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Author Topic: Current model JBL SRX boxes induce buzz in mic cables/channels  (Read 3133 times)

Greg_Cameron

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Current model JBL SRX boxes induce buzz in mic cables/channels
« on: September 11, 2018, 07:43:44 pm »

For a while now, I've been dealing with an intermittent buzz induced into my vocal mic channels on different boards in my regular venue. Avid Venue, cheap Mackie mixers, you name it. The more vocal channel you have, the worse it gets from the addition. However, if you're hearing it in the mains and you polarity flip a vox channel, it will cancel much of the buzz. So the issue is common mode from channel to channel. I thought it might be the LED PAR working lights that were installed a while back. Or maybe the Lutron dimmer controlled string lights in the venue. If you turn the the LED PARs off, it does reduce the noise somewhat. The noise lives around the 8k range, so it's some type of switching artifact.


Today I was getting it with two SM58s on short booms in front of a single JBL SRX812p wedge (self powered box with DSP using I-Tech type amplification), all plugged into a Mackie CFX mixer. Pulled the power plug on the back of the JBL box, buzz all gone. Plug it back in, buzz back. Move JBL power cable which is parallel to mic cable, buzz almost gone. For a rock show, I'll have 4-8 of these on stage. Sometimes the buzz is noticeable, other times not. It seems the high speed switching devices in these boxes are pumping out RF and it's getting rectified into the audio band into my sensitive mic cables, possibly due to proximity of the boxes AC cable to the mic cable(s). Pulling the signal cable from the back of the box doesn't change anything, so it's not a ground loop related issue with the rest of the system and the JBL box. I'm using Whirlwind MK series cables all around, they're decent. Short of replacing all my relatively new SRX monitors, any recommendations on mitigating the issue? Would ferrite beads on the SRX's AC cables help?


Suggestions appreciated.


Greg
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Riley Casey

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Re: Current model JBL SRX boxes induce buzz in mic cables/channels
« Reply #1 on: September 11, 2018, 07:56:30 pm »

Sounds like a classic pin one problem. 

Ray Aberle

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Re: Current model JBL SRX boxes induce buzz in mic cables/channels
« Reply #2 on: September 11, 2018, 10:28:03 pm »

I only have a pair of the SRX812P boxes, but have *never* had any problem like you describe in over 3 years of owning them.

-Ray
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Debbie Dunkley

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Re: Current model JBL SRX boxes induce buzz in mic cables/channels
« Reply #3 on: September 11, 2018, 10:50:35 pm »

Not that this will help you but all of my SRX boxes have always been very quiet - no buzz. I have SRX812p, SRX815p, SRX828sp and SRX818sp.
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Greg_Cameron

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Re: Current model JBL SRX boxes induce buzz in mic cables/channels
« Reply #4 on: September 12, 2018, 01:36:36 am »

Sounds like a classic pin one problem.


I don't see how. The noise is being induced into the mic cables. Mic's can't have a pin 1 problem since there's no secondary ground connection to form a loop. And as stated previously, the SRX monitors can induce this noise into the mic channels without the any signal cable plugged into them.
« Last Edit: September 12, 2018, 01:48:49 am by Greg Cameron »
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Greg_Cameron

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Re: Current model JBL SRX boxes induce buzz in mic cables/channels
« Reply #5 on: September 12, 2018, 01:37:40 am »

I only have a pair of the SRX812P boxes, but have *never* had any problem like you describe in over 3 years of owning them.


It's an odd one, that's for sure.
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Greg_Cameron

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Re: Current model JBL SRX boxes induce buzz in mic cables/channels
« Reply #6 on: September 12, 2018, 01:46:48 am »

Not that this will help you but all of my SRX boxes have always been very quiet - no buzz. I have SRX812p, SRX815p, SRX828sp and SRX818sp.


The boxes themselves don't buzz aside from when sending mic channels where the noise is being induced into a mix. They are inducing noise into the mic cables. It seems that maybe JBL didn't do a great job filtering out PSU or amp switching noise from getting back into the AC cable. I'm not going to say that for certain at this point, but it seems likely. As I said before, I was hearing the noise when the mic and power cables were parallel - they were side by side. Moving one of the mic cables away from the AC cable feeding the SRX mitigated the noise.


It just occurred to me that I heard this phenomenon this summer on an outdoor stage with these wedges. Totally different environment than my regular venue. I had forgotten till just now. So it's not lighting controllers or the like.


Greg
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Greg_Cameron

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Re: Current model JBL SRX boxes induce buzz in mic cables/channels
« Reply #7 on: September 12, 2018, 01:52:00 am »

I only have a pair of the SRX812P boxes, but have *never* had any problem like you describe in over 3 years of owning them.


Ray, do you ever use them as wedges or just as mains? When I have them on poles, I don't have this issue. Only when on the ground and the power cables are mingling with the vocal mic cables.


Greg
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Eric Snodgrass

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Re: Current model JBL SRX boxes induce buzz in mic cables/channels
« Reply #8 on: September 12, 2018, 04:32:59 pm »

Do a test with microphone cables from different manufacturers.  See if it can be reproduced. 
Is there a snake involved in this set up or just direct connections to a console? 
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Riley Casey

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Re: Current model JBL SRX boxes induce buzz in mic cables/channels
« Reply #9 on: September 12, 2018, 08:45:16 pm »

Yea, if I'd read you're whole post I'd have got that wouldn't I?  Any magnetic field around the cable is going to be proportional to the current flowing thru it thus the buzz going away when the boxes are unplugged.  Modern high efficiency amps have semiconductors connected directly to the incoming AC chopping up the waveforms.  An AC power isolation transformer might help, might not but a quick rental would answer that.


I don't see how. The noise is being induced into the mic cables. Mic's can't have a pin 1 problem since there's no secondary ground connection to form a loop. And as stated previously, the SRX monitors can induce this noise into the mic channels without the any signal cable plugged into them.
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