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Author Topic: AKG C12 with dull sounding capsule  (Read 9169 times)

Warren Huck

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AKG C12 with dull sounding capsule
« on: July 04, 2012, 09:13:57 am »

Hi there, I am a pro audio repairer and have a very old AKG C12 in the workshop at the moment. The mic was originaly dead, I traced this fault back to the centre wire on the capsule as it had broken off (seems like an odd thing to happen). There is a reasonable but not excessive amount of debris on the front of the capsule and the rear of the capsule is very clean. When I sound tested the mic I found that the rear capsule sounds great and the front capsule sounds dull. I have succesfully cleaned mics such as the U87 mics before (with water only) and wondered if this mic would respond to a clean or if there is something else going on there. The owner is nervous about having the capsule cleaned as it is very old and very special to him. Have you come across this sort of thing before and what do you suggest as a cure ?
Regards, Warren Huck
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Regards, Warren Huck

klaus

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Re: AKG C12 with dull sounding capsule
« Reply #1 on: July 04, 2012, 12:43:37 pm »

Diaphragm contamination would need to be extreme to cause severe loss of high frequency.

When a CK12 acts up that extremely, like you describe, my first suspicion is loss of diaphragm tension. This could be visually determined:

With the head basket off and the mic turned on (no audio) and set to cardioid (and, later repeating the test, to figure of eight), blow breath onto the diaphragm at a very close distance.
Viewing under a strong light: does the diaphragm come back quickly to the resting position, or does it stay sucked against the backplate? If the latter, you have the answer why there is no high end.

Electrostatic backplate suck can have two causes:

1. polarization voltage too high (check that B+ is no more than 120VDC, as measured at the 100k voltage divider inside the mic) or:

2. permanent diaphragm tension loss of the CK12 (read past posts about this  CK12-specific aging phenomenon.)

Please report back once you have found the cause for your high frequency loss.

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Klaus Heyne
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mbrebes

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Re: AKG C12 with dull sounding capsule
« Reply #2 on: July 05, 2012, 04:09:49 pm »

"..., I traced this fault back to the centre wire on the capsule as it had broken off...".
Your statement makes me concerned about what kind of capsule is really on this mic, since the wires on a C12 capsule are attached by one of the screws on the outer ring, rather than the Neumann-style center connection.

Please clarify or attach a picture.
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klaus

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Re: AKG C12 with dull sounding capsule
« Reply #3 on: July 05, 2012, 04:48:15 pm »

I think he meant the backplate wire connecting to the center electrode, whose connection is made on a C12 via two screws and one eyelet on the bottom of the capsule.

 Severance of the wire at the eyelet's solder connection is not uncommon, because of the capsule's movement in its felt mount.
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Klaus Heyne
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Tim Campbell

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Re: AKG C12 with dull sounding capsule
« Reply #4 on: July 05, 2012, 07:42:04 pm »

Dear Warren, since I almost exclusively work on CK12 capsules I have seen this problem before.
If your client wants the capsule halves matched he will at the least need one new membrane and most likely additional work beyond that.
It sounds like what Klaus describes, the front membrane is collapsing.
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Rick Sutton

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Re: AKG C12 with dull sounding capsule
« Reply #5 on: July 05, 2012, 08:55:27 pm »

 As a long time owner of AKG mics employing CK12 capsules (7 capsules in all and close to 40 years of ownership on some of the mics) I recently needed one of the capsules worked on. Tim Campbell put a new membrane on one of my capsules and I was extremely happy with the quality of the work. Tim, I hope you continue to provide this service as it is very reassuring to know that there is support for these glorious capsules.
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klaus

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Re: AKG C12 with dull sounding capsule
« Reply #6 on: July 05, 2012, 09:06:31 pm »

I respect Rick's praise of Tim Campbell's work, but Tim: let's take it easy with prescribing remedies and offering your services until we can get confirmation that the capsule is indeed defective.

And, if the diaphragms of that capsule are indeed defective, there is more than one way to skin the cat, so speak. Replacing  diaphragms on vintage AKG (or Neumann) capsules remains a hazy and hazardous undertaking. Especially when the client's expectation is nothing short of a return to original vintage capsule sound, aftermarket diaphragm replacement may not get you there.

I am always ready and eager to change my opinion on this subject, but have yet to hear a reskinned Neumann or AKG (vintage or contemporary) capsule that delivers the kind of excellence of timbre and sex appeal associated with healthy, original specimens.

Especially in the Neumann corner, reskins I have tested over the years have been universally disappointing.  None of that detailed, complex, smooth processing quality of the originals.

It's not that I would not want re-diaphragming to succeed. I have boxes full of defective original capsules gathering dust and awaiting some sort of rebirth.

I'd give Tim a chance with an original ELA M251 capsule with defective diaphragms, if that is something he wishes to take me up on.
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Klaus Heyne
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Rick Sutton

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Re: AKG C12 with dull sounding capsule
« Reply #7 on: July 05, 2012, 09:28:04 pm »

 Sorry if I overstated, but I wanted to put on the record what my personal experience was.
And to the specifics of my capsule, there was no questioning of the need for a new membrane as the old one had a puncture in it so part of my joy was the resurrection of a mic that is so important to me. Hey, these old mics become part of the family!
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Warren Huck

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Re: AKG C12 with dull sounding capsule
« Reply #8 on: July 06, 2012, 01:42:54 am »

Hi all,
I have now had a chance to test the mic as per the instructions from Klaus.

I blew breath onto the capsule while it was powered up. At first I saw nothing but under the bright light from my maglamp I could see distortion in the diaphragm and even a couple of small bumps sticking up from the back plate. When I turned the power supply off I could watch as the diaphragm let go of the backplate and return to its normal position. I repeated this process just to make sure that I was not fooling myself and also checked the rear diaphragm as well. The rear diaphragm looks fine. So it looks like the front diaphragm has lost tension as you predicted.

This mic is probably mostly used in cardioid mode and so I wondered if the capsule could simply be rotated as this could be one way to keep the "original" capsule sound. Is this a practical idea ?

Idealy the front diaphragm will need to be re-skinned or somehow re-tensioned. Its really up to the owner from here on.

Regards, Warren Huck
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Regards, Warren Huck

Tim Campbell

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Re: AKG C12 with dull sounding capsule
« Reply #9 on: July 06, 2012, 12:16:34 pm »

Dear Klaus, just to be clear please re-read my post. I never offered my services for repair of this capsule. I was really only concurring with your guess at a probable cause which in this case turned out to be correct. I could hear from Warren's original post that his client would most likely not want the capsule repaired.

Yes Warren, you can simply rotate the capsule and use the back membrane for cardioid without a problem.

Klaus, I'd gladly take a shot at any capsules you'd like to send me.
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klaus

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Re: AKG C12 with dull sounding capsule
« Reply #10 on: July 06, 2012, 12:57:17 pm »

Tim,
Thanks for the clarification. I'd rather over than understate my seriousness of keeping at least ONE forum for professional audio free from advertising, overt or covert.

Warren: you are not quite out of murky waters yet with your CK12. Indeed, try reversing the capsule's orientation, to get the healthier diaphragm to the front (as 95% of all recording with these mics is done in cardioid).

However, keep in mind that the acoustic damping of a dual-diaphragm capsule of the Braunmühl-Weber variety depends on healthy diaphragm tension all around, not unlike a snare drum, where, if the resonant head was damaged, the snare will never sound right.

If one diaphragm suffers from severe loss of tension, that will affect the overall sound of the capsule, even if the other side is in the front. But there are cases, where you can get away with that condition. Your sharp ears will tell.



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Klaus Heyne
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Warren Huck

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Re: AKG C12 with dull sounding capsule
« Reply #11 on: July 08, 2012, 12:02:30 am »

Thanks Klaus and all others concerned, I enjoyed every perspective offered here.

Now for the really tricky bit, breaking the sad news to the client, ultimately it is up to him what happens next.

Regards, Warren
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Regards, Warren Huck

klaus

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Re: AKG C12 with dull sounding capsule
« Reply #12 on: July 08, 2012, 01:44:30 am »

Before you deliver the bad news, make sure it's really solid bad news:
Did you confirm that the B+ supply voltage to the mic was indeed 120VDC?

As I mentioned: a polarization voltage where the supply's high voltage output is of more than, say, 125-128VDC will force the diaphragms to behave like their tension is weak, when that may not really be the case.
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Klaus Heyne
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Warren Huck

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Re: AKG C12 with dull sounding capsule
« Reply #13 on: July 08, 2012, 07:05:12 am »

Hi Klaus,
The 120 volt rail measures as 121.5 volts with my trusty old Fluke multimeter.
Regards, Warren
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Regards, Warren Huck

Warren Huck

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Re: AKG C12 with dull sounding capsule
« Reply #14 on: July 10, 2012, 07:11:10 am »

Hi all,

My client just turned up with another capsule. He says that he bought it many years ago for peanuts and had it stored away. It has a little bit of crap on both diaphragms but looks fairly good overall. I will let you know how it goes when I get a chance to fit it.

Regards, Warren
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Regards, Warren Huck

Warren Huck

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Re: AKG C12 with dull sounding capsule
« Reply #15 on: July 20, 2012, 12:26:02 am »

Hi all,

The last instalment of this story. I fitted the alternative capsule but it turned out to be worse than the original. It sounded fine but bottomed out on both diaphragms if I sang loudly into it. I re-fitted the original capsule but rotated it 180 degrees. It sounds fairly good here but is now a one pattern microphone. The rear still works but is dull.

I have now suggested to the client that he gets the alternative capsule re-skinned sometime. That way he can change back to the original capsule if it is found to be better for any reason.

I dont know how the serial numbers work on these microphones, this one is marked as C12, No 77 Does this make it number 77 off the production line ? Its a bit of audio history regardless.

Regards, Warren Huck
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Regards, Warren Huck

klaus

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Re: AKG C12 with dull sounding capsule
« Reply #16 on: July 20, 2012, 02:12:04 am »

Just "77"? Can you post a picture of the serial number? Sounds suspicious to me.
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Klaus Heyne
German Masterworks®
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Warren Huck

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Re: AKG C12 with dull sounding capsule
« Reply #17 on: July 26, 2012, 07:36:01 pm »

Hi Klause,
Here are a couple of photos
Regards, Warren
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Regards, Warren Huck

Warren Huck

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Re: AKG C12 with dull sounding capsule
« Reply #18 on: July 26, 2012, 07:37:11 pm »

Here is the photo I meant to send
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Regards, Warren Huck

klaus

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Re: AKG C12 with dull sounding capsule
« Reply #19 on: July 27, 2012, 12:57:50 am »

That is truly original serial number engraving- all the way to the tiny numbers that precede stamping in the big numbers
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Klaus Heyne
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Warren Huck

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Re: AKG C12 with dull sounding capsule
« Reply #20 on: July 27, 2012, 02:43:26 am »

Hi Klaus,

Would that make it number 77 off the assembly line or do the numbers work another way ?

This mic has suffered in its lifetime, some screws are missing, holes mangled, very crude engraving etc, but the owner reports that people really love using it.

This particular client has quite a few classic microphones, many of them look a little road worn, probably from previous owners.

Regards, Warren
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Regards, Warren Huck

klaus

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Re: AKG C12 with dull sounding capsule
« Reply #21 on: July 27, 2012, 03:28:01 am »

Over the years, no one I have talked to at AKG seems to have a grip on serial number logic or correlation with dates. All we know is that, usually, lower serial numbers indicate an earlier manufacture. But neither the sequential nature of these numbers has ever been confirmed, nor has there supposedly (and incredibly) ever been any documentation kept at AKG as to when a particular mic with a specific serial number was made or left the factory. To me, that's pretty hard to swallow, given that companies need to inventory stock, at least once a year, for tax purposes.

That being said, there are plenty of build features of, say, a C12 that make identifying its year of manufacture pretty easy.
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Klaus Heyne
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