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Author Topic: KM-184 RF problems?  (Read 6583 times)

jstaczek

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KM-184 RF problems?
« on: April 30, 2011, 12:41:06 am »

Hi All,

I've been chasing an RF problem in the studio and I think I've narrowed it down to my KM-184s.

I was having general issues with RF but then went through Klaus's sticky post about handling RF, corrected some shielding issues in the patch bay and mic panels and everything became dead quiet. Except for the KM-184's.

I have a matched pair that seem to randomly pick up RF. Happens on Millennia pres and Daking pres. If I swap out mics with something else, the RF will always go away. I found a post on the Neumann forums that suggested there were some KM-184s that were susceptible to RF. I tried to contact Nuemann USA and Berlin several times by phone and email and have not received a response.

I searched the forums here and didn't see anything specific about KM-184s and RF so I wanted to ask: does anyone know about RF issues specific to this mic? If so, is there a mod or a remedy?

Thanks,

Jason
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Nob Turner

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Re: KM-184 RF problems?
« Reply #1 on: May 01, 2011, 01:06:28 am »

i had an rf problem with my km184's when using a gml preamp. when used with something with transformers, the problem didn't seem to exist. a very smart tech i knew, now sadly passed, suggested i stop using my canare mic cables with these mics, but instead use something lower tech. lo and behold, the problem vanished!

klaus

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Re: KM-184 RF problems?
« Reply #2 on: May 01, 2011, 01:50:37 am »

That brings to mind: have you ever used Gotham GAC 3 cable with the mic? It also used to come as an accessory with Neumann mics, and was labelled "IC3/25" .  (Now sadly replaced by an inferior look-alike)

GAC 3 is a three-conductor cable ( separate ground return) with double Reussen-layer shield (2 corkscrew layers of bare copper stranding covering 100% of the core conductors against RF.)

Also important, and you probably know this already: connectors must have their metal casing and pin 1 connected to cable ground and shield, and on both ends!
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Klaus Heyne
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jstaczek

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Re: KM-184 RF problems?
« Reply #3 on: May 01, 2011, 02:42:17 am »

I love it! Two solutions:

- use cheaper cable
- use more expensive cable

I'm already on the cheap end, so I will try the expensive stuff and see if it makes a difference.

And yes, Klaus, I followed your RF prevention instructions for connectors and it eliminated all RF problems for everything but these mics. But you remind me that I did not check all my mic cables themselves, only snakes and patch panel.

Interestingly (and I don't have them in front of me) unlike other mics it seems like the connector on the KM-184s has a plastic insert so that the female XLR body doesn't actually come into electrical contact with the mic body. Is this so, or is there contact made from pin 1 to the mic body internally somehow? I'll test it for continuity myself next time I'm in the studio.

Thanks for the tips!

Jason
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klaus

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Re: KM-184 RF problems?
« Reply #4 on: May 01, 2011, 12:32:10 pm »

Two solutions:

- use cheaper cable
- use more expensive cable

I'm already on the cheap end, so I will try the expensive stuff and see if it makes a difference.

Canare and Gotham are on the same (relatively) cheap end of the spectrum. When it comes to cables, I find the simple equation "better vs. worse" more appropriate. Unfortunately (actually, quite fortunately!) price differences do not easily predict quality differences in cables.
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Klaus Heyne
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jstaczek

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Re: KM-184 RF problems?
« Reply #5 on: May 02, 2011, 03:42:17 am »

Ah, the Gotham sounded expensive with all those bells-and-whistles, and I thought Nob was suggesting something lower-tech than Canare. In any case, yet to be tried.

I did some more experiments and observed the same thing that Nob found: the problem goes away when using transformer input pres. Very prominent RF problem on a Millennia HV-3d and ShinyBox Si4. No problem on a Daking IV. Again, swap in a different mic on the same cabling and the problem goes away.

What do you do here? Suck it up and use tranny pres and ignore the problem? Get Neumann to acknowledge and fix it? Sell the 184s with a warning to potential buyers about RF?

Jason

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klaus

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Re: KM-184 RF problems?
« Reply #6 on: May 02, 2011, 01:03:49 pm »

You get advice and suggestions. Then you either take them or you don't. If you don't take them, you should wait with asking questions "what else can I do?" until you do take them.

Unless you experience the same level of RF with a double Reussen-layer shielded cable, properly terminated, you cannot know whether the cable is at fault. Borrow a Gotham GAC3, or call Gotham, talk to Lew Frisch, ask him to send you a 25ft length of Neutrik-terminated GAC 3 (considerably less than $50.-)  then make the test and report back.
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Klaus Heyne
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jstaczek

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Re: KM-184 RF problems?
« Reply #7 on: May 02, 2011, 07:17:06 pm »

Of course. And, so chastised, I am ordering some GAC-3.

However, I'm not clear on how to use a 25-foot GAC3 cable. Must it go directly from microphone to preamp to be effective? If it goes from microphone to connector box at end of Gepco snake to mic patch bay to preamp, is it going to tell me anything?

Jason
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klaus

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Re: KM-184 RF problems?
« Reply #8 on: May 02, 2011, 08:34:40 pm »

I would start by minimising and identifying variables:

First test: Mic into GAC 3 intro mic pre....No RF? Not the mic's problem anymore (because some mics will need stricter enforcement of anti-RF measures than others) but bad RF shielding downstream.

Then: Mic into GAC 3 into snake into mic pre: RF? Inspect the snake's ground references and shielding scheme.

Different outcome of first test: Mic into GAC 3 into mic pre...RF? Substitute the mic pre (with another high-quality transformer-less model!) Still RF? Mic goes to Neumann.





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Klaus Heyne
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Nob Turner

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Re: KM-184 RF problems?
« Reply #9 on: May 04, 2011, 04:58:04 am »

i've made a habit of using my 184's with either some GAC3 cables i made up, or with some older cables that came with akg C451's long ago.  in both cases the RF problem is gone. my old tech said the canare passed the RF too easily... not sure if that is an accurate explanation, but the main thing is that the solution is there.

jstaczek

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Re: KM-184 RF problems?
« Reply #10 on: May 06, 2011, 04:19:53 am »

And here is the report.

Lew Frisch very kindly supplied me with a length of properly terminated GAC3. Thanks, Lew, and thanks, Klaus, for the tip.

- No difference with mic -> GAC3 -> snake -> pre. RF still present.
- No difference with mic -> GAC3 ->pre. RF still present.

As before, the issue only exists if the pre is transformerless. RF was present on Millennia and ShinyBox pres. RF was absent on transformer input Daking pre.

New observation: when I touch the mic body, the RF rises considerably in volume, completely overwhelming the mic signal. This happens with either GAC3 or my stock cables, again only into transformerless pres. Touching the mic body into a txfr pre remains silent.

Also, these are a matched pair. Both exhibit the same behavior. Per Klaus's earlier post, diagnosis is "back to Neumann".

I've been scratching my head because I've had these mics for years, but never heard the problem before. I'm in a new space now, though. The former studio was built around a Trident 80B, so these mics always went into transformer pres. Now I'm without console, using outboard pres, not all with transformers.

Would anyone care to speculate on the role of the transformer in this situation?

Jason

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Jim Williams

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Re: KM-184 RF problems?
« Reply #11 on: May 06, 2011, 11:18:47 am »

A transformer is a passive coil load, non active. The base of a bipolar transistor can easily be 'tricked" into becoming a diode rectifier, hench the rf from transistor inputs rather than the passive transformer coil.

Solving this requires reworking the mic as those preamps don't create this problem with other transistor output designs. Possible solutions can include replaced surface mount output filter caps, adding ferrite rf beads and in the worse case a couple of low loss inductors. Sometimes a dual layered approach is necessary, beads into another choke.

Given the sonic complaints this particular mic has recieved in the last few years, one needs to make that decision. It's like the Gambler says:

"You got to know when to hold 'um, know when to fold 'um".
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David Satz

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Re: KM-184 RF problems?
« Reply #12 on: May 07, 2011, 05:50:44 pm »

Jason, in your first post you mentioned getting no response from previous attempts to contact Neumann USA via email. I had several similar experiences during the past few months, but then heard back from them a week or so ago. Evidently there'd been some problem in the way their email account (via their Web site) had been set up, but it seems to have been fixed now, as evidenced by the prompt reply that I got. So I'd suggest trying them again.

--best regards
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GYMusic

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Re: KM-184 RF problems?
« Reply #13 on: May 08, 2011, 02:30:10 pm »

What kind of RFI do you have?  You must identify the frequency of the offending signal before you can chase it.  AM broadcast?  FM broadcast?  CB?  Ham radio?  Each frequency range and how close you are to the source may require different methods of attenuation.

gkippola

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Re: KM-184 RF problems?
« Reply #14 on: May 08, 2011, 07:52:31 pm »

I have had this problem in the past, most of the time w/ my shoeps cmc6s,  and rectified it by using different cables,  no predicting, it's a crap shoot
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Karl Winkler

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Re: KM-184 RF problems?
« Reply #15 on: May 18, 2011, 03:29:27 pm »

Hi All,

I've been chasing an RF problem in the studio and I think I've narrowed it down to my KM-184s.

I was having general issues with RF but then went through Klaus's sticky post about handling RF, corrected some shielding issues in the patch bay and mic panels and everything became dead quiet. Except for the KM-184's.

I have a matched pair that seem to randomly pick up RF. Happens on Millennia pres and Daking pres. If I swap out mics with something else, the RF will always go away. I found a post on the Neumann forums that suggested there were some KM-184s that were susceptible to RF. I tried to contact Nuemann USA and Berlin several times by phone and email and have not received a response.

I searched the forums here and didn't see anything specific about KM-184s and RF so I wanted to ask: does anyone know about RF issues specific to this mic? If so, is there a mod or a remedy?

Thanks,

Jason

How old are your microphones? I know that a few years back, maybe close to 10 now, Neumann re-designed the KM184 in an effort to eliminate just this sort of RF noise. From what I can recall, the problem became evident when the mic was used in TV studios due to audience members having cell phones in their pockets, turned on but silenced. It was something about a ground trace on the circuit board being just the right length to act as an antenna. Further backing this theory is that with a transformer load, the problem goes away due to the very high CMRR of a good transformer input.

If your mics are relatively new, it may simply be an indicator that whatever change they made either was not fully effective, or is now vulnerable to a different RF frequency.

I'd suggest trying to reach the Neumann engineering dept. in Berlin via the neumann.com web site, and describe the problem to them and include your microphone serial numbers.
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Jim Williams

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Re: KM-184 RF problems?
« Reply #16 on: May 19, 2011, 11:14:07 am »

Further backing this theory is that with a transformer load, the problem goes away due to the very high CMRR of a good transformer input.

Hi Karl, the iron I see has a CMRR of about -70 db at 20k hz. That will usually improve at lower frequencies but drops off at higher frequencies due to mismatch and coil leakage. I don't know the CMRR of the Millenia preamp, but I suspect it's probably about -70 or better at 20k hz due to the match of the MAT 04 input transistors.
Other designs sans iron go up to -90 db CMRR, 20 hz to 20k hz, so I still believe it's transistor rectification causing the radio interference.
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Karl Winkler

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Re: KM-184 RF problems?
« Reply #17 on: May 19, 2011, 11:30:44 am »

Good points, Jim. I'm no circuit expert...
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