where the heated plastic is thick enough to retain most of its heat for a few seconds, but not so thick it's hard for vacuum to stretch it to fit the mold.)

BTW here's a video of me forming plastic over a plaster mold of my face (keep the volume low... there's no useful audio content and you'll mainly hear a vacuum cleaner):

Here's my Instructable on how to make that vacuum former: https://www.instructables.com/Make-a-good,-cheap,-upgradeable-sheet-plastic-vacu/

Here's my Instructable on how to turn a cheap bike tire pump into a vacuum pump that can pull several times as hard as a vacuum cleaner:
https://www.instructables.com/make-a-manual-vacuum-pump-for-under-$20-by-convert/ And another using a cheap electric car tire inflator: https://www.instructables.com/convert-a-tire-inflator-type-air-compressor-into-a/
 
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