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Author Topic: Radio Frequency Interference (RFI) in Mics  (Read 30969 times)

Klaus Heyne

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Radio Frequency Interference (RFI) in Mics
« on: September 13, 2004, 02:18:31 pm »

Radio Frequency Interference (RFI) in microphones can be a pest- it comes and goes, then reappears as soon as one has forgotten about it, to damage a recording in progress.
RFI can be successfully and permanently eliminated in almost all cases.
I would like to share which remedies I have found are least impacting on the mic's sound while still taking care of the problem.

RFI comes in two audible forms: distinct radio station signals mixed in with the mic's audio, and noise hash, whose presence is much harder to hear and properly identify as RFI, because it appears as featureless background white noise.

How does a microphone pick up RFI?
RFI is often a result of inappropriate cable terminations, insufficient cable shielding methods or materials and/or certain designs of mic circuitry.  More rarely, RFI may be triggered by a very strong radio transmitter close by (which in turn exacerbates some of the triggers mentioned above)

Think of the microphone, including its cable, as an antenna: unlike other audio gear, the mic, as the first link in a recording chain, is open ended on one side (towards the head of the mic) and electrically connected only on the other with an amplification device which is nicely suitable to transmit what's been captured from this antenna.

Certain configurations of the mic's processor (f. ex. Nuvistors, discreet op amps, electronically balanced, rather than truly balanced, mic outputs) are often more prone to RFI than others.


How to get rid of RFI?
I use a methodical approach to curing RFI, based on my experience of what needs to be addressed first, second, and third, regardless of other potential variables. If I do not address the problem in this order, I may miss a cure and waste time.


1. Cable Terminations
Inspect the mic cable's connector terminations. I mean visually inspect, not just assume, their correctness.  Make sure that the following terminations are present:

- 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!  

Pin #1 of the XLRs must then be connected to the cable's ground wire. If the cable does not have a dedicated ground wire, the cable's shield must be connected to pin #1. If the cable has both, ground wire and shield (Gotham, Neumann), connect both to pin #1 of the XLRs.

Beware of certain connectors (some Neutriks comes to mind) which do not have a separate ground lug termination point for the connector's housing. Do not use these connectors on mic cables, as the connector shells cannot be grounded.

When you are done, do the following test to confirm the correct connections:
An ohm meter's leads connected to both connector housings should read zero or a few single digit ohms. If it reads open, go back and inspect what's wrong, or you will get RFI!

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.

Again: install this ground/shield scheme on both sides of the cable, and when done, perform the ohm meter test, as described above.

This method of termination is always applicable for microphone cables, except for rare multi-connector hook ups (I won't get into this here.)  Please do not mix up correct RFI terminations with what you have learned about terminations to prevent ground loops- there are no ground loops with mics and their cables, as they are single ended.
The terminations described above also happen to be the official Neumann recommendation.  

Gunnar Hellquist adds:
Quote:

The microphone should be electrically insulated from the holder and hence from the stand.

If not, the mic is no longer ground loop-poof, as contaminating electricity, picked up from somewhere else, may flow through the conductive mic stand to the mic)


2. Cable material
I will not discuss advantages of certain cable brands, or their effect on sound. (I recommend and endorse, without payment, a specific microphone cable- Gotham, Switzerland)
I will share though, that in several independent cable tests for RFI susceptibility, one of them in an official AES paper, cables with double Reussen layer shields were provably more resistant to RFI than any other method of cable shielding.  

Reussen layer cable shields are nothing else but multi strands of copper wire twisted in a cork screw fashion around the cable conductors. A second layer is then twisted in an opposite direction over the first. This is visibly different from braided shields or foil shields.

To my knowledge, only Neumann, Berlin, and Gotham, Switzerland (whose Reussen cables are now made by Belden in Germany) use this type of shielding.

My experience has been that in nasty cases of RFI, this type of cable, in combination with proper terminations cured the problem when correct terminations alone did not.

3. Chokes
A choke is a wire coiled in such a way that certain frequencies of audio when it passes through the coil, are suppressed.
Typically these choke coils, which in mics are very small and look like resistors, are inserted in series at the balanced outputs of the mic.

This method of RFI suppression really works well- like a sledge hammer on mosquitoes: The deterioration of sound from choke coils ranges from subtle to unbearable, depending on the value of the coil chosen.

Typical values of coils factory installed in mics range between 40 and 100
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Klaus Heyne
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djosephson

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Re: Radio Frequency Interference (RFI) in mics
« Reply #1 on: September 29, 2004, 07:15:36 pm »

Thanks for discussing this topic, Klaus. Jim Brown (whose profession is designing large sound installations) from Audio Systems Group in Chicago and I have presented several papers at the AES on this. The work is mostly Jim's.

The problem used to be that mobile high-power two-way radios would burst in on the session. Now it is more insidious. All of the digital RF devices that may visit a studio bombard the microphone with random pulse streams that can cause a very significant degradation in microphone resolution due to elevation of the noise floor. 5 GHz signals are common, high power signals up to 80 GHz (millimeter wavelengths) are coming soon. If someone is serious about high definition recording, it is necessary for them to pay close attention to this issue and investigate how RF can get into their systems, because it won't be obvious like it is if you get a good buddy in your overhead track. We have not found a condenser microphone in existence that is RF-proof.

Of course Neumann, fulfilling the specifications of the German state radio networks, pioneered RF-resistant microphones years ago with the M 2xx and KM 2xx series mics, which had additional bypass capacitors and a carry-through of the cable shield all the way around the signal conductors to the shell of the microphone.  

While I concur that your recommendations will eliminate some forms of RFI, grounding the shell of the cable connector attached to the microphone can provoke other trouble, particularly in hand-held microphones. This can result in additional hum and potentially a shock hazard if the microphone is touched by someone who happens also to be touching something at a different power ground potential. The resulting current travels through the mic pre via the shield of the cable, and depending on the design of the mic pre can cause additional hum as this current travels through common impedances such as pc board traces and wiring inside a mixer on its way to "ground."

There is an additional problem if you ground the shells to the shield in microphone extension cables; additional points are created where power fault currents can enter the shield circuit if the connectors are for example sitting on damp concrete or a metal catwalk. As a result you need to have some cables with pin 1 tied to the shield of the female connector (only) which are used for microphones with poor RF immunity, and other cables with no such connection which are safe to use as extensions.

The solution we are recommending is that the shell be connected to the shield for RF only, that is though capacitors. Neutrik has introduced a new connector with pass-through ferrite material to present some resistance to RF flowing on the signal leads, and a concentric ring of capacitors which connects the shield to the shell for high frequencies. This is an expensive connector but is an excellent solution.

Ferrites, particularly when applied without engineering the whole solution, can cause more trouble than they solve. The problem is that most ferrite materials are partly conductive. If you put a ferrite bead on a wire and anchor it in place so it cannot touch anything else, that's fine. But if the bead touches anything else while it is also touching the signal lead, you now have a variable leakage path that can cause serious trouble, particularly at the input and output of a condenser microphone impedance converter (for the same essential reason -- a large DC potential superimposed on a small audio signal). Microphone companies who successfully use ferrites for RF suppression take particular precautions in this regard to avoid such problems.

It would be better if manufacturers paid more attention to this problem. A few companies have begun to do this, but as Jim and I documented there are several famous microphones with fairly long wires between pin 1 of the connector and the shell of the microphone. These wires form antennas that radiate shield current into the circuitry of the mic. Making this path as short and low inductance as possible will help quite a bit.
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Yannick Willox

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Re: Radio Frequency Interference (RFI) in mics
« Reply #2 on: October 04, 2004, 05:22:06 am »

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).

I think I'll try those Neutrik XLRs first.

Yannick Willox
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Doc B.

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Re: Radio Frequency Interference (RFI) in mics
« Reply #3 on: October 13, 2004, 01:26:26 pm »

Hi Klaus,

If I may offer a couple more techniques for RFI/EMI reduction that we have experimented with as regards small signal tube amplifier circuits that use tubes with VHF type bandwidth. Caveat emptor, you get what you pay for, etc., and please take this post down if it's getting too nuts and bolts for the discussion-

A small value 1/4W carbon composition resistor in the 50 ohm to 1K ohm range can be effective as a "grid stopper" and also as a "plate stopper" to block RFI. In each case the resistor must be installed with the body of the resistor right at the grid or plate terminal of the tube socket, in series. Generally the lower the value, the less influence it has on the sound, and one can thus season to taste by changing the value. I suspect it might be worth trying in a case where an RF choke is killing the RFI but also killing the sound, to see if it has less negative impact on the sound than the choke. FWIW we use 220 ohm stoppers with 6DJ8 type circuits and 50 ohm stoppers with tubes like the 417A or 6C45pi that want to see a low impedance grid load. Something like a 12AX7 or 12AT7 or 6CW4/13CW4 might even be OK with as high as 500 to 1K ohms.

Also, 1uF @ 50V X7R or Z5U ceramic caps can be effective RFI blockers when one is installed as a shunt across the heater pins, and one each is connected from a heater pin to chassis ground, as close to the tube socket as possible.
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Rob Darling

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Re: Radio Frequency Interference (RFI) in mics
« Reply #4 on: November 06, 2004, 09:47:30 am »

djosephson-

is the Neutrik product you are speaking of the xcc series xlr?
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Yannick Willox

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Re: Radio Frequency Interference (RFI) in mics
« Reply #5 on: November 10, 2004, 03:16:38 pm »

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
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recordista

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #6 on: March 28, 2005, 06:15:38 pm »

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.
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sekim

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #7 on: November 29, 2005, 04:28:46 pm »

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.
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Re: Radio Frequency Interference (RFI) in Mics
« Reply #8 on: April 15, 2006, 01:03:14 pm »

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
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Klaus Heyne

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #9 on: April 15, 2006, 07:42:43 pm »

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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Jonathan Novick

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #10 on: November 15, 2006, 10:57:41 am »

Since this thread was started, Neutrik introduced their EMC series of XLR connectors. The express purpose of these is to eliminate RF interference. I was just curious if anyone had any actual hands on experience with them.
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Uwe

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Re: Radio Frequency Interference (RFI) in mics
« Reply #11 on: January 16, 2007, 09:58:42 am »

There seems to be some fundamental misconception about the risk of introducing ground loops through the connection of the housing shell to the shield and pin 1 (XLR-connectors). Every microphone with a conductive (metal) housing shell I have come across does indeed have an internal connection from pin 1 to the housing shell already. The problem, concerning RFI, stems from the fact that this connection often is not low enough in impedance for the very high frequencies causing RFI. The direct bridge between pin 1 ond the shell inside the XLR connector simply serves to make this a more intimate and direct short for RF. You can prove this statement simply by making a resistance measurement between pin 1 and the housing shell of virtually any microphone, and you will find close to zero Ohms for the DC-resistance, even for microphones which may exhibit sensitivity to RFI. Capacitors of the proper value will work  simply because they do achieve the same objective. Capacitors exhibit increasingly lower impedance with higher frequencies, and present a near short circuit for the very high frequencies. (See the examples below.)

At the risk of opening a hornets' nest, I venture to dispel another misconception concerning the influence of chokes and/or capacitors used to su-press RFI. With proper choice of the values for these components there are practically no measurable or audible effects on the audio frequency signals, neither in amplitude or phase. A choke works simply by representing increasing impedance with increasing frequencies. Thus they do represent practically a short circuit for audio frequencies, and a high impedance for RF. A practical value used in series with the audio output leads in many microphones is 47uH. Such a choke presents an impedance of around 6 Ohm for a 20 kHz signal, but around 180 kOhm for the potentially RFI causing UHF TV-signal at 600 MHz, or nearly 30 kOhm for the potential RFI from an FM-station (unlikely), and still over 250 kOhm to put up a defense against RFI from a 1 MHz AM-radio signal. In reference to the impedances found elsewhere in typical audio circuits, I hope we can agree that the 6 Ohm at the highest audio signals can be largely ignored. Correctly valued shunt capacitors between audio leads and ground work similarly. A common value found for the very low source impedance (of 50 Ohm or less) in modern microphones may be 2.2 nF. Such capacitor will represent a 3.6 kOhm parallel impedance at 20 kHz, but the RFI from the 600 MHz UHF TV-station will be effectively shunted to ground  by the capacitor's 0.12 Ohm impedance, or the 100 MHz FM station by 0.7 Ohm, and the AM-radio at 1 MHz still by 72 Ohm.

Calculations can be made to show the negligible effect of these components on the phase for audio frequencies as well. But who are we engineers to let the physical facts stand in the way of audiophile beliefs?...
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David Satz

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #12 on: January 16, 2007, 03:09:44 pm »

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

Klaus, this statement seems to reflect a misunderstanding. Only one leg of Neumann's TLM output circuit is actively driven, but [1] its output signal is sensed between the two modulation leads (i.e. the shield is not a signal lead for the audio), and [2] equal impedance is maintained between each modulation lead and ground. Those two characteristics are what allow the balanced input of a preamp to reject common-mode interference, and they're the only criteria of whether or not a signal or circuit is balanced. (Think of a Wheatstone bridge.)

Some features that are often obtained in practice with transformer coupling go beyond the concept of balance. For example, galvanic isolation is a characteristic which a balanced circuit (e.g. at the input of a preamp) may possess, or not; a balanced output circuit may drive both of its modulation leads actively, or not. A balanced circuit may have such features, but as far as meeting the definition of a balanced circuit is concerned, it won't earn extra credit.

I just thought I should point this out so that the discussion doesn't go in circles.

--best regards
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Uwe

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Re: Radio Frequency Interference (RFI) in mics
« Reply #13 on: January 22, 2007, 04:08:24 pm »

Let's reflect on how a typical filter works to suppress undesirable radio frequency electromagnetic interference. Such a filter removes, or at least reduces to insignificance unwanted signals by providing them with a low impedance path to ground, or it blocks them with a high impedance 'obstacle' connection. The desired signals pass through the filter with very slight or no effect.

Capacitors and inductors are the primary components used for controlling the effects of RF interference. For audio frequencies and any given value, a capacitor has a far higher impedance than for RF-frequencies. Likewise, an inductor (choke or ferrite bead) has a far lower impedance for audio frequencies than for RF. For either component the difference in impedance of a capacitor or inductor between an AF-signal of 20 kHz and RF-signal of 100 MHz is 50,000!

Even with casual care in the selection of the specific values for the filter components chosen to accomplish suppression of RFI their influence on the audio signal is academic at best.

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John Monforte

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Re: Radio Frequency Interference (RFI) in Mics
« Reply #14 on: March 02, 2007, 07:53:52 pm »

Ferrite beads make inductors out of the wires that pass through them, thus Klaus' #3 and #4 are basically the same.
...
I would like to emphasise that mic level signals are small and fragile. A cable that works perfectly well at line level might not be any good at mic level. Good shielding is of paramount importance. Cables that use conductive plastic and run for a good length are often unusable for a mic but check out fine on a cable tester.

Also, the internal twisting of the pair (when the cable is so designed) must be symmetrical and consistent throughout its length or some unbalancing will occur. This will degrade active balanced circuits to the point where the benefits of balancing are not realized.

This is one reason why transformer coupling is superior (there are others). Also, a well designed transformer will not pass RF like a wideband electronic circuit might.
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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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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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Michael Aarvold
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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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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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