Canon XLR wiring

[OneWorld]

I had a problem with a mixer - now fixed. But it was, as I see it, caused by a faulty XLR cable. I plugged the mic lead into my mixer and pop - big spike on the LED and then channel stopped working.

I took the plug on the XLR off and found two of the leads were shorting together - I deduced that one of the pins takes a -ve signal, one the +signal and the third the +48v from the phantom power, and that cable shorted across to the +ve signal and thats what blew my mixer channel +48v being fed down the signal path

However, and electrical engineer tells me that couldn't have been the problem, as the 48v from the phantom power is fed to the signal as well - surely that can't be the case? And since I got a new cable, plugged it into another channel and things have been fine since. Added to that to test the old cable I plugged it into a separate mixer and sure enough I got the same result - BIG spike on the input so I unpluuged it immediately, but the new cable in - no problem.

Is the electrical engineer correct insisting that the 48v is fed to the signal side as well - he's supposed to be an audio enginner?

[dmills]

P48 is fed onto both the signal conductors (pins 2 & 3) via 6.8K resistors, the return path is via the screen (pin one).

What may have happened is that if the mixer was designed by a numpty, then it is possible for a short from a signal pin to ground occuring after the phantom power has been established to cause the capacitor that exists to block the phantom power to discharge into the preamp input. Any competent design takes precautions to avoid this causing damage, but there is still some stuff around that does not take sufficient care.

HTH.

Regards, Dan,

[OneWorld]

It's a Behringer - nuff said LOL

But I still can't make it out - +48v gets fed to the signal path. OK an ordinary Jack plug has the

'hot' tip just carrying a signal,
the 'cold' sleeve

but an XLR/Canon has
'hot signal +48v'
'cold signal'
and +48v?

[OneWorld]

Hmmm and come to think of it - I plugged in into a much higher quality Trace Elliot powered mixer and although I got the loud spike - it didn't blow the channel like it did on the Behringer so maybe the Trace-Elliot did have the better design to prevent such damage as you mentioned regarding better/non numpty designs

[dmills]

Basically the signal is the difference between pins 2 and 3 which ride up 48V above pin one.

A Dynamic mic (which connects the capsule between pin 2 and 3 will thus not see any voltage, where a condenser mic can tap some power from both pins and use the screen as a return path.

It is generally a really bad plan to plug or unplug phantom powered circuits with the phantom switched on (and worse with the faders up!).

Regards, Dan.

[OneWorld]

Thanks for the info

[Hugh Robjohns]

It is generally a really bad plan to plug or unplug phantom powered circuits with the phantom switched on (and worse with the faders up!).

I agree about the bit regarding the faders, but not that it's a bad plan to hot plug on phantom powered circuits. The preamp or console should be designed to cope with this as it is very likely to occur in normal usage. Mics get disconnected, stage boxes get re-plugged. It happens. Some desks have a global phantom power switch so there is no option if you need to re-patch something while other channels are still in use. In many BBC studios phantom power is permanently on the wall box and there is no operational provision to switch it off before re-plugging.

Apart from a very few substandard products, I've never known any problems to the mic or input device from hot-plugging with phantom -- although I have been scared by the bang on the speakers on a few occasions when the operator failed to mute the channel first!

In general, and if practical, then I wouldn't power up phantom until all the plugging is done, and I'd turn phantom off before derigging. But I wouldn't lose any sleep over having to hot plug if that was necessary.

Hugh

[dmills]

The bang is the real reason to be careful hot plugging phantom, in a non trivial rig it can be tricky to verify that you really do have all the possible routes ending up at a speaker muted.

BTDT replaced the blown tweeters (3AM, forgot to must the monitor mix to stage)..

Regards, Dan.