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
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