Simple Power Question & Phantom Powering

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statzern

Well-known member
Joined
Nov 2, 2009
Messages
52
Hey all,

I am helping a friend by building a small battery powered mic pre. He is using it for video projects, which involve a lot of field recording, away from a wall power source. Additionally, a lot of sources are quiet, such as conversations, etc with little background noise - so we wanted to keep the pre as quiet as possible. He will be using a Rode NTG-2 microphone, which is a shotgun style small diaphragm condenser, if you're not familiar. It can be powered by either external phantom power or internal batteries.

For the mic pre itself, I'm using the simple dual channel P66 Pre board available over at ESP.

I've attached my plan for the power supply and phantom powering method. First, does this look like it will work? I tried keeping things as simple as possible. The power supply is not regulated (which means headroom will sag as battery power goes down), which I chose to try and extend battery life. For resistor values, I will use 10k for the voltage divider circuit, 1%, matched as closely as possible. I'm guessing 1/4 W is enough here?

For the phantom power, I know that 6.8k is the normal value for the series resistors with P48 - does this matter if I'm using 9V instead? I have read that the value does not matter as much as the tolerance to which the resistors are matched.

Thanks for reading!

 

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ditch the resistors, the point between your batteries is ground...

will the mic run on 9v phantom? I'm assuming there's a booster inside that get's you capsule voltage, but does that work with external phantom?

check the mic specs...
 
Hey Gemini, thanks for the reply! Yeah, I didn't think of that w/ the resistors.. oops...

I'm not sure what to make of the the spec sheet - it says "P48 phantom power or 1.5 volt AA battery" I realize that 9V from a battery is neither of those, but I figured if it runs on 1.5 and 48, surely it will run on 9? Maybe it has a booster in there... how can you boost 1.5 v DC to 48?? I could see AC with a multiplier, but I don't know about DC...

Any advice on building a booster of sorts would be very helpful. I have looked around the web, but I'm kinda lost.

 
The mic will have a booster in there...I don't know of a condenser than can run on 1.5V polarization.

check out the MXL 603 (basically most cheap condenser mics use this design or similar) The upper left section is a DC-DC converter, using an oscillator to turn DC to some form of pulsed DC and then recitfied and filtered to around 60V I think (never measured)

The kicker here is, the voltage is knocked down to 12V first, and thats what the circuit runs on...

but there's no need to bash your head into a wall... they already sorted the power supply for you... use the battery power and run the preamp on +-9V as you have it.
 
Yeah, he's just gonna have to deal - it's the only mic he'll be using anyway!

Thanks again... This will simplify the design a lot.
 
Batteries are notorious for having a high output impedance so I would advise connecting a large value electrolytic capacitor with a parallel polyester across each one.

Cheers

Ian
 
statzern said:
Yeah, he's just gonna have to deal - it's the only mic he'll be using anyway!

Thanks again... This will simplify the design a lot.
The NTG2 can operate on an internal AA battery, which has a 3000hours+ life.
It can also be powered on P48 or P24, which is a variation that uses a 24V source instead of 48, and different resistors (2.2k IIRC), but no mention of P12 or less. There is an understanding of "universal" phantom power, which encompasses the 9-52V range. If the NTG2 was capable of operating at 9V, I think they would have mentioned it.
I would recommend using the internal battery.
 
It can also be powered on P48 or P24, which is a variation that uses a 24V source instead of 48, and different resistors (2.2k IIRC), but no mention of P12 or less. There is an understanding of "universal" phantom power, which encompasses the 9-52V range. If the NTG2 was capable of operating at 9V, I think they would have mentioned it.

Thanks, I think that's what I'll stick with for now.
 
If you know what mic you want to use, consider doing some simple experiments with the lower (9v) phantom voltage.  Perhaps start with 1k resistors and see how it works, If the mic sounds clean, all good.

I would also consider raising the input termination in the mic preamp since 1k phantom build outs will drop that nominal bridging termination lower than with 6.8k normal.

These 1k or whatever value, do need to be matched to each other for good CMRR.

JR
 

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