Capsule Drawings/Designs for Large Diaphragms

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abbey road d enfer

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I first followed the directions and had the distance between the mylar and the backplate at 2 thou. I reduced it to 1 thou and lowered the tension of the mylar, and the output went up by about 20db. Does anybody know if the was the distance or tension that caused the change in output?
Both. Reducing the distance and relaxing tension both tend to make the diaphragm closer to the backplate, which increases the capacitance, and thus the output level. However it also results in lowering the resonant frequency and increase damping, so the mic will lose HF and be bass heavy.
It's the recipe used to make sub-sero noise microphones, that require electronic EQ.
 

Scrappersa

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That makes perfect sense. In the original instructions by Debenham, he calls for a gap that is 2 thou between the mylar and the backplate. At that distance, the capsule is down in dB pretty significantly from other capsules. I'm running into the same issue even when I lower the gap to 1 thou.

Funny enough, both capsules I've made are consistent in sensitivity and sound. Would just be nice if it could have more output.

By the way, on the backside of the capsule, I'm using the same mylar material with aluminum. Wondering what kind of impact that might have.

Any suggestions here to get more output? I'm going to play with some more shims and see how low I can get it output-wise.
 

abbey road d enfer

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That makes perfect sense. In the original instructions by Debenham, he calls for a gap that is 2 thou between the mylar and the backplate. At that distance, the capsule is down in dB pretty significantly from other capsules. I'm running into the same issue even when I lower the gap to 1 thou.
Do you measure the intrinsic sensitivity? Or do you just compare it to another capsule in teh same circuit?
 

Scrappersa

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Are you sure of the insulation?
I'm using a coded plastic shim (.0005") that I laser cut in the same shape as the retainer ring. I measured it with a caliper. The backplate and outer acrylic ring were both machined and faced off at the same time.

I just reskinned it again with only a single .0005" shim and it's still down in output. Whereas the M7 capsule that I machined has comparable output to my Chinese and Tim Campbell CK12. And I skinned my M7 capsule the same way as the Debenham.

Any other ideas here? It's so interesting that both Debenham capsules are consistent in level. Or are the Debenham capsules just lower output? Just wondering if this is an issue on the capsule design or on the machining side instead of on the skinning side.
 

Marik

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I'm using a coded plastic shim (.0005") that I laser cut in the same shape as the retainer ring. I measured it with a caliper. The backplate and outer acrylic ring were both machined and faced off at the same time.

I never built the Debenham, so cannot comment on its output. Speaking of laser cutting the shims hope you are aware the laser leaves considerable burning buildup at the edges, so the thickness is all over the place.

The calipers are absolutely no way to measure those thicknesses (or should not be used for measuring capsules, to start with—they do not have required precision). For those purposes we use high precision dial Fowler, Mitutoyo, or Starret micrometers with precision clutches and 0.0001” graduation and set of super precision measuring reference blocks.
Best, M
 

Scrappersa

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I never built the Debenham, so cannot comment on its output. Speaking of laser cutting the shims hope you are aware the laser leaves considerable burning buildup at the edges, so the thickness is all over the place.

The calipers are absolutely no way to measure those thicknesses (or should not be used for measuring capsules, to start with—they do not have required precision). For those purposes we use high precision dial Fowler, Mitutoyo, or Starret micrometers with precision clutches and 0.0001” graduation and set of super precision measuring reference blocks.
Best, M

Hi Marik,

Yes, I totally understand. I've been using something like a Cricut to cut the shims which does not appear to be doing the same thing with burning buildup. I also ordered a small run of capsule backplates from a shop to ensure it's not my machining ability that's the issue and to see how consistent I can do it. I'm still down about 20dB consistently. The capsule sounds wonderful and the tonality is gorgeous. I really love it– but it's just too low output to be useful.

Do you have any other ideas for different tactics I can take?

I've tried tensioning the capsule between 75 and 125 grams. Tried using shims ranging from .0005" (Silver shim) to .002" (two amber shims) and everything in between.
 
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Scrappersa

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I've just reskinned a Chinese edge-terminated capsule using the shim that came with it but the mylar material I've been using for the Debenham capsules. The Chinese capsule's output is now down at the same output level as my Debenham capsule.

At this point, I'm guessing the low output is either because of my skinning technique or because of the mylar material that I'm using. I'm using the 5 micron aluminised from freeflightsupplies.co.uk.

Any suggestions?
 
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Scrappersa

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Can you explain that? Where is this tension applied?
Hi Abbey Road,

1. I start by stretching the mylar over a tensioning ring jig. Stretching it enough where there aren't any significant wrinkles but not too tight.

PXL_20210823_155630980.jpgPXL_20210823_155839464.jpgPXL_20210823_160031598.jpg

2. I place the backplate and shim on top of a holder I made to keep things aligned. Then I place the jig over the capsule backplate and shims.

PXL_20210823_155454138.jpgPXL_20210823_194133981.jpgPXL_20210823_194152580.jpg

3. Then I place round weights over the edge of the jig. In this case, it's a roll of tape. The total weight including the weight of the jig is between 75 and 125 grams depending on the weight I use. In this case, it's 75 grams (the tape weight + the actual tensioning jig = 75 grams). Then I slip the retainer ring on top of the mylar.

PXL_20210823_194219091.jpg

4. I screw the retainer ring into the backplate.

PXL_20210823_194729331.jpgPXL_20210823_194858484.jpg
 

gyraf

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If output gets low at the same spacings, your tension is most probably too high - unless other variables are introduced

Perhaps try reducing tension until the point where the capsule repeatably auto-collapses at e.g. 80V tension/bias

/Jakob E.
 

Scrappersa

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Hi Jakob,

Thanks for the suggestion. With your advice I've been able to make 6 capsules that sound consistent and are within 1.5db of output from one another (measured in REW).

However, I'm fairly certain that this capsule is about 8-10db lower in output than many other capsules. To verify this, I did the following:

1. Reskinned one side of a Chinese edge-terminated capsule. Was able to get the output within .2dB of the other side. Frequency response was almost identical. So it's not the mylar that's the issue.

2. Put different Chinese capsules in the same circuit. The Chinese capsules (K67 and CK12) were both about 10dB more output.

3. Put my Debenham capsules into my 251 and compared it Tim's CT12 capsule. Tim's had about 10dB more output than the Debenham.

4. Tried lowering the shims from 2 thou to 1.5 thou. Didn't make a significant different in the output.

5. Tried changing the tension to be as loose as possible on the Debenham. Didn't significantly changed the output.

The sound of this capsule is great. I really dig it. But I'm wonder if it inherently has less output than other capsules because of the hole pattern.

Any ideas here? I'd love to get more output out of it if I can.
 

gyraf

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Is there any excessive dead capacitance - i.e. from not-contributing parts? That's the only other influence I can come up with, if distance, tension and area are accounted for

You have the same approximate membrane area as the CT12?

/Jakob E.
 

abbey road d enfer

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Is there any excessive dead capacitance - i.e. from not-contributing parts? That's the only other influence I can come up with, if distance, tension and area are accounted for

You have the same approximate membrane area as the CT12?

/Jakob E.
Excellent suggestion! The OP could measure the residual capacitance without diaphragm.
 

Tim Campbell

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The fewer holes in the backplate the more dampning is applied to the membrane resulting in lower output.

You could try increasing the polarization voltage. If it doesn't collapse your capsule it will give you more signal.
 

Recording Engineer

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Increasing the polarization may also change the sound (for better or worse); or it may not, or with enough significance to care.

Anyhow, some designs just result in lower output; such as Sony’s C37-type. I’m looking forward to eventually hearing if this happens to be one of those cases or not. In any case, if you like the sound, as is now, but you don’t end up finding a way to increase the output without changing the sound with acceptable tolerance or changing its dependability, I’d still say it’s an extremely successful and useable project!
 
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Scrappersa

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Hi Jakob,

Can you clarify what you mean by this?

I've measured the capacitance of all of my skinned capsules and they're all around 105pf. The Chinese capsules are much lower at 58pf and 75pf.

I put the Debenham capsule into my DIY 251 which has a higher polarization voltage and it's still 10dB down from my other capsules including Tim's CK12.

Tim does make a great point about fewer holes in the backplate giving less output.

I'd also be happy to send one of the Debenham backplates (to keep) to somebody with experience skinning capsules if they'd like to take a look at it to verify whether it inherently has less output or if it's something else going on with it.
 
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