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

[e-mail frank.wood95]

Thank you for posting a link to a very informative paper. It contains
much good advice.

When our theatre designed a new sound system, many years ago, we had
to consider these problems. When I say designed, I mean right down to
component level. We were an amateur theatre, with limited funds, but,
being in West London, drew heavily on the BBC for technical staff,
and, occasionally, for resources. I was one of the design team.

We should have liked to follow BBC practice, but proper output
transformers are very costly if you want a fully balanced system, so
we had to compromise. We declared a policy that ALL inputs were
transformer-balanced, but that outputs were not, although they were
wired as though they were. This worked for us, even in the high
electrical noise environment of our theatre, and was only replaced
because there were too few channels.

On 12 March 2015 at 13:11, Steve Payne <mixedupsteve at gmail.com> wrote:
> http://www.rane.com/note151.html
>
> On Wed, Mar 11, 2015 at 3:24 PM, e-mail frank.wood95 via Stagecraft
> <stagecraft at theatrical.net> wrote:
>>
>> In my world, there are two different ground connections. One is the
>> safety ground, which comes with the mains supply from the sub-station
>> where it meets the neutral. It is connected to all exposed metalwork.
>>
>> The other is the technical ground, which is usually derived from a
>> substantial ground plate, sometimes buried under the car park. This is
>> used for the cable screens of audio equipment when balanced wiring is
>> used. If the wiring is unbalanced, it may form a part of the signal
>> path, although this is not desirable.
>>
>> It is a firm rule that NEVER the two shall meet. When doing big
>> installations, it was often the custom to connect a battery and buzzer
>> brtween the two, with the incoming technical ground disconnected. If
>> the buzzer went off, the standing instruction to the wiremen was to
>> stop, and to disconnect their work in reverse order until it stopped.
>> Then call an engineer to locate the fault.
>>
>> I remember one TV studio installation where there was a heavy hum
>> signal on the technical ground, over the whole site. Serious experts
>> were called in, who found 7A of ground fault current. By working
>> nights, disconnecting whole areas one at a time, this was traced to
>> the switching and routing centre. Detailed investigation revealed that
>> the switch panels, supplied by an outside contractor, had not used the
>> specified switches, and they all had to be re-made. After that, the
>> problem went away.
>>
>> You can see that this eliminates all the problems at the design and
>> build stages. There was also a policy that cable screens picked up the
>> technical ground from the input of the gear they were connected to.
>> Strictly applied, this eliminates hum loops.
>>
>> On some varieties of XLR connector, the shell securing screw actually
>> drove into pin 1. This is bad news, as the shell inevitably picks up
>> the safety ground from the connector panel, so linking the technical
>> and safety grounds. My advice is to check all your XLR cables for this
>> problem, and to re-terminate those that have the shell connected to
>> pin 1.
>>
>> These general policies were formulated by the BBC, which was learning
>> how to do this job back in the 1920s. They have proved sound ever
>> since.
>>
>> On 11 March 2015 at 02:49, Jon Ares via Stagecraft
>> <stagecraft at theatrical.net> wrote:
>> > On Tue, Mar 10, 2015 at 7:31 PM, tech dept <tech.dept at inbox.com> wrote:
>> >> It sounds like you bought the Beh------ unit. Well, I did and it is
>> >> holding up well.
>> >
>> > <sheepishly>  I did.  (The Beh----- cable tester, not the POS poopy,
>> > buzzy, fuzzy 4-channel mixer that started this whole mess.)  The cable
>> > tester seems to work fine.... and I like the signal generator function
>> > as well.
>> >
>> > --
>> > Jon Ares
>> > www.arescreative.com
>> > Follow my twits @jon_ares
>> >
>> > ____________________________________________________________
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>>
>>
>>
>> --
>> Frank Wood
>>
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>
>



-- 
Frank Wood