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Re: Phantom Power. Question from The Ghost Who Walks :-)

Subject: Re: Phantom Power. Question from The Ghost Who Walks :-)
From: "Gregory O'Drobinak" gmo_dunes2
Date: Sun Jan 3, 2010 11:07 am ((PST))

The circuit for supplying 48V phantom power to microphones is more complica=
ted than just running the two power leads to the pins of the microphone XLR=
 connector. It requires some 6.8K precision resistors and some filtering ca=
pacitors to ensure that the microphone gets prisitine, equal voltages to pi=
ns 2 & 3. You really need an extremely clean 48 VDC supply, preferably regu=
lated, in order to supply this voltage to the phantom circuit. That being s=
aid, using your phone line for the 48 VDC phantom supply is a recipe for di=

The 48 VDC that is found on phone lines can often have large amounts of noi=
se on it, because when your telephone is on hook, the line from the central=
 office (the CO) to your home is a high impedance circuit that picks up noi=
se from its surroundings on its way to your phone. That supply is also curr=
ent limited so that when you pick up your phone and complete the circuit to=
 the CO, the voltage drops to around 4.5 volts. The resistance at the end o=
f your phone line that makes this happen is typically on the order of 600 -=
 2500 ohms, depending on how the phone company has setup your loop paramete=
rs. So this means that your phone line may not only be unable to deliver th=
e needed 48 VDC, but it will also be subject to large amounts of noise as w=
ell. But it gets much worse!

Even though you may be only a short distance from your CO, that high impeda=
nce line to your telephone will conduct any nearby transient and delivery i=
t directly to your telephone as a voltage spike. The worst of these are due=
 to lightning strikes which could prove to be very harmful to your micropho=
nes. Telephones are designed with the appropriate circuitry to arrest most =
short-term line transients under 1500 volts or so, but microphones are not =
designed  to handle these voltage spikes, even if they are very short. But =
the clincher is the ringing voltage. When your phone rings there is a volta=
ge of 120 VAC superimposed on the phone line at a frequency of 20 Hertz! Th=
e current is substantial in order to make the older electro-mechanical bell=
 ringers work. I can say from first hand experience after being shocked by =
a ringer power supply in a CO once that it is very beafy and it hurts! The =
ringing voltage will not only destroy your microphone but also your
 preamp and/or recorder in the process, if the line transients don't do it =
first. I am sure that there is no way to predict when someone will be calli=
ng your phone, so this 120 VAC can happen at any time.

So to sum things up, please DO NOT use your phone line to power microphones=
! Use a commercial battery operated phantom supply of high quality. Deneke =
makes a good one, but it is not inexpensive. Rob Danielson has posted some =
info and test results for units that have worked well for him. I have built=
 one using four 12V "remote control" type batteries that is very clean, but=
 it is not regulated so the voltage sags a bit over the long term. Works fi=
ne for me with fresh batteries for a recording duration of a few hours.

I hope that this info helps you so that none of your equipment gets damaged=

Take care & send us some recordings when you get it all sorted out!

-Greg O'Drobinak

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