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Author Topic: MTG UM70S RFI  (Read 4112 times)

onesong321

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MTG UM70S RFI
« on: September 18, 2013, 03:58:44 pm »

I have two UM70S's - the transformer versions, but the same body style (non-modular) as the current TS versions - and while they both have always been a little bit susceptible to RFI, one of them has recently become so bad as to be unusable. I can hear the local AM country station through it louder than the source in most cases.

I've done some basic checking - making sure it's grounded properly internally, and compared most of the parts' to the other mic. The only significant difference is that, near the capsule, the working one has a 3-pin transistor where the problematic one has an older-style 4-pin. The circuit boards are the same, just one isn't using the 4th pin's pad. The non-problem mic's transistor has had it's labeling removed (or was never labelled in the first place) so i cannot tell what specific transistor it is.

Anyone have any thoughts on where I might start looking?

Thanks!
-Ben
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klaus

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Re: MTG UM70S RFI
« Reply #1 on: September 18, 2013, 04:12:52 pm »

Some microphone electronics designs are more prone to RF-pickup than others, and a simple adherence to studio connector/cable standards* will often solve RF issues, and thus avoid more intrusive remedies like installation of ferrite bands, choke coils, or mic circuit modifications. So I'd start by investigating the cable you are using, and how its connectors' grounds and shields are terminated:

Are both XLR-cable-connector sleeves grounded to both shield and ground which must be both connected to pin #1 on both ends? Start by doing just that (I assume you use a decently shielded microphone cable in the first place.

I use and recommend Gotham's GAC3 for all balanced mic and line level applications. Read about its superior RF-rejection on professional recording forum posts.

Start there; also read the RFI Sticky in the Mic Lab's Archive's section http://repforums.prosoundweb.com/index.php/topic,1152.0.html
and please report back.

* I am not having a discussion again about the efficacy of that wiring standard. It's been recommended by Neumann for four decades, and has solved a vast majority of all RF-induced microphone noise issues reported and solved by users.
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Klaus Heyne
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onesong321

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Re: MTG UM70S RFI
« Reply #2 on: September 18, 2013, 05:46:50 pm »

Klaus - thanks so much for the reply!

I have double-checked the cables I used when I first noticed the issue. I also have made sure to test both mics through the same cable/preamp/etc combinations to verify that the one is actually worse than the other, and it consistently is.

I have been planning to upgrade my cabling to Gotham soon - I'm in the middle of a studio build which will include that, thanks for the recommendation.

If the noise was at the level of the other mic, I would return to using it happily with the hope that cabling upgrades would soon be moving it further into the noise floor. But right now I am thinking that something else may be wrong with this mic.

I am going to go through the sticky (sorry for missing it) here shortly and will report back this evening.
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Jim Williams

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Re: MTG UM70S RFI
« Reply #3 on: September 19, 2013, 11:42:31 am »

Those older UM70's have that problem. It's the models with the metal can 4 legged jfet. No, it's not the fet, those are rather imune to rectification of rf signals.

I found the cause to be insufficient trapping on the output leads to the XLR. There are a pair of .022 uf ceramic caps across each output lead to ground. The capacitor leads are rectifing the radio.

The fix is to remove those caps and solder in a surface mount .022 uf cap across the pcb pads on the rear of the circuit board. Carefully clean off all the flux when done. That will do it.
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Uwe

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Re: MTG UM70S RFI
« Reply #4 on: September 19, 2013, 11:59:59 am »

... and for a reliable alternative use the Neutrik EMC-series XLR connectors:

http://www.neutrik.us/en-us/xlr/emc-series/

'The EMC-XLR Series is a specifically designed version of the XX series to give enhanced RF screening for critical applications in live performance and recording where there are particular problems with radio transmission or mobile phones.'

I am not aware of any instance, where these connectors failed to adequately ameliorate RF-interference problems
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gkippola

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Re: MTG UM70S RFI
« Reply #5 on: September 19, 2013, 09:43:50 pm »

I have an orig  Tele U47 that has had the same problem, a very low level rf audio pickup occasionally, and have posted here before on it. I'm using GAC7 cable, and all grounds are connected on both ends.  I've checked everything. The pickup is worse if I drape the cable down the mic stand, instead of just letting it hang from the mic.
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klaus

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Re: MTG UM70S RFI
« Reply #6 on: September 20, 2013, 01:37:45 am »

Quote
I'm using GAC7 cable, and all grounds are connected on both ends.

Please report how exactly you terminated the cable's shield on each connector end.
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Klaus Heyne
German Masterworks®
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Jim Williams

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Re: MTG UM70S RFI
« Reply #7 on: September 20, 2013, 11:35:34 am »

If you don't like to solder tiny caps or rewire XLR connectors you can try locking clamp-on ferrite beads. Digi Key p/n #
732-1503
www.digikey.com

Just snap those onto the cable as it exits the mic. Add another at the mic pre input. My location bag has a bunch of those, they are very handy to have around.
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klaus

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Re: MTG UM70S RFI
« Reply #8 on: September 20, 2013, 12:50:31 pm »

If the shield on a mix cable is not terminated correctly, you MUST re-solder terminals, in order to correct the problem. Ferrite bands downstream from the FET/high impedance section of the mic amp, are marginal in their effectiveness to suppress RF, I have found.

Still waiting for gkippola's information.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com
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