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Thanks Klaus!
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by klaus on August 17, 2018, 01:22:21 pm »
Figures 3 and 4 of your linked article shows increasing shrinkage of Mylar with increased heat, in all cases. What am I not getting here about your counter argument?

Besides, I've tested this with Mylarģ capsules repeatedly: resonance frequency goes up with heat applied to the tensioned diaphragms.

Get well soon!
KH
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by gtoledo3 on August 16, 2018, 11:10:50 pm »
Mylar is pre-tensioned or stetched during manufacturing. Depending on the tension used in the capsule it's more likely that heat will increase the tensioning.
But - we do not know how Neumann treats the mylar in the manufacturing process.

More than you ever wanted to know about Mylar:

http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/Mylar_Physical_Properties.pdf

The long story short is that it appears that tensile strength goes down with heat increases and elongation goes UP. At extremes it finally becomes brittle.

Mylar can be preheated, which will serve to condition it, but up to that heating point, not above. I also read another abstract about UV exposure pretreatment fwiw.

I still havenít done the test on the timbales :-) Recovering from a slip and fall, which probably accounts for more than half of the time I spent chattering about this. (Sorry Klaus!)
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by Kai on August 14, 2018, 06:22:28 pm »
Heat could possibly decrease Mylar tension ...
Mylar is pre-tensioned or stetched during manufacturing. Depending on the tension used in the capsule it's more likely that heat will increase the tensioning.
But - we do not know how Neumann treats the mylar in the manufacturing process.
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by gtoledo3 on August 13, 2018, 02:33:24 pm »
When you have a drum head under tension and heat it, a dimple stretches and the Mylar thins. You can then tighten the drum head up more, heat again. Rinse and repeat. Until the drum head gets so thin that itís shot or breaks.

I thought the pitch relaxed when it thins out on the head, with it being under tension. But it goes up more like shrink wrap? Interesting. I have some timbale heads here that Iíll do that to soon, and pay close attention.
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by klaus on August 13, 2018, 10:57:22 am »
Heat could possibly decrease Mylar tension, but the problem in this case that is that it has gold bonded to it.
Heat does the opposite to Mylar: it tightens it. That's why drum shops apply heat to dented Mylar heads; the dents come out that way.
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by gtoledo3 on August 12, 2018, 02:53:03 pm »
Klaus, did you consider variability of the caps for the high frequency shunts as another thing to examine in differences of new vs vintage? (Not naysaying your point about the tension!)

It was occurring to me that what happens with the top end there, can also really pop out to the ear.
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by gtoledo3 on August 12, 2018, 02:49:15 pm »
Very interesting review! So again, the tensioning of the capsule membranes is the biggest problem.
Could you unveil a little how it is possible to relax the tensioning of the membranes?
(Not that I am even thinking about doing this myself!!!)
As far as I remember, the membranes were glued to the tensioning rings.
(Anyway, that is what I saw when I took a damaged Neumann U87 capsule apart.)

Heat could possibly decrease Mylar tension, but the problem in this case that is that it has gold bonded to it.

Any sort of pre-age/stress like heat would be best done before application of the gold, because with heating after the fact the gold and Mylar will not respond in the same way to heat, which may theoretically cause abberations in the bond, at least according to research.

That said, itís hard to think of other approaches to tweak the tension of an assembled capsule.
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by RuudNL on August 11, 2018, 08:30:27 am »
Yes, I have seen many capsules that were destroyed by people who had attempted to 'clean' the membranes!
Gold wiped off, holes in the membranes etc.
Once I got a capsule that was cleaned so thorougly, that all the gold was gone...
The owner asked if I didn't have a spraycan, so I could apply a new layer of gold.  :D
I explained to him that it didn't work this way...
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by mikezietsman on August 11, 2018, 07:39:01 am »
It is quite easy to unscrew the two halves of a k87/k870 capsule but if you touch even a single screw that is holding the diaphragm in place, you will ruin the tension in the diaphram.

Source: I took apart a diaphragm that had been ruined by an amateur cleaning attempt.

I can say with a certain authority that these diaphragms are both tougher and more fragile than most people realise. I imagine it is a thing of working with them a lot that helps you to understand in which ways they are tough and in which ways they are fragile.

I experimented on two completely ruined diaphragms... Had I experimented on a two working diaphragms, I would have ended up with two non-working diaphragms...

Interesting. I remember I once removed the 'good' membrane from a damaged U87 capsule and used it to repair another U87 capsule. In this case, the membrane was definitely glued to the tensioning ring. (Otherwise I would never have been able to reuse it again!) The repaired microphone sounded good after this 'transplantation'.
Anyway: the owner of the microphone was very happy with the result.
Although the capsule looked as an original Neumann one, there is of course a chance that it had been replaced with a third party one. Or could it be that after many years of use the membrane just sticks to the tensioning ring? (I would be surprised if this happens.)
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