Home Studio Tip: What is 48V "phantom power" and when do I use it?

When you see "phantom power" and/or 48V on a piece of audio gear, it is for one purpose:

Powering condenser microphones that are plugged into a microphone preamp.

There are some exceptions like if you are using a Cloudlifter or other device that boosts the output coming from a low-output microphone like a ribbon mic or a Shure SM7-b. Also, some "active" (needs power supply) direct boxes or instrument preamps can also accept 48V phantom power as a power supply instead of using a wall wart style power adapter.

If you are using the line input on your interface, becuase you are plugging in an external mic preamp or perhaps a keyboard, drum machine or other line level device, you won't need the phantom power at any point.

If, on the other hand, you are using the microphone input on your interface or mixer, you may need phantom power. Just ask yourself, am I using a condenser or other device that requires phantom power to operate? If you plug a condenser mic into your mic preamp and don't turn on the phantom power, you will get no signal at all because the diapragm of the mic isn't activated.

If you plug a dynamic or ribbon mic into your mic preamp and don't turn on the phantom power, the mic will work fine, just as it should. In fact, turning on the phantom power when you don't need it can actually be a bad or damaging thing.

Is there any reason to turn it off if I don't need it? YES. The 48V coming from the power supply can add noise to the signal, or worse yet, it can damage ("fry") the ribbon of a ribbon mic. Also, 48V is a fairly high voltage, and is often a somewhat awkwardly "tacked on" feature to an overly complex circuit and power supply, making this section of the power supply succeptible to failure or causing problems elsewhere in the signal chain.

A deeper dive: 48V is applied to both the hot and cold side of a microphone cable (pins 2 and 3). Since a mic cable only has the hot and cold wires, there is no convenient place for the voltage to return, so it must return on the cable shield. The shield is not intended to carry signal, much less a voltage, so it is truly a design compromise where if there are any shield or grounding problems with the cable, the circuit or some other piece of gear in your studio, you may encounter problems in your audio. A hum, buzz, noise etc. So please keep phantom power off if you are not using it. It's also highly preferrable to have the mic plugged in before turning on phantom power and to turn off phantom power before unplugging the condenser or other phantom powered device.