Going Direct: The Ins & Outs Of DI Boxes

Mikail Graham, sound engineer at the Grass Valley Center for the Arts, tells me, “I tend to use active DIs for bass and guitars, with passive units primarily for keys and various processors.” He adds that “keyboards and rhythm boxes, as well as the ever encroaching array of vocal effects processors, also benefit greatly when used with a DI.”

Nick Malgieri, AV manager and audio engineer at Stanford’s Bing Concert Hall, states, “Generally speaking, I prefer active DIs. I find the higher output and high-presence tone to be better for most applications. Passives work as well and are often preferred by rock ‘n’ roll engineers who prefer a softer, rounded tone, although I find them to sound a bit flat and the low output can drive up noise floor.”

The design decision of whether an active DI will allow battery powering or use phantom power from the console relates to the potential limitations that battery power poses to the maximum signal level the DI can handle, as well as the fact that batteries deliver less voltage as they’re used up, and that they can run out of juice in the middle of a show. Units such as the Radial Engineering PZ-DI and the Klark Teknik DN100 deliberately forego the battery option.

There may be circumstances where a convenient source of phantom power is not available at a particular location, and an active DI is necessary; for example to connect mixing consoles in different locations while using the ground lift, or acting as a line balancer. The Countryman Type 10 active DI works with both phantom and battery power, and it also includes a power monitoring circuit with a pair of LEDs and a power-test switch that tracks the relative levels and can transition between sources to maintain the best performance.

I/O Impedance
The output impedance of a passive electric guitar or bass pickup can be in the hundreds of kilohms, and that of an under-saddle piezo pickup even greater – and the impedance varies with the frequency of the note being played.

The Klark Teknik DN100 foregoes battery operation.

In order to transfer the audio signal correctly, rather than attenuating the instrument’s lower or higher frequencies, the input impedance of the next device in the signal chain must be considerably higher. In this regard there are numerous choices; for example, the impedance of the BSS AR-133 is 1 megohm while the Leon Audio Mk 2A is 33 megohms. (Both are active units.)

When connecting an acoustic guitar with piezo pickups, Conley notes, “I like the Radial PZ-DI because of the impedance button on the side of the unit; it helps to match the impedance better and therefore warms the tone.” The PZ-DI has impedance settings of 220K, 1M, and 10M to accommodate devices from lower to very high output impedance.

Passive units have an input impedance about an order of magnitude lower, with typical units in the range of 50 to 150 kilohms. The lower output impedances of keyboards, effects, CD players, and the like – ranging from a few hundred ohms to several kilohms – means that the input of a passive DI will be more than sufficient to accept the audio signal without introducing frequency-response problems.

Some makers offer DI boxes with single and dual channels, as seen here with the Countryman Type 10 and 10S (“S” stands for stereo).

Also, the passive DI can be less prone to overload distortion with high signal levels, since transformers saturate at higher levels rather than distort, which can be more pleasing to the ear.

The XLR output of a DI box is low impedance, similar to that of a microphone, so that it properly interfaces with the mic input of a mixing console. The output level is also closer to that of a microphone, so that the normal range of the channel’s trim control is able to make any fine level adjustments, rather than seeing a signal that is too low or too hot.

Attenuation buttons or “pads” on a DI are often available to make larger adjustments to the signal level before it is sent to the console, with a variety of values depending on the DI, in the range of -10 dB to -40 dB.