Phantom - Trying to design a microphone

[überfuzz]

I'm trying to get my head around phantom power supply. When I measure my sound card I get a 46 voltage drop over the two leads, zero everywhere else. How is this potential normally tapped out of the leads? Is the signal shielded by condensers and super imposed +V -V leads..?

Any insight on this would be appreciated!

[Jazz P Bass]

Without having to type it all, here is a good read.
Link: Phantom Power

[überfuzz]

By courtesy of wikipedia: Phantom_power ala wikipedia

[überfuzz]

I tried to design a circuit capable of harvesting the 48V phantom power of a normal 3pin XLR. Any help or pointers would be highly appreciated!


Edit, messed about with png-files.

[Enzo]

Here is an example:
http://www.naiant.com/images/MSH_Kit_Mic.GIF

Or google "phantom mic schematic" for various responses.

[überfuzz]

Thanks enzo but I had something different in mind. I think I got a working circuit as of now.

[Enzo]

Maybe I misunderstood. I thought you were trying to get the phantom out of the powered line. That is why I showed a mic circuit that extracts the voltage for use. Are you instead trying to insert power into the line? Your drawings appear the same and should work as a supply feeder. Or steal the almost identical circuits from any mixer with phantom.

[überfuzz]

It's tapping power from the 48V phantom. (Don't be fooled by the funky left to right order.) Next step would be testing it to see how much juice I can get. A saw somewhere that maximum current in a phantom feed is 14.1mA. Approximately 0.7 Watts... hmm

Edit: The spice-ware I use can mirror schemes. Here you go.

[Jazz P Bass]

It's tapping power from the 48V phantom. (Don't be fooled by the funky left to right order.) Next step would be testing it to see how much juice I can get. A saw somewhere that maximum current in a phantom feed is 14.1mA. Approximately 0.7 Watts... hmm


[ATTACH=CONFIG]25350[/ATTACH]

Would not the maximum current be a function of the 48V source?
Is that 14.1ma a design rating? (ie: safety)

[Enzo]

48v through 6.8k. Dead short to ground yields: 48/6800 = 7ma on one leg, or with a low resistance between hot pins, two 6.8k in parallel for 3400 ohms, so 48/3400 = 14ma. SO unless the 48v cannot even source 14ma, that would be the max draw. All that is on the supply end.

If you are trying to power something off phantom in your board, by reversing the circuit as you have, you have added more 6.8k in series, so assuming the same premise, you now have two 6.8k in series on each side, or for low resistance between a pair of 3.4k in series, so the max dead short current your powr thief can draw is 7ma. That is enough for a microphone, but little else.

[Enzo]

48v supplies in mixers don't need to be hefty. Even at 14ma dead short loads on every channel, even a 32 channel mixer needs less than half an amp to power it.

And remember, since it comes through 6.8k resistors, or 3.4k in parallel, you only get 48v unloaded. Draw even 5ma and you are down to about 30v.

[überfuzz]

48v supplies in mixers don't need to be hefty. Even at 14ma dead short loads on every channel, even a 32 channel mixer needs less than half an amp to power it.

And remember, since it comes through 6.8k resistors, or 3.4k in parallel, you only get 48v unloaded. Draw even 5ma and you are down to about 30v.

Quite right, with my set up it's in the realms of 32V / 4.5mA which makes about 0.15W. I can definitely power a little pre-amplifier with that. Like the one you linked i an earlier post. I'm going to tinker a bit with that source to see if I can power other stuff... to.