A computer's built-in sound card is designed for listening, not for quality recording. To connect a professional microphone, an instrument or studio speakers you need an audio interface, an external device that contains preamps, converters and dedicated outputs. Understanding what its various parts offer helps you choose the right one without paying for features you will never use.
What's inside an interface
An audio interface, even a small one, performs four tasks:
- Amplifies the microphone's weak signal up to line level, through the preamp.
- Converts the analog signal to digital (ADC) for recording.
- Converts digital back to analog (DAC) for listening on headphones and speakers.
- Communicates with the computer over USB, managed by a driver.
Inputs
Combo XLR and jack
Many interfaces use combo connectors that accept both an XLR cable, for microphones, and a 6.3 mm (1/4") jack, for instruments or line sources. It is worth counting how many microphones you will use at the same time: a two-person podcast needs at least two microphone inputs.
Instrument input (Hi-Z)
An electric guitar or bass plugged in directly needs a high-impedance input, often labeled Hi-Z or INST. Connected to a regular line input, they sound weak and dull.
Line input
Keyboards, mixers and other devices with line-level outputs do not need preamplification. Line inputs are designed to receive these signals without overloading.
The preamp
The most important figure, often overlooked, is the preamp's maximum gain, expressed in dB. Low-sensitivity dynamic microphones need a lot of gain for spoken voice. With a preamp that offers little gain, you end up running the knob at maximum, where hiss increases.
Other useful parameters to read in the specs:
- EIN (Equivalent Input Noise): indicates the preamp's own noise; more negative values mean less noise.
- Maximum input level: how much signal you can apply before saturation.
- Pad: an attenuator, useful with very loud sources.
Phantom power
Condenser microphones require 48 V phantom power. Almost every interface provides it, sometimes with a single switch for several inputs. In that case, turning it on powers all the inputs in the group. Modern dynamic microphones tolerate it without problems, but some older ribbon microphones can be damaged: check the microphone's manual.
A good habit is to switch phantom power on and off with the speaker volume down, to avoid sudden pops.
Direct monitoring
When recording, listening to your own voice on headphones through the computer introduces a delay: latency. Many interfaces offer direct monitoring: the input signal is sent to the headphones straight from the interface, without going through the computer, with virtually no latency.
Often a "mix" knob sets the balance between the input signal and playback from the computer. With direct monitoring on, you should turn off track monitoring in the DAW, otherwise you hear your voice twice, once delayed.
Outputs
- Main outputs: for monitor speakers, usually on balanced jacks.
- Headphone output: check that it has enough power for the headphones you own; high-impedance models need more power.
- Additional outputs: useful for sending a different mix to another person or another pair of speakers.
Orientation table
| Use | Inputs needed | Important features |
|---|---|---|
| Solo podcast | 1 microphone | High gain, direct monitoring |
| Podcast with in-person guest | 2 microphones | Two headphone outputs or a splitter |
| Singer-songwriter | 1 microphone + 1 instrument | Hi-Z input, phantom power |
| Small band | 4 or more | More preamps, separate outputs |
Drivers and compatibility
Many interfaces work in "class compliant" mode, meaning without installing specific drivers, using those of the operating system. On Windows, to get low latency, the manufacturer's ASIO driver is still recommended. Before buying, it is worth checking that the manufacturer regularly updates drivers for your operating system.
In my work I have seen many crackling problems solved simply by connecting the interface to a USB port directly on the computer, instead of an unpowered hub.
In short
- An audio interface amplifies, converts and connects microphones and instruments to the computer.
- The preamp's maximum gain is decisive, especially with dynamic microphones.
- 48 V phantom power is needed for condensers; check ribbon microphone compatibility.
- Direct monitoring eliminates latency while recording.
- Choose the number of inputs based on how many sources you will record together.
