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Author Topic: 48v phantom power question?  (Read 3146 times)

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Offline pdastoor

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48v phantom power question?
« on: July 18, 2015, 09:11:45 PM »
I have a microphone Rode NTG2 which requires 48v phantom power.  However the mic can also be powered by a single AA battery.

1) My question is, is this safe? How can a AA battery which is only 1 v supply 48v of phantom power? I read a lot of people use the battery with the mic but where does it get the rest of the 48v phantom power

2) I also have a preamp that can be powered using the AC adapter or a 9v Battery and also supplies phantom power. Once again, how can that 9v battery supply 48v phantom power.

Is it safe to use? Do those two batteries supply all the power for the mic and pre amp? Just making sure cause if that's the case then I can save battery on my recorder (which supplies 48v phantom power) and just plug in the AA battery in the mic

Offline DSatz

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Re: 48v phantom power question?
« Reply #1 on: July 18, 2015, 10:42:11 PM »
These things are not only possible, but commonplace today. Just because a microphone can be driven with standard 48-Volt phantom powering doesn't necessarily mean that its circuit requires (or even uses) 48 Volts as such.

When DC voltages higher than the supply voltage are needed in a microphone, a "DC converter" can be used. This type of circuit (also available as a prepackaged component) takes a given DC voltage and uses it to drive an oscillator, at a frequency which, in audio circuitry, ought to be well above the audio range so as to avoid interference. The high-frequency AC can then be stepped up to a higher DC voltage either with a tiny transformer (followed by a conventional rectifier and filtering), or with a diode-and-capacitor network called a "voltage multiplier" (plus filtering).

DC converter circuits reliable enough for professional microphones started to be possible in about the early 1970s; older designs avoided them with a few notable exceptions, but newer designs use them a lot.

Condenser microphones may have pre-polarized diaphragms (i.e. electrets) or they may be "traditionally" polarized with external DC (or a third option, which isn't so relevant here, is to use the diaphragm and its backplate as a variable capacitor in a radio-frequency circuit). The Rode microphone is most likely an electret, which means that it doesn't need any high-voltage DC to polarize the diaphragm of the mike; it only needs enough to run the FET impedance converter circuit which drives the output. Again, that might be obtained by stepping up the 1.5 Volts of the battery to, say, 5 Volts, which should be more than enough for the audio circuitry.

Historically, 48-Volt phantom powering was devised for use with capsules that had been used in tube microphones with 60 Volt polarization. But even when you are feeding 48 or 60 Volts to a capsule to polarize it, the capsule is, after all, a capacitor--which means that once it is charged, no more current flows into it from the supply, except to replace whatever is lost from leakage. The usual audio-frequency circuit has this voltage applied to the capsule by way of an extremely high-value resistor (hundreds of megohms to a few gigohms), so the amout of current is absolutely miniscule. As a result, a DC converter can easily generate enough output to polarize the capsule without drawing much current from the preamp. The main consumer of power is the output stage of the microphone, which is usually class "A" circuitry (in which the output devices are always conducting).

I hope this also answers (by implication) your question about the preamp that can run from a 9-Volt battery and yet offer 48-Volt phantom powering. In a way, the arrangement is a little inefficient because 9 Volts is being converted to 48 Volts but in many cases (e.g. Neumann, Schoeps, and some AKG mikes) the microphone takes that 48 Volts (or the somewhat lower voltage that it actually receives, since when current passes through the pair of 6.8 kOhm phantom resistors, the voltage drops in proportion to the amount of current being drawn), drops it to some lower voltage with a Zener diode, and then runs that into another DC converter, e.g. to get 60 Volts to polarize the capsule.

Does this help?

--best regards
« Last Edit: July 18, 2015, 10:59:43 PM by DSatz »
music > microphones > a recorder of some sort

Offline pdastoor

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Re: 48v phantom power question?
« Reply #2 on: July 19, 2015, 02:18:26 AM »
yes thank you that helps. so just to make sure, I should be okay with using the battery rather than the ac adapter? I'm not so good with science and math. Thanks once again

Offline DSatz

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Re: 48v phantom power question?
« Reply #3 on: July 19, 2015, 09:11:01 AM »
It depends. I suggest that you look at the specifications for the microphone. It may perform identically with internal battery power vs. phantom power, or it may have different specifications for the two modes of operation. Rode's specifications are usually pretty clear.

--best regards
« Last Edit: October 23, 2015, 10:36:05 PM by DSatz »
music > microphones > a recorder of some sort

 

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