Understanding Phantom Power For Microphones

Phantom power is normally described as electrical current running over the same cable that carries the mic’s audio signal. All non-battery powered professional microphones with active circuitry need this power to operate. But as with most pro audio topics, it’s a bit more complicated. Here is a description of phantom power and how to stay out trouble when using it.

Phantom power, usually 12 to 48 volts DC, can be generated from within mixing consoles, mic preamplifiers, in-line power supplies or high-end portable audio recorders. Most professional gear can now deliver phantom power to condenser or active ribbon microphones. The IEC, the international standards body, specifies that a device be capable of delivering 48 VDC at 10mA per input. However — a gotcha — some USB and battery-powered audio interfaces will not deliver the full 10 mA.

Condenser mics need power for impedance converters, preamplifier circuitry and, in some cases, polarized capsules. Ribbon mics, with active circuitry to boost low mic levels, need phantom power for this circuit.

Balanced XLR-3 mic connectors that carry phantom power have two signal conductors relative to a ground conductor. With such connectors, pin-2 and pin-3 are the signal conductors and pin-1 is the ground conductor. The definition of phantom power is an equal voltage applied to pin-2 and pin-3 with respect to pin-1.

To verify that a mixer or preamplifier provides phantom power, voltage measurements taken between pin-2 and pin-1, and pin-3 and pin-1 will read identical levels. No voltage is present between pin-2 and pin-3.

Popular phantom voltages are 12, 15, 18, 24 and 48 volts. The most popular by far is 48 volt. Because phantom voltage is the same on pin-2 and pin-3 with respect to pin-1, phantom power has no effect on balanced, dynamic microphones. A properly connected balanced, dynamic microphone will operate with or without phantom power present.

When using passive ribbon microphones, phantom power must be turned off because the power can damage the sensitive ribbons. However, an increasing number of new ribbon mic designs demand phantom power for the active circuits. These active circuits insure that ribbon mics work with modern preamps and computer interfaces.

High-quality AC powered gear can provide phantom current without issues. But battery-operated portable gear can be more problematic. Sound Devices, a maker of pro audio mixers that feed 10 mA of phantom power to each mic, uses batteries and clearly notes that phantom power reduces the overall battery life of their products.

Each microphone varies on power consumption due to the mic’s design. Even mics that need 48 volts vary on the amount of their current draw, which is always reflected in milliamps. Take these mics, which all use 48 volts. A Sennheiser MKH800 draws 3.0 mA, a Neumann U87 draws .8 mA, while a Shure KSM 32 draws 4.6 mA and all Earthworks models draw 10 mA.

Some mixers, even AC powered ones, specify a total milliamp load for all their combined mic inputs. This means users need to add the various requirements of all their microphones and then make sure the mixer will support the combined load. It can get complicated.

Denecke PS-2 double 48 volt, 10 mA power supply. Click to enlarge.

Denecke PS-2 double 48 volt, 10 mA power supply. Click to enlarge.

Phantom powered condenser microphones can be categorized into four general types:

— Phantom or self-powered mics, meaning the microphone has the option of using 48 volt phantom power or an internal battery.

— Mics that operate on any external voltage ranging from 9 to 48 volts. There are no performance benefits with higher phantom voltages on these mics.

— Mics designed to operate on 48 volt phantom power, but will work at lower voltages with decreased headroom.

— Mics which require 48 volt phantom only and do not operate at all on lower voltages.

AEA R44ACE with active circuitry. Click to enlarge.

AEA R44ACE with active circuitry. Click to enlarge.

For active ribbon mics, the phantom current draw for active AEA ribbon mics is 7 mA. The specs may vary for other brands of ribbon mics. Some lower-cost USB and battery-powered audio interfaces may not deliver sufficient amperage.

Though 48 volt phantom power may sound simple, it is not, especially when using lower cost equipment with sketchy specs. It is important to check not only the voltage of your mics, but the milliamp requirements as well. And then check the mixer itself. Having 48 volt phantom power is not enough, especially when simultaneously using multiple different types of microphones.

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