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-   -   Help a newbie with phantom power (https://www.dvinfo.net/forum/canon-xl-gl-series-dv-camcorders/51901-help-newbie-phantom-power.html)

Andrew C. Stewart September 28th, 2005 12:29 PM

Help a newbie with phantom power
 
Can the XLR connections on the XL2 be used to power something other than audio devices. Like say a small lcd monitor? Noticed that most 6-7inch lcd monitors require 12 v DC and consume less than 6w. Could this be done???? Or am I a complete idiot for asking?

Jeremy Davidson September 28th, 2005 12:43 PM

Not to worry, you are not a complete idiot.

Unfortunately, however, I don't believe it can be done. If I remember correctly, phantom power is typically 48v at only a few milliamps, so it would not be able to actually power such a device as it was never intended to run more than a tiny mic capsule and preamp. At 12 volts and 6 watts the current draw would be 500ma. Your best option for the LCD is probably to use a separate battery pack.

'Hope this helps, and keep asking questions! We tend to learn things faster that way.

William Putnam September 28th, 2005 04:36 PM

Quote:

Originally Posted by Jeremy Davidson
Not to worry, you are not a complete idiot.

Unfortunately, however, I don't believe it can be done. If I remember correctly, phantom power is typically 48v at only a few milliamps, so it would not be able to actually power such a device as it was never intended to run more than a tiny mic capsule and preamp. At 12 volts and 6 watts the current draw would be 500ma. Your best option for the LCD is probably to use a separate battery pack.

'Hope this helps, and keep asking questions! We tend to learn things faster that way.

You are absolutely right!

Phantom power is applied, usually, across both lines of a balanced microphone input via well matched 10 to 12 Kohm resistors, such that it is perfectly balanced in regards to the two active audio lines. You can better visualize this if indeed, the microphone input was applied to an input transformer primary winding, where the 48 volts phantom power applied to a center tap on the winding - this power thus is of the same polarity and voltage going back to the microphone in perfect balance. (This method harks back to the days of vacuum tube amplifiers.) This method is seldom used today since most microphone amplifiers have eschewed the input transformer as such transformers are quite expensive and heavy. High quality solid state opamps do this job quite nicely today. Even so, when a transformer is used with a center tap application of phantom power, a rather high value resistor is used at the center tap so that if one side of the microphone line happens to get shorted to ground, little harm is done, as little current can thus flow in the short circuit.

In the microphone, similar resistors are then used to render this very low power to power the tiny amplifier needed to amplify the very tiny signal produced by the condenser head in the microphone. Older microphones may also use a transformer with a center tap to then recover the phantom power to power the condenser mics amplifier. Other older condenser microphones had a separate power supply, rather then the phantom power set-up, which applied upwards to 300 volts to the condenser head! Some of the older German Telefunken condenser mics were this way. But I ramble here, sorry. :)

In any case, attempting to use the 48V phantom power for other uses that require greater current would be out of the question as you say.

Look at me, this old retired electronics technician reveals his age here!

William Putnam


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