[Elecraft] K3/K3S Pin 1 question

[Jim Brown]

On Mon,9/14/2015 10:12 AM, Dan Atchison via Elecraft wrote:
> I am building a switchbox to allow use of a Heil PR-781 microphone to 
> be used by my K3S and K3, the switchbox using XLR connectors.  I note 
> that in the Heil XLR cable ended in an 8-Pin Foster has mic low (-) 
> tied to ground and (+) tied to mic high.
>
> Knowing that the XLR pin 1 should be chassis ground, my question is, 
> "At the switchbox, should I tie XLR pins 1 and 2 together and only 
> switch XLR pin 3 between radios?"  IOW, should XLR pins 1 and 2 be 
> chassis grounded or should I chassis ground pin 1 and switch both XLR 
> pins 2 and 3?

The answer is that it depends on the kind of cable used -- single 
conductor plus shield with mic low on the shield, or twisted pair plus 
shield, with the mic low not connected to the shield.

If twisted pair plus shield, carry mic high and mic low to their 
designated pins on rig, and connect the cable shield to the shielding 
enclosure (the chassis, in a K3 or K3S). For any shielded twisted pair 
going to an unbalanced input, the connection between one side of the 
audio and the chassis should be at the radio input. This allows audio to 
stay on the twisted pair, which inherently rejects noise by virtue of 
its twisting.

If a single conductor plus shield, connect mic high in the cable to mic 
high in the radio, and the cable shield to the chassis. In your plan, 
you would want to switch everything but the shields.

NEVER connect a cable shield to a return pin unless that pin has a 
direct short to the chassis at the connector. To do so is, by 
definition, a Pin One Problem, and a major cause of hum, buzz, and RFI.

73, Jim K9YC