Help me rid myself of this annoying hum

November 22, 2010 9:10 PM

Why am I picking up a hum when recording?

I have been recording in a 2 mic setup for quite some time but lately I have picked up some sort of electric hum. I have spent 2 hours trying different configurations and now, frustrated, I am turning to you Mefites for assistance!

I have two Shure SM-57 microphones. They are connected by XLR to a Behringer X1204USB 12-Channel Mixer, one on Input 1, the other on Input 2. The mixer is then connected to my Macbook pro through an RCA to 1/4" adapter cable.

The hum is something new the past few months.

My troubleshooting steps:

*Plugged Headphones directly into the Phones jack of the mixer, yep I hear the hum.

*I checked all the mixer board settings. I set everything to 0 and the hum went away (though I did hear a bit of ambient static). As I brought up the sound to either microphone the hum came back. I tested some recording levels, and my recordings were overly soft. By the time I got up to where I was hitting 0.5 on the Mac (with the Mac's recording input maxed out) the board's settings were all pretty high and the hum was very audible.

*I tested this with one mic then the other. Both mics demonstrated the hum.

*I then replaced the XLR cable between one mic and the board. Hum still there.

At this point I'd tried 2 mics, 3 XLR cables, and 2 outputs all with identical results and thought perhaps my board was bad. So I pulled out my old Nady MXE-612, hooked up the two mics, plugged my phones in the jack...and there's that god damn hum!

Now I am listening to my room. I do NOT hear this hum. I have turned off everything in the room including the lights. I have UNPLUGGED everything in the room except the mixing board. STILL THIS HUM...

The hum is NOT constant but it is rhythimic. In the recording software I see it ebb and flow like a sine wave. I don't know what to make of it.

I have uploaded an MP3 of this sound to http://cilff.com/wtf.mp3 The beginning is the hum when the levels are set at a recording volume where my words can be picked up. The end of the WAV is where I turn everything WAY up to get the maximum noise and show the way it comes in and out ever so slightly.

At this point I am willing to buy whatever needs to be bought to get rid of this hum, but I'm not even sure if it's a "replace the part" problem as I have replaced every single component now and still get the hum.

I'm losing my mind...please help!

posted by bodgy to technology (16 answers total)

Sounds like 60 cycle hum to me; is there possibly a new fluorescent light in the room? Maybe you put fluorescent bulbs in one of the fixtures in the room? Not sure if those will do it, but older fluorescents with ballasts will surely do it.

The fact that you don't hear it unless you listen to your equipment indicates to me that you've got something funky going on either with the power to your gear (broken third prong or a bad ground maybe?) or a light, speaker magnet or other electronic device giving you grief. Have you tried moving into another room?

Good luck! I've chased this kind of stuff before.
posted by littlerobothead at 9:15 PM on November 22, 2010

I will admit that while typing this very post I started to wonder about the flourescent bulbs above my head, and so I turned them off...still had the hum. It shouldn't matter if they're off, right?

The MXE has only a 2 pronged, ungrounded, plug...the Behringer is a grounded socket...but yes I'm going into the same outlet. I will try another room and post results back
posted by bodgy at 9:18 PM on November 22, 2010

Get a mic preamp. That way you can run your levels lower.

I dislike Behinger products, generally shoddy workmanship$
posted by Ironmouth at 9:19 PM on November 22, 2010

The fact that the problem didn't exist before but has suddenly manifested itself, assuming you are using the exact same configuration, indicates that something has changed. I suspect a ground has lifted somewhere. This could be in the building wiring (isolate by moving to a different location where you're sure the ground is solid), or it could be in the grounded piece of equipment itself.

You can test the equipment by going from the ground prong of the XLR input to the ground prong of the AC connection (make sure you don't have ground lift engaged, of course). Testing to make sure the building ground is solid is tougher. I've seen it done by feeding a known-voltage source in and looking at the current flow (though it seems like the reverse would work fine too), but that's not a trivial test if you don't have the right equipment.

60Hz hum problems almost always come down to grounding, though, and using balanced lines wherever possible (which you seem to already be doing). Getting rid of fluorescents is fine, but if your mains connection has a floating ground, so that everything is floating, you're sunk. You need to get a solid ground and tie everything into it; that's pretty much the only sure route.
posted by Kadin2048 at 9:30 PM on November 22, 2010

Another room, same hum...gah!

Ironmouth, any suggestions for a specific preamp? In looking it seems many preamps would replace the mixer in my sceneario... Both of my mixers offer "phantom power", is this different than a preamp in a fashion that would make a difference here?
posted by bodgy at 9:31 PM on November 22, 2010

RCA to 1/4" adapter cable.

This could be the problem. The quality is decaying or a connection here is funky. Seems like the only thing equipment-wise to check.

Have you tried moving the whole set-up to another location (either inside your house/apt. or somewhere a few minutes by car away? Maybe it is some crazy outside-radiation.
posted by irishcoffee at 9:33 PM on November 22, 2010

I also recommend a pre-amp. My studio has bad wiring and I get all sorts of funky noise if I have my levels too high. The phantom power on my interface doesn't do much to abate the hum, so you may be experiencing the same with the Behringer.

ART makes a nice cheap tube pre-amp that works pretty well for me. If I crank it way up, the hum is there but at a much lower level.
posted by palacewalls at 9:50 PM on November 22, 2010

Are your microphones using phantom power or batteries? Dying batteries can cause buzzing. If you're using phantom power but still have batteries in the microphones, it can also cause buzzing.
posted by dorkydancer at 10:15 PM on November 22, 2010

Have you tried plugging the mics into channels known to be quiet when other things are plugged into them?
posted by rhizome at 10:28 PM on November 22, 2010

Phantom power is for powering condenser mics. Don't use it unless you have condenser mics. Most good preamps have it. You'll need it if you ever want to use them.
posted by Ironmouth at 10:40 PM on November 22, 2010

Note that SM-57's are dynamic mics and do not require batteries or external power.
posted by Ironmouth at 10:52 PM on November 22, 2010 [1 favorite]

Have you tried listening with *nothing* connected to the mixer inputs, and only the headphones connected to the outputs, though with the gain still up as if you were recording?

If you get hum then, you're proven its either the mixer itself that is the problem, or that its getting bad power.

(Outlets with bad grounding are disappointingly common in residences. You can get grounding testers at home improvement stores, I believe.)
posted by Hither at 10:53 PM on November 22, 2010

There are any number of things that could be causing this. The most likely scenario, I believe, is interference to the power supply.

Turn off the circuit breaker to find out what's connected to it. Unplug everything that's on that circuit that doesn't need to be and plug it in somewhere else. Then get a good surge protector, plug it in to one outlet, plug the mixer and the laptop and any power amps for speakers into the same surge protector and make sure you're using the 3-pronged PSU for the laptop. Make sure the transformer box for the laptop power supply is as far away from the transformer box of the Behringer's power supply as possible and that the cables from the two do not overlap ever. Turn OFF phantom power. Connect the mixer to the laptop. Try to keep all audio cables from overlapping each other. Now test to see if the hum or buzz is still there. If it is, try moving the power supply for either the laptop or mixer or both while listening to the source to see if it makes any difference. This MIGHT help.

If not, it's likely a grounding or more advanced shielding problem which could likely be solved by using a higher quality mixer.
posted by and1 at 1:01 AM on November 23, 2010

Have you tried it without the laptop ac adapter plugged in? I get awful noise when my adapters are plugged in.
posted by gjc at 7:12 AM on November 23, 2010

I don't think it's the laptop as for the testing last night I plugged headphones directly into the phones jack and disconnected and even powered off the laptop.

And the sound isn't there if the mics aren't attached/turned up. The other jacks don't have it. It is specifically something coming from the mics.

Thanks for all the suggestions folks!
posted by bodgy at 10:14 AM on November 23, 2010

A follow-up question has been posted here:

http://ask.metafilter.com/176577/Glass-table-hum-Why

Thanks again for the help!
posted by bodgy at 10:15 AM on January 24, 2011