Thursday, May 24, 2012

Can phantom power damage your microphones?


There have been a lot of speculations on this topic, so I felt responsible to give you my take on it, which comes from my experience as an audio engineer as well as a studio tech.
Phantom power or +48V DC seems like a high enough voltage to do some serious damage to microphones that don’t need it (like ribbons, dynamics and tube mics). Some people shut phantom power off when plugging and unplugging condenser microphones as well. The truth of the matter is that in reality the chances of damaging a condenser microphone because it was unplugged while phantom power was on, is absolutely none. Of course if you are used to shutting it off, every time you unplug a microphone, there’s nothing bad about it. For the 1 in 10 millions or so chance of it actually damaging a condenser mic keep following your routine.

How it actually works:
You don’t really need to know much of electronics to understand the following diagram:





Phantom power is usually 48V DC at very low current – (micro amps up to about 10 miliamps). It starts with the phantom power switch (far right on the diagram) then through a decoupling resistor R3 charging a capacitor C3, which acts like a filter, making sure there’s no ripple (ac component) to the +48V DC power. Notice how resistors R1 and R2 in a way split that power to two destinations - pins 2 and 3 of the XLR plug. So technically there’s 2 independent phantom powers running through your microphone cable. One between ground (pin1) and Hot (pin2) and the other between ground (pin1) and Cold (pin3), which automatically means the voltage difference between pins 2 and 3 is exactly 0 volts. In fact resistors R1 and R2 have an exact 1% tolerance to ensure both pin 2 and 3 carry exactly the same voltage – therefore no voltage difference between them. As you would notice on the microphone side the dynamic moving coil is connected between pins 2 and 3. Since the voltage between pins 2 and 3 is zero the coil would not even “know” there’s phantom power applied to it. Since the ground in any dynamic microphone is simply connected to the body of the mic and has nothing to do with the coil inside of it, the coil is not affected by the 48 Volts between ground and pin 2 nor the 48 volts between the ground and pin 3, unless we have a bad cable where the ground is shorted to either pin 2 or pin 3. A lot of microphones use Isolation transformers as shown on the diagram below. Transformers work on the principle of induction, which does not allow DC power to pass through it and in a way protects the moving coil from receiving phantom power even if the cable is shorted.


Most modern ribbon microphones have their ribbons isolated either electrically or with a transformer so phantom power could not damage them; in fact a lot of ribbon microphones designed in the past 10 year require +48V.
Tube microphones on the other hand have their own power supplies due to the high voltage and current demands of vacuum tubes and do not need phantom power either. In fact all outputs of tube microphones have isolating transformers, which stop the very high DC voltage (150V to 350V depending on the design) component of the tube audio output in order not to damage the microphone preamp. The same transformer obviously would protect the electronics of the microphone from phantom power as well. So tube microphones are OK with phantom powers as well, assuming there are no shorts in the XLR cable.
When it comes to dynamic mics. I’ve only come across a few models by “Heil Sound” out there that actually use phantom power. Surprisingly enough they actually work without phantom power, but +48V turns on an active circuit inside the microphone and makes the output level of it go higher. So if you have the choice of sending phantom power to a dynamic mic, don’t do it. Again the chances of damaging a dynamic mic with phantom power are very slim or rather none unless we have wiring problems.
The moral of the story is: Phantom power is generally harmless, even though it sounds very unpleasant while switched on or off and it could damage your speakers, headphones and more importantly your ears, if you mess with it while listening to the output of the microphone preamp. Having said that, your expensive microphones deserve good care and you shouldn’t risk damaging them by a freak coincidence and a 20-year-old shorted cable.
Here are some scenarios where phantom power could cause some damage:
1. Unbalanced electronics connected to a microphone input – For example an MP3 player or an output of a consumer wireless receiver, a laptop headphones output, etc. The unbalanced nature of those sources naturally shorts the Cold signal to ground, making +48V present on Its output between Hot (pin 2) and Ground (pin 1). In those cases it is possible to damage the output of the device but not necessarily. My iPod for example has seen plenty of phantom power on its outputs over the years (it wasn’t me sending it) and it never got damaged.
2. Sending Phantom Power to an old (prior to 1970 design) ribbon microphone without an isolating transformer, while using a bad cable, which has the ground (pin 1) shorted to pin 2 or pin 3 of the XLR. This is the one classic example why everybody says - do not send phantom power to ribbon microphones, but the chances of this “perfect storm” to happen are really not that big.
Theoretically, this scenario would damage a dynamic microphone too, but in reality the current of the phantom power is low enough so that even if it flows through the coil of a dynamic microphone it wouldn’t inflict any permanent damage to it, at least not in the first couple of minutes or so. R1 and R2 from the first diagram are limiting the current to maximum of 0.007 Amps or 7 miliamps (from ohms law: Current = Voltage/ Resistance) or Current in this case is 48volts divided by 6800 ohms = 0.007 Amps. Definitely not the best thing to do to your dynamic microphones, but at least they won’t blow up like ribbons would under the same conditions.
3. All those examples deal with having Phantom power present. The moment of switching Phantom Power ON is a whole different issue.
For example you should NEVER patch anything carrying phantom power on a TT or ¼ inch patchbay. The nature of these connectors and the fact that the Tip, Ring and Sleeve are physically lined up behind each other makes them short out and/or send Phantom power to all the wrong pins of the XLR on the other end of the microphone line, at the moment the cable is inserted in to the female patch point.

Besides damaging your microphones, Phantom power could damage some microphone preamps as well. In the above described case of patching Phantom power on patchbays and in some cases of simply plugging a condenser microphone to a line with already existing phantom power, from diagram number 1, C1 and C2 are 2 decoupling capacitors stopping phantom power from entering the first stage of the microphone preamp and damaging its own active components. In some microphone preamps where no input transformers are used, the initial surge of current from a microphone being plugged in, or from cross-patching using TRS plugs – C1 and C2 get charged to 48 volts, but the side connected to the input of the preamps draws some current through the input stage of that amplifier. Over time and repeatedly doing this, the active components of the first amplification stage (transistors or ICs) start to “ware out” and become noisier than the usual.

In conclusion: Most microphones designed in the last 30 years either require Phantom Power or don’t care about it, assuming no wiring problems exist. So using ribbon, dynamic, tube, wireless, and condenser microphones on the same board with a master Phantom Power ON at the same time is just fine. Simply watch out for those odd cases described above.

4 comments:

  1. How about -power mic will it damage a 416T, I am buying an adaptor
    but I might forget one day to put it
    David , http://www.freshcutsoundeffects.com

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  2. Thanks for this article. I have a question that you may have covered but I'm a bit fuzzy on.

    I'm using an Audix VX10 that requires a full 48V of phantom power. It is running into a processor that only supplies 24V. If I put a power supply between the processor and microphone, will the 24V damage the power supply?

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  3. 48V Phantom power is provided on pins 2 and 3, with reference to ground (pin1) through 6.8k Ohm resistors, which limit the current a mic can draw. A more “heavy duty” microphone like the C414, for example, loads the 48V down to about 12V while operating. So if your processor provides 24V on the correct pins and the VX10 doesn’t need much of a current it may actually work just fine. In the case it doesn’t - all you need is isolation capacitors (33uF/63V or 47uF63V) or a transformer between the inputs of your processor, your external phantom power supply, and the mic.

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  4. What is your recommendation in connecting an Ipod/mp3 player to a mixer with phantom power switched on? Will a passive direct injection box helps in isolating it from possible damage?

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