Mixed Signals

Mixed Signals
Our Guided Tour of Mixer Topography Helps You Go With the Signal Flow

Page 1, 2, 3, 4, 5, 6, 7, 8, 9

Going Through Channels
The number and type of input channels govern the number and type of devices that can connect to a mixer. For example, if you want to record a large ensemble using a dozen microphones simultaneously, you’ll need a mixer with at least 12 mic inputs. Similarly, if you have a 16-track recorder, and you want to hear all the tracks played back at once, you need 16 inputs capable of handling the type of signals output by your recorder.

FIG. 2: This simple block diagram illustrates the signal path through one channel of a typical analog mixer. The blocks with extra arrows represent the aux send, group, and main buses fed by all mixer channels.

If you are intimidated by the sheer number of knobs, buttons, switches, and faders on larger mixers, take comfort in the fact that most of them are channel controls. If you know how one channel works, you understand them all. Typically arranged in vertical “strips,” mixer channels have controls for modifying and routing input signals. The controls are organized in a manner that more or less follows the signal’s path through the channel. This path varies from mixer to mixer, but there are more similarities than differences, so we’ll refer to a generic “channel” (see Figs. 1 and 2).

The In Crowd
All sorts of things can be connected to a mixer’s inputs, from cheesy high-impedance microphones to state-of-the-art samplers. But if a device has analog outputs, it usually falls into one of two groups: line-level sources and microphone-level sources. Channel strips typically have inputs for both types, usually accompanied by a mic/line selector switch. In many mixers that lack a mic/line switch, plugging a device into the line input automatically disables the mic input. However, some mixers allow you to use both types of inputs at once.

FIG. 1: This generic channel strip shows the controls for the mic preamp, auxiliary bus, EQ, and fader sections.

Line-level inputs are high impedance (see the sidebar “Impeding Progress”) and typically employ 1/4-inch phone jacks or RCA phono connectors. Keyboards, samplers, drum machines, the outputs from most recording decks, effects processors, and instrument preamps can all connect to line-level inputs. For the most part, you use mic inputs only for microphones and direct boxes.

Because many line-level sources have stereo outputs, it is increasingly common for mixers to have one or more stereo input channels—that is, a single channel strip that handles two input signals. The two inputs in a stereo channel are most often treated as a single source—with one fader, one EQ section, and so on—making it unnecessary to dial in duplicate settings for the left and right signals. Stereo input channels sometimes have fewer features than regular channels because they are most often used for modern synths and samplers that have onboard effects, EQ, and level controls. The idea is to get the instrument’s sound into the mixer and route it with little or no processing.

An additional type of line-level input is the tape return. This term originated when all recordings were made to tape, but it now designates inputs that route playback signals from any type of multitrack recorder back into the mixer. I’ll deal with how signals get routed to and from multitrack recorders shortly.

Microphone-level inputs are low impedance and typically employ female XLR connectors. Because mixers operate at line level, low-impedance microphone signals must be boosted to line level using a microphone preamp. Channel-strip mic preamps generally include a trim (or gain) pot that adjusts the input sensitivity, a pad (or attenuation) switch for reducing very hot signals (typically by 10, 20, or 30 dB) to the range covered by the trim pot, and a clip (or overload) LED that lights as the signal approaches the upper limit of the input circuit’s capacity, usually around –3 dB.

While these characteristics are the same for the majority of mixers used in professional applications, some confusing exceptions exist. First, microphones can be high impedance, and these must be patched to high impedance inputs, usually on 1/4-inch phone jacks. Second, some older mixers and compact newer models use 1/4-inch jacks for low-impedance mic connections because 1/4-inch connectors are smaller and less expensive than XLRs. Some mixers use 1/4-inch, switchable mic/line inputs on certain channels; these mic/line channels include mic preamps that are in the circuit only when the input is set for mic-level operation.

If your mixer offers only 1/4-inch mic inputs, check the manual to determine their impedance and mic/line status. This is important, as low-impedance microphones function improperly when connected to high-impedance inputs, and vice versa. You can circumvent this problem by using “step-up” and “step-down” transformers. Besides altering the level, they also function as adapters, converting one type of connector into another. However, they can also affect the sound.

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