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Author Topic: Buzz in monitor mix  (Read 5870 times)

Taylor Phillips

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Buzz in monitor mix
« on: February 22, 2015, 09:49:34 PM »

We suddenly have a buzz in Mon 1 mix of our Carvin C3244 mixer.  Mon 2 is fine, and we're using it for now.  No issues in the main mix either.  Any ideas what the buzz could be and can it be fixed?  We've been only using one monitor mix for right now, but I would like to get the drummer his own mix since he need it louder and I don't like turning up ALL the monitors on stage.  The mixer has 4 monitor outputs, but with Mon 1 out and using both built in effects, it only has one in practicality because Mon 2 and Mon 3 are either/or sends and you can't send the same input to both (who thought that was good idea?). 
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Scott Holtzman

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Re: Buzz in monitor mix
« Reply #1 on: February 22, 2015, 10:52:54 PM »

We suddenly have a buzz in Mon 1 mix of our Carvin C3244 mixer.  Mon 2 is fine, and we're using it for now.  No issues in the main mix either.  Any ideas what the buzz could be and can it be fixed?  We've been only using one monitor mix for right now, but I would like to get the drummer his own mix since he need it louder and I don't like turning up ALL the monitors on stage.  The mixer has 4 monitor outputs, but with Mon 1 out and using both built in effects, it only has one in practicality because Mon 2 and Mon 3 are either/or sends and you can't send the same input to both (who thought that was good idea?).

Please share the steps you took to isolate the "buzz" to the board.  My first instinct would be upstream or downstream.  What frequency is the buzz (hint, use a spec an on your phone)?  If it is centered around 60hz half the balanced signal has been compromised somewhere.

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Scott AKA "Skyking" Holtzman

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Taylor Phillips

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Re: Buzz in monitor mix
« Reply #2 on: February 23, 2015, 07:34:50 AM »

I first changed the cable from the amp to the wedge,  then the cable from the snake, then changed the snake channel, then the adapter cable from the mixer to the snake. The output on the mixer is 1/4 inch and the snake is all XLR. Buzz disappeared completely when changing mixer outputs.
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Steve M Smith

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Re: Buzz in monitor mix
« Reply #3 on: February 23, 2015, 09:32:31 AM »

Swap 1 and 2 sends from the mixer.  If the buzz moves, it's the mixer.  If it stays in the same place, it's from the mixer onwards.


Steve.
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Scott Holtzman

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Re: Buzz in monitor mix
« Reply #4 on: February 23, 2015, 11:23:08 AM »

Swap 1 and 2 sends from the mixer.  If the buzz moves, it's the mixer.  If it stays in the same place, it's from the mixer onwards.


Steve.

Yes, Steve is correct.  Need that center freq. of the noise please.
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Scott AKA "Skyking" Holtzman

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Jerome Malsack

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Re: Buzz in monitor mix
« Reply #5 on: February 23, 2015, 11:27:55 AM »

Another question would be is the monitor a powered speaker? 
or is this a passive connected to an amp? 

1/4 inch TS unbalanced   or TRS Balanced? 

If powered speaker  where was the AC plug  plugged up in relation to the mixer power and FOH power.
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Taylor Phillips

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Re: Buzz in monitor mix
« Reply #6 on: February 23, 2015, 03:14:56 PM »

Switching mixer outputs eliminated the buzz. Balanced TRS output from mixer via TRS-XLR adapter,  XLR to amp, banana to 1/4 TS cable from amp to first wedge. 1/4 TS cables between wedges.

We have 3 passive 60W, 8 ohm wedges daisy chained from one side of a QSC USA400 amp rated at 250W for a 2 ohm load. Sound equipment is all on its own dedicated circuit. 

Probably wont get a chance to see frequency is buzzing until Sunday.
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Jerome Malsack

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Re: Buzz in monitor mix
« Reply #7 on: February 24, 2015, 10:55:47 AM »

If this is a problem with in the system it should happen at home also.  You should test mixer amps and monitors in another location to remove the equipment as a failure. 

If the equipment works at home in a normal state than we need further investigate at the site and for items like light dimmers, CFL lights, and such that can create problems. 

Looking at Carvins owners manual for the mixer it appears to have line drivers (op amp) on pin 2 and pin 3.  If this low detail document provides.  If someone used a TS unbalanced and the output has an op amp to drive the balanced signal than the Ring op amp may have blown and causing the hum on aux 1 output permanantly until repaired.  You may have to move to aux 3 or 4. 
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Michael Williams

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Re: Buzz in monitor mix
« Reply #8 on: February 27, 2015, 09:35:47 PM »

Switching mixer outputs eliminated the buzz.

You said swapping outputs eliminated the buzz in aux 1, but did it then show in aux 2 on the stage? 

A couple thoughts...

If it did then as you suspect it would seem to be be coming from the board on aux 1.  Maybe double check that there are no unmuted channels with noise being sent to aux 1.  (I have seem engineers chasing output problems only to find an input channel had noise and was routed that output.)

If after swapping the aux 1 and aux 2 connections from the board you find that both channels are quiet then you have an interesting puzzle.  The noise should follow either the output channel or the down-stream channel.

As a side-note, while using an unbalanced TS connector in a balanced TRS output is a poor system-deign configuration it will not harm anything.  Commercial mixer circuit designs use series resistance on the op-amp outputs to prevent damage from a direct short like that.   It sounds like you have it properly configured for balanced through the whole path so you are fine anyway.
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Taylor Phillips

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Re: Buzz in monitor mix
« Reply #9 on: February 28, 2015, 08:29:35 PM »

You said swapping outputs eliminated the buzz in aux 1, but did it then show in aux 2 on the stage? 

A couple thoughts...

If it did then as you suspect it would seem to be be coming from the board on aux 1.  Maybe double check that there are no unmuted channels with noise being sent to aux 1.  (I have seem engineers chasing output problems only to find an input channel had noise and was routed that output.)

If after swapping the aux 1 and aux 2 connections from the board you find that both channels are quiet then you have an interesting puzzle.  The noise should follow either the output channel or the down-stream channel.

As a side-note, while using an unbalanced TS connector in a balanced TRS output is a poor system-deign configuration it will not harm anything.  Commercial mixer circuit designs use series resistance on the op-amp outputs to prevent damage from a direct short like that.   It sounds like you have it properly configured for balanced through the whole path so you are fine anyway.
No buzzing came back, but we'll see tomorrow if that's still the case!  A bad unmuted channel was my first thought after changing cables didn't affect anything. 

While using the wrong connector type shouldn't have affected the electronics inside the board, a broken op amp would seem like the most likely culprit causing the buzz at this point.
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Michael Williams

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Re: Buzz in monitor mix
« Reply #10 on: March 14, 2015, 12:34:25 PM »

While using the wrong connector type shouldn't have affected the electronics inside the board, a broken op amp would seem like the most likely culprit causing the buzz at this point.

FWIW, when an op-amp fries the op-amp's output usually rails high or low, and occasionally floats, but will never buzz.  The only way for an op-amp to buzz is if there is a buzz in its input or the power source is buzzing.  If all other inputs/outputs work on a board that rules out internal power supplies and you are almost always left with ground loops caused by an external input/output.
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-Michael

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Re: Buzz in monitor mix
« Reply #10 on: March 14, 2015, 12:34:25 PM »


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