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Author Topic: Radio Frequency Interference (RFI) in Mics  (Read 32021 times)

johnR

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Re: Radio Frequency Interference (RFI) in mics
« Reply #15 on: April 01, 2007, 12:41:56 pm »

Yannick Willox wrote on Mon, 04 October 2004 04:22

The BIPT that came to measure the RFI in the concert hall where my problems started (they measured no abnormal RF, just very high levels of the transmitter at 104 MHz) state that the best way is connecting a ceramic capacitor between the conductors of the mic cable and the earth (10 nanofarad or 100 nanofarad).


That is bad advice for two reasons:

1 - A 100 nF cap has an impedance of around 159 ohms at 10 kHz and will shunt out high frequencies in the audio range.

2 - Ceramic caps produce distortion due to the piezoelectric effect. The thickness of the insulating layer in the capacitor (and hence the capacitance value) varies with the signal voltage. The result is harmonic and intermodulation distortion. Ceramic caps shouldn't be used in a high quality signal path.

John Rigg
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brett

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Re: Radio Frequency Interference (RFI) in mics
« Reply #16 on: July 29, 2007, 07:54:16 pm »

Any tricks for a smooth wireless system free of RF, besides the above mentioned comments?

Regards

Brett
Hollywood Ca
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Mike Cleaver

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #17 on: August 26, 2007, 10:26:39 pm »

Your friend here is a simple piece of wire with an alligator clip on each end; probably a couple of them.

Try the different grounding schemes, tying pin one to the shell at one end, both ends, or all ends of the cable.

Every connector you add increases the probability of a noise problem (generated through mis-wiring or a wrong connection scheme)

You have a connector at the console/preamp end, one at the wall going into the studio, another at the wall coming out in the studio then two on the mic cable.

Now, find out where the EMI is coming from.
Start with only the console or mixer or preamp fired up and start switching things in.
If it's coming in on the power line, you'll hear it on the board before anything else is turned on.

Are you in your own building or sharing space with other businesses?
Try some of the above.
Let us know what you find out.
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brett

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #18 on: September 09, 2007, 02:45:46 pm »

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nob turner

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #19 on: March 04, 2008, 03:03:15 pm »

I have a Neumann TLM193 which is more susceptible to hum than any other mic I own.  This is made much more obvious when you touch the basket of the mic--the hum gets significantly louder.  I live not far from the tv tower that broadcasts to the entire area.

A message to Neumann USA got a response suggesting that my problem was indeed RFI, and specifically the vertical sync signal from the tv transmissions.  They say there is no update to the mic to improve this issue.  I have checked out my cables, and they are properly wired and grounded.  I've read through this thread, and it appears that chokes would need to be installed in the mic by the manufacturer, if i've read this right.  The info on ferrite beads is perhaps a bit beyond my technical understanding.  Nonetheless, if there is a possibility that they'd improve my situation, I'd like to give them a try.

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Klaus Heyne

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #20 on: March 04, 2008, 04:46:58 pm »

Before you move to sound-deteriorating chokes and filters:
Are you using Neumann/Gotham GAC 3 cable, where ground and shield are terminated at both ends to pins #1 of the XLRs and to the sleeves of the connectors?

Please double check, as most RFI problems disappear when the above steps are taken.
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Klaus Heyne
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nob turner

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #21 on: March 21, 2008, 10:35:39 pm »

Yes, Klaus, I've now acquired some GAC-3, wired it as per your above instructions... and the 60-Hz hum is still there.  

Keep in mind I am about 2 miles from the Sutro broadcast tower in SF.  I've tried patching directly into my mic preamps so as to avoid any other cable in the loop.  I've tried three different preamps.  

What is the next step?
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Klaus Heyne

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #22 on: March 22, 2008, 06:24:36 pm »

I am sorry that I did not read your post carefully enough. My cable suggestion was in reference to your RFI/radio tower suspicions.
60Hz hum is by definition unrelated to RFI, but is either due to wrong cable terminations, or the mic pre's grounding scheme or a problem in the mic's amp itself. In rare cases, 60Hz hum can already be present in a deficient AC grounding scheme coming to your studio from the power pole/distribution box.
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Klaus Heyne
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nob turner

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #23 on: March 25, 2008, 03:39:08 pm »

ok, i'm ready to go about checking ground issues... but want to mention that NO other mic in my collection exhibits this problem, including other transformerless mics like km184.  the hum increases  significantly when i touch the basket of the mic, but NOT when touching other portions of its body or the mic connector.  in fact, the hum commences to increase as my hand nears the body, but doesn't touch it.  this latter fact was sighted by the neumann tech i spoke to as strong evidence that the problem was rf related.  

mean time, i will look again at grounding here.
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maarvold

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #24 on: October 11, 2008, 12:11:20 am »

Klaus Heyne wrote on Mon, 13 September 2004 11:18

...On phantom powered mics (and other mics with XLR three-pin connectors):
Pin #1 and the ground lug of each connector's housing must be connected together, on both cable ends!


At one of the 3 big scoring stages in Los Angeles, connecting Pin 1 and the XLR shell's ground lug at both ends produced ground hum that made the approach mentioned above unusable.  Connecting the shield and shell ONLY at the female end (in conjunction with using cable with double Reussen shielding) completely cured RFI problems in an environment where the very same microphones had always exhibited problems in the past--the type of problem where one can hear a radio station, at low level, coming in on the mic feed.  It cured the problem so completely that I was able to remove the mic's capacitor/inductor RFI filtering networks and still be free from the RFI problem.  And the mics sounded noticeably better in this configuration.  

I was recording rhythm tracks at a studio in North Hollywood.  I was using my '47-ish' Pearlman TM-1 on  the upright bass.  The first day everything was fine.  The second day, we came in and the bass player immediately noticed some low level 'fuzz' that was only present when the instrument was playing.  When there was no note being played, there was no fuzz.  I asked my assistant to put up the house U47 (tube).  It exhibited the same fuzz behavior.  Then I asked him to use one of my Gotham double  Reussen-shielded mic cables, with Pin 1 and XLR shell's ground lug  connected on the female end, between the power supply and the wall panel.  Problem solved: no more fuzz.  I asked my assistant to put the Pearlman back instead of the U47: the problem was solved there as well.  
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Klaus Heyne

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #25 on: October 19, 2008, 12:22:33 am »

Your key sentence which makes your exception to the absolute RF suppression rule plausible is this:
Quote:

...Then I asked him to use one of my ... cables... between the power supply and the wall panel.  Problem solved: no more fuzz.


I have no doubt that some of the connections inside the wall panel were compromised.

There is no plausible scenario in any straight mic-to-cable-to-power supply-to mic pre setup where an open shield on one cable end would ever be advantageous for RF-suppression.

The whole concept of RF suppression, as stated many times before, rests on the complete shielding and grounding of all microphone-related components, from mic housing to final XLR connection at the mic pre. Leaving even one shield and ground connection open in this chain invites trouble.  I bet, if you were able to bypass the wall  panel, and use a straight wire connection between mic and mic pre, the traditional, complete, shielding scheme would yield superior RF rejection.
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Klaus Heyne
German Masterworks
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Alixander III

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #26 on: October 28, 2010, 03:58:34 am »

Having major problems with RFI in my studio, but only with Neumann mics!

I solved the problem in my U47 with what Klaus suggested:

Quote:

On tube mics:
Connect ground wire and cable shield together to the pin of the connector that is dedicated to ground and install a wire from ground to make contact with the connector's housing. This connection is often conveniently made at the cable strain relief, where a clamp is screwed into threads of the connector housing.


For some reason, this doesn't work on my M249! I tested for continuity between mic body, ground pin and cable connector, and continuity is there.

Any idea why it would work on the 47 but not the M249?????
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ALPHONSE ALIXANDER LANZA III

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radardoug

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #27 on: January 17, 2011, 03:13:16 pm »

Just to clarify a couple of things.
The reason that r.f. interference has a 60Hz buzz is that often this interference is a TV transmission, and the frame sync signal is transmitted at 60 Hz. What we hear is this modulation being demodulated by the microphone.
The problem occurs because rf signals travel up the cable, and into the microphone's active circuitry. The methods of shorting pin 1 to the cable sheild tend to bypass this rf to ground before it can enter the microphone.
Certain Neumann microphones are very sensitive to rf. One model of the U87 and the Fet 47 come to mind.
One way to eliminate rf from mikes is to install a ground plane in the bottom of the microphone. Pass the audio leads through a small hole in this ground plane. Make sure that the audio wires are well twisted and try and keep them equal in length. The ground plane can be made from flexible pcb material, which is insulated on one side. Make sure you gound it well to the body and pin 1 on the connector.
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Klaus Heyne

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #28 on: January 31, 2011, 04:36:26 pm »

Some early U87s had a problem in extreme RF environments, until Neumann upped the filtering near the connector.

Fet47s are a harder nut to crack, though here too Neumann increased filtering. The nature of a discrete op-amp makes RF suppression harder.
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Klaus Heyne
German Masterworks
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