KM184's receiving Electromagnetic Interference

[bjorn400]

Hi,
I just got a pair of lightly used KM184's. I plugged them into my system, and I find that they are receiving EMI(electromagnetic interference), like the way a single coil pickup buzzes when you are facing a specific direction(or 180 degrees opposite that direction.

None of my other mics are humming this way(studio projects c1, sm 58, Microtech Gefell M930), and both of these pair are being affected in the same way. I have tried many a cable, so thats not the problem.

Only when I clutch the mic body in my hand does the buzzing go away totally no matter which direction it is pointing.

What is causing this? Why are the km184's experiencing this and no other mic. Are they bum? They do sound great when the buzzing isn't there, but that only happens when I'm holding it. Is it a grounding problem in my building?

Are there any suggestions for how I can stop this.

[MichaelJoly]

Klaus Heyne has a very good general tutorial on RFI in mics here:
http://recforums.prosoundweb.com/index.php/t/1894/0/

He dosen't address the KM184 specifically in his post but you could probably get a good answer from him if you asked.

[bjorn400]

Thanks for the link.

Here is an update on my tests.

I have now wrapped my km184 in tin foil. This stops all EMI. But I really don't want to have to do that.

any more ideas anyone?

bjorn

[philbo]

Very likely a loose or broken ground wire inside.
________
REINSURANCE FORUMS

[Meriphew]

I own a pair of KM-184's, and though I do experience EMI with my pup's in my Strats/Tele's, I've never had ANY EMI problems with the 184's. It sounds to me like the mics are damaged/defective.

[vsr600]

yea definitely sounds like a grounding issue if it goes away when you touch it.

[A-Barr]

Yes, grounding!
Use a multimeter and check for continuity between an exposed metal part (the grill?) and the ground pin on the xlr connector on the mic.
Did you try different cables? Ground connection could be lose in there too. I find it very odd that both mics have this problem though. Did you get them from ebay? Perhaps someone has a connection to Neumann factory seconds or something...

[bjorn400]

Thanks everyone for your input!
But I have an important update...
I have tried the mics at a separate location with the same xlr and I got no EMI, the mics worked perfectly. I have also tested the continuity of the ground pin to the windscreen, and I found it to be continuous.
But when I use it in my apartment, I can't avoid picking up random buzzing frequencies depending on where I point the mic, but I have no problem with my other mics. Could this mic be sensitive to large magnetic fields such as a nearby frieght elevator shaft, or maybe a loose ground in the building's(not the mic's) wiring? And it still goes away when I clasp the mics, or even just when I hover my hand around the mics, or wrap them in tin foil.

What is up. This is so weird, I can't explain it.
I did get these mics on ebay, I'll ask the seller if he got them refurbished or something.

thanks
bjorn

[A-Barr]

Did you try different preamps?

[bjorn400]

yes, though maybe I'll try more.

I tried it into my FMR RNP straight into my monitors and I also tried it directly into a behringer mixer with nothing but headphones plugged in, and I got the same results.

[scott anthony]

Shot in the dark...

Look out your window. Is there a big transformer on a telephone pole?

[bjorn400]

Ok, I'm at work now(guess I shouldn't be surfing tapeop), but when I get home, I'll survey the suroundings.

I am suspicious of the frieght elevator across the hall from me.
thanks for your help!!

[bjorn400]

I checked around for large power transformers outside my window. I didn't see anything. The freight elevator seems to be the only thing of suspicion.

I have checked this problem with a number of preamps, I have shutoff all of my breakers except the one I am using, I still get the same results. The only thing that reduces the buzz is wrapping the mic in tin foil or moving away from my apartment. But if I plug into a hallway outlet close to my apartment, I still get buzz, and I get a lot of buzz near the elevator. The further i go to the opposite end of the building, the less the problem is. When I bring the mics and my preamps to work, I get no noise. And once again, my other condensers are not picking up interference.

I have checked the mic's ground pin, and its hooked up right. it passes continuity testing.

It seems these mics are sensitive to EMI or RF, but I get no info from Neumann or any forums that anybody else has ever had this problem.

any more ideas? I'm starting to think the only thing I can do is cover my apartment walls with aluminum tinfoil.

-Bjorn

[A-Barr]

Well here's the thing, wrapping your hand or tin foil around it is effectively the same thing as what the grounded mic body is supposed to do. I see that you are sure that there is continuity between the chassis and ground, but could there be continuity between chassis and signal? Test the output pins for continuity with the chassis, maybe one of them is grounding out somewhere, you could still get signal (I think) in this situation because a balanced mic has 2 low-level outputs. If that doesn't get you anywhere, I'd take the sucker apart and see if something in the circuit is grounding, as this could happen and still test not continuous with ground at the pin.
Good luck!

[bjorn400]

thank you,
All the pins check out good. The chasis is only connected to the ground and nothing else is shorting.

I'm getting the feeling that there is nothing wrong with the mics at all and that my apartment is bombarded with intense EMI or RF, and that these mics' capacitors are not designed to cancel out such interference? I have no way of proving this though unless Neumann tells me this is possible.

Do you strongly believe that this is not the case? Should I have Neumann 'repair' them? Not too sure if I should open them up myself.

thanks so much for your help.

Bjorn
http://bjornquenemoen.com