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Author Topic: KM-184 RF problems?  (Read 5377 times)

Karl Winkler

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Re: KM-184 RF problems?
« Reply #15 on: May 18, 2011, 03:29:27 pm »

Hi All,

I've been chasing an RF problem in the studio and I think I've narrowed it down to my KM-184s.

I was having general issues with RF but then went through Klaus's sticky post about handling RF, corrected some shielding issues in the patch bay and mic panels and everything became dead quiet. Except for the KM-184's.

I have a matched pair that seem to randomly pick up RF. Happens on Millennia pres and Daking pres. If I swap out mics with something else, the RF will always go away. I found a post on the Neumann forums that suggested there were some KM-184s that were susceptible to RF. I tried to contact Nuemann USA and Berlin several times by phone and email and have not received a response.

I searched the forums here and didn't see anything specific about KM-184s and RF so I wanted to ask: does anyone know about RF issues specific to this mic? If so, is there a mod or a remedy?

Thanks,

Jason

How old are your microphones? I know that a few years back, maybe close to 10 now, Neumann re-designed the KM184 in an effort to eliminate just this sort of RF noise. From what I can recall, the problem became evident when the mic was used in TV studios due to audience members having cell phones in their pockets, turned on but silenced. It was something about a ground trace on the circuit board being just the right length to act as an antenna. Further backing this theory is that with a transformer load, the problem goes away due to the very high CMRR of a good transformer input.

If your mics are relatively new, it may simply be an indicator that whatever change they made either was not fully effective, or is now vulnerable to a different RF frequency.

I'd suggest trying to reach the Neumann engineering dept. in Berlin via the neumann.com web site, and describe the problem to them and include your microphone serial numbers.
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Jim Williams

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Re: KM-184 RF problems?
« Reply #16 on: May 19, 2011, 11:14:07 am »

Further backing this theory is that with a transformer load, the problem goes away due to the very high CMRR of a good transformer input.

Hi Karl, the iron I see has a CMRR of about -70 db at 20k hz. That will usually improve at lower frequencies but drops off at higher frequencies due to mismatch and coil leakage. I don't know the CMRR of the Millenia preamp, but I suspect it's probably about -70 or better at 20k hz due to the match of the MAT 04 input transistors.
Other designs sans iron go up to -90 db CMRR, 20 hz to 20k hz, so I still believe it's transistor rectification causing the radio interference.
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Karl Winkler

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Re: KM-184 RF problems?
« Reply #17 on: May 19, 2011, 11:30:44 am »

Good points, Jim. I'm no circuit expert...
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