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
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