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.
