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22nd February 2008   #34
Testing123

Joined: Nov 2007
Location: CT, USA
Posts: 69

Quote:
 Originally Posted by huub With long cable runs ( 300 meters or more) something is lost; on a Qbox the fantom power lights become less bright, and some mics, that are more critical to phantom power, stop working, or become problematic..
I'd be curious 300 meters of what? I grabbed the spec for a typical snake cable (Belden 1514C - 16 shielded pairs) and it states 23 ohms per 1000 feet for the conductors. The shields are only 16 ohms per 1000 feet. Since both conductors are used to pass the phantom power, the effective conductor resistance is half of the resistance of one. That gives us a loop resistance of (23 * 0.5) + 16 = 27.5 ohms for 1000 feet.

The spec that gives the paramaters for phantom power is IEC 61938, and it specs a max of 10mA current draw for a phantom powered device (which most mics are well under). So at the max current draw we'd have a 27.5 * 0.01 = 0.275 volt drop. Our phantom powered device would see 48 - 0.275 = 47.725 volts. Doesn't seem like that should make much difference.

As for the QBox, I'd wonder if they are exceeding (maybe by a lot) the 10mA current draw to light the LEDs and do whatever else it is they do in that box.