Understanding Preamps and Digital Audio Interfaces

If you’re thinking about getting into digital recording, you’ll need more than just a microphone, a computer, and recording software. This article is an introduction to two necessary and closely related pieces of gear–microphone preamps and interfaces. These are often the missing links in people’s understanding of digital audio. So, what do they do and why are they important?


A microphone’s job is to convert sound vibrations into electrical impulses (or a signal), which can then be recorded, either on analog tape or to a computer hard drive. The initial signal coming from the microphone, however, is very weak, and if you somehow tried to record it directly to disk, you probably wouldn’t hear much of anything on playback.

This is where a preamp comes in. A preamp simply boosts the signal to a usable level (line level). Preamps are found on all kinds of live and studio gear–PA heads, guitar and bass amplifiers, and mixing boards all have preamps, or preamp sections. By turning a knob, you can dial in the appropriate amount of amplification (or gain) from your preamp to get an acceptable recording level. Too much gain, and you introduce noise and distortion (or clipping), too little and your signal will be too weak to record. Most preamps have an onboard LED meter that lets you know when you have boosted the signal to the appropriate level.

Audio Interfaces

After you’ve boosted the microphone signal, it’s time to capture it to disk. There’s just one problem–the signal from the mike preamp is analog and your computer is digital. The two formats don’t mix, so you’ll need a mediator to bring them together. The answer to the problem lies in an audio interface, which has an analog to digital (A/D) converter.

The soundcard your computer probably came with is an A/D converter, but you need a special kind–one up to the task of converting multiple analog signals to at least CD-quality digital audio–which most PC and Mac soundcards are not capable of doing. A recording quality A/D converter accepts the analog signal from your preamp, encodes it in real time as computer-friendly binary code, and then transfers that data to your computer via either USB or FireWire.

FireWire is much more powerful than USB, meaning you can record a lot of tracks simultaneously. (More than 48!) While much more limited in the bandwidth department, USB interfaces are considerably less expensive, plus every computer made in the last decade comes standard with USB capabilities. The same is not true for FireWire. That means, if you go this route and your computer isn’t equipped with a FireWire port, you may have to make a trip to the electronics store to buy an aftermarket PCI card.

Smart Shopping

Luckily, most interfaces have preamps onboard, so you should only have to buy one piece of gear. For even more value, most interfaces come with recording software, so almost everything you need to get started is right in the box.

To find the best fit in a preamp/interface combo, figure out what you want to use your studio for. If you’re a solo acoustic singer songwriter, then a USB interface with two or four mike preamps should fit your needs perfectly. If you want to track your entire band, drum set and all, consider a Firewire interface with at least eight mike preamps.

Before you make a purchase, it’s important to check for compatibility with your computer. This information should be readily available on the manufacturer’s website. Check the interface’s minimum PC/Mac requirements, as well as what equipment and software it either does or does not work with.

It is highly recommended that you check consumer reviews of any gear that you’re thinking of buying. There are many experienced audio gear professionals and serious hobbyists that post comments and answer questions on audio forums. Common problems they can clue you in to are noisy preamps that don’t provide enough gain, unreliable converters that introduce popping and clicking into your signal, and durability issues (e.g., poor quality knobs, buttons, and connectors). By doing your research, you’ll become an expert on which products give you the best bang-for-your-buck, and which ones are better left on the warehouse shelf.


Decoding the Specs

When you’re shopping for an interface, reading the specs to get an idea of the unit’s capabilities is the best way to start. Of course, it helps if you know what you’re looking at. As an example, let’s take a look at M-Audio’s Fast Track Ultra and break down some of the key features.

M-Audio Fast Track Ultra

A/D Resolution: 24-bit/96kHz

This is the quality of the analog to digital conversion.
A good interface should be at least 24-bit/44.1kHz.


Number of Preamps: 4

This means you can attach up to four microphones directly to the unit. The first two inputs can also be used for instruments.

Audio Output Types:
6 x TRS,
2 x TRS (Headphone)

TRS (tip, ring, sleeve) are stereo jacks. You can plug headphones directly into the headphone jacks, and the six outputs can be hooked up to your playback monitors, with outputs to spare for additional headphone mixes.


Digital Input Types:
1 x S/PDIF (Coax)

TRS inputs accept analog signal, but S/PDIF jacks accept digital signal. You can take an S/PDIF output, which carries two tracks of audio, from another A/D converter and route it through the Fast Track Ultra into your computer.


Phantom Power: Yes

Condenser mikes need phantom power, so make sure your preamps have this capability onboard.


Computer Connection Type: USB 2.0

After the audio is digitized, the information is sent to the computer via USB cable.


Audio Inputs:

6 x TRS, 2 x 1/4″ (Insert)

4 x XLR refers to the microphone preamp inputs; 6 x TRS jacks are set up to receive analog signal from another line level source such as preamps. The inserts allow you to patch outboard effects into the signal.

This article is from our March-April 2011 issue. Click to order!


what preamp/interfaces are good out there , one that has that packaged 2 in 1, i would want to have one that is good and at least 8 channels, any suggestions?

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