After microphones, direct boxes are the most common audio input devices on stage. Though seemingly simple, direct boxes are vital to the clean transfer of audio from source to system. Originally termed DI (Direct Injection), direct boxes usually accept an unbalanced hi-Z signal from a bass guitar, acoustic pick-up, keyboard, or laptop and convert it to a low impedance balanced signal suitable for a console or stage box. Understanding how they work and when to use each type is a handy skill to possess, so here are some steps toward that goal:
1. DIs are typically used to interface stage instruments with the sound system.
A standard unbalanced ¼” TS (Tip Sleeve) instrument output will not transmit more than about fifteen feet without succumbing to noise and interference, making the DI a perfect solution since it converts the output into a balanced mic signal able to travel 100’ or more to the console without interference, thanks to common mode rejection.
2. Initially direct boxes were passive, meaning they require no external power to operate.
A transformer inside the DI is responsible for matching the impedance the mixer requires with that of the instrument. While passive DIs worked with most instruments, the sound of Rhodes electric pianos and single coil basses were altered, leading to the introduction of active DIs with additional circuitry. As its name implies, an active direct box contains electronics to gain and shape the signal, and those circuits need power to operate. While battery power was an option, most current active DIs use 48V phantom power coming down the XLR cable from the console to provide the energy needed. A general rule of thumb is to use passive DIs on active instruments such as keyboards and active DIs on passive instruments such as a P-Bass.
3. Good direct boxes are worth their cost.
While a $25 DI will function, it will not transfer the true character of a $4000 guitar. Think of a direct box like a tire on a car. A Ferrari 488 will simply not perform as designed on $59 Pep Boys tires. For passive direct boxes, the low-cost standard is the Whirlwind IMP2. It has a decent transformer to prevent saturation on most instruments and is built tough enough to survive years on the road. The Radial PRO series is the most common step-up model with both mono (D1) and stereo (D2) versions available. High end passive DIs can run hundreds of dollars and often boast Jensen brand transformers. Active DIs start around $100 for decent units and go up from there. Some high-end versions include pass filters and tonal compensation. Finally, tube-based DIs, such as the REDDI, are popular in studios and have some application in live environments, if given proper care.
4. Consider remote and re-amping situations.
The Radial SGI (Studio Guitar Interface) is used to create what is essentially a really long guitar cable. A balun is a balanced/unbalanced device and makes it possible to send short-range signals (HDMI, guitar) over distance without issue. The SGI uses a transmit (TX) and receive (RX) module to allow the remote use of a guitar on stage while the amp is in the back room. The primary worship application is to remove the stage volume of a cranked tube amp to an unpopulated area. Re-amping is running a signal through an alternative signal chain and can be helpful when trying to recreate Gateway, Bethel, and Hillsong tonality with limited gear.
5. Use a stereo direct box to fix media audio problems.
Audio from laptops and other media players is notoriously unstable. The tiny 3.5mm jack and cheap sound card behind it always have noise and hum as part of their output. A solid DI designed for stereo line level signals will eliminate almost every issue. It is a good idea to have one on the stage plus one in the tech booth.
Direct boxes can transfer clean audio from stage to console and do so with little cost relative to the system. Keep several on hand and remember to use active DIs on passive signals and passive DIs on active signals.