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

Radio Frequency Interference (RFI) in Mics

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Klaus Heyne:
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.

Alixander III:
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?????

radardoug:
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.

Klaus Heyne:
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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