Audio Mixers Buyer's Guide: Mix it Up

Paul Suchecki
April 2006

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It can be something as critical as taping an interview at home with the new governor, when she suddenly invites her husband to join her. It could be a garage band that persuades you to tape a performance; or it could be something as intimidating as taping several gang members exaggerating their exploits. To combine these diverse sources you'll need an audio mixer.

Take the street gang as an example: One thug's voice is barely a growl; another's cracks like an excited teen; a third is so angry that he practically shouts; and our interviewer's speech is as resonant as an anchorman's. An audio mixer isolates multiple input sources in separate channels where you can shape each sound. Shouts can be subdued, whispers boosted, nasality cut as you mix down to the two channels most of us use in video, producing a harmonious whole.

The most common mixer set up features a series of vertical strips, also known as buses or channels. Although we'll look at compact portable mixers later, this studio style is best for shooting musicians, doing full-scale electronic field production or linear post production. For non-linear editing, a mixer can act as an instant patch bay when you can route audio quickly by pressing a few buttons.

When looking for a mixer, determine how many and what type of inputs you need. Devices like tape decks and DVD/CD players output a line level signal. It is strong enough to not need shielding and typically uses a ¼" mono phone or RCA, phono, plug. Many mixers will also accept line level through balanced XLR connectors. Professional microphones, however, output a less powerful mic level signal which benefits from shielding and requires more amplification. To fend off noise, mics usually output through balanced male connectors. These are either three pin XLR or stereo plugs, both quarter inch and mini, and are readily identified by the two bands of insulation separating the tip, ring and sleeve. Phonograph inputs require pre-amplification, are mainly found on disc jockey mixers and have RCA jacks labeled "phono." A DJ mixer should have at least two pairs. If you plug a phonograph into a tape deck jack, you probably won't hear it. If you plug a tape deck into a phonograph jack, the audio will distort. Newer mixers include 3.5mm stereo input jacks for iPods.

Digital input/outputs can include FireWire to interface with edit systems, Pro Tools, MIDI, and camcorders; or USB for digital disc recorders. These mixers convert digital to analog to combine with other analog sources, then back to digital for output. When possible, choose at least 96kHz digital processing over CD quality (44.1kHz). For post, two mic level inputs will be adequate; the rest could be line or digital I/Os. For field mixing you should have at least four microphone inputs.

Since a microphone's output varies depending on its construction, the first bus control on many mixers is a volume control known as a trim pot. This stage generates most of the mic noise. The built-in mixer preamp will provide a variable gain to a microphone's signal. Some mixers have pads instead of trim pots which allow you to attenuate devices with too strong an input signal. Other mixers, however, have pots that pad the signal when turned to the left and add gain to the signal when turned to the right.

Many mixers have phantom power for condenser mics, which eliminates the need for an onboard mic battery. The mic is powered in the way that a computer's USB devices are, down the same wires that carry the signal. It's called phantom power because the current is unseen by dynamic mics which generate their own power.