TWA and microphone input polarity on ELK-M1WS

[Jozza]

Hi All,
 
I was playing around with a TWA and speaker/microphone M1WS connections on the weekend. What I noticed was that if you had the microphone inputs hooked up one way, I would get a buzz on the phone line whenever I used telephone remote control of the M1 panel. If I reversed the microphone inputs, the buzz went away. Each input only had a single M1WS connected
 
Has anyone else noticed this?
 
At any rate, if you're getting a buzz / hum on the phone when using telephone remote control and you're using a TWA (or I assume a TWI as well) this could be your issue.
 
Jozza

 

[RAL]

Did you use shielded, twisted pair wire to connect the microphone to the TWA?

 

[Jozza]

I have shielded cable installed, though the cable is RG6 quad shielded, not twisted pair. For some reason when I had three of these runs put in for the microphones it was in RG6Q. However, as I stated, as long as its connected the "right" way, it works perfectly with no noise on the line at all. Very clear reception from the microphones - a little too clear sometimes! 

 

[RAL]

It's best to use twisted pair for a microphone like this, as the signal level is much lower than you'd have for a video signal. Without the twisted pair, it's much more susceptible to picking up hum, as you've experienced.  At least you were able to eliminate the hum by reversing the leads.  Sometimes, you get lucky.   

 

[Del91574]

Coax is a far different cable than UTP or STP.
 
If it works, you got lucky. If it works for a while, you got very lucky.

 

[Jozza]

Yep, unfortunately when I did all the wiring runs these three were the last ones and I just got completely mixed up on the page regarding the type of cable to use. At first I was going to just forget about it since they shouldn't be used anyway. However, its working perfectly now - so yes - I must be lucky!
 
Does anyone have any idea technically why reversing the connection (on RG6 quad shielded cable) was able to remove the hum for this?
 
Jozza

 

[Del91574]

More than likely the braid picking up hum vs. a ground connection.