Choosing a Preamp for an XLR Microphone

An XLR microphone cannot plug straight into a computer or a phone. Its signal is mic level, which is far weaker than what a computer or recorder expects, so it first needs a preamp to raise it. The right preamp depends on the type of microphone, how much gain it needs and where the audio goes next, whether that is a video call, a recording app or a camera.

Before you start

Choosing and setting up a preamp is a local hardware task. No online service or account is involved. Some USB audio interfaces work with the operating system's built-in drivers, while others need a driver or control software from the manufacturer. Check this on the product page or in the manual for your operating system before buying.

Gather this information first:

• Microphone type: dynamic, condenser or ribbon. This is listed in the microphone's specifications.
• Sensitivity or output level from the microphone's spec sheet. Low-output microphones need more clean gain than average.
• Whether the microphone needs phantom power. Most condenser microphones need it, and dynamic microphones usually do not.
• Where the signal goes next: a computer over USB, an existing audio interface or mixer, or a camera or field recorder.
• How many microphones you need to use at the same time.

Ribbon microphones need extra care. Some models can be damaged by phantom power or by a faulty cable while phantom power is on. Follow the manufacturer's guidance before connecting one to any preamp that supplies phantom power.

Step-by-step instructions

1. Pick the type of preamp that fits your setup

There are four common ways to get a preamp into the chain. Choose the one that matches the device you already own:

• USB audio interface with built-in preamps: the simplest option for computers. It provides the preamp, phantom power, conversion to digital and headphone monitoring in one box, and appears as both an input device and an output device on the computer.
• Standalone preamp: a dedicated unit that outputs a line-level signal. It still needs an interface, mixer or recorder with a line input to reach a computer. It suits people who already own an interface and want a different sound or more gain.
• Inline gain booster: a small device placed between the microphone and an existing preamp. It adds clean gain for low-output dynamic or ribbon microphones and usually draws its own power from phantom power. It does not replace a preamp.
• Mixer or field recorder with XLR inputs: useful for several microphones, for live sound or for recording to a camera. Check whether it can also send audio to a computer over USB.

2. Compare the specifications that matter

• Maximum gain: compare it with what your microphone needs. Low-output dynamic microphones are the most demanding, and a preamp near the end of its gain range often adds audible hiss.
• Equivalent input noise (EIN): a lower figure means a quieter preamp. Manufacturers measure it under different conditions, so compare it with care.
• Phantom power: required for most condenser microphones. Check whether it can be switched per channel or only for all inputs at once.
• Number of XLR inputs: buy for the number of microphones you need at the same time.
• Sample rate and bit depth: relevant for interfaces. Common sample rates for voice and video work are supported by most current interfaces. Check that your recording or calling software supports the same rate.
• Monitoring: a headphone output with direct monitoring lets you hear yourself without the delay added by the computer.

3. Connect the hardware safely

• Turn the gain to minimum and switch phantom power off.
• Connect the microphone with an XLR cable to the preamp or interface input. If you use an inline booster, place it between the microphone and the input.
• If you use a standalone preamp, connect its output to a line input on the interface or mixer, not to another microphone input. Feeding a line-level signal into a mic preamp causes distortion.
• Connect the interface to the computer and install the manufacturer's driver if your operating system requires one.
• Switch on phantom power only if the microphone or inline booster needs it.

4. Set the input level

Speak or play at the loudness you will actually use, at your normal distance from the microphone. Raise the gain until the loudest parts come close to the top of the meter without reaching the clip indicator. Leave headroom for laughter, emphasis or sudden loud sounds. If you reach maximum gain and the level is still low, move closer to the microphone before adding more gain, because extra gain also raises background noise.

5. Select the device on the computer

Open the sound settings of your operating system and choose the interface as the input device. If you want to listen through its headphone output, choose it as the output device as well. Then open the same settings inside your video call or recording app, because many apps keep their own device selection. If the interface has a control panel, set the sample rate there and make sure the operating system and app use the same rate. To undo the setup, switch the input and output devices back to the previous ones.

Check the output

• Record a short test in the app you will actually use and play it back on headphones.
• Watch the meter in the app as well as on the preamp. The level should move with your voice and stay below clipping.
• Record a few seconds of silence and listen for hiss, hum or buzz. Steady hiss usually comes from high gain or the room, and hum often comes from the power supply or ground.
• Check whether the voice plays from both sides. Many interfaces send input 1 to the left channel only, and some apps record it that way.
• In a video call, use the app's microphone test or ask another participant to confirm the level and clarity.

A preamp cannot fix every problem. Room echo, distance from the microphone and the microphone's own character shape the sound as much as the preamp does. Calling apps may also apply their own noise suppression and automatic level control, so what others hear can differ from your local recording.

Common problems and next steps

The microphone is too quiet even at maximum gain

This is common with low-output dynamic and ribbon microphones on interfaces with modest gain. Move closer to the microphone first. If that is not enough, add an inline gain booster or choose a preamp with more maximum gain. Check also that the app's input volume is not turned down and that you selected the correct input channel.

No signal from a condenser microphone

Most condenser microphones produce no signal without phantom power. Switch it on for the correct channel and check the cable. Some interfaces need phantom power enabled in their control software rather than with a button.

Distorted or harsh sound

Lower the gain until the clip indicator stays off. If you use a standalone preamp, confirm its output goes into a line input. Two preamps in series with high gain on both is a frequent cause of distortion.

Hiss, hum or too much background noise

Reduce gain and move closer to the microphone, so your voice is louder relative to the room. For hum, try a different cable, keep audio cables away from power cables and connect the interface to a different USB port or power outlet. A dynamic microphone often picks up less room sound than a condenser, which can help in noisy spaces.

Audio in one ear only, wrong device or crackling

For one-sided audio, set the track or app input to mono or use the interface's mono or direct monitor setting if available. If the app ignores the interface, select it again in the app's audio settings and restart the app. Crackles and dropouts often come from a sample rate mismatch between the interface, the operating system and the app, or from a buffer size that is too small. Match the sample rate everywhere and increase the buffer size in the interface software or recording app.