Choosing a DI Box for Your Recording Session

Choosing and using a DI box for the studio and stage
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Fig. 1. The Demeter VTDB-2b Tube Direct produces a full, sparkly tone that’s great for recording acoustic and clean-sounding electric guitars

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IF YOU’RE not using a direct box to record some of your instruments and in live performance, your productions are probably coming up short of their true potential. A direct box—also known as a direct-injection or DI box—with additional processing will often produce the best-sounding track, especially if your tracking room’s acoustics sound crummy. And on stage, using a direct box can mean the difference between pristine sound and muffled trash polluted by noise.

Some DI-circuit designs will give you better results than others for a given application. In other cases, using a DI box isn’t even necessary. Use the following tips to decide when to go direct and which DI model is best for you. For the uninitiated, I’ll begin with a 30-second primer.

Avoid Line Inputs To ward off dull, choked, and lifeless sound, never plug your electric guitar, acoustic guitar (assuming it has a pickup), or passive electric bass into a line input. Plug your instrument into the unbalanced, high-impedance input for a DI box instead, using a standard guitar cable. Use a mic cable to connect the balanced, low-impedance XLR output of the DI box to a mic input on your I/O box, mixer, or external mic preamp. If your instrument is fitted with magnetic pickups, the DI box will maximize its high-frequency response, producing a more present and sparkly sound. It will also dramatically extend the bass-frequency response of an acoustic guitar fitted with piezo-electric pickups, yielding a fuller and rounder tone.

By repelling noise, a DI box will also help you record cleaner tracks. The DI will reduce pickup of hum and buzz from lighting fixtures and AC outlets. It will also muzzle static-y RFI (radio-frequency interference) from your computer, outboard digital effects units, and nearby radio or TV stations.

Check the Impedance The higher the DI box’s rated impedance, the more extended your passive instrument’s frequency response will be. All other things being equal, a DI box with an impedance of 10 Megaohms will produce more sparkly highs than a DI box that has a 1-Megaohm impedance, when used with a passive instrument fitted with magnetic pickups. Similarly, the higher the DI box’s impedance, the more extended the bass-frequency response will be for an instrument fitted with piezo-electric pickups.

Very high impedance is obviously important when recording acoustic guitar, where a full tone with sparkly highs is of paramount importance. It’s not always necessary or desirable, however, when recording electric guitar with an amp-simulation plug-in. The speakers in guitar amps generally roll off highs above 5kHz. Extending your DI’d electric guitar’s highs far above this characteristic roll-off point could potentially make overdriven tones sound fizzy or icy. On the other hand, clean amp-sim patches sound terrific when you feed them your guitar patched through a DI box with sky-high impedance. Owing in large part to its stratospheric 27-Megaohm impedance, the Demeter VTDB-2b Tube Direct sounds killer on Strats and Teles, producing chime-y tones that stand on their own even without an amp sim or other processing.

Check the Design Before You Buy Most of the tube-based DIs I’ve used over the years sound great. But if you perform live, there’s another reason to use tubes: Assuming the DI’s unbalanced output follows the tube input buffer (which is usually the case), the unbalanced output will sport a constant impedance that will preserve high-frequency response over a long cable run to an amp. If you need a 30-to-40-foot leash to run around on stage, choose a box that uses this type of circuit design; models include the AMB Tube-Buffered Direct Injection Box (distributed in the U.S. by The John Hardy Company) and the aforementioned VTDB-2b Tube Direct.

Some tube DIs use electronically balanced outputs, placing semiconductors in the signal path. All-tube boxes, on the other hand, use transformer-balanced outputs. If you need to run a long cable to an amplifier, choose an all-tube DI to drive the line. The transformer will reduce the pickup of hum.

Skip the Box? Synthesizers and active electric bass guitars don’t require a DI box. You can plug these instruments directly into a line input without compromising their tone. But for all passive instruments that use a magnetic or piezo-electric pickup, a well-designed DI box will give you cleaner tracks with much higher fidelity.