New Microphone Hummmmmm

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timinbovey
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New Microphone Hummmmmm

Post by timinbovey » Tue Nov 12, 2013 7:05 pm

Very small production studio, equipped with a Yamaha MG124cx mixer, feeding recording audio to a Macbook Pro. Present mic is a cheap MXL condenser -- don't remember the model, but it's a low end mic. There's a CD player in on one channel, and audio in from another computer. Very basic. Over the past couple years hundreds of voice overs and spots have been recorded on this basic outfit.

This week I decided to upgrade the cheap mic and put in a brand new EV RE-20. Instant HUMMMMM on the mic channel. (Yes I remembered to turn off the mic power from the condenser). First thought, bad cable. Switched out for a fresh cable, no change. thought -- maybe mic defective -- swapped out for a handy SM-58 -- same hum remained. Reconnected the cheap condenser, hum gone, all sounds great. Reconnected RE-20 -- hum. Reconnected SM-58 -- hum. Back to condenser, no hum. (No hum was present on any other channels). Did the usual checking of grounds, etc. Building is old with very old electrical system. Mixer is plugged in to a UPC UPS unit. Disconnected from power so board was running on the internal batter disconnected from the mains -- hum remained. Unplugged everything else from the board -- hum remained -- but always with the RE-20 and SM-58 but never with the condenser. Connected long cable walked around room with RE-20, hum remained although I was able to orient the mic in certain positions where the hum would *almost* disappear. Same with the SM-58. Plugging in the condenser, didn't make any difference -- no hum at all. Tried plugging in the mixer power into different power circuits, reversing the AC plug, etc no difference. This mixer has an outboard PS so it's just DC going into it from the mains.

Finally pulled out a new, spare Behringer mini-mixer. Same thing exactly. No hum with condenser, hum with SM-58 and RE-20. Apparently there is something in this studio that's generating this hum and for some reason the condenser is resistant to it.

I can't remember exactly, but I swear at one time a while back we had several mics plugged in to this board for a multi voice recording and didn't have this problem.

Is there something obvious I'm overlooking? Is there some reason why a powered condenser isn't susceptible to this hum, but a SM-58 and an RE-20 are? I think my next idea will be to shut off things in the room one at a time until there's nothing left running but the board and seeing what happens. Is it possible the problem is from the UPS unit? I think it's the only new thing in there in the past few months, but still doesn't affect the condenser mic, and I have similar units installed in several studios with no issues. I did take the new RE-20 to a different studio, in a completely different radio station miles away, no hum when plugged in there -- replacing their normally installed SM-58. So it's not the mic.

It's possible to find a spot in the room to hold the mic where the hum is nulled, but it's very small spots where the slightest movement brings the noise back. There's gotta be something obvious I'm not thinking of.

Why is the cheap condenser completely unaffected and how do I convince other mics to lose the hum?

Tim in Bovey

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Deep Thought
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Re: New Microphone Hummmmmm

Post by Deep Thought » Tue Nov 12, 2013 7:23 pm

RE-20 and SM-58 are electro-mechanical moving-coil devices which respond to small changes magnetic field produced when the coil moves as the diaphram vibrates, but also respond to external fields if they are strong enough. The condenser relies on vibrating capacitor plates to vary the output voltage and generally immune from magnetic fields.

Sounds like you've got a big electromagnet somewhere nearby. Anything above or below you that could generate a major alternating field? Did you unplug the UPS completely? If not, that may be the culprit.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

timinbovey
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Re: New Microphone Hummmmmm

Post by timinbovey » Tue Nov 12, 2013 7:36 pm

I tried to think through other gear in the room (the electro-magnet concept crossed my mind, but was dismissed). Of course it's near the laptop, and an imac (not wired to the mixer) on an adjacent counter. Two non-powered studio monitors above the mixer on a shelf, that's about it. Yes, I literally pulled the plug from the mains on the UPS unit so it was off and supplying power on battery. I'm going to shut down computers, etc and see what difference it may make. I did move both the SM-58 and RE-20 across the room -- small studio so maybe 8 feet away from the "gear" side of the room and could still hear the hum but could position the mic to null it. Must experiment some more, first with shutting things down to see what's up. The laptop and imac, along with the mixer itself were, I believe, the only things running. I'll share further testing.

Tim in Bovey

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Deep Thought
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Re: New Microphone Hummmmmm

Post by Deep Thought » Tue Nov 12, 2013 11:14 pm

"Yes, I literally pulled the plug from the mains on the UPS unit so it was off and supplying power on battery"

This is a very big transformer running on some very dirty power if it was what was powering the mixer. It needs to be completely off with nothing plugged in at all.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

beaniecopter
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Re: New Microphone Hummmmmm

Post by beaniecopter » Wed Feb 12, 2014 12:40 am

The hum problem you are experiencing results from a near-by electromagnetic field. The source may be the ballast of a fluorescent lamp, the transformer of a desktop high-intensity lamp, the external power supply of a laptop computer, a light dimmer, the igniter of an oil-fired furnace, or other electrical apparatus. An incandescent lamp with a plain on/off switch and no dimmer in the circuit should not produce noticeable hum.

If the hum is present only in a small region, look first for near-by transformers, whether large or small. Here, "near-by" means within a foot or two.

If the hum is present everywhere in a large region, suspect something such as a lamp dimmer which is generating interference which is being radiated through the electrical wiring.

A five-dollar receptacle tester (the kind which has three neon lamps) can verify that the problem is not a broken ground conductor or a broken neutral conductor. Of course, all studio-grade gear expects to be powered from a three-conductor receptacle with a proper ground; this is a must for safety.

A condenser microphone with a transformerless output inherently is immune to hum. But if the microphone has a transformer, hum can be induced into the transformer. Some high-quality condenser microphones have a transformer output; other high-quality condenser microphones are transformerless. Likewise, an inexpensive condenser microphone may have a transformer output or may be transformerless.

A dynamic microphone has a coil of wire (a "voice coil") attached to the diaphragm. The coil sits within the field of a magnet which is stationary. A signal is generated as the coil moves relative to the magnetic field. If the coil is exposed to an external magnetic field such as the field produced by a transformer, hum is induced into the coil.

A studio-grade dynamic microphone such as the ElectroVoice RE-20, the Shure SM-7B, the BeyerDynamic M-99, or the Sennheiser MD-421 has an internal hum-bucking coil which is connected in series with the voice coil. The intent is that an external magnetic field which induces hum in the voice coil induces a like but opposite hum in the hum-bucking coil, so that the two hum signals cancel one another. However, not every hum-bucking design works well, and some designs are better than others.

Typical hand-held dynamic microphones intended for use by stage performers have no hum-bucking mechanism; the SM-58 is an example.

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KPJL FM
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Re: New Microphone Hummmmmm

Post by KPJL FM » Thu Feb 13, 2014 2:17 pm

Sounds (pun intended!) like big transformer or bad wiring on fluorescent lamps.

Bovey as is Bovey MN, home of the painting 'Grace"?
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beaniecopter
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Re: New Microphone Hummmmmm

Post by beaniecopter » Thu Feb 13, 2014 3:38 pm

My own informal A-B testing (same book, same cable, same preamp) of the ElectroVoice RE-20, the Shure SM-7B, and the BeyerDynamic M-99 indicates that the same source (the transformer in the base of a 20-Watt 12-Volt halogen desktop lamp ) induces in the RE-20 hum which is about an order-of-magnitude greater than the hum which the same source induces in the SM-7B or the M-99. In ordinary terms, the hum induced in the RE-20 is intolerably loud, while the hum induced in the SM-7B and in the M-99 is almost inaudible.

Informal testing in the repair shop of a nation-wide distributor of professional audio gear found another RE-20 to be very susceptible to hum from sources such as transformers and variacs (variable transformers); this was a result which surprised the technician performing the experiment. The fact that two unrelated RE-20 microphones behave similarly when in proximity to a transformer indicates that neither unit is defective, but, rather, that susceptibility to hum is inherent to the design of the RE-20.

Clearly, some humbucking designs perform better than do others; and clearly, advertising copy is not always credible. Always look for an engineering specification regarding susceptibility to induced hum.

If the source of the hum is localized, such as a transformer, the intensity of the electromagnetic field decreases with the square of the distance; this is the "inverse-square law". Accordingly, increasing the separation between the microphone and the transformer from four inches to eight inches (a factor of two) should reduce the level of hum by a factor of four. A further increase in separation from eight inches to sixteen inches (another factor of two) should result in another reduction by a factor of four. Conversely, reducing the separation from four inches to two inches (a factor of two) should increase the hum by a factor of four.

However, as previously noted, if the hum is radiated by the 120Vac wiring (this can be the case with an electronic lamp dimmer), the only reasonable solution may be to switch to a different microphone.

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