[kingkorg]

I had this capsule lying around, so i decided to do a mod to it based on b&k paper. The capsule is a k67/k47 hybrid, but the principle is the same for almost any capsule.

This one can "easily" be modded, as the diaphragm is glued to the tensioning ring, and won't collapse when detached.

The science behind is that increasing diaphragm damping will smooth out the resonant frequency peak. Which was the goal here. Like with 3U audio's smooth k67 or Neuman's TLM103 the goal was to reduce the count of blind holes on the backplate, which increases damping. I closed 28 blind holes (if i counted correctly) and made the capsule almost flat! I used epoxy. I dropped tiny drops using a needle. I tried to do it as random as possible. But nothing scientific. I polished it with sandpaper, and needless to say removed any excess dust remaining.

Here's the capsule link:
https://www.ebay.com/itm/34mm-mylar-capacitor-microphone-large-diaphragm-capsule-mic-cartridge-core-/303850450014?_trksid=p2349624.m46890.l49292

B&K paper:
https://www.bksv.com/media/doc/be1447.pdf

[kingkorg]

Here is the response before and after. Notice that response above 15k is improved!

[kingkorg]

There is a caveat though. The graph above  is level matched. The modded capsule is 3.5db less sensitive. By increasing damping, the capsule stifnes is increased, and sensitivity reduced. As the original goal of u67 design was to reduce noise, it is quite logical why they went with brighter and louder capsule and then further reduced noise by adding HF deemphasis part of the circuit.

Unadjusted graphs:

[kingkorg]

And the 3U audio super smooth capsule, notice it has missing holes on the backplate.

[abbey road d enfer]

The science behind is that reducing diaphragm compliance will smooth out the resonant frequency peak.

Actually it's not the compliance that is being changed, it's the damping.
Compliance is the result of diaphragm tensining, which admittedly you have not changed.
Indeed, increasing the damping flattens the response, extending both the HF and LF cut-off points.
The fact that the LF extension is not visible is probably due to the 20Hz test setup limitations.
Interestingly, the second graph illustrates perfectly how a simple EQ can provide significant S/N ratio improvement, as it is done in the super low noise B&K.

[kingkorg]

Actually it's not the compliance that is being changed, it's the damping.

100% right, i'll update that.

There were a lot of discussions that EQed capsule is just as good as flatter one. However this experiment shows EQed bright capsule is actually better.

I might do another one with thinner diaphragm-backplate spacer, highest possible polarization voltage, wider holes (less damping - more HF) and EQ to even it out. It would be interesting to se how quiet it can get.

[RuudNL]

Next: a mod for better low end! 

[kingkorg]

Looser giaphragm tends to give more low end, and more output of a capsule. Hovever in this case, with blind holes closed, i am afraid it might negate the effect of closing the holes, and might bring back some of the high end. But i am just guessing. Haven't tried with k67.

MXL603 type of capsules (cardioid and hyper) tend to have way to much tension on diaphragm, and therefore poor low end. I've modified them before by stretching diaphragms gently in circular motion with a q-tip against non coated side of diaphragm. I do it until i reach between 1-2khz. I check resonant frequency by blowing gently against them at an angle. They resonate clearly at fundamental frequency that way.

Keep in mind if you decide to try it, they gain back some tension after some minutes, or hours, so you might want to do it in stages.

[RuudNL]

"Microphone guru" Klaus Heyne mentioned that he is able to 'relax' the diaphragm tensioning of bass shy K87/K870 capsules. No idea how he does this.
I am not sure if the membranes are glued on the Neumann capsules.
Once I created a 'good' K87 capsule out of two damaged ones, and at the time I had the impression that the membranes were glued. Or maybe they have been so long under the tensioning rings, that the membranes simply were stuck to the rings...
Anyway, the 'reconstructed' capsule worked, but I won't try to do this on a good capsule!

[abbey road d enfer]

"Microphone guru" Klaus Heyne mentioned that he is able to 'relax' the diaphragm tensioning of bass shy K87/K870 capsules. No idea how he does this.

A book could be written with  KH's technical citations. It would be a work of fiction. He's good at history telling, though.

[RuudNL]

 

[rogs]

I am not sure if the membranes are glued on the Neumann capsules.......

Not according to this clip (see from 2:15 ) ..  https://www.youtube.com/watch?v=JedLeRCG-Is

[kingkorg]

According to some mylar spec sheets i've read, exposing it to heat for period of time stretches and stabilizes/ages the material. Knowing type of material used it could be done by sticking a capsule in an oven, but no need to mention how risky it is.

Here it is. I might try it with some cheaper capsules.

For instance, if heated at 220°C (428°F) for 30 min, the film loses about 10% of its tensile strength; moreover, the film becomes brittle and shatters after heating at 235°C (455°F) for less than 1 min.

https://www.google.com/url?sa=t&source=web&rct=j&url=http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/Mylar_Physical_Properties.pdf&ved=2ahUKEwiirO_5wOruAhWp-yoKHcT3B9wQFjACegQIDRAC&usg=AOvVaw2X3_wCZBNIkPTzYPhCwJeQ

[Khron]

How (well / closely) does tensile strength correlate with elasticity / compliance, though?

[RuudNL]

I don't think I am going to try this with the capsules in my Neumann microphones!
In the past, someone suggested that you (maybe) could stretch the membranes with vacuum.
Also in this case a lot of care should be taken. (In order not to suck the membranes out of the capsule...)

[kingkorg]

Here are two backplates, both come from same model of Alctron mc410. Not a bad mic btw.

One mic was brighter, the other one had almost u87 response in the flat circuit. I don't think it's capsule production variation, as the brighter one had golden screws, flatter had silver screws. They probably mark them this way.

Brighter one is to the right, with considerably deeper ring groove, and slightly deeper holes. Flatter to the left is shallower altogether.

[bockaudio]

A book could be written with  KH's technical citations. It would be a work of fiction. He's good at history telling, though.

Sadly no "like" button on this forum.
Perhaps my favorite KH insistence was that paralleling triodes (in the same envelope) would lead to a time delay between them. Or any other parallel circumstance you might think of.
In his defense, he did advise me to listen carefully to a new version of a tube I was assuming was "the same" as the old one, he was correct they were nothing alike.   

[abbey road d enfer]

Perhaps my favorite KH insistence was that paralleling triodes would lead to a time delay between them.

Arf!

[rogs]

......... Alctron mc410. Not a bad mic btw.
One mic was brighter, the other one had almost u87 response in the flat circuit. I don't think it's capsule production variation, as the brighter one had golden screws, flatter had silver screws. They probably mark them this way.

I have 2 Alctron MC410s which, as you say, are not bad mics - (especially for the price when the Dollar exchange rate is good!) .
One has 'gold' screws the other 'silver'... I have not taken the capsules apart, but the backplates look a little different. What does look different is the gold on the front face of the capsules. The 'gold' screws capsule is almost opaque, whereas the 'silver' screws one is almost 'see thru'.....



I had originally intended to use them for an ORTF stereo pair experiment, but I ended up trying out a Mid/Side mic using 3 x EM204s.  so I never have actually compared the two.
I must check how similar (or different?) they actually are from each other now you've drawn attention to the fact they vary...

[GeorgeToledo]

I think I remember many years ago Klaus writing on some forum...maybe the George Massenburg forum, about using a hair dryer without heat to create a puff of air on the capsule, in order to test something. Maybe low end, maybe if the capsule diaphragm would easily “bottom out”? I can’t quite remember.

But...it’s a stab in the dark, but it made me wonder if maybe the theory is some sort of “burn in” of the capsule using a similar air blast?

Moisture will cause mylar to lower tension, but I can’t really think of any way that could be used to manipulate overall tension permanently, post diaphragm mounting.

I am not sure if he has been so specific to limit it to actually doing anything to the diaphragms, the wording could possibly allow for manipulating the backplates? Or not.

I have never personally encountered a *new* Neumann capsule that I thought was problematic though, only used ones.

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