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

 Subject: Re: Phantom Power. Question from The Ghost Who Walks :-) "Gregory O'Drobinak" gmo_dunes2 Sun Jan 3, 2010 11:07 am ((PST))
 ```Greg: 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= saster. 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 ```
 Current Thread Phantom Power. Question from The Ghost Who Walks :-), greg.winterflood Re: Phantom Power. Question from The Ghost Who Walks :-), vickipowys Re: Phantom Power. Question from The Ghost Who Walks :-), greg.winterflood Re: Phantom Power. Question from The Ghost Who Walks :-), vickipowys Re: Phantom Power. Question from The Ghost Who Walks :-), Chris Edwards Re: Phantom Power. Question from The Ghost Who Walks :-), Scott Fraser Re: Phantom Power. Question from The Ghost Who Walks :-), Randy Perretta Re: Phantom Power. Question from The Ghost Who Walks :-), Dan Dugan Re: Phantom Power. Question from The Ghost Who Walks :-), Gregory O'Drobinak <= Re: Phantom Power. Question from The Ghost Who Walks :-), Richard L. Hess Re: Phantom Power. Question from The Ghost Who Walks :-), greg.winterflood