From the May 2017 issue of Acoustic Guitar | BY DOUG YOUNG
If you’ve used microphones for live performances or in the studio, you’ve almost certainly encountered phantom power—that slightly mysterious-sounding button found on many mixers and preamps. Phantom power provides a source of electricity to some microphones. Most of the time, the phantom-power feature just does its job silently, but a bit of knowledge can help when things go wrong or when choosing the gear with which you work. As a guitarist, you may also encounter some less common scenarios related to phantom power.
These pointers will help demystify phantom power.
Now You See It, Now You Don’t
Phantom power is a way to provide power to microphones that require electricity to operate, typically condensers. On the other hand, dynamic mics—the ubiquitous Shure SM57 and SM58, for example—do not require power. Phantom power involves a clever scheme that leverages the multiple wires in a typical balanced-XLR cable to provide voltage to mics that need it without affecting those that do not, in most cases. You can usually plug either type of mic into a mixer that provides phantom power, and the condenser mic will detect and use the power, while most dynamic mic will ignore it. This trick is why the scheme is referred to as phantom—it’s there for mics that need it, but generally invisible to others!
One complication with phantom power is that there are multiple standards for voltage levels. Most modern mixers provide 48 volts, which has become so common that many people assume that phantom power automatically means 48 volts. However, the ANSI standard (IEC 61938) that covers such things allows for 12, 24, and 48 volts. As a result, you may encounter gear that supports any of these voltages. Several popular acoustic guitar amps provide 24 volts. One popular acoustic preamp even provides just 15 volts.
In most cases, these different levels should not cause concern. Most mics work fine over the voltage range of 12-48 volts, although many mic manufacturers specify 48 volts for best performance, and some mics may be more problematic than others when encountering lower levels of phantom power—you may notice some loss of output or sensitivity with lower voltages. In any case, a lower voltage won’t damage the mic, so you can always see for yourself if a lower voltage will work.
Phantom power is a complex subject, but for most of us, it’s a matter of matching the requirements of our mics—or other gear—with the features of the mixer.
Some DIs or preamps can be powered by phantom power (the popular L.R. Baggs ParaDI, for example), and these may not work with lower voltages. The amount of current provided by the phantom power source is another factor. Most mics require only a few milliamps of current, which any modern mixer should easily support, but some mics—as well as phantom powered preamps—may have higher current demands. Some studio microphones, most notably tube mics, require so much power that they use their own dedicated power supply instead of relying on phantom power.
Although it is usually safe to plug a dynamic mic into an input that provides phantom power, there are exceptions. Some ribbon mics (usually used only in studios) can be damaged by phantom power if a cable or the mic is mis-wired. It’s a good idea to be wary of any vintage microphone or one that has been modified in any nonstandard way. It’s also best to plug microphones in before turning on phantom power. At the very least, doing so while phantom power is active can cause a loud, audible pop from the speakers if the volume is up. In addition, plugging in other electronic devices—a keyboard or an effects pedal that has an XLR out, for example—is less certain. Check your instruction manual to be sure that the device is safe for phantom power, and assume that it is not safe if the manual is silent about the issue. For multiple reasons, it’s a good practice to use a DI between such devices and a mixer, including isolating them from phantom power.
Other Powering Schemes
A frequent source of confusion for guitarists is that some microphones use different powering schemes. One such approach is known as bias power—also called plug-in-power. Bias power is frequently required by internal guitar mics, as well as mics used with some portable recorders, wireless systems, and more. Bias power uses a two-wire system, unlike the XLR 3-wire system supported by most mixers. A voltage, usually around 5-9 volts, is applied directly to the mic’s “hot” wire. This voltage is not “phantom,” and you cannot use bias-powered mics directly with a phantom power supply, or vice versa.
Acoustic guitarists often encounter bias-powered mics as part of dual-source pickup systems. In commercial dual-source systems with onboard electronics, such as the L.R. Baggs Anthem or DTar Multi-Source, the mic is powered by the onboard electronics and battery, so you don’t even have to be aware of it. But many guitarists like to build their own systems by adding a mic, such as K&K Sound’s Silver Bullet mic, to an existing pickup. In these cases, the mic can be powered by wiring it to the ring terminal of a stereo jack in the guitar, and then using a stereo guitar cable to plug into an acoustic guitar preamp that provides the bias power on the ring of the cable. The Grace Felix and Headway EDB-2 preamps are examples of guitar preamps that support this feature. Rolling your own dual source pickup system can be daunting, and requires planning out your entire system. A good guitar tech can clear up any confusion and help you set up such a system correctly.
Unfortunately, many preamp and guitar-amp manufacturers incorrectly label bias power as “phantom” power—they are not the same thing! However, the difference is usually obvious from the connectors: If you have an amp or preamp that provides power for a mic, an XLR-mic connector almost certainly indicates real phantom power. A system that provides power through a stereo ¼-inch guitar jack generally indicates bias power. The mics are similar—a mic with an XLR connector expects phantom power, a mic with two wires, or a simple non-XLR connector, probably expects bias power.
Phantom power is a complex subject, but for most of us, it’s a matter of matching the requirements of our mics—or other gear—with the features of the mixer. When in doubt, consult your instruction manual or get in touch with the manufacturer.