Phantom Power

Condenser microphones have long been the first choice among engineers for many different applications in the recording studio. They tend to exhibit high
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Condenser microphones have long been the first choice among engineers for many different applications in the recording studio. They tend to exhibit high
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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 instruments.

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.

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 "phantom" power.

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.

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 similar consequences.

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 nontransformer coupled.

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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).

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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.

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.