microphone buzzing with RF interference

I have a couple of mics that always gives me buzzing and radio interference troubles. It is always the same radio station and by moving the cable (between mic and pre) i can get rid of that or the buzz or sometimes both. It is always the same mics that that have the problem:
neumann tlm 103 (the one that is supposed to have really low self noise),
The royer mic mod for the mxl 603 (small dia), An Audio tecnica, and most recently a pair of stapes omnis( the avensen sto 2 ). I really just wanted to plug in the new stapes and start getting sounds, but i allready wasted the first two hours moving the mic cables around trying to get rid of the buzz. I could either get rid of the buzz then radio would play through or get rid of radio then the buzz would come back or sometimes both, but only temporarily. And its really too noisy to be useful.
This is the only stage of the signal chain that i have trouble( between mic and pre). Nothing else buzzes, and i have plenty of mics that are 100%
problem free.
I can't figure this out and could really use some help if anyone has any ideas.
Thanks

I would check you wall outlet ground. Radio shack sells a outlet tester with 3 lights that you can test your outlet wiring. You might have a bad ground.

Is all of your equipment on the same phase leg.

Maybe your mike body is not grounded. I think the body is at earth potential in most mikes.

[quote author="Gus"]I would check you wall outlet ground. Radio shack sells a outlet tester with 3 lights that you can test your outlet wiring. You might have a bad ground.

Is all of your equipment on the same phase leg.[/quote]
I have one of those radio shack testers and the outlet is ok. Also everything i have is plugged into the same outlet.

See the ferrite rf bead thread also.

On checking the resistance on the avensen (stapes omni) mic I did not get continuity when probing the actual mic body and pin one at the preamp.
But the mic is painted black,which may be causing resistance, and i dont want to scrape away paint on brand new mics. So i probed a metal ring that fits between where the male xlr is inserted into the mic body. That checked OK.

Why is it that some mics I have are prone to this buzzing and RFI while others are just fine? All pluged into the same signal path, cable etc.

Mics with transformers will pick up noise.

Im just wondering. Are the buzz and the radio interference two symptoms
of two different problems or are they two symptoms of a single problem?

Buzz is most commonly a ground problem. Radio stations can be something else. Some microphones, by design, will also just act as RFI magnets. I spend a good amount of time at work walking around cities waving a mic around wearing headphones, the fet neumann mics will pick up any kind of shit that is in the air, two way radio interference especially. A sennheiser 416 on the other hand is totally bullet proof... If you've been over the grounding scheme in your studio and its solid, there is a possibility that you might have to live with the inconvenience that is built into the mic if you are in a really bad RF environment. If you have a static buzz and then occasional interference in my experience points to two different things. One solution that has almost always worked for me is, as CJ mentioned, using ferrite beads on the input to your mic pre's if you have blasts of RFI coming through. On the jensen website you can order a bag of 100 beads, slip them over the leads that go to the input connectors on your mic pres, it can help quite a bit. If you dont want to open your mic pre's up, they make much larger beads, I think I got some at digikey, that will slip over the mic cable itself so there you just have to rewire the cable, much less of a big deal, the only drawback is that the beads are ferrite and they chip on the cables if you dont protect them somehow. That often works as well, and if you look in the catalog you'll see they come in different frequency dampening ranges. That might help with the RF interference, might not. If you have a static buzz, start lifting AUDIO GROUNDS ONLY to see if you can make the problem go away. Leave your power grounds connected always, they are there to potentially save your life... Make sure that your path from the mic to the pre has a ground connected on both ends, then try lifting the shield on the line outputs from the box or the line inputs on the next box to see if you can make the buzz go away. If you are using a patchbay thats another source of grounding that can play, a big source. There is a lot of theory to learn about audio grounds, much of which I still do not understand, but if you dont mind the time it takes to go through your system, if you unplug everything, start plugging stuff in until you hear the buzz (learned that little tidbit from the sslpirate) then once you find which piece of gear is buzzing, start lifting the audio shields at either end to see if you can make it go away. And as mentioned earlier, if you have transformers on your gear, they can also act as the buzz sponge, if you have a transformer in one piece of gear and it is in the rack sitting directly above or below the power transformer of another piece of gear, sometimes the transformer can pick up a hum that way, good thing manufacturers never put the power transformer in the same place in a box...

yet another thing to do, when you hear the noise is to switch out the mic quick, if you switch mics and the problem persists, than you know its not the mic, if you have a 421, thats always been a pretty bullet proof mic for me so far as intereference goes.

dave

This is a classic case of RF interference in a microphone. The buzzing (which imitates the power line variety) is caused by RF interference from a TV transmitter. Peak power occurs during vertical blanking of the video - and this occurs at a frequency of 59.94 Hz for US television. Therefore the sound is virtually indistinguishable from power-line buzz. You can verify the TV origin by looking at the signal with an oscilloscope whose trigger is line-locked ... the waveform will slowly crawl horizontally. It would be stationary if the buzz source were the power line.

This particular microphone is badly designed from the RF immunity standpoint and has been documented in a study by Jim Brown (he and I have co-authored a couple of recent AES papers). The problem is internal to the microphone and is caused by a relatively long pigtail connection of pin 1 to the mic housing. This wire appears as a high impedance to VHF frequencies and allows coupling into the active electronics in the mic. The cure is to make the shortest possible connection between pin 1 and the housing.

Wow! It's cool to have Bill here. It sounds like he's pretty much got the RF thing figured out too. I hope you will enjoy hanging out here Bill!

Ian

Thanks Ian (almost a neighbor ... my home office is in Oxnard)!!

I really enjoy this forum but don't have nearly enough time to keep up with it. I tend to check it irregularly and then go for topics where I feel I can contribute something useful. I'll be teaching a grounding and interfacing tutorial seminar at AES in San Francisco ... and teaching a couple of seminars at CEDIA in Indianapolis next month - just in case some of you might want to attend.

Hi Bill

:thumb:

[quote author="wowandflutter"]On checking the resistance on the avensen (stapes omni) mic I did not get continuity when probing the actual mic body and pin one at the preamp.
But the mic is painted black,which may be causing resistance, and i dont want to scrape away paint on brand new mics. So i probed a metal ring that fits between where the male xlr is inserted into the mic body. That checked OK.[/quote]

Well since I manufacture these mics, I thought I would chime in here and say that indeed the microphone bodies are grounded. The outside is anodised, so you won't get a reading, but inside it is grounded from pin 1 with 1/4" piece of wire.

Thanks for all the tips guys.

Bill,
I understand(I think) what you mean by buzz caused by the rfi from a TV transmitter. But the radio station that is there in addition to the buzz is definetely radio and not TV. In fact, with all of my mics that have ever given me this trouble,
it is always the exact same radio station (NPR 94.9 fm). So is this just two seperate interferences? If so is there a relationship between the two?

Anyway im thinking about just trying the ferrite beads for the NPR interference and just "tune out" the buzz. The buzz is fairly easy to get rid of by rearranging the mic cable, but the rfi is a bit more persistant.
Thanks again

you mite try some beads in the mic itself, it is always best to supress the noise at it's source, blah blah blah...