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Author Topic: Noise Problem - Possible Power Issue? or Gain Structure?  (Read 7863 times)

Aaron_McQueen

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Noise Problem - Possible Power Issue? or Gain Structure?
« on: March 10, 2011, 03:07:22 pm »

We have 8 Shure ULX wireless units going into a Yamaha LS9.  Whenever we have these units turned up in the system there is audible noise.   The noise is like a white noise.  This really isn't a huge problem during the worship time, but if it's quiet then you can hear the noise.  Turning off the mic channels at the board gets rid of the noise.  Turning of the transmitters or the receivers does not.  We also have an Aviom system for the musicians and the noise from these mics is also audible through that all the time, which makes since because the mutes on the board don't affect the direct outs to the Aviom.

I had a consultant that suggested that it was a power issue. Specifically he stated that the board and the mics are on different legs.  However they are plugged into the same outlet that is controlled by a power sequencer.  The only difference is that the board is connected to a UPS.

Right now I'm looking for things to test to try to narrow down the source of the problem.  Any help is appreciated.
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John Roberts {JR}

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Re: Noise Problem - Possible Power Issue? or Gain Structure?
« Reply #1 on: March 10, 2011, 03:18:59 pm »

White noise is not likely power related.

It seems turning off the receiver, should kill the mic noise. From your description your mic preamp, which isn't really amplifying the mic, is adding noise.

There will always be noise in any system, but you may want to review how mic receiver is fed to the console input (line input or padded down to mic level?). 

JR
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Mike Spitzer

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Re: Noise Problem - Possible Power Issue? or Gain Structure?
« Reply #2 on: March 10, 2011, 03:21:50 pm »

A couple of thoughts and questions.

I wouldn't think it's a ground loop issue if it occurs when the receiver is off. That's odd right there, and indicates that the problem is with the channel somewhere, or the cable. The LS9 has a digital gain which, in my experience, introduces some noise, but not much. What happens when you pull it down?

Are you sure this is only the wireless? Move the cable to another input on the board and see if it follows. If it does, can you still hear it when you unplug the cable from the board? It's possible that the cables from the mics are picking up some noise somewhere. If it stops when you unplug the cable, use the cable from the receiver to plug into a DI with some kind of signal (keyboards, maybe). See if the noise is still there. Leave the cable in the same spot, though, in case it's picking up interference somewhere.

Also, what else is on that circuit? I had a similar problem once that ended up being our lighting system. When we unplugged the stage lights, the noise went away. From then on, we just made sure they were on another circuit.

One last thing. There could be something wonky with the board config (one of the reasons I avoid digital). Reset to factory (scene 00? or something) and see if that changes anything.

That's all I can think of off the top of my head.

-mS
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Kent Thompson

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Re: Noise Problem - Possible Power Issue? or Gain Structure?
« Reply #3 on: March 10, 2011, 05:27:00 pm »

We have 8 Shure ULX wireless units going into a Yamaha LS9. 
Something I have noted dealing with some digital mixers is using the pad provided on the preamp can result in more hiss. I don't recall if the LS9 has one or not but if you have one and its engaged try removing it and re adjusting your gain appropriately.
 
edit:
Yes it sounds like gain structure. It could be that the signal from the transmitters is padded down too much causing you to have to open up the gains too much or the reciever gain is too low etc.
 
 
« Last Edit: March 11, 2011, 10:55:06 am by Kent Thompson »
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Jonathan Johnson

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Re: Noise Problem - Possible Power Issue? or Gain Structure?
« Reply #4 on: March 11, 2011, 12:47:03 am »

Are you using balanced or unbalanced cables between the receivers and the mixer?
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Brad Weber

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Re: Noise Problem - Possible Power Issue? or Gain Structure?
« Reply #5 on: March 11, 2011, 08:24:05 am »

I agree that a 'white noise' sound is more likely to be related to a noise and gain problem than a ground loop problem.

I don't know if you're using the unbalanced 1/4" or balanced XLR output on the ULX but the combnation of connection and output level setting could be a factor.  If you're using the XLR output on the wireless mic receiver then it has a nominal +3.9dBV/+6.1dBu output level for the line level out setting and -17dBV/-14.8dBu nominal output for the mic level setting.  The LS9 input levels are -62dBu nominal and -42dBu maximum for the mic level -62dB gain setting and +10dBu nominal and +30dBu max for the line level +10dB gain setting.  So it appears that the best match in terms of levels and gain structure might be to have both the ULX and LS9 set for the line level settings.
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Arnold B. Krueger

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Re: Noise Problem - Possible Power Issue? or Gain Structure?
« Reply #6 on: March 11, 2011, 09:55:36 am »

We have 8 Shure ULX wireless units going into a Yamaha LS9.  Whenever we have these units turned up in the system there is audible noise.   The noise is like a white noise.  This really isn't a huge problem during the worship time, but if it's quiet then you can hear the noise.

Audible noise from good equipment is almost always a gain sructure problem.

In the case of a wireless system, the usual gain structure problem is insufficient signal going into the wireless transmitter.

A sympotom of this would be low audio level as shown on the reciever's display. Another symptom would be excessively high gain setting on the console.

I found Shure's user guide for the SLX at http://www.shure.com/idc/groups/public/documents/webcontent/us_pro_ulx_wireless_en_ug.pdf.

It sounds to me like you need to revisit the transmitter gain adjustment prodcudure contained in that guide.

Quote
Turning off the mic channels at the board gets rid of the noise.  Turning of the transmitters or the receivers does not.  We also have an Aviom system for the musicians and the noise from these mics is also audible through that all the time, which makes since because the mutes on the board don't affect the direct outs to the Aviom.

That all makes sense, given what your problem is.

Quote
I had a consultant that suggested that it was a power issue.

Sounds like strange advice.

Quote
Specifically he stated that the board and the mics are on different legs.  However they are plugged into the same outlet that is controlled by a power sequencer.  The only difference is that the board is connected to a UPS.

If that were really a problem it should be simple enough to power the wireless receivers from a plug strip that was plugged into the UPS. Wielress mic receivers draw minimal power so they wouldn't burden the UPS.

But, I don't believe his diganosis. If there were power problems like he seems to be suggesting, I would expect hum, not hissing.
[/quote]

To get hiss like you seem to be describing, I would expect the audio level meters on your SLX receivers to be barely indicating anything.  Grab a wireless mic and take it back into the booth, and see what sort of indications you get on the reciever when you are talking into it.
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Jonathan Johnson

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Re: Noise Problem - Possible Power Issue? or Gain Structure?
« Reply #7 on: March 11, 2011, 11:57:29 pm »

Audible noise from good equipment is almost always a gain sructure problem.

In the case of a wireless system, the usual gain structure problem is insufficient signal going into the wireless transmitter.

A sympotom of this would be low audio level as shown on the reciever's display. Another symptom would be excessively high gain setting on the console.

I found Shure's user guide for the SLX at http://www.shure.com/idc/groups/public/documents/webcontent/us_pro_ulx_wireless_en_ug.pdf.

It sounds to me like you need to revisit the transmitter gain adjustment prodcudure contained in that guide.

That all makes sense, given what your problem is.

Except that Mr. McQueen said:
Turning off the mic channels at the board gets rid of the noise.  Turning off the transmitters or the receivers does not.

This seems to indicate that it may not necessarily be a gain structure issue at the transmitter. If the receiver's output is set too low, then you'd be turning up the input gain on the mixer, and that could be where the noise is coming from -- in the mixer's preamp (it could be faulty). Mr. McQueen hasn't returned to tell us if he's done any other troubleshooting.

It would be interesting to know if the noise goes away when unplugging the cable from the mixer.
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Mac Kerr

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Re: Noise Problem - Possible Power Issue? or Gain Structure?
« Reply #8 on: March 12, 2011, 12:32:41 am »

It would be interesting to know if the noise goes away when unplugging the cable from the mixer.

Why would that be interesting. As others have said it is almost certainly a gain structure issue. It may start with not enough signal getting into the transmitter, so everything else has to be turned up to compensate, it may be somewhere else in the signal chain. Starting at the first point of gain control, in the transmitter, the OP should make sure that there is plenty of level at each step in the chain. Step 1, mic into transmitter. Step 2, transmitter output, is it line level, or set for mic level? If it is mic level, set it for line level. Step 3, mic pre level, checking with the PFL is there plenty of signal getting into the console?

After the gain structure has been straightened out, and the system tested at real operating levels, then it will be interesting to find out if the noise problem is still there.

Mac
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Arnold B. Krueger

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Re: Noise Problem - Possible Power Issue? or Gain Structure?
« Reply #9 on: March 12, 2011, 11:36:27 am »

Except that Mr. McQueen said:

Except that Mr. McQueen said:
Quote
Quote from: Aaron_McQueen on March 10, 2011, 03:07:22 pm
Turning off the mic channels at the board gets rid of the noise.  Turning off the transmitters or the receivers does not.

This seems to indicate that it may not necessarily be a gain structure issue at the transmitter.

Except I never said that it was a gain structure issue at *just* the transmitter.

*every* gain structure issue I've seen has 2 parts:

(1) Some signal that is not big enough *or* too big

(2) Some other  that is too big or not big enough without some heroic gain setting someplace else.

My guess is that (1) is probably due to not enough modulation on the RF due to insufficient audio gain at the transmitter.

Then (2) is too much gain being applied to the output of the receiver. When this happens, the receiver's output appears to be too noisy.

Quote
If the receiver's output is set too low, then you'd be turning up the input gain on the mixer, and that could be where the noise is coming from -- in the mixer's preamp (it could be faulty).

Do you now see how we may be on the same page?  Yes the mixer's preamp could be faulty, but then it would be simple to solve the problem by simply moving the receiver's output to some other channel. (I hope this has already been done with no improvement. If not, this would be a good first step.)

I think that the real problem is that too much gain is being applied to the mixer's input, and the noise is simply residual noise from the receiver. The excess channel gain is needed because the receiver's gain control setting is too low, or because the RF coming from the transmitter does not have enough modulation.  Now low gain in the recevier could be the result of a mic-level output of the receiver being applied to a line input on the console. Or, there could be a defect in the receiver, or the receiver's gain setting could be way too low.

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Mr. McQueen hasn't returned to tell us if he's done any other troubleshooting.

Indeed.

Quote
It would be interesting to know if the noise goes away when unplugging the cable from the mixer.

Agreed.

My approach here has been to suspect that the cause of the problem is something that I did to myself along the way. One of my early mistakes was to have the gain on the transmitter set too low.
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