Help me kill the hum

January 15, 2008 7:31 PM

MusicGearFilter: I've got a hum in my recording/performance setup. I think I know how it's getting in -- a balanced mic cable going into a piece of gear with an unbalanced jack. How do I fix it? Full details within.

My headset mic has a permanently attached cable with a XLR plug on the other end. I connect this to an extension cable, which has an XLR jack on one end and a TRS plug on the other. So far, it's a balanced signal.

The next step is where I think the problem occurs: I'm plugging that TRS plug into my Zoom G2 guitar effects unit, which has a 1/4" (mono) input jack. Then I send the signal out to my mixer.

I only get the hum when I have the G2 in the loop. When I plug either the mic's XLR plug or the TRS plug from the mic extension cable directly into my mixer, the hum disappears.

The hum also disappears if I touch any conductive part of the signal chain -- the mic cable connectors, the G2, the G2-to-mixer cable connectors, or the mixer itself. Apparently I am grounding everything with my body.

As far as killing the hum at its source, I've tried that. A lamp dimmer was the worst offender, but I'm still having some problems with the power adaptors for my music gear and some of my computer peripherals.

Any suggestions on how could I fix this? Is there something I could insert somewhere along the chain to ground everything? This is my first experience dealing with this kind of issue, and my Googling hasn't produced any useful answers so far. The ideal solution will be workable both at home and when I play out.

posted by Artifice_Eternity to technology (6 comments total)

From my husband: "he needs an XLR to 1/4" mono adapter."


(husband here) - The TRS is essentially a stereo connection. When you plug it into the Zoom, one side of the connection is open and ungrounded, hence the hum.

You can get an XLR to 1/4" mono cable, or a TRS to 1/4" mini adapter. Either way, it should kill the hum. Good luck.
posted by kellyblah at 8:03 PM on January 15

Okay, here's the thing. Mics aren't meant to play nice with guitar pedals. You've got this nice, balanced signal, which you're forcibly unbalancing, and then plugging into a jack that's expecting a balanced (yet still 1/4") input. That's just no good.

Do yourself a favor - avoid this entirely. Does your mixer have an FX send/return? Plug the mic into the mixer, then use the send (which could be balanced) to go to the pedal, and then bring it back to a return. Consult your mixer's manual to figure out whether the send/receive are balanced or un. This is the standard way of mixing in vocal effects. If you're looking for an entirely wet signal, make the send prefader, and pot the dry mic channel all the way down.

If that's not a reasonable solution, then you're looking at using transformer isolation to to make the connection terminate correctly. Transformer isolation is expensive. There's also ways to make it work by terminating cables in a very specific manner - This Rane document should be of assistance.

Notably, in this instance, the TRS isn't a stereo connection of any sort. Two of the pins are signal, but one of the signal pins is phase-reversed 180 degrees, and the third is ground. In a stereo connection, 5 pins are required for proper balancing.
posted by god hates math at 8:39 PM on January 15

Sounds like a ground loop? Make sure all your stuff is grounded properly so that potential isn't flowing through your signal.

God hates math might have it- guitar cords are mono, are you plugging a stereo plug into the G2 when its expecting mono?
posted by gjc at 8:59 PM on January 15

Mr. kellyblah: Thanks. I will look into an XLR-to-1/4" or TRS-to-1/4" adapter.

god hates math: I tried the mixer's send/return already. It's crap -- the signal is so hissy it's useless. (Cheap mixer.)
posted by Artifice_Eternity at 9:02 PM on January 15

You want to look specifically for an impedance matching transformer. Sam Ash has them for about $15. It has a balanced XLR on one end and a 1/4" unbalanced phone plug on the other. Your microphone is a low impedance device. Your effects unit is expecting a high impedance device (guitar pickup). So you need a transformer to match them.
posted by leapfrog at 9:30 AM on January 16

UPDATE: Kellyblah's husband made the right call.

I bought an XLR-to-mono-1/4" cable, and it works like a charm! No more hum. An added benefit: My mic signal seems to be stronger.
posted by Artifice_Eternity at 9:21 AM on January 17