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Author Topic: Grounding/Shielding  (Read 5149 times)

Phillip Graham

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Re: Grounding/Shielding
« Reply #15 on: May 26, 2004, 08:45:24 pm »

weihfool wrote on Wed, 26 May 2004 11:47

Hey Josh,
Thanks for being part of this thread. Very, very informative. I have a question and if this is something you were going to tackle later on, then I'm sorry for jumping the gun. I'm experiencing the electric interference you spoke about. There is a radio tower a few miles from where I'm setup and the interference isn't constant. It comes and goes. I don't have it at all with dynamic mics. It's only there with condensers (as you mentioned earlier). Is there ANYTHING I can do to combat this, barring moving my studio's location? It seems as if the radio transmitter is cranking up the power only at certain times of the week. Thanks again.

-Sean


Sean,

This is covered nicely in a Synergetic Audio Concepts Tech Topic.  There is an online version of that tech topic hosted on Prosoundweb, as a part of the excellent "Study Hall:"

http://www.prosoundweb.com/install/synaudcon/tt25_2.php

And, yes, radio stations are constrained to different power outputs at different times (generally lower at night).
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Phillip Graham

weihfool

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Re: Grounding/Shielding
« Reply #16 on: June 08, 2004, 03:46:10 pm »

Hey Josh and Phil,
   THANKS!!!  I very much appreciate the assistance.

Josh, to answer your request for more info, I've done a bit more investigating.  I get the noise from all of my condenser mics (qty:2, BPM CR-73 LDC, qty:2, AKG C451E).  

I've noticed the RF gets worse when I use 4 conductor twisted pair wire over standard 2 conductor+shield.  

I've also noticed that the noise is present in the console when the 48V is turned on with NOTHING connected to the board.  

However it is CONSIDERABLY louder when the mics are connected to those inputs.  

With nothing connected, I have to pretty much pin the mic gains to hear anything, which I figured might be something that can't be eradicated.

I've tried placing the mics in different parts of the building (the place is about 3000sqft.), varying proximities from the console, etc.

Let me know if you need any other info.

Phil, I'm reading that article you linked me to as soon as I post this.

Thanks to the both of you again!!!

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ebeam

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Re: Grounding/Shielding
« Reply #17 on: June 10, 2004, 04:45:31 pm »

gtphill wrote on Thu, 27 May 2004 01:41



Part of your problem is that you have unbalanced your setup by the use of T/S cabling.


Yes, I realize this and it was intentional.  The Lynx outputs can be switched +4/-10 and connected using the cabling I described.  The reason I did this is that to maintain balanced signal through my passive volume control would require a 4 gang pot and I'm using short cable runs anyhow.

Quote:


The extra cable capacitance of star quad is not worth it for your unbalanced interconnects.


OK, but its what I already have from making mic cables and such and its really not much more expensive.  There's not really any harm is there?

Quote:


If the shield is left unterminated, is can be possible to create an internal amplifier feedback loop, where very high frequencies on the speaker output cables couple back into the signal inputs, resulting in ultrasonic oscillations.  It is rare, but known to exist.  Connecting the shield provides a shunt for most of this.


Interesting...I'm planning on wiring it with the shield connected.

Quote:


Start by reading Rane Note 151:
http://www.rane.com/note151.html

Then read Rane note 110:
http://www.rane.com/note110.html

Then properly balance your setup utilizing 1:1 isolation transformers and proper cabling.

Then go to AES.org and download Preprint 5747, "Common-Mode to Differential-Mode Conversion in Shielded Twisted-pair Cables (Shield-Current-Induced-Noise)"

Finally, build the device in the posted figure and test all of you equipment to see if it is Pin 1 compliant.



Good reading, I already read 110 and I'm printing 151.  I think I'm going to stay unbalanced for the time being until I have a lot of time and more money to invest.  I still have quite a bit of unbalanced gear and my patchbay is unbalanced...

Thanks for your help.
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Ben Myers
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