[SML] XLR mic cables: Pin 1 to case/ground... yes? no?

[Dale Farmer]

On 3/10/2015 8:04 PM, Jon Ares via Stagecraft wrote:
> Hokay.... so I picked up a cable tester.... has your basic continuity
> testing of XLR, TRS, RCA, 1/8", etc (but no Speakon) -and also has
> test tone, and ability to show intermittent shorts.  So far, so good.
> I've tested about 20 cables so far, and two show "grounded shield"
> (pin 1 to shield, AND metal case) - and all the others do not. And
> indeed, I can see almost none of them have soldered across the case,
> and the pin one/shield. So the question is: do we not ground pin 1 to
> the case now?  The two cables that indeed are grounded in this fashion
> are pretty old-school - I can tell by the feel of the cable, and by
> the connectors (tiny screws, rather than screw-together connectors).
> The soldering is good inside these old cables, but indeed they jumped
> together Pin 1 and the case. Right or wrong?
>
The answer is, sometimes.    And it is a matter of tradeoffs. Generally, 
the higher end the system and users of the gear are, you will connect 
those shells. Mostly this is in recording studios. Expert audio folks 
will recognize the ground loop when it shows up and have the skills and 
tools to quickly solve the problem. Inexperienced folks will waste piles 
of time chasing the ground loops and often doing more damage and 
dangerous things such as breaking off the ground pins on power cords to 
'solve' the problem.

When you connect the metal shield of the connector to the pin one 
ground, you do close that gap in the RF shielding that is the mated 
connector.  But, you have also potentially connected that shield wire to 
whatever it happens to be laying on. If that thing happens to be 
energized, then the electrical potential there will flow through the 
microphone shield wire to the system ground at your mixer or whatever.  
Thus creating a ground loop that induces noise, and if your gear has the 
pin one design fault (still pretty common in lower end gear) then 
inducing loads of noise into your sound system, even if you have that 
channel muted.

     If you do not connect those connector shells to the cable shield 
connector, then every place you connect a pair of cables together to 
make a longer one, you are leaving a window open in your RF shielding on 
that mic cable.  This lets in a small but measurable amount of RF 
interference.   Not enough to care so much for most applications, but 
for high end recording situations and the like, it is enough to care 
about.   The connectors plugged into mixers or microphones will pick up 
a ground connection for the shell from the mixer or microphone, making 
the question moot.

   Since you are asking the question, you probably should not have them 
connected to the shells.   The worst situation is to have a mix of 
connected and unconnected cables, making troubleshooting really painful 
and time consuming.

--Dale