The choice of DI, and even its switch settings, can make a big difference in tonality. Look for a DI that offers several impedance loading selections such as the PZ-DI from Radial Engineering.
Many situations in professional audio call for a direct feed from an instrument, even when that instrument (or the instrument amplifier) has already been mic'd. The specific reasons for wanting a direct feed vary depending on the instrument and the musical texture that's desired. We'll look at a few common situations below.
But first, to make sure we're all on the same page, let's define what a DI can (and can't) do for you. “DI” stands for “Direct Injection,” a term that goes way back to the formative days of the sound and recording industry. The concept is simple: the DI is intended to interface the output of an instrument, or that of an instrument's amplifier, to the input of the sound system…usually through the mixing console.
DI's can be exceedingly simple – or laden with features and intended to support a certain type of sound or usage. The typical DI is capable of subtracting gain with one or more passive ‘pad' switches that avoid overdriving the DI's input stage. Some active designs can add gain as well, useful when working with very low output instrument pickups. But first and foremost a DI's overriding purpose is to match impedances between the source and the next stop in the signal chain, which is usually a mic preamp or a line level input on the mixing desk. The DI provides a balanced signal output derived from a typically unbalanced input source: that of a guitar, keyboard, or other instrument. The balanced signal is necessary when driving long lines from the stage to the FOH console, when adjacent AC power lines are prone to causing EMI (Electro Magnetic Interference), and in many other situations in which interface or interference problems need to be solved.
In order to balance an unbalanced line, active electronics, a passive transformer, or both must be used in conjunction. Most high quality DI's will employ a purpose-built audio transformer to provide galvanic isolation and high CMR (Common Mode Rejection). CMR is a measure of how well ‘balanced' a balanced line is. The higher the value, the better the rejection of Common Mode Interference. While active electronics can enhance a DI's usefulness, an active DI is not always necessary. Moreover, unless the active DI is also equipped with an isolation output transformer, it will not provide freedom from ground loops (see “Active vs. Passive” below). Worse, some inexpensive DI's are nothing more than a nice looking box with a ¼” phone connector on one side and an XLR on the other, and probably a ground lift switch. No balancing, no galvanic isolation. Buyer Beware!
Let's look at how, and why, DI's are used with some common instruments:
Bass Guitar
Very often associated with a “DI,'” a direct bass signal works well because bass amplifiers are difficult to mic properly and often sound wooly or muddy, especially with low or medium quality mics. By taking a direct signal the engineer will have a solid source to work with. EQ and dynamics can then be applied to enhance tonality.
Acoustic Guitar
Most modern acoustic guitars have built-in pickups, many of which sound very good. By taking a direct feed, mic placement issues are eliminated, feedback becomes a non-issue, and the musician is free to move about as desired. If the artist is using an acoustic guitar amplifier as one of the means of shaping her tone, then it's best to take the direct feed from the amplifier's line output or speaker output, though you may want a second DI on the direct output of the guitar itself. We'll discuss this in more detail below.
Electric Guitar
Most guitarists use their amplifier as a principal element in obtaining a specific tonality; in other words…'that's how they get their sound, dude!' This means you'll want to mic the amplifier so that you're getting the same sonic signature at the FOH console as the artist hears on stage, but you may also want to take a direct feed from the guitar itself - and also from the output of the guitar amplifier - if the situation permits. The purpose might be to round out the low frequency (LF) if it's too thin from the mic'd amp, or perhaps to send the DI channel to the effects bus instead of the mic'd amp channel. There are numerous possibilities but the basic idea is simple: the more signal options you have available, the better your chances are of dialing in a perfect sound.
But let's not kid ourselves…this is only true if you have adequate time and a genuine command of the tools you're using. Otherwise, too many signals coming at you all at once can be a recipe for a very poor tasting meal. But fortunately, in a church setting, it's usually possible to build gradually, making incremental improvements from service to service until full satisfaction has been achieved.
Keyboards
Almost all keyboards are routinely taken as straight DI feeds. There is little value in mic'ing the keyboard amplifier – if one is present – because the PA system is normally higher in fidelity and wider in frequency response than most keyboard amplifiers. Many keyboardists prefer to play solely with stage monitors, though they may want some control over each keyboard's level if multiple keyboards are used. However, some notable exceptions exist.
Hammond Organ
Actually, pretty much any kind of organ. Unless the artist is seeking a 1960's “Doors” type of sound, most organs are played through a rotating ‘Leslie' amplification system. To capture the Leslie sound a pair of microphones should be placed around the Leslie speaker…and in an isolated room if at all possible. A direct feed from a Hammond organ sounds rather dull and uninspiring, but is useful as an emergency backup if the Leslie speaker were to fail during a show. Just don't expect a Hammond DI feed to sound very good.
Electric Piano
Some EP playing styles obtain their sonic signature from overdriving an amplifier, just as guitarists do. In such case it's best to obtain the DI signal from the electric piano's “speaker level” output. That said, if enough channels (and DI's) are available, it's also useful to have a ‘straight' signal from the piano itself. This gives you an extra tool to work with when finessing the instrument's tonal properties.
Strings
Some string instruments have built-in pickups. This is often seen on violins, violas, and cellos that are owned or used by players who regularly perform through sound systems. Pickups on strings do not usually sound as good as properly placed high-grade microphones. But instead of chasing feedback all through the service you can have all the gain-before-feedback (GBF) that you need –– by using pickups whenever they're available. A small sacrifice in tonality is usually worth the benefit of greater control. You'll want the best quality DI's to achieve acoustically realistic results, especially with higher register instruments like the violin.
Contrabass
Also called “upright bass,” “double bass,” and “concert bass,” this instrument needs a very good microphone to capture its full tonal range. Popular choices are 441's. 421's, SM-7's, RE-20's and plenty more. But as important as a mic can be, many contra bass instruments are also equipped with some form of pickup that will require a DI to interface to the mixing console. Some older pickups, especially early piezoelectric types, exhibit very high impedance. The choice of DI, and even its switch settings, can make a big difference in tonality. Look for a DI that offers several impedance loading selections such as the excellent PZ-DI from Radial Engineering. The company calls it a ‘Variable Load Active DI” that combines active technology with a precision transformer to provide galvanic isolation for eliminating ground loops. The potential improvement in sound quality, by choosing among the Radial's 1M, 10M, and 220K impedance load settings can range from subtle to profound, depending on the nature of the pickup.
I recently encountered an older piezo pickup on an acoustic bass that displayed a surprising change in tonality and sustain as the load switch on the PZ-DI was adjusted to different settings. Obviously, some thought and research went into this product. In addition to the load switch, the PZ-DI features a frequency adjustable lo-cut control to eliminate LF contamination from upper register instruments, and a switchable hi-cut filter with a knee at about 3 kHz for reducing ‘edginess' from an instrument's internal pickup or preamp.
