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Neumann Mic problem

Discussion in 'Microphones' started by Gin Ridge, May 22, 2003.

  1. Gin Ridge

    Gin Ridge Guest

    Greetings to all,

    I'm in the middle of recording a project in my home studio and have run into a baffling problem with my Neumann KM184s. For reasons neither I or the techs at Sweetwater or Neumann can figure out I am getting some form of interference with the Neumann mics. It seems to be a high end noise the modulates, almost "swooshes" rather than an continuous steady sound. It is more pronounced in certain area of the room and less so in others. Sometimes a matter of a few inches makes a big difference.

    My signal path is rather straight forward. The Neumanns go into Grace 101 pre-amps then into a TASCAM DA-38 via the ballanced inputs. I do not have this problem when I use a Blueberry, AKG 460 or AKG414 mic. The Neumann tech said he had heard of their mics being suceptible to RF interference and suggested buying their cables, which I did, and that lessoned but did not remove the problem.

    I've used these mics in the past but didn't have this problem. I've switched cables, lifted grounds and moved equiptment around to no avail. I've bypassed the Graces and used the preamps in a Mackie 1202 and a Mackie D8B and the noise is still there. Thinking I might have magnetic field problems, I've shut off various electrical curcuits to see if that would help,
    also to no vail.

    I'd appreciate any suggestions for dealing with this, it's making life miserable!

    Thanks!
     
  2. Some things to consider:

    Are you recording near power lines?

    Are there flourescent lights in the room you are recording in?

    If you can, try using the mics in a different room. Dose the problem change?

    Can you try them in a friends studio?

    Cheers
    Kevin.
     
  3. Gin Ridge

    Gin Ridge Guest

    Hi Kevin,

    Thanks for the response.

    There are no florescent lights in the room or the house. I do have a vintage neon sign downstairs on another curcuit, but I've turned that off and still have the problem. There are no significant power lines in the area, it's residential in a small town.

    I'll be going over to a friends studio later tonight and to a brand new studio (the regional public radio station) next week and will take the mics there to check out. As I mentioned, I've used these mics before in the same room and in other locations with no problems. I'm just guessing, but I don't expect to have the problem outside this room. Since recording my last CD here, I have upgraded equiptment to include a Mackie D8B and Grace 101 pre-amps, but currently do not record with the D8B since the fan noise is excessive. I keep everything I'm not using off (monitor screens, outboard equiptment, etc), when I record to avoid any chance of extra interference.

    I spoke with the chief engineer at the public radio station who is an extremely smart guy (don't ask him a question unlessyou want to know ALL there is to know about any subject to the smallest detail). He just set up their new station and he thinks it sounds like some form of RF interference. There are microwave transmitters that were placed on a water tower about 2 blocks away. I'm wondering if that might be the source of the problem, but I'm confused by the fact that it only occurs with the Neumann mics.

    All thought and suggestions are greatly appreciated! I'll keep you informed as I try the mics in different locations.

    Thanks

    Gin Ridge
     
  4. Gin Ridge

    Gin Ridge Guest

    Hi Kevin,

    Thanks for the response.

    There are no florescent lights in the room or the house. I do have a vintage neon sign downstairs on another curcuit, but I've turned that off and still have the problem. There are no significant power lines in the area, it's residential in a small town.

    I'll be going over to a friends studio later tonight and to a brand new studio (the regional public radio station) next week and will take the mics there to check out. As I mentioned, I've used these mics before in the same room and in other locations with no problems. I'm just guessing, but I don't expect to have the problem outside this room. Since recording my last CD here, I have upgraded equiptment to include a Mackie D8B and Grace 101 pre-amps, but currently do not record with the D8B since the fan noise is excessive. I keep everything I'm not using off (monitor screens, outboard equiptment, etc), when I record to avoid any chance of extra interference.

    I spoke with the chief engineer at the public radio station who is an extremely smart guy (don't ask him a question unlessyou want to know ALL there is to know about any subject to the smallest detail). He just set up their new station and he thinks it sounds like some form of RF interference. There are microwave transmitters that were placed on a water tower about 2 blocks away. I'm wondering if that might be the source of the problem, but I'm confused by the fact that it only occurs with the Neumann mics.

    All thought and suggestions are greatly appreciated! I'll keep you informed as I try the mics in different locations.

    Thanks

    Gin Ridge
     
  5. rmburrow

    rmburrow Active Member

    Reply to old post but RFI occurs often and many don't know how to identify it. Do you live near the transmitter of a FM or TV station? Do you have access to a RF spectrum analyzer? The mic problem sounds like RF from some nearby VHF or UHF source. FM RF into a mic can be hard to identify without a spectrum analyzer...a good instrument will provide a relative signal level and frequency of nearby RF sources...one last hint...keep cell phones away from solid state mics...rectification of the carrier by the FET or other solid state components in a condenser mic can have surprising results....

    My regular line of work involves RF systems and interference control or abatement....hope this helps...
     
  6. DonnyThompson

    DonnyThompson Distinguished Member

    Thanks for the info. :)
     
  7. rmburrow

    rmburrow Active Member

    A follow up to my earlier comments: A while back, I recorded a concert at a church in Washington DC and encountered TVI from a nearby VHF television station (approximately one mile away) in the pre-digital TV days. The source was obvious since the sync buzz was demodulated and audible, and 100 kw ERP from the TV antenna at a distance of a mile is a strong field. Isolation transformers on the AC power to the mic supplies and the recording gear took care of that one. The new digital TV waveform is much different from the old analog waveform, and either a spectrum analyzer check or a check of your venue's geographic coordinates against FCC broadcast data bases will identify nearby broadcast transmitters. (You can get geographic coordinates of your home or venue from DeLorme's Street Atlas or other suitable on line software.)

    Another interesting case involved signal leakage from a FCC Part 15 device (a MIDI controller for a pipe organ). Finding the RFI source required a spectrum analyzer and sniffer antenna...the offending energy was both radiated and conducted...the conducted component of the energy was present on the AC outlets, and the radiated energy was high in the vicinity of the organ. (The offending MIDI device was a FCC Class A device, for use in commercial venues, without the higher RF suppression required for FCC Class B devices for general consumer use.) The use of isolation transformers, along with the "Hum-X" RFI suppressor, kept the RFI out of the mics and recording equipment. The conducted RFI was confirmed when the background noise level of a house amplifier rose when the organ was switched on (conducted interference). Unfortunately, the organ power was hard wired into 230 volt AC so no ready means of filtering the organ's incoming AC supply or the MIDI device was possible. (RF filters for various 230 (and higher) volt incoming power configurations are commercially available, but most are for permanent installation by a qualified electrician.)

    Keeping an isolation transformer and a couple of the "Hum-X" suppressors in the tool box may be the difference between a marginal (or no recording) and a decent recording at a venue polluted by RF or dirty AC power. The "Hum-X" devices run around $65 or so each from B&H in New York City; they are plug in devices and good for around 6 amp apiece. The cost of isolation transformers depends on the required wattage. Small transformers (i.e. 50 va) for isolating mic supplies are less expensive than a 1 kVa unit powering a complete recording setup. Last but not least, pay attention to your wiring (audio and AC) and avoid ground loops.
     
  8. sdelsolray

    sdelsolray Active Member

    Perhaps it is Wi-Fi interference.
     
  9. rmburrow

    rmburrow Active Member

    Really, any Part 15 FCC device near a solid state mic could be a problem...especially any clocked device...
     
  10. Klaus

    Klaus Active Member

    This thread is limping along through the decades, but hopefully it will end with a satisfying solution.

    I'd like to get a concrete description of the actual cable that was used with the mic when the RF occurred, as almost all RF mic problems are solved with good cable material and proper connector termination:
    How are your XLR connectors on the mic cable terminated? Please unscrew the sleeves and describe how ground and shield are connected, and please inspect both ends.
     
  11. rmburrow

    rmburrow Active Member

    Back to the Mr. Ridge's original symptoms...the "swooshing" sound being demodulated by a mic or ancillary equipment... that sounds like FM broadcast interference to me.... any nearby radio transmitting towers? Depending on where you are, the FCC permits FM stations to radiate up to 50 kW (depending on class in most of the NE states), and 100 kW in other US areas not within the Class B zones. The FM blanket contour is 115 dBuV; 115 dBuV is a strong signal. Mr. Ridge, do you have the call sign of the nearby FM station? If you do, you can search on the FCC web site (www.fcc.gov) for FMQUERY. If you know the geographic coordinates of your home/studio, you can "plug" those into FMQUERY, use a 10 km search radius, and list nearby FM stations. Those FM stations with higher effective radiated power ("ERP") and distance closest to your location could be likely contributors.

    Klaus is absolutely correct about cable termination and shielding. Check your cables like he requests and verify there are no broken or intermittent connections. A broken shield connection will let RF in to wreak havoc.
     
  12. Klaus

    Klaus Active Member

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