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Author Topic: Neumann KM64 decca mod  (Read 2634 times)

Offline hugovanmeijeren

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Neumann KM64 decca mod
« on: February 12, 2015, 11:13:23 am »

Here's a question about a microphone about which not much information is available. It is (what I believe) a Neumann KM64 that has been modified by the Decca Studio's from tube to FET. (Apparently they discarded the AC701k  :-\).

The problem however is that the electronics are faulty. If I connect the microphone and apply phantom I do get sound, but almost only low frequency (the bass response is extremely exaggerated), it is extremely noisy and I need to apply an enormous amount of gain.

I have the idea that the internal amplification is faulty and that I'm using the preamp in the mixer directly to get some sound off the capsule.

Internally, the body relies on a Texas Instruments BC264 FET (which has also been used by Schoeps in various microphones if I'm not mistaken) and another transistor with the following labeling: TI 850 953A (probably also Texas Instruments). However when trying to search for the latter, I can't find any information or datasheets.
See also this picture: https://www.dropbox.com/s/5ax5wsi5z9vjss1/IMAG1624.jpg?dl=0

Can anyone give some hints on where to be looking for the problem? Personally I suspect that BC264, does anyone know of a source in Europe to find this?

As a side note, the capsule itself is fine. I also own 4x KM84's and since the capsules of the KM64 and KM84 are the same, by swapping them, I can confirm that the capsule works perfectly.

Some other pictures of the interior of my KM64 (these have been previously posted to Gearslutz, before finding out that it was a KM64, since there is no labelling on this microphone):

Offline Gabor A. Erdelyi

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Re: Neumann KM64 decca mod
« Reply #1 on: February 27, 2017, 02:44:43 pm »
Hello there!

It's a bit late reply, but maybe useful. Have you tried to contact Mr. Mick Hinton at D-A_V Electronics? He was an engineer at Decca and he knows everything about this mods. He can help you, and he is quite a nice person. I would give it a try.


Offline klaus

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Re: Neumann KM64 decca mod
« Reply #2 on: February 27, 2017, 04:16:44 pm »
Sorry, I must have missed the thread when it was started.
The BC264 is a standard edition J-FET, suitable as impedance converter in condenser mics. You can conveniently substitute it with a 2N3819 which Neumann uses and used in almost all of their FET mics.

Besides, I do not think that the low pass response in your mic is caused by a defective FET.
Usually, severe high end attenuation and low output points to a defective capacitor and/or that second transistor which may be there for biasing. I would start by replacing it.
Klaus Heyne
German Masterworks®

Offline hugovanmeijeren

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Re: Neumann KM64 decca mod
« Reply #3 on: March 01, 2017, 07:52:45 am »
Ha, it's been several years later and finally a response.  ;)

Actually yes, through another forum I'd found out that Mr. Hinton was responsible for this mod. He was very friendly and helped me back in 2015 to identify some of the parts that were used. It turned out that the transistor, a standard BC547, alongside the BC264C FET was defective. I replaced it and re-biased the FET and the microphone is working fine again!

Sorry for not getting back to this topic afterwards...