R/E/P > Klaus Heyne's Mic Lab Stickies

Radio Frequency Interference (RFI) in Mics

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Yannick Willox:
I just bought some xcc series xlr, to modify the cable that comes with my RFI sensitive mic.
However, it seems my friendly manufacturer put ferrite beads on all output leads/pins inside the mic, and the RFI (in the concerned concert hall) is completely gone.
So I cannot test the other solutions, nor these special xlrs ...

As a side note ref Klaus's first post :
nrs 1 and 2 were always correctly implemented, moreover in point two there is a dilemma concerning multiple capsule mics, with special 4ch mic cable ...

nr 4 seems to work very well, without the sonic compromises that might arise from the chokes mentioned in point 3.

Yannick Willox

recordista:
I highly recommend AES preprint #5720 (to which David alludes,) available from the AES website


There's also a 1985 Neumann paper on the subject.

sekim:
Ferrite bead is very effective when used properly.  It was interesting to see that Schoeps had early development using it.  I used very similar techniques to iron out rf issues with a prototype capsule amplifier not so long ago based on my prior knowledge from electronics design work in an unrelated field...  It's also very cost effective, much more so than an active warranty program and reputation for susceptible gear...

[edit] btw - simplistic explanation of how bead works: an electron traveling thru a straight (or curved for that matter) wire produces a magnetic (B) field that is circular about the wire at the point of the electron.  A ferrite sleeve around this wire alters the B field thus creating the phenomenon of induction.  So in the case of ferrite bead, an inductor is formed tuned to reject rf.

To really understand rf issues in audio it is helpful to better understand rf itself: google on "foxhole radio" and "antenna theory" for good primer info.  A basic understanding of radio and antenna theory (just an evening or two of reading) makes it very apparent what the root problems are and thus the effective ways to deal with it.

bjorn400:
HI
I was recommended to this thread from another.

Thanks for the good info.  I do have a more specific question.

Can you think of any reason why my neumann km 184's are picking up EMI buzz and none of my other mics are?(studio projects c1, sm 58, Microtech Gefell M930). I've troubleshooted chords, preamps, outlets.  The only thing I haven't tried is plugging them in outside of my electricity grid.  But why would my other mics be unaffected?

Is there something about that mic that makes it more sensitive?  

thanks
bjorn

Klaus Heyne:
The KM184, like other TLM mics, is not a truly balanced mic, but an electronic approximation of a balanced system.

Occasional RF problems in red-badge (TLM) Neumann mics, especially when used in heavy RF exposure environments, have been reported.

Check similar inquiries on the Neumann Pinboard, read Martin Schneider's answers, and, if your problem persists, send it into Neumann for investigation.

Best regards,

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