Many everyday devices need electricity to function and connect to a power outlet via a power cable. Microphones, similarly, plug into microphone inputs via a mic cable in order to transfer electrical audio signals.
Do microphones need power to function properly? Some microphones require power to function (active mics) while others do not (passive mics). Microphones that have built-in active circuitry or non-electret electrostatic (condenser) capsules require power. Dynamic mics with no internal preamps do not require power.
So microphones do require power to function properly while others do not. Let’s take a closer look at which microphones need power, the type of power they need, and the ways in which power is supplied.
Microphones That Require Power And Those That Do Not
Although usually correct, it’s actually a common misconception (even in the audio industry) that dynamic mics do not require power while condenser mics do.
Though the above “misconception” is typically true, it’s active microphones that require power and passive microphones that do not. It just so happens that all condenser microphones are active and the great majority of dynamic microphones are passive.
What is an active microphone? An active mic requires external power to function. Mics with internal preamps, FET/JFETs, vacuum tubes, externally-polarized capsules or A/D converters are active since these built-in devices require power. Every condenser mic is active and some dynamic mics with internal preamps are also active.
What is a passive microphone? A passive microphone does not need power in order to function. Dynamic mics are passive since they work on electromagnetic induction, which requires no power. Mics without internal preamps, FET/JFETs, vacuum tubes, externally-polarized capsules or A/D converters are likely passive.
Passive Microphones Do Not Require Power
Nothing in a passive microphone design requires power to function. The vast majority of dynamic microphones are passive (some modern ribbon microphones have been designed with active circuitry, which we’ll get to later).
In basic moving-coil dynamic microphones (which provide the best example of passive mics), a diaphragm with an attached conductive coil of wire moves in reaction to the sound waves around it.
The Shure SM57 (pictured) is an excellent example of a commonplace moving-coil dynamic microphone.
The conductive coil is suspended in a tiny cylindrical space between magnets. The diaphragm and conductive coil move inside a magnetic field and an electrical mic signal is created in the coil via electromagnetic induction.
The induced audio signal is then sent to a step-up transformer and outputted from the microphone as the mic signal.
All of the parts of this moving-coil dynamic microphone are passive. Here is a list of the parts:
- Diaphragm (moves in accordance to sound waves)
- Moving-coil of conductive wire (attached to the diaphragm)
- Magnets (provide the magnetic field)
- Step-up transformer (improves signal strength at the output)
For more information on moving-coil dynamic mics, check out my article Moving-Coil Dynamic Microphones: The In-Depth Guide.
The step-up transformer actually blocks DC from entering the microphone (DC voltages are most often used to power active microphones). So not only does the typical passive dynamic microphone not require power, it actually is designed to reject power!
Active Microphones Do Require Power
A microphone is considered active if it has any of the following active devices in its design:
- Internal preamp/impedance converter: These are typically transistor circuits (often JFETs) that convert the low voltage high impedance signals of condenser capsules to a higher voltage, lower impedance signals. Other types of internal preamp circuits exist for ribbon microphones.
- Non-electret condenser (capacitor) capsule: Before electret materials made their debut on condenser capsules, the capsules had to be powered externally. These “true” condenser capsules require an external voltage to become properly polarized and transduce energy.
- Vacuum tubes: Vacuum tubes require electricity to heat up the tube’s cathode and to send a positive change to the anode.
- Analog-to-digital converter: ADCs need power to convert analog signals into digital information.
Each of the parts listed above requires varying amounts of power to operate and it only takes one of these devices for a microphone to be considered active.
First, let’s list out the common types of active microphones along with the active pieces that are incorporated in their designs.
- Electret condenser mics: electret condensers require power for their internal active preamps/impedance converters. These converters are typically some sort of field-effect transistor. Their capsules are pre-polarized with electret material and do not require external power.
- Examples: Sennheiser ME2 Lavalier Mic and Rode NT1-A Studio Mic
- “True” condenser mics (non-electret): “true” condenser microphones are solid-state condensers that came before electrets and after tube condensers. These mics require power for their internal preamps/impedance converters and to polarize their condenser capsules.
- Example: Neumann U87AI Studio Mic
- Active ribbon mics: Because ribbon microphones typically output very low-level signals, some modern ribbon mics have internal preamplifiers, which require power to function.
- Example: AEA R84A
- Tube condenser mics: Vacuum tubes are the “vintage way” of amplifying the signals of condenser mics to usable levels. Tube mics require power for their tubes and to polarize their “true” condenser capsules. As far as I know, there’s no such thing as an electret tube microphone.
- Example: Neumann U47
- USB mics: USB microphones have built-in analog-to-digital converters that require power in order to function. A great percentage of USB mics are also electret condensers (though not all), meaning most USB mics also require power for their internal preamps/impedance converters.
- Example: Blue Yeti
As we can see, there are many types of active microphones. On top of that, we see that condenser microphones are not the only mics that require power!
Now that we know of the active parts of active microphones, let’s look at effective methods of providing the required power to these active parts:
- Phantom power.
- DC bias voltage.
- Battery power.
- External power supplies.
- USB power.
Rather than explain each of the listed powering methods briefly, let’s get into each in detail.
What is phantom power? Phantom power (P48 or +48V) is a DC voltage of 48V +/- 4V (professionally). P48 is provided by mic input preamps and works with balanced audio lines, sending +48 volts through 6.8 kΩ resistors up pins 2 and 3 (relative to pin 1). P48 is “unseen” in a balanced line, hence the name “phantom power.”
Phantom power is the most common method of powering professional studio microphones.
The studio mics that require phantom power are designed to take precisely the amount of power they need while studio mics that do not require phantom power are designed to effectively reject it.
Note that some vintage ribbon mics may be damaged by phantom power, especially if the mic is disconnected from phantom power abruptly (removing a connected cable or a brownout).
For more information on phantom power and ribbon microphones, check out my article Will Phantom Power Damage My Ribbon Microphone?
Phantom power is supplied by pretty much every modern mic preamp worth its salt (in audio interfaces, mixing consoles, audio recorders, etc.). However, not all phantom power supplies output the full +48 V. Therefore, it’s important to use quality P48 sources with mics that need it.
You can check the phantom power of your mic inputs with a voltmeter.
Some mics will perform at any amount of “acceptable” phantom power (11 V to 48 V). Other mics will still perform, but at a hindered capacity, if the full 48 V is not provided. Yet other mics will not perform at all unless receiving the full 48V +/- 4V phantom power.
DC Bias Voltage
What is DC bias voltage? DC bias voltage is typically between 1.5 – 9.5 V DC. DC bias is generally used to power the JFETs of electret mics (the preamps/impedance converters). Bias voltage does not require a balanced audio line to work and it can be supplied on either the same line as the audio or on a separate line.
DC bias is perhaps the most common method of powering microphones in general (considering most microphones are not studio mics, but mics in cell phones, laptops, etc.).
DC bias voltage effectively powers the JFETs of many electret condenser microphones, which is required for proper mic functioning.
Electret condenser microphones output low-level signals with incredibly high impedances. These signals are unusable as audio and are only capable of travelling short lengths of cable before losing their quality.
JFETs (Junction Gate Field-Effect Transistors) are put inline directly after the electret condenser capsule (to minimize any cable length the signal must travel through). JEFTs work on the provided DC bias voltage to amplify the electret signal (see preamp). More importantly, though, the JFET converts the extremely high impedance to a usable mic signal impedance (see impedance converter).
What is microphone battery power? There are two main scenarios where a mic may be battery powered:
- Wireless mics will need batteries to power the wireless transmitter and oftentimes the mic preamp/impedance converter (often with a battery supplied DC bias).
- Studio mics that have an option for battery power but will also work with phantom power.
Battery power may supply the aforementioned DC bias voltage for lavalier and other wireless microphones.
It may also supply the polarizing voltage or preamp power for studio microphones with an option for batteries. In this scenario, the batteries provide a similar voltage to phantom power.
External Power Supply
What is an external microphone power supply? An external microphone power supply provides mic specific power to a mic (oftentimes more than phantom power’s 48V). These mic specific power supplies have no standard and use various connectors including the 7-pin XLR (carrying signal, polarization voltage, heater, and providing ground).
External power supplies are very common with tube condenser microphones. Tubes require more energy than transistors and often this need for power cannot be met by the 48 volts of phantom power.
Therefore, microphone specific external power supplies have been designed to heat and power the vacuum tubes while also supplying the proper polarizing voltage to the condenser capsules.
Note that some transistor microphones also use external power supplies (namely the DPA 130V microphones). These are solid-state microphone that simply require more power than phantom power could provide them.
What is USB microphone power? Active USB microphones are designed to draw the power they need from the +5V DC pin 1 of the USB connector. This +5 volts is almost always exclusively used to power the analog-to-digital converter of the USB mic along with its JFET mic preamp/impedance converter.
As mentioned, USB microphones require power for their analog-to-digital converters (and for their JFETs if they’re condenser USB mics). This power is supplied by pin 1 of the USB connector.
USB mics are designed to take what they need from the +5 volts provided by the USB connector (and ultimately by the connecter computer).
Do wireless microphones need phantom power? Wireless microphones do not require phantom power. Wireless mics run on batteries, which provide power to the wireless transmitter and, if the microphone is a condenser mic, a DC bias voltage a microphone’s JFET impedance converter (internal preamp).
For more information on wireless microphones, check out my article How Do Wireless Microphones Work?
How do you check phantom power? Typical mic inputs that supply phantom power will have an indicator that phantom power is on (usually a light). To check the exact voltage of your phantom power supply, used a voltmeter. Full phantom power should show 48V between pins 2 and 1 as well as pins 3 and 1 and 0V between pins 2 and 3.