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[A Man]

As some of you know, I've made the jump from FOH Noise Boy to Ear Boy for a major artist. We're carrying a little Crest XRM mixer to do the ears and use the supplied 'house' console(s) for the wedge mixes on a spate of club/theatre runs before the new album release in January.
The XRM is set up like this:
A 12 channel W-1 subsnake to panel mount handles the upstage inputs and a little 6 channel subsnake to fanout handles the downstage, it's split out of the console pass-thoughs via a 20 pair Whirlwind XLR to XLR fanout (which goes to the house splitter)

on a recent gig,I noticed that I could hear the tech touching the cables on the whirlwind fanout when he was patching other cables into the house splitter. We traced it down to one input (ch. 18) and tried a number of 'fixes' like pin 1 lifting @ the XRM, pin 1 lifting @ the house split, reversing the FOH/MON snake heads @ the splitter, none of them worked..
On a lark, techie pulled the input out of the house monitor console ( a old SM20 this day) and it went away, we stuck it in another open channel on the console and did the show sans problems.

Did 3-4 more gigs without the mystery gremlin and then on the last show of this Canadian run,  it reared it's ugly head again, this time on numerous channels.

I happened to be listening to my 'Canadian Bought iCom Scanner' ( a whole new thread on that soon) as I was patching the house split and noticed the more I plugged in, the louder it got...
(ie; when I scratched the 'armor' on the Whirlwind cables with my fingernail, I could hear it on the scanner, which was set on one of the IEM frequencies) So, thinking back to the previous 'fix', I undid the Iso multi out of the splitter, still there...pin 1 lifted, still there..reversed the snakeheads...nota, still there..Undid the W1 and pulled the downstage subsnake out of the XRM...still there.
I THEN pulled EVERYTHING out of the XRM and started plugging in the Whirlwind on the XRM split side one channel at a time (it wasn't plugged into the house split on the other end)
The gremlin was still there. I grabbed a couple of regular XLR mic cables and plugged it in to see if it was the Whirlwind.
Still there....  

Called Meridian MS. and talked to a tech who was as baffled as I so I now come to the "LAB Brain Trust" seeking help.

What could cause this and why?

TIA.



 

[Marc Schwartz]

Triple J,

I am not sure, but I suspect that the pass through mic spilt on the XRM is making the signal path vulnerable to grounding issues. I have heard about others having the same problem.  The signal path for the mic inputs is pretty long when they have to pass through the ear mixer, then to house spilt, and then to house monitor and FOH consoles. There is a lot of potential for problems with that much copper involved. You are on the right track trying to isolate it and lift suspect grounds on the channels, but the problem is in knowing if all the snake channels and splits actually have continuity pin to pin on every channel, especially when dealing with someone else's stuff. It would take a lot of time to test all the lines, more time than you are likely to have during set-up for a one-off during a national tour anyway. Another problem could be that the ground potential where you have your ear mixer power plugged in is not the same as at FOH. It could certainly be a venue power issue. I would bet the problem is in the venue stuff. I doubt if it has anything to do with your gear.

-Marc

[John Roberts {JR}]

Jon Martin wrote on Sat, 01 October 2005 23:29

As some of you know, I've made the jump from FOH Noise Boy to Ear Boy for a major artist. We're carrying a little Crest XRM mixer to do the ears and use the supplied 'house' console(s) for the wedge mixes on a spate of club/theatre runs before the new album release in January. The XRM is set up like this: A 12 channel W-1 subsnake to panel mount handles the upstage inputs and a little 6 channel subsnake to fanout handles the downstage, it's split out of the console pass-thoughs via a 20 pair Whirlwind XLR to XLR fanout (which goes to the house splitter) on a recent gig,I noticed that I could hear the tech touching the cables on the whirlwind fanout when he was patching other cables into the house splitter. We traced it down to one input (ch. 18) and tried a number of 'fixes' like pin 1 lifting @ the XRM, pin 1 lifting @ the house split, reversing the FOH/MON snake heads @ the splitter, none of them worked.. On a lark, techie pulled the input out of the house monitor console ( a old SM20 this day) and it went away, we stuck it in another open channel on the console and did the show sans problems. Did 3-4 more gigs without the mystery gremlin and then on the last show of this Canadian run,  it reared it's ugly head again, this time on numerous channels. I happened to be listening to my 'Canadian Bought iCom Scanner' ( a whole new thread on that soon) as I was patching the house split and noticed the more I plugged in, the louder it got... (ie; when I scratched the 'armor' on the Whirlwind cables with my fingernail, I could hear it on the scanner, which was set on one of the IEM frequencies) So, thinking back to the previous 'fix', I undid the Iso multi out of the splitter, still there...pin 1 lifted, still there..reversed the snakeheads...nota, still there..Undid the W1 and pulled the downstage subsnake out of the XRM...still there. I THEN pulled EVERYTHING out of the XRM and started plugging in the Whirlwind on the XRM split side one channel at a time (it wasn't plugged into the house split on the other end) The gremlin was still there. I grabbed a couple of regular XLR mic cables and plugged it in to see if it was the Whirlwind. Still there....   Called Meridian MS. and talked to a tech who was as baffled as I so I now come to the "LAB Brain Trust" seeking help. What could cause this and why? TIA.

I have seen micro-phonics from all kinds of passive components including wire. It seems capacitors are the most common suspect, and most wire micro-phonics occur in combination with phantom power.

While I haven't done a rigorous analysis I suspect when current is flowing through the wire's distributed resistance any change in this resistance will cause a modulation of the resultant voltage.  Stranded wire may make and break conduction between sundry adjacent strands in response to vibration. While this voltage may be sub micro-volt, at the input of a sensitive mic preamp it could be audible.

I would investigate the mic cable, i.e. type, gauge, length, wrap, etc, and phantom power interactions such as plugging into another console does provide a small but real drain on phantom power, causing current to flow.

There may also be changing capacitance effects and even "tribo-something-or-other" (similar to piezo) effects in wire insulation, where just bending the wire creates a voltage, but this is more obscure and less likely.

JR  

[Michael 'Bink' Knowles]

The first half of your description makes me think the old SM20 had an pin 1 problem on the one channel. Something like shield termination wierdness or an RF-blocking cap losing its spec. You fixed it easily for your gig though the house guy ideally would take the console out of service and have it brought up to spec by a skilled tech.

The second half of your post where you are using the handheld RF scanner makes me think of the old vaudeville joke where the patient says "Doc, it hurts when I do this" and the reply is "Stop doing that!" I think the RF scanner experimental results can be safely ignored since they aren't being heard in the IEM or PA.

[Eric Snodgrass]

I'm with Marc on this one.  I think there was some ground potential difference between you and FOH that was causing it.  I've seen similar things like this happen at our venue where a monitor rig was brought in and hooked up to the house split on an ISO transformered output.  As we plugged in more XLRs to the monitor console a noise started to occur at FOH an got worse with each XLR plugged in.  The noise lessened as we pulled out each XLR.  After pulling out my hair for hours on this one (FOH and monitor land were on the same power) we came to the conclusion that there was some grounding issue with the console itself (wihich wasn't a hard conclusion to have with a beat-up Yamaha M3000).  
While it wouldn't hurt to replace your W1, I think you experienced a similar thing.  There was just some ugly mismatch between systems.  

[Jon Halverson]

Jon Martin wrote on Sat, 01 October 2005 23:29

I happened to be listening to my 'Canadian Bought iCom Scanner' ( a whole new thread on that soon) as I was patching the house split and noticed the more I plugged in, the louder it got... (ie; when I scratched the 'armor' on the Whirlwind cables with my fingernail, I could hear it on the scanner, which was set on one of the IEM frequencies)

 Was the IEM transmitter turned on?  That is, was it audio noise being transmitted from the IEM, or was is RF noise you were listening to?

There might be a valuable clue here.

jon

 

[Baron Gray]

I'm going with JR's theory.

I have experienced a similar thing with a snake that had come into contact with some quantity of water (sprinkler system due to flaming curtains).

We were testing everything after the big dry out when we noticed we could here footsteps, on the stage, through the system. This would have been expected if there had been a microphone connected to anything but the drop boxes were bare.
The snake was extremely microphonic and it actually sounded like it would have made a half decent kick mic. How do you get 20 yards of snake into that little hole?

Baron

[Matthew Knischewsky]

cables with a foil sheild are vulnerable to becoming microphonic. the foil may develop a small amount of resistance between itself and the ground wire over time, or with heavey use. add a bit of moisture and you've gone microphonic.

i had this happen to me with a poorly built sub snake. it could be heard by tapping the head, but only on certain channels. I opened up the head, and trimmed away a bunch of excess foil, and the problem was gone.

perhaps your problem could be something like this?

[Tim Padrick]

How old is the W1?  Perhaps its pinblock has the "getting conductive as it ages" syndrome that older W4s have.

[Al Keltz - Whirlwind]

Question, what is plugged into the other end of the cable(s)? We get a lot of tech calls from people that have a snake plugged into their mixer with nothing attached to the other end. Then when they open up the "empty" channel, they hear all sorts of weird things like radio stations, buzz or microphonics. As soon as they plug in a mic - it goes away.

This is because an unterminated cable possesses a very a high impedance characteristic and will act like an antenna or appear to be microphonic. So the unterminated channels need to be kept muted.

We had a situation a while ago where an outdoor venue had a two-way hardwired split - one leg to FOH and the other 500' to a recording truck that wasn't always there. Problem was that when the recording truck wasn't there, there was an unterminated 500' snake laying underground and still connected to the system. This was causing RF interference. The only fix - either add disconnects to get rid of the unused cable or make termination plugs with 150 Ohm resistors for EACH connector on the open end (and remember to plug them in!) NOT what they wanted to hear.

- Al

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