U87 diaphragm tension

I read on Klaus Heyne's Microphone Lab forum, that a lot of U87 capsules have a too high tensioning of the diaphragms, resulting in a poor LF response. I think I can agree with that to a certain extend. As far as I know, the 'older' U87 microphones had a better LF response. In order to compensate this, I already changed the (fixed) LF rolloff filter. (C104 in the U87ai.)
Now Klaus Heyne writes that he has a way to "relax the tensioning of the membranes".
I don't have an idea what he could mean with this. The membranes are pretty fragile, so I can't think of a way to accomplish this.
(Especially because the membranes are not glued to the tensioning rings, it seems.) Any ideas?

He is claiming the same for the k870 capsules, but we made a test with vintage u67's and untouched NOS/reskinned k67/new k870 equipped  KK67 heads, where we could not hear any difference between the NOS k67 vs the new K870 (the reskins were not up to par unfortunately) so I still prefer ordering from sennheiser if the budget allows that.

Thanks. The biggest problem he (Klaus) mentioned, is the +/- 2 dB tolerance used by Neumann.
This could theoretically mean a difference of 4 dB between two capsules.
I used to have a couple of 'old' U87 microphones in the past, and from what I remember they had a richer low end than my current U87ai microphones. But, the U87ai is also a bit brighter, so that may influence my impression.
But, as always... how far do you want to go? He also claims he can hear the difference between microphone cables.
To me, most good quality cables sound the same. After all, it is 200 ohms we are dealing with; the capacity of the cable doesn't have an audible influence (when using a reasonable length) and resistance isn't an issue. But maybe he has better ears than I have...

RuudNL said:

Thanks. The biggest problem he (Klaus) mentioned, is the +/- 2 dB tolerance used by Neumann.
This could theoretically mean a difference of 4 dB between two capsules.
I used to have a couple of 'old' U87 microphones in the past, and from what I remember they had a richer low end than my current U87ai microphones. But, the U87ai is also a bit brighter, so that may influence my impression.
But, as always... how far do you want to go? He also claims he can hear the difference between microphone cables.
To me, most good quality cables sound the same. After all, it is 200 ohms we are dealing with; the capacity of the cable doesn't have an audible influence (when using a reasonable length) and resistance isn't an issue. But maybe he has better ears than I have...

Click to expand...

The U87A, and the U87ai have different circuits, and the old Neumann capsules can "slow down" due to spits and dust contaminations. (ck12's also in the vintage c12's) Maybe he thinks about this, or simply advertising his company? I admit I don't know a lot about capsule tuning but we are doing lot of AB tests between the originals and the different remake products.

RuudNL said:

He also claims he can hear the difference between microphone cables.
To me, most good quality cables sound the same. After all, it is 200 ohms we are dealing with; the capacity of the cable doesn't have an audible influence (when using a reasonable length) and resistance isn't an issue.

Click to expand...

The cable is a load on a fairly sensitive and somewhat organic Hi-Z circuit.  It may be audible, as great an affect as a simple tube change has on the response due to the interaction with the capsule, the reflected load differences may too.  I don't know. 

Maybe he thinks about this, or simply advertising his company?

Click to expand...

The man does not lack for work, I don't think so. 

i would only touch to the center screw if it's too tightly screwed (or vice versa), it's the only way to release or tighten ( a little bit) the diaphragm uniformly.

i always try to find the right screwing force, when the diaphragm is probably the most parallel to the backplate, mostly when it sounds the best.

i repeat the process every 2 or 3 years of use (even before it the mic was dropped or manhandled) as the capsule's cables coupled with the vibrations, produce loose with time.

i wouldn't touch to the peripheral screws as the diaphragm is not glued on K87, that could loosen it anarchically and kill the sound.

i don't see how a diaphragm can be "largely" loosened correctly without reskinning...

granger.frederic said:

i don't see how a diaphragm can be "largely" loosened correctly without reskinning...

Click to expand...

That was exactly what I thought!
Loosening the screws in the tensioning ring is IMHO a guarantee to destroy a capsule!
Would the influence of temperature have a long lasting influence on the membrane tensioning?

Mr. Heyne wrote about membrane tensioning:
"I am working on a device that makes my previous efforts more predictable and easier to repeat and to dial in more precisely."

"relax the tensioning of the membranes" could be some kind of heat treatment?

Could it be by adjusting the polarization voltage ?

It seems heat will have an influence on the tensioning, but this process should be carried out under well controlled circumstances.
Especially the cooling down time seems very important.
A member of this forum has sent me some very interesting links about the effects of heat on mylar (BoPET or biaxially-oriented polyethylene terephthalate), but I have to study this further.

That sound interesting. Let us know your conclusions.

would a higher than normal bias voltage applied and modulated stress and stretch in a uniform manner?

A higher polarisation voltage on  a capsule certainly has an affect on the tensioning of the diaphragm(s), with the risk of collapsing.
(Some people think that this is the major difference in sound between the Neumann U87a and the U87ai.)
But I am not sure if this effect on the membrane(s) will be permanent.
Also, if you 'stretch' the diaphragms, the bonding of the gold is something to take care of.

My experience in messing with capsules tells me that if you change low end response the high end response will change as well. When i managed to boost low end, high end got boosted as well, and shape of the curve changed as well.

The capsule with best low end response on an u87 i ever heard had 100% busted back diaphragm, and it sounded wonderful. Not that i sugest busting yours though

Capsule, where the diaphragm isn't glued, can be tuned manualy. I wouldn't expect any fancy method, especially with use of heat.
Like with drum tuning, on a cross, with ultra small steps. Of course this can be done with really small tolerance.

A capsule manufacturer like Tim , could explain the story at lot better than me, but...
my advice is : don't touch to the peripheral screws if the diaphragm isn't glued.
you cannot add or remove some tension that way.
the drum's skin mechanism is completely different.

ideally, that screws are tightened with a torque wrench.
the only thing that will happen is to compress the spacer and change the gap between the diaphragm and the backplate, thus the capacitance, not uniformly.
you can ruin the capsule.
if you already have tightened the peripheral screws , do not unscrew them quickly, or you will risk to decompress the spacer too quickly, and ruin the diaphragm tension.

the only safe screw is the center one, but all the screws if the diaphragm is glued.

if you're dismantling a capsule, you must screw all the peripheral screws with the same initial torque.
for the center one, screw it very very slowly, to not crumple the diaphragm, then listen and process gropingly.

Here is some more information about the properties of mylar:
http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/Mylar_Physical_Properties.pdf

Maybe it is adjusting the time delay with the spacer between the backplates for an increased figure 8 output