Chris Holmes wrote:
snip
We have a microphone (XLR) multicore running to the front of the
church, and also a coax line (future proofing for use with video?).

If I made up a suitable cable to feed the 12V down one or the other of
these;

a. Would it work?

b. Would the cable go bang?

c. Would it b*gger up the sound system?

Ignoring all the OT posts about Dr. Who....

Given that a multi-core system will deliver 48v "Phantom Power" to
microphones that need such a thing (e.g. powered dynamic condenser mcis),
then passing 12v though ought to be fine... The only thing I am not sure
about is the ampage, and whether this would be a problem.

I would suggest researching a bit on the phantom power bit - which of the 3
wires in the XLR takes the feed, and what the usual ampage is.

after some very quick research..
phantom power is usually pretty low - up to 60mA. 2A sounds rather high
Take a look at http://www.shure.com/support/technot...-phantom1.html or
http://www.sounddevices.com/phantom.htm

The idea on using the multi-core to flick a relay is a good one. Maybe the
coax one is OK too, but you'll need to check what the cable can handle.

HTH,
AlexL