Phantom Powering Basics
"Phantom powering" is a method of providing power to microphones by applying a voltage to the same wires that carry the audio signals. Phantom power can be generated from mixing consoles, mic preamplifiers, or in-line phantom power supplies. All of Sound Devices microphone inputs supply phantom power.
In general, phantom voltages are used to power electronics within condenser microphones. Condenser microphones require power for various parts of their operation, including impedance converters, preamplifier circuitry and, in some cases, to polarized microphone capsules. Phantom is usually a DC voltage ranging from 12 to 48 volts. Microphones draw current from this voltage based on their needs.
Balanced microphone interconnections have two signal conductors relative to a ground conductor. With XLR-3 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 V, 15 V, 18 V, 24 V, and 48 V. 48 volt phantom is the most common and many microphones require 48 V for proper operation. New specifications from the AES outline five specifications for phantom voltages and current levels - P12L, P12, P24, P48, and P48H.
Because phantom voltage is exactly 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 present. With ribbon microphones turning phantom power off is recommended.
Microphone Current Draw
Condenser microphones draw current from the phantom voltage based on their electrical needs. Because Sound Devices products are portable and run from batteries, power used to operate microphones reduces battery runtime.
Microphones vary widely in the amount of current they draw. The chart below shows a sampling of microphones from various manufacturers. The voltage and current specifications are from manufacturer's literature.
|Microphone||Voltage Specified (volts)||Current Draw (mA)|
|Neumann USM69||48||2 x 0.7|
|Neumann KU 81 I||48||2 x 0.7|
|Neumann RSM 19||48||2 x 1.9|
|AKG C414B ULS||9||2|
|AKG C414B TL II||9||2|
|AKG C522MS||9||2 x 1.5|
|AKG C451E (old)||9||10|
|B & K 4007S||48||3.5|
|Shure VP88||9||2 x 1.3|
|Earthworks - all||48||10|
|Beyer - most||48||3 - 5|
|Schoeps - most||48||4|
|** Per AKG tech support, the SE300B Blue Line preamplifier functions properly at any phantom voltage from 9 to 52 V. There is no performance benefit running at 48 V.|
Battery Runtime with Portable Products
Sound Devices products provide phantom power for condenser microphones. To increase runtime when powering from batteries use the lowest phantom voltage necessary. (Note: The USBPre has 48 V phantom power only.)
Phantom powered condenser microphones can be categorized into four general powering classifications.
- Phantom / self-powered (internal battery) microphones which operate on phantom voltages from 9 - 48 volts.
- Microphones which operate on voltages from 9 - 48 volts. These mics have no performance benefit with higher phantom voltages.
- Microphones designed to operate at 48 volt phantom power but will work at lower voltages with decreased headroom.
- Microphones which require 48 volt phantom and do not operate at lower voltages.
Consult your microphone manufacturer's documentation for specifications.
Since power draw is in direct proportion to voltage level, power draw is significantly increased when running at higher phantom voltages. For instance, running the AKG SE300B Blue Line electronics at the voltages below shows the difference in phantom power consumed.
15 V (from the MP-2) x 0.002 A = 0.030 W (30 mW)
48 V (from the MP-2) x 0.002 A = 0.096 W (96 mW)
Since there is no performance benefit to running some condenser microphones, like AKG Blue Line mics, at 48 volt phantom, running at 15 volts reduces power draw considerably.
Sound Devices products provide up to 10 mA of phantom power per microphone input at 48 volts. With the MixPre, MP-2, and USBPre - all two channel products - a maximum of 960 mW (nearly a watt) can be drawn from the batteries. Phantom powering condenser microphones is the single largest power drain on the product, and battery life is directly affected by the microphone type and voltage powered.
Sound Devices thanks Jim Brown of Audio Systems Group in Chicago, IL for providing the data for most of the listed microphones. He has measured many of them to verify the manufacturers specifications and reports that their current draw is generally within 10% of the listed value.