For many levels of nonprofessional producers, one of the common misunderstandings — if not total mysteries — is signal level. When it comes to cable, it's common to think, “If the shoe fits, just wear it.” In the audio world, however, the concept of trying to fit a square peg in a round hole definitely applies. Technical terms such as ohms and impedance do matter very much. Here's a peek at some common signal differences.
THE TOP THREE
Let's begin with the three most common analog signal types: microphone, instrument and line. About 90 percent of microphones direct a mic-level (aka low-impedance or Low-Z) signal to their destination. The cables used for such signals are XLR-to-XLR type, balanced and typically shielded to help prevent signal interference. Hi-Z microphones exist, though they are usually inexpensive and not of professional-grade quality. Those typically function with an unbalanced ¼-inch cable (often a cheap one), making them extra susceptible to noise and interference. Instrument-level (aka high-impedance or Hi-Z) signals typically refer to electric guitars, basses and other instruments retrofitted with magnetic or other pickups. These signals are almost exclusively carried through unbalanced ¼-inch cables that, like XLR cables, are almost always internally shielded (all quality ones will be), despite their unbalanced status. Line-level signals can further muddy the waters of understanding because they make use of several cable types, from balanced and unbalanced ¼-inch to RCA and XLR. Furthermore, the strength of the signal, usually rated at either +4 dBu (professional-grade) or -10 dBV (consumer-grade), can be different from one piece of gear to another. Line-level signals are also Hi-Z, typically the highest of the three. In addition, turntables are a special case; those require a particular type of preamp. Though most turntables' connector cables (usually RCA) may be the same as other line-level devices, due to the limitations of vinyl as a recording surface, phono preamps make use of technology known as the “RIAA compensation curve.” This compensation EQ radically boosts low frequencies while attenuating highs.
WHAT GOES WHERE
All microphones need preamplification. Just about any multichannel mixing console has at least a few built-in mic preamps, and there are an abundance of stand-alone preamps out there. Any respectable mic preamp will have a volume gain (or “trim”) on it, though the amount of gain provided varies widely from preamp to preamp and console to console. I caution against purchasing high-impedance mics and their respective accessories; generally speaking, Hi-Z mics are bundled with, or targeted to, cheap systems such as consumer-grade PA and karaoke systems. If you already are committed to one of those, however, your accessory bag should also contain a step-down transformer, which converts the mic's ¼-inch plug into a balanced XLR output, while simultaneously lowering the impedance. As any live-sound engineer knows, sometimes a console does not have adequate line-level inputs or the engineer has easy access to (or simply prefers) only Low-Z XLR inputs. For Hi-Z instruments such as electric guitar or line-level signals such as keyboards, once again a step-down transformer is used, typically in the form of a direct box, also known as a DI (direct interface or direct input). A direct box usually converts a mono Hi-Z signal (usually via unbalanced ¼-inch cable) into the same signal in low-impedance form, by way of standard balanced XLR jack. In cases of stereo signals, you will need either a dual or two discreet (preferably identical) direct boxes. There are two types of DIs: passive and active. Passive requires no external power supply to function, while active requires batteries or standard +48V phantom power. Passive types can do the job fine, yet active types contain preamplifiers, which can be helpful for signals that need to be carried long distances.
What if you need the signal to go in reverse? “Step-up” transformers do exist for converting lower impedance signals to higher impedance (while typically also changing the connection type in reverse, from XLR to ¼-inch or, less commonly, to RCA). But these transformers are less common for a number of reasons. Impedance (Z) is a name for the amount of resistance to an electrical current in a transmission line and is rated in ohms. If there is less resistance in the line, you'll get a purer signal. Also, balanced cables (almost all XLRs) are far better at rejecting hum introduced into the signal, making them a more suitable choice for long cable runs (where this induction usually occurs). And because XLRs use two internal cables to carry duplicate copies of the audio signal (one inverted specifically for the purpose of rejecting hum), a side benefit is that when those signals are recombined at their destination, the signal automatically receives a +6 dB boost due to a law of physics. It's important to note that cable adapters are not the same as transformers. Just because an accessory converts an XLR into a ¼-inch jack does not mean it alters the impedance. How can you know the difference? If it's in the form of a box (such as a DI), it probably will change the impedance. Smaller adapters often have a switch if they are also transformers; if there's no switch but it's still a transformer, it will likely be labeled as such.
With their unique RIAA EQ curve, specialized turntable preamps are always built into phono-accessible DJ mixers, as well as many vintage and modern stereo receivers. Fewer standard line mixers have dedicated phono inputs, but dedicated phono preamps can be had for a small cost. As with microphones, this preamp is a necessity. Finally, familiarize yourself with the ohms rating of your gear, get to know the different cable and connection types and know the inputs and outputs of your gear.