"What is phantom power for mics?" is one of the most common questions, followed immediately by a concern about damaging a microphone by delivering that sweet 48 volts of DC power when it's not needed. There's a ton of questions regarding this topic and I'm about to fly through them all for you...
What is phantom power? How does it work? Why does it have that name? Which mics need it? Anybody who's been in this industry hears these questions a million times a year, including me, which is why I'm writing it up. Now I can just send people here and save myself the trouble.
Usually what happens is a newcomer gets their first microphone and preamplifier or audio interface and see that little button that says +48v on it. They look in the manual and for the first time see this mysterious phrase and are off on an investigation.
Hopefully they, like you, have landed here. This is the most complete but down-to-earth and easy to understand guide on the topic. Let's not waste any more time. We'll go in the same logical order that the questions tend to come up in conversation.
Phantom Power is the name for a specific amount of voltage required by some microphones to function properly. Specific types of microphones, mainly condenser mics, contain active circuitry that needs a power source to drive it, as opposed to passive circuitry which does not.
This power is transmitted as DC electric power (direct current, as opposed to alternating current) to the microphone typically through the microphone cable. It is used to power the active circuitry as well as for polarizing the transducer element, which is the part that turns sound waves into an electrical signal.
Early condenser microphones from the 1930's to 1950's were designed in such a way that each needed its own power supply. These power supplies were quite bulky. You'll still find microphones like this to this day, like my favorite vocal mic, the Rode NTK.
When I use this mic, I have to have this big box out on the desk, eating up space. I also have to use an extra mic cable. You can see how annoying this could be if every mic we used was like this. That was the problem back then, and in the 1960's audio engineers sought a solution.
You've probably heard of Neumann and Schoeps, both German microphone companies. They led the way to creating a standard for powering condenser microphones using 48 volts of direct current. Pretty much everybody hopped on board because it benefited us all by reducing complexity.
This meant that future condenser mics would be designed for this amount of power and all phantom power supplies would deliver that amount. It also meant that manufacturers that made mixing boards, preamplifiers, and audio interfaces could start to refine the process and make the parts tiny.
And that's why you see a +48v button on certain pieces of gear. We no longer have external power supplies, but tiny and efficient ones hidden away inside of other necessary recording equipment that is always coupled with a microphone.
Because the power source is hidden away, it was decided that it should be called a phantom, as in an invisible ghost. That's the summary of the history of this power source and how it got its name. It being a phantom circuit played a role, too, I'm sure.
Earlier I mentioned a transducer inside condenser mics, commonly just called the capsule. What's happening inside this transducer is that there is a capacitor made up of two plates. One of these plates is stationary but electrically charged. The other is the diaphragm that vibrates due to the sound waves hitting it.
When the diaphragm moves closer and further from the other plate, it causes that plate to release varying amounts of voltage. This released charge is your audio signal, later converted from an electrical signal to a digital one at the analog-to-digital converter.
None of this works if the non-diaphragm plate isn't electrically charged. It gets that charge from the 48 volts we're sending up the mic cable. In some cases it charges vacuum tubes and powers other parts of the microphone, but powering the transducer is the main job.
The global ANSI standard (IEC 61938) for the amount of DC power to deliver is 11 to 52 volts, though the studio community settled on 48v. In most cases, you'll look for a button that says +48v, and if you press it a red LED bulb lights up to let you know it's engaged.
Usually that button is either on a channel on your mixer that features an XLR input that leads to a preamplifier or near the XLR connector inputs on your audio interface. If you're using a standalone preamp then you'll find it there. You can buy standalone phantom power units as well, though they're rarely needed.
Our 3-pin XLR microphone cables (a balanced audio connector) are designed not only to carry audio signals in the form AC electricity on pins 2 and 3 (pin 1 is the ground) but can also allow equal amounts of DC power to piggy back on it without interfering with audio quality. That's how the 48v moves through the mic cable.
In general, you'll probably only run into two types of mics: condenser and dynamic microphones. Ribbon microphones are making a come back and are a special case I'll also mention. Also, let me state:
Pro-Tip: You should try your best to make sure the 48v button is turned off before plugging in or un-plugging your mic cable from the power supply. In very rare cases it could lead to damage from a power surge, but generally you'll just hear a loud pop that could hurt your speaker monitors, headphones, or your ears.
Yes. Some may get it from their own power supply or even from a battery inside the mic's chassis, otherwise they receive the 48v from the mixer, interface, or preamp. Condenser mics need phantom power to reduce their high impedance output and due to their use of active electronics as described above.
No, dynamic mics don't need this form of power, but you won't harm the microphone or affect your audio quality if you do use it. Since the DC voltage on pin 2 and pin 3 of the XLR cable is equal, there will be no current flow. Modern dynamic mics are even designed to protect against any disasters.
In the most rare cases of a broken XLR cable or phantom power unit, there can be a voltage difference between the pins causing a current to flow across the diaphragm of the dynamic mic. Typically that won't harm the mic unless it's extremely sensitive, such as in the case of old ribbon mics.
Dynamic mics use passive electronics, including a magnet and a coil to generate the electrical signal that makes up the audio signal. The sound waves push on the diaphragm which vibrates the magnet within the coil and generates the current all without the need for pre-charged electricity.
Old ribbon mics can be damaged in the manner described above, where a voltage differential in the cable can cause a current to flow over the sensitive ribbon, damaging or destroying it entirely. Modern ribbon mics are much more resilient and designed to be safeguarded from these catastrophes.
The various types of power supplies have boiled down to the following:
A DI box and preamplifier (What is a Preamplifier?) is technically what you're plugging into when you're using an audio interface or mixing console as well. If your microphone asks for a battery, then you don't need to engage the +48v button, or you can remove the battery instead.
Though we've taken the full plunge into the topic, all you really need to know is that you need to use it for condenser mics only and to turn it off before you plug in or disconnect your mic cable. The rest or technical details that don't affect how you use it.
But if you're like me you want to know everything about anything you're involved in. I hope you enjoyed the answer to "What is Phantom Power?" and all of the related questions. Now get back to recording!
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