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Author Topic: Neumann TLM 67 picking up RF/computery noises  (Read 4488 times)

Dinogi

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Neumann TLM 67 picking up RF/computery noises
« on: July 01, 2014, 05:32:45 pm »

We have a year old TLM67 in our VO booth. Just recently we began hearing odd electronic noises coming through the monitors and on our recordings. I switched to a Senny 416 and we're back in business. I'm going to bring in some of my personal mics (AT4050 & Bock 195) tomorrow to see if they do the same thing. The IT folks did recently install a huge WiFi antenna right outside the booth. Could this be the problem? and why doesn't the 416 hear the noise?
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Nob Turner

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Re: TLM 67 picking up computery noises
« Reply #1 on: July 02, 2014, 02:00:12 am »

Could be the fact that it is transformerless. I've had problems with other transformerless Neumanns depending on what preamp and mic cable I use with them. Specifically, I've had trouble using Canare cable and GML pre's, which are also transformerless. I get RFI - I'm about a mile as the crow flies from a large broadcast tower. Switching to older, less "hi-fi" cables, or Gotham GAC, has helped. Also, switching to a transformer-input pre takes care of it. YMMV.

klaus

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Re: TLM 67 picking up computery noises
« Reply #2 on: July 02, 2014, 03:14:04 am »

To expand on Nob Turner's mention of cables as an issue:
I have solved 90% of all RF noise issues by using a cable with a decent shield construction, and terminating it properly.

Gotham's GAC3 and some of its copycats are still the cable to beat in this regard, and prove it on a purely objective basis: no other cable construction has the ability to keep external noises out of condenser mics that a double-Reussen layer of shielding provides.

And terminating your mic cable properly is vital in your situation: shield and ground (applicable in three-conductor shielded cables like the GAC3 must be connected to pin #1 and the connector housing (metal) at both connectors.

This is the recommended termination scheme of Neumann, yet I often find shields not being connected to the connector housing or to ground on one of the connectors (in the erroneous assumption that a ground loop could occur if both connector ends had a shield connection).

Unless their body were to touch a device that has its own power source (and grounding configuration), mics, by definition, are at the beginning of the connection chain of the recording devices, thus, the issue of a ground loop is practically moot.
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Klaus Heyne
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Kai

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Re: TLM 67 picking up computery noises
« Reply #3 on: July 02, 2014, 02:53:23 pm »

shield and ground (applicable in three-conductor shielded cables like the GAC3 must be connected to pin #1 and the connector housing (metal) at both connectors.
I don't connect the housing here.
It's not necessary if no extension is use, as the connection is made on the mic body and preamp / patchbay housing.

I don't connect it to avoid ground loops in case the connector comes in contact with other metal parts, e.g. stage floors.

Regards
Kai
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klaus

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Re: TLM 67 picking up computery noises
« Reply #4 on: July 02, 2014, 03:47:10 pm »

Strongly disagreed. This is one area of otherwise democratic forum discussion where benevolent dictator dictum trumps. Metal stage floor contacts are such an extreme outlier of a mic/cable application that they bear no relevance for the 99.9% of situations where the rule I previously posted (connector sleeves to ground/shield) is valid and helpful in solving RF problems.
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Dinogi

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Re: TLM 67 picking up computery noises
« Reply #5 on: July 02, 2014, 05:34:04 pm »

Thanks guys
We're putting together some cables to see if we can get a handle on this. Even so, I'm beginning to get the feeling that the microphone may have issues beyond RF getting in.
When testing the system with another transformer-less mic the computery/ cell phone sound was way down compared to the 67. There was also some other lower thumping, farting, popping noises that don't show up at all on the other mic. There may be something wrong with the capsule or circuitry itself. I try to tell the folks not to plug the mic in while the PP is turned on, and to handle it with care, but you never know what folks do when you're not there to watch over their shoulders. I hate to think that our warrantee is voided by rough handling. Incidentally, my 195 didn't seem to be affected at all under the exact same circumstances.
more to follow, and thanks again...Dean
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Jim Williams

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Re: TLM 67 picking up computery noises
« Reply #6 on: July 03, 2014, 11:46:26 am »

A quick check with a DVM will tell you if the XLR shell is connected to the mic's chassis ground or the mic preamp chassis ground.

Every mic/preamp combo tested here does connect that ground but only when patched in.
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Dinogi

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Re: TLM 67 picking up computery noises
« Reply #7 on: July 08, 2014, 05:50:35 pm »

Hey guys,
We discovered that the cable feeding the wifi antenna was strung directly over our VO booth.
I switched over to our Sound Devices battery operated mixer just to see if it made a difference. It didn't. I may bring in my UA M610 just out of curiosity to see if that makes a difference.

Well, after a day trying different cables, rewiring connectors, and physically moving the mic around it appears that the TLM67 is just more susceptible to whatever that cable is putting out. Moving the mic within the booth does seem to make it better or worse but it never completely gets rid if the problem. My AT4050 also has the issue but its far less. When moved a few feet the AT also becomes quiet and usable. My 195 doesn't seem to care where its placed. It's quiet as a mouse no matter where its located.

I was finding it hard to believe that there's not something wrong with the 67 until I took it out to our sound stage where it became perfectly quiet. I still think the Neumann has something wrong with it, whether a design flaw or something is broken. Now we're stuck with trying to get the IT folks to move their box. That's going to be political nightmare.
I thank you all for your input.
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Kai

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Re: TLM 67 picking up computery noises
« Reply #8 on: July 09, 2014, 08:05:59 am »

My AT4050 also has the issue but its far less ... something wrong with the 67 until I took it out to our sound stage where it became perfectly quiet.
The HF disturbance in you VO booth must be quite strong.
How does the mic handle HF from cell phones close to it?

- Check if ALL outside metal parts of the mic are connected to ground / XLR Pin1.
It should really be low Ohms, below 0.1 Ohm or so.

If yes:

- Shielding the top of the booth with alumium foil could help and is an inexpensive fix.

- To make the mic less sensitive to stray electric fields you would need to add little ferite rings to the input and output of it's electronic:
The point where the capsule is connected to the circuit board, and where the cables are soldered to the XLR connector.
The cables should go through the rings 2 times (1 full turn)
This does not alter the sound of the mic.
Be careful not to contaminate the input side of the circuit board when soldering.
Best not to use extra soldering tin, just what's already there and use a clean soldering iron.
Flux remains can cause noise problems.

Regards
Kai
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klaus

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Re: TLM 67 picking up computery noises
« Reply #9 on: July 09, 2014, 10:27:55 am »

Check if ALL outside metal parts of the mic are connected to ground / XLR Pin1.

To expand on this suggestion, I'd like to suggest a simple method to do so: put one probe of your ohm meter firmly against an unpainted section of the mic's head basket. The other probe should be placed against the connector sleeve that is usually plugged into the mic pre/mixer/phantom power unit. BUT for this test, the cable connector should not be plugged in.
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Dinogi

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Re: TLM 67 picking up computery noises
« Reply #10 on: July 09, 2014, 05:51:14 pm »

Quote
The HF disturbance in you VO booth must be quite strong.
How does the mic handle HF from cell phones close to it?
Yes from day one it seemed susceptible to cell phone interference.

Quote
Shielding the top of the booth with aluminum foil could help and is an inexpensive fix.
I was thinking along those line myself. I even am considering wrapping the WIFI cable in foil and running a ground drain from it. I don't want to do any mods to the mic itself until the warrantee runs out. I still think there is something wrong with it because it is the only mic making the popping, sputtering noises.

I'm going to recommend to the tech staff to go over the mic with an ohm meter as suggested.

Thanks again guys,
I'll let you know how it works out
Dean
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Kai

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Re: TLM 67 picking up computery noises
« Reply #11 on: July 10, 2014, 06:08:18 am »

I even am considering wrapping the WIFI cable in foil and running a ground drain from it.
Wrapping the WIFI cable can help a lot.
It's even unneccesary or contraproducive to ground this shield. For the 2.5 or 5 GHz WIFI signal grounding is ineffective, as the wavelenght of the signal is much too short to make it's way to the ground. A grounding cable could even work like an antenna, sending more HF energy to the air.
Consider double shielding:
Wrap one (multi-)layer of (household-) aluminium foil round the cable, then some isolation like sticky tape or cloth, then a 2nd layer of foil.
Do this for the entire accessible length of the cable, not just above the booth.
This should do the trick, costs a few bucks and an hour of work.

Regards
Kai
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Uwe

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Re: TLM 67 picking up computery noises
« Reply #12 on: July 10, 2014, 03:47:49 pm »

Klaus' advice to connect pin 1 on both ends of the microphone cable's XLR connectors is absolutely correct and should be standard procedure. After all, there is a reason why most XLR connector manufacturers provide this fourth terminal for the connector shell! There is zero possibility for a dangerous ground loop, since practically all microphones do have an internal connection from pin 1 to the housing already. This can be easily confirmed by measuring the resistance between pin 1 and the housing. However, and this is the crux of the problem, while it may show a near zero DC-resistance, for very high frequencies the internal length of wire in practically all cases of RFI behaves as an inductor (choke) with a relatively high impedance and fails to harmlessly shunt the interfering RF to ground. This RF is picked up by the microphone and/or cable doubling as an antenna. The effectiveness of this unintended antenna varies with the microphone location and cable position. A direct connection between pin 1 and the connector shell will greatly improve the direct path for RF to ground. Even better is the use of either double shielded cable like the Gotham or Neumann cables, with the one shield connected to pin 1, and the other to the shell. An effective alternative solution would be using the EMC Series XLR connectors from Neutrik: http://www.neutrik.com/en/xlr/xlr-cable-connectors/emc-series/
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Dinogi

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Re: TLM 67 picking up computery noises
« Reply #13 on: July 31, 2014, 05:46:09 pm »

Well folks, as it turns out it wasn't the WiFi antennae after all. We had the WiFi contractor come in with his equipment and do a scan of the area. That in itself was amazing but I'll cut to the chase. The culprit as it turns out was a cell phone antennae about six feet away from where the WiFi unit was that we didn't even notice. When it was disconnected the interference completely vanished. The contractor relocated it about eight feet further away from the booth (He used the cable from his test antennae which was all he had on hand) and depending on where I positioned the microphone I could get things pretty quiet. The interference appears to be directly proportional to how busy the cell unit is. He's coming back to replace the short cable and I'm planning on asking him to move the darn thing another twenty feet away. I'll bet that takes care of the problem. For the VO I did today I grabbed a ladder and (don't tell anyone) disconnected the cell unit for the duration of the session and all went well.

Thank you all for the input. If nothing else, I have definitely learned a lot about the proper wiring of microphone cables.
Respectfully, Dean
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klaus

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Re: TLM 67 picking up computery noises
« Reply #14 on: July 31, 2014, 06:46:40 pm »

Thanks for the update, Dean,
But you never confirmed that you installed the proper grounding/shielding scheme in your cable connectors.
The fact that you are still getting some RF interference mixed into the mic's signal at certain mic/cable positioning begs to ask that question once again.
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Klaus Heyne
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Dinogi

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Re: TLM 67 picking up computery noises
« Reply #15 on: August 01, 2014, 05:27:49 pm »

Oops... I guess I should have mentioned that.
We spent the week rewiring every different brand of cable we had in house. One of the cables we tested but didn't alter was the one that came with the mic. It was actually marked Neumann on the cable itself. It was already connected to the   connector at both ends. We tried them grounded to the connectors at both ends. We tried grounding the connectors only at one end. We tried new cables right off the reel. We tried OEM cables from Shure that came with Beta 58's. We tried new connectors and older style ones. We tried older cables. We tried four conductor + shield cables wired several ways at the suggestion of this and other websites. I'm still looking into acquiring some of the EMC series connectors to see if they make a difference. So yes, we really tried to do our due diligence. This all brings me to the question of why the Neumann TLM67 was so much more susceptible to this interference than my AT 4050 or my Bock 195? We also had no problem with the Sennheiser 416. I was almost at the point of shipping the 67 off to Sennheiser to get checked out, but now I know I can temporarily disconnect the cell antennae and it will work perfectly. We're requesting that the antennae be moved about thirty feet further away from the booth. When I previously tested all these mics out in the sound stage about forty feet from the booth there was no interference there.
Thank you again Klaus and company for helping this old guy out. I've been doing this stuff since the seventies and have never run across anything this befuddling.
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klaus

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Re: TLM 67 picking up computery noises
« Reply #16 on: August 01, 2014, 07:20:38 pm »

Neumann's TLM mics don't have truly balanced outputs, but are "electronically balanced", with some reports of the same kind of interference noise trouble you are having.

Read through the Neumann Pinboard (their brand-specific forum) and you will see what the issue is, and what Neumann's recommendations are for alleviating RF from these mics.
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Klaus Heyne
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Dinogi

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Re: Neumann TLM 67 picking up RF/computery noises
« Reply #17 on: August 22, 2014, 06:01:00 am »

I finally had the opportunity to get back to the RF issue we were having. As it turns out, removing the cell antennae from the equation revealed that there is indeed an issue with the microphone. Once the cell phone noise was removed there was still a popping and sputtering sound coming from the microphone. The rf interference was making it impossible to discern whether the issue was caused by the cell antennae or a problem with the mic. I packed it up and sent it in for repair.
Hopefully it's something covered by the warrantee and not the result of one of my heavy handed co-workers.
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