What's Phantom Power?

You have seen it mentioned. You have buttons or switches on your mixer or recorder that say +48. But what is it and why do you need it?

Many high end microphones usually condenser type have a preamp inside the microphone. Most do not use batteries to power the preamp but rather get their power from the mixer or recorder. The +48 power is fed up both lines, pins 2 and 3 of an XLR type mic cable from the mixer. The shield is the ground return. When using phantom power you should not connect or disconnect the mic from the cable or mixer until you turn the phantom power off. You could damage the mic or mixer, speakers if they are being amplified, and you could damage one's hearing if using headphones.

The above shows a simplified drawing of a phantom power circuit. The microphone is on the left and the mixer input stage is on the right. A typical XLR microphone cable is in the middle. The phantom power switch, when turned on, applies +48 volts to both R1 and R2, two matched 6K8 (6800 ohm) resistors. The value of the resistors is not as important as the fact that they are closely matched to give exactly the same voltage on both pins. They are usually low noise metal film type resistors. C1 and C2 block any power from getting into the amplifier stage but allow the audio to get to IC2, the mixer input preamp. The +48 volts appears on pins 5 and 8 of the audio transformer. Power for the microphone preamp is pulled off pin 6. The ground and the negative supply for the preamp comes from pin 1. The audio output of the preamp appears across the primary of T1. Since the power is tapped from the center of the transformer, any audio is cancelled and the result is pure DC for the preamp.

Phantom power should only be used with XLR type connectors because you need three wires to establish connectivity and it is an industry standard.

+48 volts is used for most mic's although some older consoles use +24 volts. +48 volts gives the most headroom and better signal to noise ratio.

Non powered microphones can be plugged into an input that has phantom power without damaging them. The explanation below will show how this works without damage.

As you can see, +48 volts is applied to pins 2 and 3. There is no flow of current to complete the +48 volt circuit to ground, because there is no DC circuit path. The dynamic microphone element is completely isolated from the ground. The ground provides a shield around the microphone element.