Recording - May 2010 - 48

from several sources like sound effect libraries, and musical keyboards or drum machines. We’re frequently asked by people who use outboard mic preamps (or no mics at all) but have a lot of synths, why they have to pay for mic preamps that they don’t need. There are also those who believe that analog summing adds something organic to a purely digital system and are seeking a mixer for the line level outputs of their multi-channel DAW audio interface. There are a few line-level mixers around, but they’re pretty rare. If all your sources are line level, take a small degree of comfort in the fact that the mic preamps in many of today’s budget priced mixers add very little to their cost, so you’re not really paying very much for something you don’t think you need. Today, the most common mixer front end design consists of a mic preamp with an attenuator (or pad) between the line input jack and the mic preamp stage. Intuitively, starting with a high-level source, attenuating it to mic level, and then amplifying it back to its original level seems like it would be both inefficient and noisy. This really isn’t as silly as it sounds, however. Since the output impedance and level of the source aren’t easy to predict (there’s no standard, just a fairly large

ballpark), the designer would need a low (near unity) gain buffer for the line input to match it to the innards of the mixer circuitry. This doesn’t really cost significantly less to build than a mic preamp. Padding the mic preamp input with a few cents’ worth of resistors gives you a balanced line input at about the right gain and impedance. Since mic preamps are designed to be very quiet and transparent (even the cheapest of them aren’t all that bad), the extra stage of electronics following the line level source doesn’t really do any harm.

Purists can, and often do, use a channel insert return as a mic-pre-free line input. Since most insert inputs are unbalanced, the elimination of one potential noise source (the mic preamp stage) must be juggled with the possibility of increased noise pickup through the unbalanced input. It’s usually a wash. Since most of today’s line-level sources are stereo it’s fairly common for mixers to have a few stereo line-only channels. Stereo channels have two input jacks, but a single set of “strip” controls. The main fader adjusts the level of both inputs, the equalizer applies the same curve to both channels, and aux busses, which are almost always mono, get the sum of the left and right channel inputs. The pan control works like the balance control on a home stereo receiver. Stereo line-level channels are great for connecting synths or bringing the output of a stereo effect processor into the mix. It’s fairly common for the two input jacks to be “normalled,” allowing a mono source to be plugged into just one of the jacks. In this mode, the channel works like a conventional mono channel with the pan control moving the source from left to right. (Be careful when counting channels and inputs on a mixer like this. A “12 channel” mixer might have only eight sets of channel controls, four for mic/line inputs and four for stereo line inputs.)

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Recording - May 2010

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