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Old 07-16-2013, 07:59 AM   #1
GaryBillington
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Weird PA feedback issue

I recently had a couple of auditions with a band who need to replace their current guitarist. I've got the job but there's a weird issue with feedback from the PA that I need to resolve.

Basically, everything is all fine until I actually use my mic. Once I've used it, a constant feedback starts which can only be resolved by turning off the mic. I've tried 3 different mics so far:
My old SM58 (that I've always suspected may be a fake)
My old Altai mic (similar to an SM57)
The other guitarists spare mic (think it was an SM58 style Behringer)

We've also tried plugging me in to different channels of the PA, the same thing happens - it sits there nice & quiet until the time comes for me to actually use my mic, after which the feedback starts again.

Both times I've played with these guys, we've been set up in mock gig positioning - the mic was a few feet behind the PA speaker (which was obviously pointing away from me), with my amp about 6 or 7 feet behind the mic. Obviously I'm stood between the mic & my amp, with my pedalboard on the floor under the mic (see my sig for the guitar/amp/pedals).

Surely it couldn't be my amp/pedals etc? Obviously they're on all the time, but as they're several feet away from a mic that isn't directly pointed at them and I can play without issue until I actually need to sing into the mic (which sets the feedback off), I don't know how they could be causing the problem.

I'm planning on getting a new (genuine) SM58 in the near future, but as the problem has occurred with 3 different mics so far I don't believe that will resolve the problem (and my old mics have never caused a problem before through years of gigs & recording). The only other thing I can think of that we haven't tried yet is a different cable, but surely that couldn't be causing the feedback?

This has happened both times I've played with this band. They've never experienced it with the guy who's leaving, and they tried another guy in between my auditions without experiencing the same problem.

Any ideas what it could be?

My next rehearsal with them is Sunday night, so I'd like to be able to resolve the issue then - or at least have some sensible ideas for what might be causing the problem. Obviously I've given as much info as I can think of, but if you need anything else I'll do my best to answer.
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Old 07-16-2013, 10:38 AM   #2
Blackwaterdraw
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Do you have a wedge close by? I have run into the same thing and it turned out to be the wedge sitting a little too close to the mic.
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Old 07-16-2013, 10:46 AM   #3
tim_mop
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Are you holding the mic when you use it?

If you are and you're covering the bottom of the grill you could be turning it into an omni, which would explain the feedback.

If the mic is pointing towards any of the speakers it will feed back, but it would also feedback when you weren't using it as well, so we can rule that out.

Is the feedback constant in volume as you sing into it? Normally as the pitch and transients change as you sing, the feedback will change too.
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Old 07-16-2013, 11:00 AM   #4
GaryBillington
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Quote:
Originally Posted by Blackwaterdraw
Do you have a wedge close by? I have run into the same thing and it turned out to be the wedge sitting a little too close to the mic.

Depends how you define "close by". There's one just the other side of the lead vocalist a few feet away, but his mic is between the wedge & my mic.

Quote:
Originally Posted by tim_mop
Are you holding the mic when you use it?

If you are and you're covering the bottom of the grill you could be turning it into an omni, which would explain the feedback.

No, it's on a stand. I just do occasional backing vocals while playing guitar so that isn't it.
Quote:
Is the feedback constant in volume as you sing into it? Normally as the pitch and transients change as you sing, the feedback will change too.

Not really, it sort of comes in gradually - basically there's not feedback for however long, then I sing a line and the feedback starts, so usually I'm not actually singing when it becomes a problem, it's just that my occasional vocals seem to kick it off.
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Old 07-16-2013, 11:15 AM   #5
Arby911
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Troubleshoot, don't guess.

Turn everything but the PA off, see if it feeds back.

Add things to the mix one at a time until you've found the culprit.
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Old 07-16-2013, 11:18 AM   #6
tim_mop
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Quote:
Originally Posted by GaryBillington
Depends how you define "close by". There's one just the other side of the lead vocalist a few feet away, but his mic is between the wedge & my mic.


No, it's on a stand. I just do occasional backing vocals while playing guitar so that isn't it.

Not really, it sort of comes in gradually - basically there's not feedback for however long, then I sing a line and the feedback starts, so usually I'm not actually singing when it becomes a problem, it's just that my occasional vocals seem to kick it off.



Okay, that makes sense. So what that suggests is that your mic is in an unideal position, and while it's not quite loud enough to cause feedback on it's own, when something loud like your voice goes through the system your mic picks it up.

My advice would be to experiment using different layouts, different positions for the mic. If your layout is a traditional performance layout make sure the speakers are actually in front of the mics. This will help a lot.

Another thing: When you sing BVs how close are you to the mic? If you're further than a couple of inches away you'll get much less gain before feedback. So make sure when you sing you're right in the mic. My general guideline is to have my nose touching the grill (not always actually touching, but that sorta distance!) that way you can turn the mic down a lot and get more of your voice before it feeds back.

Last edited by tim_mop : 07-16-2013 at 11:19 AM.
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Old 07-16-2013, 11:23 AM   #7
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Quote:
Originally Posted by Arby911
Troubleshoot, don't guess.

Turn everything but the PA off, see if it feeds back.

Add things to the mix one at a time until you've found the culprit.



No feedback will happen with no mics on.

Also, this method isn't as helpful in this situation as the feedback can be affected with every new mic that is added, not just one at a time.
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Old 07-16-2013, 11:44 AM   #8
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Quote:
Originally Posted by tim_mop
No feedback will happen with no mics on.

Also, this method isn't as helpful in this situation as the feedback can be affected with every new mic that is added, not just one at a time.


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Old 07-16-2013, 11:49 AM   #9
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^ Care to elaborate?
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Old 07-16-2013, 11:55 AM   #10
lucky1978
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^ read Arby's post again

Last edited by lucky1978 : 07-16-2013 at 11:56 AM.
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Old 07-16-2013, 12:05 PM   #11
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Quote:
Originally Posted by tim_mop
^ Care to elaborate?


i am just wondering why it would not be a 'helpful' idea to simplify the issue and isolate a problem. imo Arby's advice was dead on, if you can't understand the more complex issue then simplify the setup and isolate the problem.

to ignore arby's advice could set someone up for some serious confusion, while following his advice wouldn't take that long and could lend some insight into the problem. i pretty much do as Arby said whenever i have a sound issue like the TS: i simplify and isolate.
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Old 07-16-2013, 12:14 PM   #12
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^ As a general principle I agree wholeheartedly, but in this case the feedback is most likely compounded by the use of multiple microphones which would make isolation hard with this method.

For example, it could be that the combination of the two mics (lead and BVs) leads to feedback whereas having only one open does not. If this is the case, troubleshooting using the normal method could result in either one of the microphones being the problem, depending on which one is turned on first.

Again, I do agree with the troubleshooting method. Perhaps in this case though the way to do it would be to start with all mics on, but no speakers. turn each speaker on individually, and that might isolate which speaker is causing it to feedback. Try moving the speaker then repeat.

Also I might have misinterpreted part of what Arby said, so feel free to ignore that post entirely!
EDIT in fact I completely misinterpreted what Arby said. My bad!
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Old 07-16-2013, 12:15 PM   #13
Arby911
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Quote:
Originally Posted by tim_mop
No feedback will happen with no mics on.

Also, this method isn't as helpful in this situation as the feedback can be affected with every new mic that is added, not just one at a time.


I'll give you the benefit of the doubt here and say that what I think you're trying to say is that it might not be a single point that is causing the feedback, but a combination of equipment?

In that case the troubleshooting is exactly the same, you simply add items until the problem appears, then remove one and see if it quits. If it does, that piece of gear is ONE aspect of the problem. You then turn it back on and remove another. If the problem then quits, that piece of gear is ALSO one aspect, but if the problem remains we can reasonably assume that particular piece of gear isn't material to the concern at hand.

And so on and so on....until we have the gear or combination of gear that is causing the problem.

It's not rocket surgery, it's basic fundamental step-by-step troubleshooting and it's hardly a new concept, nor was it developed by me.

Edit: Just read your post above, it's all good!
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Old 07-16-2013, 12:17 PM   #14
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Quote:
Originally Posted by tim_mop
^ As a general principle I agree wholeheartedly, but in this case the feedback is most likely compounded by the use of multiple microphones which would make isolation hard with this method.


that is quite true, but the simplification part will at least indicate at what complexity the PA starts acting up, which in itself may be very valuable information. the isolation will let you know where/when crosstalk or interference or overloading my occur.
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Old 07-16-2013, 12:17 PM   #15
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Good point!
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Old 07-16-2013, 12:45 PM   #16
GaryBillington
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Quote:
Originally Posted by Arby911
Troubleshoot, don't guess.

Turn everything but the PA off, see if it feeds back.

Add things to the mix one at a time until you've found the culprit.

I know logically this is the right answer, I did turn all of my kit off and the problem was still there, so I did suggest it at our last session - the only problem with it is the band politics, I didn't want to push it too much as the last weekend's session was still only an audition. First official practice is this weekend coming, so I'm hoping we can sort it then.

The PA is owned by the bassist/vocalist, who the other guitarist (about my age, bassist & drummer are both about 10 years older) described as "a bit of an old fart", especially when it comes to his gear! Apparently he usually figures things out eventually, but he likes getting there his way and at the moment all he can see is that it wasn't a problem before, so it must be my kit.

As far as I'm concerned it probably isn't because we tried different mics & I also tried turning my amp off (although admittedly I didn't turn my pedals off, but surely they couldn't be causing it?).

I think it probably will come to that in the end, but I'm mostly asking what other factors could come into it - I know very little about PAs & why they feedback, so any suggestion for more subtle things to try first would be welcomed.
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