Weird Phenomenon that occurred during band practice..

T

TheAquired

New member
Hi all,

So this really weird thing was happening during our band practice yesterday, and I was hoping somebody would be able to tell me what it was, so I can stop it next time.

Our Lead singer plugged his acoustic guitar in (He uses an XML cable) and then his mic. We have a little mixer that then hooks up to the speaker. This is where things got weird. There was a hum coming through on the speaker, and it seemed it was being caused by the mic. Well, its my mic, and I knew for a fact it never caused any kinds of hum. So after some trying to sort it out, we came upon this weird thing. If the lead singer touched his acoustic guitar (Anywhere, the wood or strings or anything) and then touched the mic, the humming would instantly stop. if he touched the mic without touched the guitar, then the humming wouldn't stop. I also tried this, and i could stop the humming in the same way.

Ok, so what on earth is this? I do live and studio audio engineering as a hobby, so I'm pretty familiar with these kinds of things normally. But this was beyond my comprehension. Any thoughts would be great! :)
 
Hi,
Sorry if this is patronising you, but when you say XML cable, do you mean XLR?
I assumed XML was a brand, but I couldn't find it on google.

Just to rule out the obvious first, guitars plug in with a TS to TS jack.
Both ends look like this.


The mic plugs in with an XLR female to XLR male cable, like this.

TheAquired said:
Ok, so what on earth is this?
Click to expand...

Hehe. Probably an earth issue.
Let us know about your cabling and we'll take it from there.
 
Hi, thanks for the fast response! Haha I'm so sorry.. My head was not quite in the right place obviously as I wrote this.. yes, XLR is what i was talking about. (XML is a file format:facepalm::facepalm:)

His guitar has both an XLR and TS jack output (the XLR is a bit nicer as it directly outputs a balanced signal to the desk)
 
It's a ground loop problem. Try plugging the mixer/amp (whatever you are plugging the mic and guitar into) in to a different wall outlet that is on a different circuit. This assumes the mixer/amp has a 3-prong (grounded) plug.
 
Yep ground (earth in the UK) issue. My first guesses would be mic cable or the connection from the mixer to the speaker. What mixer? Is the mic cable XLR at both ends? Are you using a balanced connection to the speaker (assuming it's a powered speaker)? Does it have XLR or TRS output? Note that with powered speakers you don't want to use a speaker cable, you need a line or mic cable.
 
Thanks for all that guys, my first assumption would be something to do with the mixing desk being plugged into a power socket without proper grounding. Its at my friends house, and its just a basement, so its quite basic and probably doesnt have great electrical wiring. Its definitely not the cable, as thats mine as well and I use it all the time.

I thought it must be some kind of ground loop, but what doesnt make sense is why it affects his guitar and the mic, yet not my guitar or the bass that is also plugged in. Also, it was the first time we had that issue, yet we have practiced there before.

Well thanks for the info everyone, if it happens again, I'll check out his wiring and everything. :)
 
Touching the wood (and mic..) it stopped? This seems more like a) an ungrounded (+ 'high impedance?) + b) gain, and the mic is providing some sort of coupling path to ground..?
 
Hi Mixsit, yes, touching the wood and mic did stop it. If just the wood was touched or just the mic though, it wouldn't stop it at all. I'm not too sure what you mean, could you please elaborate? thank you :)
 
Yeah I'm just winging it. Everyone's saying ground loop -ie shields are there but there's a slight voltage diff on the neutrals between two wall plug and/or breaker paths back to the common ground ('earth ;)

In that case I suppose the 'connection 'wood to hand to mic' shorting' his difference out (canceling the noise) is possible too, and maybe as good as where I was leaning- The guitar, or cord, missing a shield can be noisy like that, and.. 'wood to hand etc providing a missing grounding effect back to the guitar.
But really, I don't know in that case why wouldn't your body just wearing the guitar do it?

The 'high impedance thing, ac guitar / high impedance/very low power and voltages. This 'coupling' effect, also very high impedance stuff too I would guess.

Let us know what you find about the cables, guitar, whatever ?
 
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