Using the phantom power

Gabevee · 29th July 2005, 02:25 AM

Hello all!

I want to make a preamp to put into my microphone that will be powered by the phantom power.

If I recall correctly it comes through a 6.81K resistor on each leg.

My problem:

Does the 48 volts drop through those resistors? IOW, if I need to draw 4 mA, will it drop the voltage? That means it drops 27 volts, and I only have 21 volts to play with.

Is that so?

Thanks so much in advance for your replies.

Gabe

Enzo · 29th July 2005, 04:43 AM

Yep, Ohm's Law. Those resistors limit current to a handful of mas. So 4ma will drop it to about half as you describe. But with "only" 21 volts to use, there is still plenty of headroom for any mic signal I can imagine.

Gabevee · 29th July 2005, 05:32 AM

Thanks so much. I was afraid so... I am using MOSFETS and the ones I use require 3 volts just to turn on.

I have the circuit designed to get 12 volts... so I think I will be OK. Just wanted to be sure.

Thanks!
Gabe

29th July 2005, 02:25 AM	#1
Gabevee diyAudio Member Join Date: Feb 2002 Location: Richmond, VA.	Using the phantom power Hello all! I want to make a preamp to put into my microphone that will be powered by the phantom power. If I recall correctly it comes through a 6.81K resistor on each leg. My problem: Does the 48 volts drop through those resistors? IOW, if I need to draw 4 mA, will it drop the voltage? That means it drops 27 volts, and I only have 21 volts to play with. Is that so? Thanks so much in advance for your replies. Gabe __________________ Gabe CGV Electronics Home of the CGV-300B amplifier on a budget

29th July 2005, 05:32 AM	#3
Gabevee diyAudio Member Join Date: Feb 2002 Location: Richmond, VA.	Thanks so much. I was afraid so... I am using MOSFETS and the ones I use require 3 volts just to turn on. I have the circuit designed to get 12 volts... so I think I will be OK. Just wanted to be sure. Thanks! Gabe __________________ Gabe CGV Electronics Home of the CGV-300B amplifier on a budget

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29th July 2005, 04:43 AM	#2
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	Yep, Ohm's Law. Those resistors limit current to a handful of mas. So 4ma will drop it to about half as you describe. But with "only" 21 volts to use, there is still plenty of headroom for any mic signal I can imagine.