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Post by drbill on Jun 22, 2017 11:11:54 GMT -6
I posted this 10 years ago, elsewhere, and ran across it again, as a fellow AE was having a problem with his UM70's. Thought it might be worthwhile posting it here. This simple fix completely saved me thousands and almost dumping a BUNCH of unbelievably great sounding mics. If you're into vintage Gefell's, file this away.....
If you've ever had RF or grounding problems with your Gefell mics, you need to read this.
I bought 9 used Gefell's about 6 months ago and have had intermittant strange grounding and RF noise with them. Out of the 9 mics, (UM70's, M71S's, 692/M70's), 8 of them were problematic, and strangely enough 1 was always OK. AND ONLY IN MY STUDIO. Everywhere else they were OK. All my other condensers and dynamics are fine at my studio. (Neumann, AKG, Shure, Audix, EV, Sennheiser, etc.)
I tried everything. Got many suggestions here. None worked out They still made the noise - and intermittent to make things worse. I talked with Jim Williams and he told me that they were junk and told me to off them on ebay. I didn't want to give up on them yet, so I kept hunting. I don't want to get into all the details about what I tried right now, but a search would give you the background.
Anyway, I talked with a scoring engineer buddy of mine and he did some research for me and talked to a guy that made "pigtail fixes" for gefell's.
Basically, on the F XLR that plugs into the mic, you remove the shield (Pin 1) and tie it to the little loop on the chassis of the plug at the top. (Switchcraft style) I tried it and was amazed. It dropped the noise floor into the basement. It made the mics useable again. Needless to say...I am one happy camper.
File that away in case you ever have a problem with your Gefells at another studio. Well worth soldering up a few pigtails just in case.
Thanks Bobby!
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Post by bradd on Jun 22, 2017 15:39:28 GMT -6
Interesting. Is this a problem with new Gefells or just vintage ones?
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ericn
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Post by ericn on Jun 22, 2017 16:30:08 GMT -6
Interesting. Is this a problem with new Gefells or just vintage ones? Old ones. |
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Post by hadaja on Jun 23, 2017 0:30:11 GMT -6
I have an original M70 on 692 with original power supply and leads and have not had this problem touch wood but I did store Dr Bills info in the back of my mind when I read it first on "the other site".
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Post by Ward on Jun 23, 2017 5:27:08 GMT -6
That's fantastic, Dr Bill. Do you, or anyone else, know of any fixes for the old NG tube microphones?
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ericn
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Post by ericn on Jun 23, 2017 11:19:40 GMT -6
That's fantastic, Dr Bill. Do you, or anyone else, know of any fixes for the old NG tube microphones? Shannon has done some excellent stuff with them. |
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Post by popmann on Jun 23, 2017 12:56:08 GMT -6
Hasn't there been a retail solution for this? I know the guy who sold me mine years ago lent me his little adapter--said he needed it back for another mic....but, it didn't make any difference (then) in my studio. I've since run into some issues-once out of no where Garth Brooks was singing in my ear as I started to do a vocal take. Few minutes later.....gone. Silent again. Nothing moved....nothing changed in the gear....
So did you buy it from someone? You weren't clear....or did you buy some female to female adapter and rewire it inside for the one pin? I'd like to do that now....not that I don't like Garth as much as the next guy, um, but....
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Post by popmann on Jun 23, 2017 12:57:14 GMT -6
The thing this guy had looked like an inline pad....just female to female "plug extender" kind of deal. FWIW. No pigtail wires--though I can see how that would be easier to DIY fashion.
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Post by jayson on Jun 24, 2017 12:21:14 GMT -6
I can definitely report that I've NEVER experienced anything like this on the UMT800.
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Post by drbill on Jun 24, 2017 12:35:58 GMT -6
That's fantastic, Dr Bill. Do you, or anyone else, know of any fixes for the old NG tube microphones? Thanks! But sorry, I don't have any experience with those mics. |
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Post by svart on Jun 25, 2017 22:23:29 GMT -6
80's UM70S here, never had an issue.
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Post by drbill on Jun 25, 2017 22:47:59 GMT -6
80's UM70S here, never had an issue. How come your mics are always quiet?!?!?  Actually, seriously, it all depends on the RFI in a particular area - and the grounding of the chassis of the mic. (One of my 8 Gefells never made a single noise, while the other 7 could be chattering away - and that particular mic was the mid serial number in a group of three consecutive serials. Go figure) My conclusion is that audio ground somehow got tied to chassis internally. I know the original owner of all 8, so I know their history, and none of them were ever modded. Very strange. These days, mine are dead quiet, but I moved and theres no significant RFI interference near me (evidently). Back where I lived before, it was about a 50/50 chance of horrific noise ruining a take depending on the day, phase of the moon, humidity, angle of incidence or  ?. Sometimes horribly noisy, sometimes dead quiet, always different - and often VERY strange. I could put on headphones and by pointing the mic around the room, it was almost like a Mexican Radio Station geiger counter or metal detector - up and down, and changing depending on the day, hour, and direction and angle the mic was pointing. Maybe you don't have any rogue mexican radio stations nearby? LOL j/k |
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Post by svart on Jun 28, 2017 10:04:01 GMT -6
80's UM70S here, never had an issue. How come your mics are always quiet?!?!? Actually, seriously, it all depends on the RFI in a particular area - and the grounding of the chassis of the mic. (One of my 8 Gefells never made a single noise, while the other 7 could be chattering away - and that particular mic was the mid serial number in a group of three consecutive serials. Go figure) My conclusion is that audio ground somehow got tied to chassis internally. I know the original owner of all 8, so I know their history, and none of them were ever modded. Very strange. These days, mine are dead quiet, but I moved and theres no significant RFI interference near me (evidently). Back where I lived before, it was about a 50/50 chance of horrific noise ruining a take depending on the day, phase of the moon, humidity, angle of incidence or ?. Sometimes horribly noisy, sometimes dead quiet, always different - and often VERY strange. I could put on headphones and by pointing the mic around the room, it was almost like a Mexican Radio Station geiger counter or metal detector - up and down, and changing depending on the day, hour, and direction and angle the mic was pointing. Maybe you don't have any rogue mexican radio stations nearby? LOL j/k I dunno but my strat is really good at picking up local AM stations! If you have to unhook a ground to stop a loop, then that's not right. Audio and power ground should always be at identical potential, and any small difference will cause current flow and noise. Since it seems to be a common theme for older Gefell mics, it's possible that they have aging components or tarnish building up that cause potential shifts. Most come-and-go noise is usually traced to bad connections or oscillations. |
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Post by drbill on Jun 28, 2017 12:53:23 GMT -6
You're talking about something completely different. It's not a ground loop issue.. It's RFI getting into the mic from external sources that are not constant or consistent - i.e.: radio stations. There is/was no ground loop noise in the signal of my Gefell mics. Only rogue AM radio stations. The fix is not lifting the ground. It's tying audio ground to the chassis. "Technically" right or wrong, it's still grounded and who cares? It's fixed dozens of mics from having an issue.
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Post by svart on Jun 28, 2017 13:19:34 GMT -6
You're talking about something completely different. It's not a ground loop issue.. It's RFI getting into the mic from external sources that are not constant or consistent - i.e.: radio stations. There is/was no ground loop noise in the signal of my Gefell mics. Only rogue AM radio stations. The fix is not lifting the ground. It's tying audio ground to the chassis. "Technically" right or wrong, it's still grounded and who cares? It's fixed dozens of mics from having an issue. Long cables form the antennas, and the nonlinearities from current flowing through impure interconnects form the diodes that are needed to mix AM radio down into audio frequencies. So yes, ground loops can cause radio interference.. There's a saying in the RF design world I work in.. When is ground, ground? It isn't! That's because ground is seen by most as some kind of constant, but it's just a current carrying conductor same as any power or signal conductor and can carry as much or more (un)wanted signals as any other cable. Currents power your devices, but they don't simply disappear into the chips. Ground carries the equal of all forward currents back to the source. In the technical world we tend to call them "returns" rather than grounds. But I see what you're describing now. The initial vision I had was of someone disconnecting the mic chassis like those who unhook shield wires in mic cables to stop hum propagation (also one of the many ways a loop can manifest itself). I guess I don't have any issues because in my cables I make, I always hook the XLR/TRS body to the shield ground if they aren't already. |
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ericn
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Post by ericn on Jun 28, 2017 20:44:14 GMT -6
When we first started selling Gefell besides Transformer/ Transformer less they had a regular and low noise distinction.
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Post by Deleted on Jun 30, 2017 6:04:33 GMT -6
Some time back--20 years or more--I had to record a choir in a terrible room that called out for cardioids. I didn't have enough mics to cover, so David Griesinger loaned me a handfull of Gefells (I think they were 300s or the equivalent of the day). I only needed a couple, but he gave me four or five. He said that two of them would work, but I wouldn't know which two until the day. He was spot on. I found a pair that were nice, with very much a KM84 sort of vibe. The rest of them sounded like a raccoon coughing up phlegm.
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Post by standup on May 3, 2018 17:11:55 GMT -6
Today I got a UM70S in the mail. My studio is packed away for another few weeks, I think, so I can’t really listen to it. So I’m reading about Gefells instead.
Mine was a later one, I think, black mesh screen, pretty new looking, serial no. under 2000, I couldn’t find a Gefell serial listing online. The older ones appear to have a silver metal basket.
Anyway, I’ve read they’re a little light in the bass, which is fine because close mic’ing is something I do a lot of in my home studio. It’s treated, but still not a stellar space. I also read the bass-lite trait is because of aging PVC capsules. We’ll see. All I can do right now is confirm that it works.
I look forward to comparing it to a u195, which has often been my best mic for vocals, guitar, etc. up to now.
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Post by adamjbrass on May 3, 2018 20:56:38 GMT -6
I love the Gefell's
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Post by mrholmes on May 4, 2018 2:43:41 GMT -6
In such a case it maybe is a wise idea to write a Mail to MG-Germany - asking for help.
I know my fellows from the east they are always glad to help for free as far as they can.
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ericn
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Post by ericn on May 4, 2018 6:25:44 GMT -6
Hard not to ! Every time somebody tries to sell me a mic I measure the price/ value in used UM70’s . |
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Post by adamjbrass on May 4, 2018 6:59:25 GMT -6
Well, I probably wouldnt buy an old UM70 from ebay. M7's dry out over time. People ask WAY too much for them. $600 is what I'd pay. Not $1000. I like factory fresh ones. I have a UMT70s and love it. And before anyone says its worse then a UM70, I pre-disagree
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Post by ericn on May 4, 2018 7:17:15 GMT -6
Well, I probably wouldnt buy an old UM70 from ebay. M7's dry out over time. People ask WAY too much for them. $600 is what I'd pay. Not $1000. I like factory fresh ones. I have a UMT70s and love it. And before anyone says its worse then a UM70, I pre-disagree Yeah at this point as much as I love the old version of the M7 I would probably buy a used modern UM70 for consistency, I think in terms of $700ea. What can I say I’m cheap and got spoiled paying cost ? |
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Post by adamjbrass on May 4, 2018 11:16:02 GMT -6
Well, I probably wouldnt buy an old UM70 from ebay. M7's dry out over time. People ask WAY too much for them. $600 is what I'd pay. Not $1000. I like factory fresh ones. I have a UMT70s and love it. And before anyone says its worse then a UM70, I pre-disagree Yeah at this point as much as I love the old version of the M7 I would probably buy a used modern UM70 for consistency, I think in terms of $700ea. What can I say I’m cheap and got spoiled paying cost ? Perhaps. I like to pay full price to the company I work for. Dealing with myself can be a pain in the ass. |
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Post by iamasound on May 4, 2018 12:27:55 GMT -6
A Gefell UMT70S and their m930 combo is mid/side heaven and makes me a happy camper.
I do need to tell folks to get up close and personel with it, to me it is not a big old bottem end affair and I use proximity to tune in what is needed for the song to mitagate the use of eq for the most part. It is a fantastic mic to me on lots most everything. The low end of the m930 begins like three feet away and is an animal of different stripes. They bond well together.
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?. Sometimes horribly noisy, sometimes dead quiet, always different - and often VERY strange. I could put on headphones and by pointing the mic around the room, it was almost like a Mexican Radio Station geiger counter or metal detector - up and down, and changing depending on the day, hour, and direction and angle the mic was pointing.
