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Author Topic: The Instability of M7 PVC Capsules  (Read 2572 times)

J.J. Blair

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The Instability of M7 PVC Capsules
« on: January 03, 2017, 11:49:23 am »

In 2008, I had Udo at MTG reskin an M7 capsule for me. Since then, it has sat unused in its airtight container that they shipped it back in.  In 2009, I bought a blue line M7 from Thiersch, which has resided in one of my U47s. 

This week, I happened to take a look at the MTG capsule, as well as the Thiersch, and they are already developing the familiar PVC cracks.  (pics soon)

This leads me to the question: Why on Earth would anybody use PVC in this day and age, if the rate of decay is that fast? 

I have some original Berlin M7s, and have been thinking of sending them to Thiersch, but at this point, I wonder if it's better to do them in Mylar.  (Thoughts welcome on that.)

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J.J. Blair

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Re: the instability of PVC
« Reply #1 on: January 03, 2017, 12:00:22 pm »

Here's the MTG.
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klaus

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Re: the instability of PVC
« Reply #2 on: January 03, 2017, 01:45:39 pm »

Thanks for sharing.
Of course, "Udo" does not reskin capsules, he is MTG's marketing guy who was hired away from BMW a few years back, and seems to be doing a good job in that capacity.

Why would anyone even contemplate using PVC with its time bomb potential as membrane material?  Because it can sound like god on an M7 backplate. At least it could, in the distant past, before ca. 2000, when the PVC formula had to be changed, due to environmental concerns. So why would anyone use it today? Ignorance, and the hope that someone will some day soon stumble upon the chemical formula that can revive the magic.

I have had Thiersch and Gefell reskin original Neumann/Berlin-made M7 with Polyester/Mylar and the sound was in all cases limper and less engaging in the crucial upper midrange than the similar-sized Neumann Mylar K47.

So what would be the point of reskinning an M7 with Mylar? In an age when you don't even have to deface the original M7 mount of a vintage M49 or U47 anymore (Neumann and Thiersch make ready-fit K47 capsule columns for these mics): none.
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Klaus Heyne
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Dan Popp

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Re: the instability of PVC
« Reply #3 on: January 03, 2017, 02:45:42 pm »

Mr. Heyne, since it seems you're recommending replacing aged M7 capsules with K47 capsule assemblies, would you describe how that will change the sonics?  Thanks very much. 
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J.J. Blair

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Re: the instability of PVC
« Reply #4 on: January 03, 2017, 03:38:02 pm »

Thanks for your thoughts, Klaus.
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klaus

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Re: the instability of PVC
« Reply #5 on: January 03, 2017, 03:42:11 pm »

First, to be clear: ALL alternatives to the genuine Neumann M7 PVC will change the sonics. The relevant question: which is the most acoustically and emotionally pleasing, option?

Thiersch's M7 PVC lacks in speed - that three-dimensional quality which lends the original its "reedy" quality, and it is also quite congested, rather than transparent, in the upper mid range, resulting in sibilance.

MTG had excellent PVC capsules in all of their mics until ca. 2000, which were equal in listening satisfaction to those made by Neumann in Berlin. But as the company currently does not sell stand-alone capsules, it's not even worth discussing sonic shortcomings of their current crop in detail, but they are in the same general direction as those of Thiersch.

Neumann's K47 - when they are at their best*- are equal in emotional draw to a healthy Neumann or pre-2000 MTG M7 PVC, but they draw from a different sonic terrain: their authority rests on the superb rendering of the lower mid range octaves, and they don't quite have the pointed "liquid gold" high end of the M7, yet make up for that with their rich yet robust lower register. 

Given all currently available choices, I recommend to collectors, who continue to use their vintage Neumanns as superior recording tools as well as investments, to go the K47 route.

* It is worth mentioning that all capsules discussed here can stray significantly in response, and therefore quality, from sample to sample. Neumann, which has probably the tightest capsule manufacturing quality control in the business, specs most of its LD mics ±2dB across the response range. From my measurements, the mic amps are usually within ± 1dB from mic to mic, even back in the tube days; therefore, the 4dB spread is almost entirely due to capsule tolerances, which remain unavoidable, given the handmade nature of the product.
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Klaus Heyne
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klaus

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Re: the instability of PVC
« Reply #6 on: January 03, 2017, 03:49:18 pm »

In 2008, I had Udo at MTG reskin an M7 capsule for me. Since then, it has sat unused in its airtight container that they shipped it back in. (...)

This week, I happened to take a look at the MTG capsule, as well as the Thiersch, and they are already developing the familiar PVC cracks.  (pics soon)

Curious to learn how it sounds, compared to the Thiersch? Or even compared to a still-healthy Neumann M7, if you have such a thing?
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Klaus Heyne
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Dan Popp

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Re: the instability of PVC
« Reply #7 on: January 03, 2017, 04:02:39 pm »

Neumann's K47 - when they are at their best*- are equal in emotional draw to a healthy Neumann or pre-2000 MTG M7 PVC, but they draw from a different sonic terrain: their authority rests on the superb rendering of the lower mid range octaves, and they don't quite have the pointed "liquid gold" high end of the M7, yet make up for that with their rich yet robust lower register.

Thanks!
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AusTex64

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Re: The Instability of M7 PVC Capsules
« Reply #8 on: January 04, 2017, 10:37:30 am »

JJ, how bad does the Thiersch PVC M7 look? I have one that's about two years old in an AMI kit I bought from Oliver. Been contemplating installing a Neumann K47 in the mic, turns out I might have to in a few years, based upon your findings.
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soapfoot

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Re: The Instability of M7 PVC Capsules
« Reply #9 on: January 05, 2017, 09:11:51 am »

Question/speculation alert:

I wish we had a plastics expert to readily consult with the question-- perhaps Klaus knows someone? But having a bit of limited experience with plastics breakdown in other contexts, I wonder if the "airtight container" could've actually contributed to the accelerated demise of the PVC?

For instance, I know that celluloid plastic, which also has an unfortunate auto-destruct feature built in, can last MUCH longer if exposed to the open air than if sealed in a tight container.

One classic example is vintage Gibson guitars-- the pickguard/fingerrest releases toxic vapors as it breaks down, and the vapors themselves actually accelerate the process, resulting in a feedback loop. Pickguards from guitars kept on stands exposed to the open air have been demonstrated to maintain their integrity for decades longer than those who have spent their lives in closed cases where air doesn't as readily circulate.

I wonder if something similar could possibly be at play here? 8 years does not sound like a very long time for a PVC capsule to break down, particularly when there are still decades-old original examples in good working order.
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boz6906

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Re: The Instability of M7 PVC Capsules
« Reply #10 on: January 05, 2017, 11:03:40 am »

It's called 'outgassing', emitting volitile organic compounds, causing the PVC to lose flexibility and become hard & brittle.

I would speculate that, like paint, you would want an air-tight container to prevent the PVC from deteriorating.

http://www.consumer.org.my/index.php/products/106-household/206-pvc-plastic-products-outgas-poisons
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Kai

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Re: The Instability of M7 PVC Capsules
« Reply #11 on: January 05, 2017, 12:45:20 pm »

The container itself can be the reason for problems, by outgasing destructive chemicals.
And don't forget the electrolytic capacitors, ALL of them do outgas to a certain amount.
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J.J. Blair

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Re: The Instability of M7 PVC Capsules
« Reply #12 on: January 05, 2017, 12:50:38 pm »

I'll take a picture later of the Thiersch, which was not in an airtight container.  It also has the problem.

I never listened to the MTG, which is why it's still on the UM57 mount. 

I actually have an old K47 that I was going to put in the U47 with the Thiersch, but I seem to be short a K47 mount, and can't find any online.  When I get around to it, maybe I'll try the MTG also, and see how they all sound. 
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J.J. Blair

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Re: The Instability of M7 PVC Capsules
« Reply #13 on: January 05, 2017, 12:52:51 pm »

The container itself can be the reason for problems, by outgasing destructive chemicals.
And don't forget the electrolytic capacitors, ALL of the do outgas to a certain amount.

There's no electrolytics in any of my U47s. 
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J.J. Blair

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Re: The Instability of M7 PVC Capsules
« Reply #14 on: January 05, 2017, 05:58:21 pm »

Here's the Thiersch.  Those pock marks in the crack are interesting, too.

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