Condenser microphones have long been the first choice among
engineers for many different applications in the recording studio. They
tend to exhibit high sensitivity with good pickup characteristics, and
their extended frequency response generally provides a crisp, accurate
reproduction of the sound source. They also have excellent transient
response for accurately reproducing sudden sonic impulses, such as
those produced by the human voice, piano, or percussive
Of course, condenser mics must have a source of electrical power in
order to operate. (For more information on condenser microphones, see
"Square One: Microphonic Machinations" in the May 1995 issue of EM.)
Some models are powered by an internal battery, but most condenser
microphones receive their power through the mic cable from the input of
the device to which they are connected, such as a mixer or mic preamp.
This arrangement is called phantom power. Unfortunately, the issue of
phantom power often confuses musicians and recording engineers, so
let's take a closer look at this critical subject.
WHAT IS PHANTOM POWER?
Phantom power, also known as simplex
powering, is a DC voltage (generally ranging from 11 to 48 volts) that
powers a condenser mic's electronics and also provides a polarizing
voltage for the capsule. In addition, different mics draw between 1 and
12 milliamps (mA) of current. Under most conditions, phantom power is
supplied by a mixer, but it can also be supplied by a separate,
dedicated power supply.
Most contemporary condenser mics will work with phantom power
voltages from 9 to 54 VDC. These mics include an internal power
regulator that makes the mic operate successfully at whatever voltage
you give it.
Phantom power requires a balanced connection between the mic and
power supply. This connection uses a three-conductor cable with XLR
connectors on each end. The DC voltage is applied equally to pins 2 and
3 relative to pin 1, which is at ground potential. For example, if a
recording console supplies +48V of phantom power, pins 2 and 3 each
carry +48 VDC relative to pin 1. Of course, the microphone cable
transmits the audio signal as well as this voltage, hence the name
Generally speaking, a phantom power supply that gets its juice from
a wall outlet is recommended to ensure the optimum performance of any
condenser mic. Battery-powered supplies should be considered only when
AC power is unavailable, such as in field recording.
TYPES OF PHANTOM POWER
Three common types of phantom power exist. (I
will also discuss a less-common type, called T-power, shortly.) The
three common types use different voltages: 12, 24, and 48 volts.
The 12- and 24-volt varieties are fairly common in battery-powered
portable mixers. Until recently, these mixers suffered from significant
limitations because of their power source; many of the earlier models
provided only 12 or 18 volts of phantom power and very little current.
Battery-powered mixers with 12- and 24-volt phantom power, such as the
Shure FP33, are still available.
Studio consoles traditionally provide 48 VDC of phantom power to
each individual mic input. Because these consoles are powered from a
wall outlet, there is no practical limit to the amount of phantom power
they can provide. As a result, many studio-oriented condenser
microphones are designed to operate at 48 volts. In fact, some mics
operate only at 48 volts with a specified amount of current draw.
Even if a mixer supplies 48 VDC of phantom power per microphone, you
still need to pay attention to the current draw. Some consoles are not
capable of providing 12 mA of current per microphone across the board.
After you connect several mics, the phantom power supply might not be
able to provide sufficient current to each of them-or it might just
crash altogether. This rather unpleasant possibility can occur with
less-expensive consoles as well as battery-powered mixers, so you need
to know the current requirements of each mic you connect to the mixer
and the total current available from the mixer.
T-power is also known as A-B power. Unlike traditional phantom
power, which puts equal voltage on pins 2 and 3 with respect to ground,
T-power systems put a 12-volt potential difference between pins 2 and
3. In some systems, pin 2 is 12 volts above pin 3, and in other
systems, pin 3 is 12 volts above pin 2. What's more, the DC voltage on
these pins is called a floating voltage, because it's not referenced to
ground. Some equipment, such as Nagra recorders and the Shure mixer
mentioned earlier, have switches that select T- power or phantom power,
which lets you determine the configuration of the pins.
T-power was invented primarily for use by location film recordists,
who frequently need to run long microphone cables. Not too long ago,
both T-power configurations were common in the United States and
Europe; they were associated primarily with Sennheiser and early
Schoeps microphones. However, T- power isn't used much anymore.
It's important to know that only T-powered microphones should be
used with T-power supplies. Connecting a T-powered mic to a more
conventional phantom power supply is likely to damage the mic, the
power supply, or both. Conversely, connecting a microphone intended for
use with 48-volt phantom power to a T-power supply will result in
Unfortunately, because there are several ways to
implement phantom power, some combinations of mic and power supply can
behave strangely. Basically, the two methods for delivering phantom
power from a microphone input are known as transformer coupled and
FIG. 1: One of the methods of delivering phantom power is from a transformer-coupled input, which uses a center-tap transformer to deliver equal voltage and current to pins 2 and 3.
A transformer-coupled microphone input generally uses a center-tap
transformer and runs the phantom power voltage through a resistor into
that transformer (see Fig. 1). This provides equal voltage and current
to pins 2 and 3. (A transformer consists of two or more coils of wire
wound around a central core of magnetic material. It is typically used
to convert voltages from one value to another or match impedances. For
more on transformers, see "Square One: Going Direct" in the July 1997
issue of EM.) A nontransformer-coupled input uses resistors that are
closely matched to provide equal voltage and current to both pins (see
FIG. 2: In a nontransformer-coupled input, a pair of resistors, which must be matched to within 1 percent, delivers equal voltage and current to pins 2 and 3.
Because no real standards exist for implementing phantom power in
mixers, it is almost impossible to know what products will work
together reliably. Unfortunately, microphone manufacturers do not
provide a list of compatible products, so there is no way to know for
sure. You simply need to plug in the mic and try it.
Phantom power is often switched globally across groups of mixer
inputs (typically eight channels at a time), so you need to know what
might happen if you connect another type of microphone, such as a
dynamic mic, to an input that has phantom power applied. In most cases,
you will have no problem with a dynamic mic.
Condenser mics with their own power supplies should not be connected
to an input with phantom power applied. In addition, you shouldn't
connect tube mics to phantom-powered inputs. Tube mics require higher
voltages and currents for their electronics and capsule, so they
generally use their own external power supplies.
If you attempt to connect a ribbon microphone to a phantom-powered
input, things can get ugly very quickly. In such a case, the ribbon
acts like a fuse and pops instantly. A ribbon mic should never be
connected to a microphone input that has phantom power applied.
PRACTICING SAFE MIKING
It is always a good idea to regularly inspect
and maintain all of your mic cables and connectors to ensure good
continuity, proper polarity, good solder connections, and clean
contacts. Any degradation of any of these elements can adversely affect
the audio. If the phantom power going to the microphone is not
consistent and clean, the microphone becomes noisy, loses headroom and
dynamic range, and even makes crackling and popping noises.
The issue of phantom power can be quite maddening. During my
interviews, one common theme emerged: there are no real standards. As a
result, it's important to know where the bumps in the road are, which
can help you steer clear of trouble.