Topic: Microphone Problem - Neumann KM184

[cmoorevt]

I am having some problems with one of my Neumann KM184 mics.  We were hanging some cables in a local venue last night and hooked everything up to make a test.  I noticed that I wasn't getting any sound from one channel.  In listening back to the test recording, one mic does not pick up anything other than making this weird sound.  If you've ever listened to mics when the phantom power is turned on and off-that is the sound it is making.  Sort of an air raid siren type thing.   I've attached an mp3.  Listen to the left channel.  The odd thing is that the sound comes and goes.  Like phantom is being turned on then off then on.  There is also some weird static.

I've switch pre amps/power supplies, recording decks, cables, everything, so I know the mic is the problem.  At this point I'm going to send it back to Neumann USA for a repair, but wanted to know if anyone here has ever heard anything like this before. 

Did I royally screw something up?

Thanks.

[DSatz]

cmoorevt, like many other transformerless condenser microphones, the KM 184 doesn't polarize its capsule directly from the DC of the phantom power supply; it uses a DC/DC converter to step that voltage up--in this case, to around 60 Volts--for greater sensitivity and dynamic range. (A microphone drawing 3.2 mA would get only around 37 Volts; that capsule was designed for 60.)

Since transformers can't step DC up directly, there's a little oscillator in the amplifier circuit which provides an AC signal (normally well above the audible range), which can then be run through a tiny transformer and converted back to DC again. What you're hearing is leakage from that oscillator which is unable to get up to speed, so to speak. In most cases this indicates not that the microphone has a problem, but that your phantom power supply isn't putting out enough current for the microphone.

Your signature lists an Edirol interface. In any of their portable recording interfaces that I've ever tested (I'm the obnoxious guy who brings a set of phantom power testers to the Audio Engineering Society convention and tries them out in all the exhibit booths if people will let me), the phantom powering has been marginal at best. It may be around 48 Volts when nothing is connected to it, but plug in a microphone and the supply itself begins to sag--I don't mean the normal voltage drop across the 6.8 kOhm resistors; it's more than that.

Many modern, transformerless condenser microphones require from 2 to 6 mA apiece. Your KM 184 only wants to draw 3.2 mA (2.3 if it's the older model, but by now yours probably isn't), though during startup it may need a little more, and it seems as if that wasn't available. Check this microphone out with a known good phantom power supply, and the chances are very good that the microphone will prove to be OK.

--best regards

[cmoorevt]

Thanks DSatz.  I always learn something helpful from your posts.

My initial guess was the Edirol pre amp, but I've been able to replicate this problem using a Graham Patten DMIC 20 preamp as the power source and also using a Studio Technologies Mic-PreEminence preamp as the power source.  I won't even pretend to know how the phantom powering in either device compares to the Edirol, but I would think that between the three of them, one would be able to provide enough current?

Unfortunately I don't have a true dedicated power supply such as BS48 but I may be able to track one down.

I've also cross posted this problem to the Neumann forums (which you've probably seen) and mentioned something there which I failed to mention here; that being that in attempting to troubleshoot this I made a dozen or so small recordings.  All of them include the phantom powering sound, but occasionally the mic works properly.  Usually for only a few seconds and then the phantom power noise returns.


I purchased these mics new in 1998 if that makes any difference.

[DSatz]

Hmm--if the symptoms are about the same with three different preamps/power supplies, then unfortunately that tends to throw more weight onto the side of "there may be something wrong with the microphone."

Add to that the fact that a 10-year-old KM 184 is likely to be of the older type that drew less current (2.something mA instead of 3.something), and I do begin to think that it might be advisable to send the mike to Neumann USA for checkout.

--best regards, and hoping I'm wrong ...

[cmoorevt]

Last night I removed the small screw on the body of the mic and slid out the circuit board.  Martin on the Neumann forum suggested that there might be a broken contact/solder joint on one of the xlr pins.  Upon removal, the xlr pins and soldering looked fine so I looked over the rest of the circuitry to see if anything else was broken.  Everything looked ok.  I then reassembled and plugged the mics in to the preamp and everything now seems to be working properly.  I can't get the noise to return. 

The only thing that I can think of is that when we were testing the cables when this noise first surfaced, one of the Neutrik xlr connectors on the cable was a very tight fit into the mic body and never clicked in completely.  I removed the connector and then reinserted it and finally got the click that it was fully seated.  We then began recording and the noise was present.  Perhaps that first pull out of the connector caused the circuitry to separate from the capsule a hair??? Even though the screw that goes thru the mic body is supposed to hold the circuit board in place?  Does this even seem plausible?

[DSatz]

cmoorevt, it is absolutely plausible and I'm sorry I didn't think of it sooner. Neumann includes a wrinkly black circular rubber washer in many of their microphones, which generally sits in the deepest inside part of the XLR connector on the microphone; it is there to prevent rattling with shorter XLR connectors on cables, but it prevents some other cable connectors from making good contact or, as you said, from "snapping" into place.

You should remove that rubber washer if you haven't already. It may explain this whole thing.

--best regards

[cmoorevt]

No apologies necessary.  Thank you very much for the advice. 

Ever since I first purchased these mics I've always found that any model of Neutrik xlr connector is a tight fit.  I normally use them because a.)they almost always snap into place, albeit with a little force and thusly feel like a solid connection (for whatever that is worth) and b.)whenever I make cables, I find them easier to work with than the Switchcraft connectors.  But after 10 years and who knows how many connections and disconnections with Neutrik hardware, I can see how things may have become a bit loose.

I'll remove that rubber washer tonight and see if that makes any difference with the connector seating.

Thanks again.

[John Willett]

The rubber washer is there for Switchcraft connectors - Neutrik connectors don't need it; so take it out if you use Neutrik XLRs.

Yes, I should have thought of this as well. 

[ArchivalAudio]

DSatz
John Willett
thanx for always having something to contribute
I have learned a lot from reading
this sounds like an interesting delemma

has it been solved?
or do they need to go back to Neumann...
I really prefer the neutriks to the switchcraft myself


any way thanx
-- Ian

[cmoorevt]

has it been solved?

I removed the washers this afternoon and the Neutrik connectors fit *much* better.  Still a nice solid fit, but inserting and removing the connectors requires less force than before. I wish I had known about this 10 years ago. 

As for the odd noise, I've tried these with all three of my preamps/power supplies and different cables and both Neutrik and Switchcraft connectors and can't get the sound the return.  I'm hoping that the problem was a small gap between the circuit board and the capsule, which was created by me trying to remove a tight fitting Neutrik connector.  I don't think it was a coincidence that my removal and reassembly of the circuit board immediately proceeded the mic functioning properly. 

I'm obviously going to monitor the situation but as of now, I don't plan to send them back to Neumann.

[Frequincy]

Iv'e not encountered this problem but I always felt when inserting a Neutrik connector into my KM184's that it took way too much force. I always appreciate the information here. thanks.

[wvmaine]

does the rubber washer have the serial# printed on it? Would like to confirm before I start messing around?

[John Willett]

does the rubber washer have the serial# printed on it? Would like to confirm before I start messing around?

The rubber washer is just a wavey rubber ring that sits in the bottom of the XLR connector on a Neumann mic.

There is nothing printed on it.

It is only there to prevent Switchcraft and similar short XLRs from rattling and should be removed for when you use a Neutrik connector.

[Chuck]

The XLR connectors inside my Microtech Gefell M300's rattle when there is no cord attached. When I sent them to Germany for service I mentioned it and the told me it's they way they are supposed to be. None of my other mics do that, but I tend to believe that the folks at Gefell know what they are doing. 

[wvmaine]

Thanks for your reply, I do use neutrik and it's a tight fit. On my km 184's the only thing in the bottom of the connector is a thin piece with the serial # on it in white. I bought them new so nobody's taken anything out. If this would help out the fit I would remove it. If I do remove it, should I just pull it out with tweezers?