In the audio world, phantom power is an option available on devices with audio inputs. Phantom power can usually be switched on or off globally or, on more expensive hardware, in banks or for individual channels. Various microphones and DI Boxes require phantom power to operate.
In a nutshell, phantom power is a DC voltage (usually +48V) (although it varies from 9-52v) applied equally to pins 2 and 3 relative to pin 1 of an XLR input to a Mixing Desk or pre-amp. This provides a power supply for Condenser Microphones and DI boxes to use.
Originally, phantom power was designed for provide a constant voltage bias for capacitor (condenser) microphones to operated, in such a device you have a charged plate and another ground plate, vibrations cause an alternating current to be set up and be sent to the mixing desk. Phantom powered DI-boxes use active low-current circuitry to change the impedance and balance incoming signal. Phantom power is very low current so it cannot provide power for active amplifier such as those found in valve pre-amps.
For a more in depth explanation and information about the standards that exist for Phantom Power see the excellent article here: The Feeble Phantom
Phantom power is a useful tool for professionals as it cuts down the amount of hardware and cabling needed since individual power supplies and cabling are not required. However the use of phantom power requires some user input as using incorrect wiring can damage systems; It is VITAL that proper balanced cabling is used when Phantom power is supplied. Using incorrect cabling or unbalanced devices can damage both the console/pre-amp and/or the input device.
How it Works
There are two common methods of supplying Phantom Power in both cases a positive DC voltage is connected to pins 2 and 3 of an XLR connector. The 'ground' or 0V connection for the supply goes to Pin 1 of the XLR. Thus there is no potential difference between pins 2 & 3 of the XLR and if something, e.g. the coil of a dynamic microphone, is connected across pins 2 & 3 then no current will flow and no damage should result.
In older equipment this is achieved by using a transformer with a centre tapping on the primary circuit. The transformer is usually there to perform the function of converting from a balanced input signal to an unbalanced signal anyway so adding the phantom power here makes sense. The DC supply voltage is fed to the center tapping of the transformer via a current limiting resistor, typical value of 3.3k Ohm. The 'ground' or 0V connection for this supply goes to Pin 1 of the XLR.
On newer equipment with electronically balanced inputs the following arrangement is more common. The DC supply is fed through two equal resistors to pins 2 & 3 of the XLR. For the standard 48 Volt Phantom, a resistor value of 6.81k Ohms is used. The 'ground' or 0V connection for this supply again goes to Pin 1 of the XLR.
For a more in depth explanation and information about the industry standards that do exist for Phantom Power see the excellent article here: The Feeble Phantom
Operating Phantom Power
All mixing consoles are different but the operation of phantom power is usually very similar. On smaller consoles the switch is often global so that all mic inputs can supply phantom power or not, in this situation, all inputs should use balanced cables.
Larger mixing consoles may have phantom power switched per bank or per individual channel.
When using a separate monitor split, phantom power only needs to be sent from 1 console. There is no standard to this, some people prefer to send from the FOH console others, the monitor console.
In this scenario it is important to note that a direct link is required. A transformer splitter will remove a phantom power supply. It is therefore important that the console supplying phantom power is from a passive split. When using an active splitter it is usually possible to apply phantom power from the splitter itself.
Some desks do not have a phantom power option. In this scenario small single or multi channel units can be purchased which are placed in-line with the mic input, these supply phantom power where necessary. Rack mounted mic preamps typically have phantom power available.
Phantom vs. Bias
Radio mic transmitter packs, PC sound cards and consumer recording devices such as portable mini disc recorders have a low power (5V or less) bias supply. The bias voltage is for use with a microphone designed for this type of device and wired accordingly. Plugging bias voltage microphones into a console with phantom power may result in damage. Conversely, plugging in a microphone that REQUIRES phantom power into a bias voltage device may also result in damage. For normal use this should not be a problem since very different types of connector are used.
Phantom vs T power
Most comon in the Film and broadcast industries, but also still found in theatres (e.g. mics used on show relay systems), T power, also know as AB power, was once common on professional condenser microphones - and can still be found today - some manufacturers producing both Phantom and T powered versions of otherwise identical microphones. T Power is not compatible with Phantom Power. T power uses a 12V supply but unlike Phantom power it is applied differentially across the two signal wires and damage will result to dynamic microphones if they are connected to a T power source. Converters are available to enable T powered mics to be used on Phantom power e.g. Canford Audio PHANTOM TO T-POWER CONVERTER .