the Phantom Power Menace

By in 56knobs, audio, podcast on June 9, 2010
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the amount of mythology that surrounds Phantom Power is staggering. mostly the problem is that people don’t have to understand anything about electronics making lots of the information the stuff of magic. there are claims that say it’s bad to mix mics, that having Phantom Power on will hurt a mic that doesn’t need power and that you can “hear” the 48v. this subject is confusing enough and should not be glossed over with a blanket statement… don’t do that.

Phantom Power is a DC voltage used to power a capacitive plate that sits behind a diaphragm. when the mic is spoken into the plate discharges making small electrical changes which become an audio signal. this is different the passive signal generated by a diaphragm moving a magnet inside of a coil.

the spec for Phantom Power is that it’s 48v DC at 10 milliamps. this is the maximum amount of power an mic needs to work. if the power is off the mic will not work. while 48v is the defined spec less voltage can be used.

a balanced Microphone cable uses a “differential” signal to ensure that there is no “noise” introduced to the line over the distance of the cable. any noise that is picked up is automatically canceled out when the audio is recombined. pins 2 and 3 carry the audio signals. pin 1 is ground. Phantom Power put power on pins 2 and 3. if a mic is dynamic there is no connection to ground (pin 1) required to complete a circuit. if you measure voltage on pin 2 to pin 3 there will be +0v or nothing. measuring between pin 1 and pin 2 or pin 1 and pin 3 will reveal 48V DC.

what kind of mics need Phantom Power? Dynamics do not need power. they generate their own signal power. mics like the Shure SM58, the Heil PR40 and the Electro Voice RE20 are dynamic mics. condensers require Phantom Power. mics like the Perception 01, the Neuman M and the raft of cheap chinese mics all require power. there are other mics that also require power. like ribbon mics. but these have different power requirements and are beyond this discussion.

so here are the myths and questions that come up.

will phantom power hurt my dynamic mic?
No. all modern mics follow a design that simply ignores power if it is present. if you look a the schematic for a dynamic mic you can see that even though power is present on pin 2 and 3 of the mic because neither line is connected to ground there is not a complete circuit.

so a bad cable can ruin my mic if Phantom Power is turned on?
yes or maybe or no. YMMV. depends on the mic. if you are in the habit of making your own cables and don’t do a very good job of making them you could easily damage your mic. you should ALWAYS test your cables with a multimeter or a cable tester to make sure it’s good. if you are setting up and tearing down on a regular basis always test your cables before you connect your mics.

how do I know if I need Phantom Power?
if your mic isn’t working at all that’s a good sign it needs power. if the mic is big and heavy and the box that it came is says “condenser” on it somewhere then it needs power. you should always check the specs before you plug your mic.

what about a mic that has a battery?
some mics contain their own power. lav mics (Lavaliere), wireless mics, and shotgun are all types of mics that may have a different source of power. there is no need to use Phantom Power with this type of mic. in face the presence of Phantom Power might ruin this mic.

I have a vintage mic. what happens to my mic if it’s not compatible.
too bad, so sad. seriously. if you have something very old chances are you need to care and feed it. don’t just plug it into your board without knowing exactly what it needs. lots of these old mics required a pre-amplifire in order to work. and just because it has a 1/4″ jack doesn’t automatically mean you can just plug it into your modern board.

can I hear if Phantom Power is on?
no. you cannot hear DC voltage.

can I mix condenser and dynamic mics on the same board?
yes. but… as long as all your dynamic mics are made within the last 40 years you won’t have a problem. however a mic that is shorted or a cable that is bad can cause problems. but you’ll be able to hear that problem.

I have a Neuman MLM149 can I mix it with other…
if you have that mic you don’t have a budget problem. you very likely have that connected to a pre-amp or mic processor to help it along. you could think of the pre-amp as a very expensive fuse that is protecting the mic from your board. which by the way won’t need Phantom Power turned on because the pre is doing that job for the board.

is there some “best practices” that I can follow to insure I don’t blow up my stuff?

    verify, verify, verify. check your cables. cut bad cables in half. recycle the wire.
    take your head phones off before powering on your board after hooking up mics.
    the power to the board should be off when plugging in. turn on Phantom Power after turning on the board.
    mute the channel before powering on. this prevents feedback loops.
    you can mix modern mics but if you don’t know, look it up the specs.
    if you have expensive or vintage mics use a preamp. this makes it separate from the rest of the board.

check your cables
there are several very inexpensive cable testers. you can also make your own. it takes about 30 minutes and 5 bucks in parts. the big question on the DIY is what you want to test for which could be the presence of power or that the cables are wired correctly. it’s two different tests.
DIY tester – nifty 4 LED tester shows what exactly is wrong.

when Phantom Power doesn’t work no matter what! meaning it has to be OFF!
an unbalanced mic is present. which is Mic wired between ring and tip plug. power can damage this mic.
some old mics have grounded center-tap outputs. they may use an XLR connector but they they aren’t wired to carry signal on pins 2 and 3.
high impedance mics.
mics that are damaged having leaks or shorts between pin 2 or 3 and pin 1. the mic will crackle with Phantom Power turned on. and it might stop working altogether because the power short caused the coil to melt.

other tips if you want to be a freak about not mixing mics with power

  • that mic you traded the Cadillac for… that mic gets treated special no matter what.
  • there are higher end mixers that allow you to turn Phantom Power on/off per channel. the Mackie Onyx boards have this feature.
  • you can always unwire the switch on your board so it is impossible to turn on Phantom Power. it would be about a fix minute fix to open the board and clip the wire from the switch.
  • there’s no reason that you have to use just one board.
  • if you can possibly help it. don’t mix mic types. freak.