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Author Topic: Partyline buzz, line impedance, termination  (Read 51 times)

Diogo Nunes Pereira

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Partyline buzz, line impedance, termination
« on: Today at 04:28:08 AM »

Hi.

I've been chasing an intermittent buzz in a 2-wire comms line for 3 days now. Main station is a FreeSpeak 2 Base II. Comms are pretty clean most of the day: only a very tiny buzz in the background, but totally acceptable by my A1, imperceptible by the rest of the crew. Strangely enough, there's a timeframe where the buzz gets louder (around 8am to 9 am). Loud enough to bug me (and the A1) at least.

I've changed cables, Y's and packs, and haven't been able to fully understand the nature of the problem. I've also been checking all my Y's and cables for Pin1/Chassis continuity.

I'm attaching a comms diagram. I've measured the 3 lines coming out from the FS2 Base and read 4000 ohms on lines A and B; 2000 ohms in line C, symptom of a double terminated line, but there's no other power supplies in the system, just the FS2BaseII. Removing line A from the system cleans the channel, so I changed runs to this area a couple of times - I measured a lot of RF noise (with my RF Explorer) coming from the CCU rack - and it helped reduced the small noise to tiny noise. Still, at 8 am it gets louder...

FreeSpeak 2-wire Ports A and B are in RTS mode to feed ClearCom dual channel stations. 2-wire Port C is in ClearCom mode to feed single channel stations. Termination is enabled in all 3 ports.

I've been reading old posts here and elsewhere, but still decided to start a new topic on the subject of intercom buzz, line impedance and termination, to get ideas and suggestions from your brains.

Old topics and other links:

https://forums.prosoundweb.com/index.php?topic=75330.0
https://forums.prosoundweb.com/index.php/topic,156166.10.html
https://www.gravitymedia.com/assets/Uploads/product-downloads/A-Comprehensive-Guide-to-CC-Analog-Digital-Partyline-Systems.pdf

Andy Leviss has an good article on the topic; I found his website to be down, but still available in the internet archive:

https://web.archive.org/web/20080606004048/http://www.onefromtheroad.com/index.php?p=29
« Last Edit: Today at 04:36:19 AM by Diogo Nunes Pereira »
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Diogo Nunes Pereira
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Brian Jojade

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Re: Partyline buzz, line impedance, termination
« Reply #1 on: Today at 02:10:01 PM »

If it's time specific, chances are there is something in the building that kicks in at a certain time that's generating significant noise.  If you know when it's happening, maybe you can move stuff around during that time and see if you can pinpoint a direction it is coming from.  Removing the source of noise, if possible, can often be more effective than trying to shield it from getting into your system.

As far as 'replacing cables', what kind of cable are you using?  Different types of cables may make a difference in noise rejection, depending on where the noise is entering the system.
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Mac Kerr

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Re: Partyline buzz, line impedance, termination
« Reply #2 on: Today at 02:34:25 PM »

If it's time specific, chances are there is something in the building that kicks in at a certain time that's generating significant noise.  If you know when it's happening, maybe you can move stuff around during that time and see if you can pinpoint a direction it is coming from.  Removing the source of noise, if possible, can often be more effective than trying to shield it from getting into your system.

As far as 'replacing cables', what kind of cable are you using?  Different types of cables may make a difference in noise rejection, depending on where the noise is entering the system.

Yes, try to identify any equipment that cycles on when you are having the issue. While there is a small possibility that something like StrQuad might help, it is generally not recommended for use in 2W PL which is unbalanced by nature, and very impedance sensitive as well. StarQuad type cables have significantly higher capacitance than plain vanilla mic cables.

I assume you have removed everything from the system one item at a time to discover what area the noise may be coming from.

Mac
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Re: Partyline buzz, line impedance, termination
« Reply #2 on: Today at 02:34:25 PM »


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