[RFI] Low pass filter opinions

[Jim Brown]

On 1/12/2011 8:26 PM, Dale Svetanoff wrote:
> The trick here is twofold: 1, get Pin 1 tied back to chassis ground via the
> shortest possible route; 2, apply common mode chokes (clamp-on beads, or
> similar devices) to eliminate or reduce the currents of unwanted signals
> flowing on the shield.  Some XLR connectors have the option to tie Pin 1 to
> the connector shell, a process that may help to reduce the coupling of
> external RFI into the equipment.  However, as I have found recently when
> doing some audio work on PA systems, not all XLR connectors have conductive
> shells.

In pro audio (including broadcasting, news gathering, sound 
reinforcement, concert sound, and studio work, there are VERY important 
reasons why cable shields must NEVER have a DC connection to the shells 
of CABLE-MOUNTED connectors.  Rather, the connection between pin 1 and 
the shell and the shielding enclosure MUST be made at the connector 
within the equipment. Both Switchcraft and Neutrik, the major 
manufacturers of XL-type connectors, made versions of their connectors 
designed to accomplish this. AES Standards AES48, AES54-1, AES54-2, and 
AES54-3 spell this out in considerable detail.  I led the writing group, 
and am vice-chair of the Working Group, that produced these Standards. 
The Working Group included representatives of BBC, ABC-TV, many major 
manufacturers, several of whom are hams.

As part of our work on those Standards, I proposed, AES54-1 recommends, 
and Neutrik eventually developed and manufactured, a new family of 
cable-mounted XL-connectors that makes a capacitive connection between 
the shield and the shell, and a DC connection of the shield to pin 1 
through a ferrite bead. These Standards can be downloaded (for a small 
fee) from the AES website, and much of the rationale behind them is 
discussed in various tutorials on my website.

73, Jim Brown K9YC