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

[e-mail frank.wood95]

I agree with Andy.

The important principle is that there are two different grounds. There
is the safety ground, which dictates that all exposed metalworl must
be connected to it. Certified double-insulated gear is exempt from
this provision.

And then there is the audio signal ground. The two must, ultimately,
meet, but only under strictly controlled conditions. The Rane Note is
a very clear explanation of this, and worth your study.

Unfortunately, some XLR connectors automaticallyl connect pin 1 to the
shell. This is unequivocally bad news, and few, if any, cable testers
will show this up. Only measurements or visual inspection will reveal
this fault. You probably can't do this witha visiting company's gear,
but it is the first place that you should look at for hum problems.

It has been said many times, but will bear saying again. NEVRR, NEVER,
NEVER, LIFT THE SAFETY GROUND CONNECTION. It may provide you with a
temporary solution.  It may also provide you with a dead performer.

On 14 March 2015 at 17:01, Andy Lang via Stagecraft
<stagecraft at theatrical.net> wrote:
> On Tue, Mar 10, 2015 at 8:51 PM, Dale Farmer via Stagecraft
> <stagecraft at theatrical.net> wrote:
>> 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.
>
> I'm going to have to respectfully beg to differ with Dale here. For
> the longer version, see the Rane Note that Steve Payne linked to, it
> does a great job of explaining things. For a slightly shorter
> version...
>
> As far as cables go as a general rule, NEVER tie pin 1 to the shell
> for a general purpose cable. The ONLY time it should be done is to
> solve a specific problem, and in many of those cases, the shield
> should then ONLY connect to the shell, and not to pin 1 at all. And,
> in either case, it should be very, very clearly and permanently
> labeled as such. And, ideally, you should use one of those fancy
> Neutrik connectors with the capacitor and choke in it to make the bond
> so that it only happens at RF frequencies, and not audio frequencies.
>
> For detailed explanations, see that Rane Note, or search Google for
> writeups on the "Pin 1 Problem" by the late great Neil Muncy or Bill
> Whitlock, but for the capsule version, a properly designed piece of
> audio gear should have pin 1 connected to the chassis of the gear AT
> THE INPUT. The idea is that the shield of the cable and the chassis of
> the enclosure connect together to form the RF shield and prevent noise
> from getting into the device.
>
> Many pieces of gear are instead designed so that pin 1 goes to the PCB
> ground, and then, via the long and winding path therein, to the
> chassis. This is bad, and is called the Pin 1 Problem. It means that
> any noise injected into the shell gets into the circuit board, and
> then into the audio path. In these devices, it can sometimes be fixed
> by connecting the cable shield to the shell instead of pin 1, but not
> always.
>
> But connecting pin 1 and the shell as a matter of course provides all
> sorts of opportunity for induced noise to get into the circuit, and is
> generally a bad thing on any circuit. On unbalanced circuits (say, if
> your cable gets used with a  ClearCom pack), it's a near guarantee
> that you'll get buzz in the the com, and spend far too many hours
> trying to track down the responsible cable. When on tour, we once had
> to put an entire local audio crew plus our touring crew on a delayed
> dinner break on load-in trying to troubleshoot a buzz in the spot com
> line that made it unusable, only to eventually track it down to a
> single Y cable with the shell and pin 1 tied together. One. Damned.
> Adapter.
>
> -Andy
>
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-- 
Frank Wood