Help me catch a ghost- SV in our venue's PA?

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jasonallenh

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Hi everyone,

Since we're experiencing extended downtime due to the coronavirus outbreak (in Florida,  :eek:  ) we're trying to squash a noise issue that has baffled us for quite some time. Let's see who in the lab can win this game......

Our venue's PA system has a constant, high-frequency, and rhythmic noise with a consistent, repeating pattern. We have a GLD-80 with the AR24 box, both on their own dedicated breaker on a subpanel. We started by only turning on the console and ar24 on this subpanel, with no other parts of the PA system on. There are no lights on this subpanel- it *only* powers the PA, and the only parts of the PA that are powered on are the console and its i/o box.

The noise itself starts around 5kHz and bounces around above that- up to (and probably beyond) the 20kHz point on the console's RTA. Disconnecting the i/o box does make it go away, however I don't believe it is to blame because....

...the noise we are hearing is a noise that we used to hear ambiently in the room with the entire PA off. The noise came from fire alarm strobes in the building- something I think may be a 'Supervisory Voltage' that monitors the power supply wiring for the fire alarm when it is in normal (not alarming) mode. The noise we heard is *identical* to what were hearing in the console now, and that sound was present in our building before we installed the current console. Because of this, we know that the noise is created by our fire alarm system and not within the PA itself.

With *nothing* plugged into the console inputs, the sound goes away, however, it returns with something as simple as a passive di plugged in via an xlr cable (but nothing connected to the DI input) or an xlr output of a wireless lav receiver that is disconnected from power completely. If I take a 6' xlr and plug it in directly to the i/o box and the output of a DI (once again, no input) then the sound goes away. The only difference is that I am not using the installed multichannel cabling from the stage to the control room.

My feeling is that the fire alarm noise is somehow being induced into our cabling between the stage and control room (our AR24 is in the control room and connected to the returns from the stage as opposed to being the actual snake...i know, i know). I want to figure out what is causing it. I am already fairly certain that just locating the AR24 on the stage would be an immediate solution, but I want to catch the ghost in our current system to better inform our decisions when we renovate.............. please chime in with your theories and opinions ;D

Happy hunting!
 

Gold

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My guess is the fire alarm wiring is not correct . Have you had an electrician who knows technical installation look at it? I’d think that would be money well spent. I’d want to measure EMI radiation.

Have you tightened all the connections at the breaker panel?
 

jasonallenh

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JohnRoberts said:
cell phone?

JR

Definitely not. The noise has a signature that we have long since confirmed is associated with the fire alarm strobes. It's like hearing a jingle you've known all your life. The strobes were out of sync for a long time (they are supposed to flash together- not independently) and before it was fixed, the sound would come from the strobes audibly. That has since been fixed but now it's in the audio line.
 

JohnRoberts

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Should I ASSume the fire alarm system is using RF?

Sounds like time to apply basic RF troubleshooting...

Mic preamp rectification is pretty well known issue.

Of course advice to check the fire alarm makes sense.

JR
 

jasonallenh

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Gold said:
My guess is the fire alarm wiring is not correct . Have you had an electrician who knows technical installation look at it? I’d think that would be money well spent. I’d want to measure EMI radiation.

Have you tightened all the connections at the breaker panel?

I have not touched the breaker panel, but we moved all the equipment on to a separate subpanel and the issue remains.  I went up into the attic/catwalk and found something new- there is a wire (looks like console cable,  or Mogami W2944) running from the abyss of our building into a 15' section of conduit that passes our stage mults to the control room. It then lands at an anonymous conduit box. Upon opening it up, I found a relay...which is connected to the external switch contacts on our power sequencer.

This is the fire alarm relay that shuts down the audio system when the alarm goes off.

I think you're on to something.....
 

jasonallenh

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JohnRoberts said:
Should I ASSume the fire alarm system is using RF?

Sounds like time to apply basic RF troubleshooting...

Mic preamp rectification is pretty well known issue.

Of course advice to check the fire alarm makes sense.

JR

See post #6. Does not appear to be RF based, as the wire for the alarm's audio system shut-off relay is *sharing a conduit* with my stage mults. FFS who does this???
 

Gold

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I’d look for arcing at the relay.  I’d assume the point where the mult happens is unshielded. That might be where the problem is.
 

jasonallenh

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Gold said:
I’d look for arcing at the relay.  I’d assume the point where the mult happens is unshielded. That might be where the problem is.

At least it *would* have been shielded if not for the fact that they stuffed it all into a conduit together  :mad:
 

jasonallenh

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It turns out that this relay specifically is a special fire-alarm relay that utilizes a 16-30V square wave to communicate with pull stations, strobes, etc throughout the building. Absolutely the culprit. I'm sure the alarm guy that installed this line through my audio conduit had no idea what he was doing, so I'm not going to be hard on him.  ::) ::)
 

Whoops

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jasonallenh said:
Absolutely the culprit. I'm sure the alarm guy that installed this line through my audio conduit had no idea what he was doing

Most of them don't know what they're doing,
you as a client are a checkmark in a paper for them, they install it anyway they can put the Check mark and move on,
had a similar situation with contractor electricians and fire alarm installation guys in a place my girlfriend opened.

I was amazed by how bad and ignorant most "Pros" were

 

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