What is a DI Box And How to Use One in the Studio

We’ve all been there. You have a question about a piece of gear that you’re too afraid to ask. You figure you’ll just fake ’til you make it, but it goes on for so long you decide you finally have to ask…

“What the hell is a DI box and how do I actually use one?”

Don’t worry, we’re here to help!

DI Basics
DI stands for Direct Input. A DI box is essentially just a box for connecting instruments to a console or multitrack recorder.

Its main function is to convert impedance, but realistically, DI boxes are also used for the following reasons:

  • To isolate an instrument and remove noise bleed
  • To record a direct guitar signal for use in mixing
  • To re-amp a guitar signal with a different rig later

DIs are a great way to capture the vibe of a live performance without all of the noise bleed. Bassists and guitarists can use DI boxes to capture a direct, isolated copy of their instrument, while still sending a duplicate of the same exact signal to an amp. This works particularly well for concerts and live recordings.

Many modern engineers choose to record a direct signal when tracking guitars. Some use it during the mix, others simply use it as a safety net just in case the amp mics happen to be unusable. In any such event, there’s always the DI to fall back on.

The direct signal can be run through digital amp simulators, or “re-amped” where the signal is sent through a different guitar amp and re-recorded. Both are excellent options for musicians who want to perform the parts themselves, but don’t have access to a proper recording rig.

How DI Boxes Work
Most DI boxes are all built very similarly. One end features an unbalanced 1/4” input, and the other features a balanced XLR output. Inside, there are basically just transformers.

These transformers are used to convert the high impedance (“HI-Z”) instrument signals, to low-impedance (“low-Z”) signals that can be amplified with a preamp. Impedance is a specialized form of resistance found in analog circuits.

Unbalanced signals use two conductors: “positive/hot” and “ground”. These are the 1/4’ TS cables you’ve seen used on instruments and speakers.

Balanced signals use three conductors: “positive/hot”, “negative/cold”, and “ground”. These are the 1/4” TRS and 3-pin XLRs you’ve seen used in professional studios.

Direct boxes also convert unbalanced 1/4” TS instrument outputs to balanced XLR outputs. This allows you the instrument to be placed as far away from the console or recorder as needed without any signal loss.

Passive vs Active
DI boxes come in two forms — passive, or active. Passive DIs do not require a power source. No batteries, no phantom power from the console; just plug in the instrument to start passing signal. They typically have very basic features:

  • Input Pad — Typically reduces the incoming instrument signal by ~15dB to avoid clipping
  • Polarity Reverse Switch — Reverses the polarity of a signal by flipping the pins 2 and 3 on the XLR. Often used to interface with old pre-AES audio equipment, or to avoid phase problems when combining the signal with a live mic.
  • Ground/Lift Switch — Removes 60Hz hum caused by AC power by lifting pin 1 (ground) on the XLR.

Active DIs, on the other hand, do require a power source like phantom power from the console or internal batteries. However, with great power comes great features!

  • Gain Control — Active DIs are literally preamps and some even offer knobs for gain control.
  • Low Cut Filter — The use of additional power allows active DIs to manipulate the frequency response of a signal.

Decisions, Decisions
Searching for the right DI can be overwhelming. There are a lot of options, and the price ranges can vary dramatically. Some engineers choose to go for the least expensive DIs available, since they don’t technically “do” anything to the signal. But, as anyone with a modded mic will tell you, transformers can have a big impact on the sound.

Radial Engineering has made a name for themselves as "the world’s finest direct box,” by creating durable, high-quality DI boxes for decades. Their J-series features large Jensen transformers, known for extended low-frequency responses. Their less-expensive ProD series features a proprietary Radial-designed transformer, and a slightly more compact design.

Countryman DIs are also staples in professional recording studios, known for their pure, warm and natural sound. Telefunken has also recently made their way into the DI market with Active and Passive boxes that come in both mono and stereo configurations. 

However, some DIs on the market feature as many bells and whistles as a preamp, because they actually are preamps. The A-Designs Audio REDDI use an actual 6N1-P tube, inspired by the classic Ampeg B-15 tube amps.

The Acme Audio Motown D.I. WB-3 inspired by "The Detroit Sound” of Motown’s Hitsville studio. It features a variable input attenuator with a range of -20dB to -60dB, which is perfect for high-level inputs like guitars and synths, as the attenuator reacts to the changing volume of the signal.

Little Labs Redeye 3D Phantom features built-in re-amping capabilities. In DI mode, the instrument output works as a thru signal so you can simultaneously feed your guitar amp and mic pre. In re-amp mode the instrument output signal comes from the line level output of your DAW (converted to HI-Z) to feed your guitar amp, assuring your re-amped guitar sounds exactly the same as it did when you were laying it down.

Another popular choice on the market is Rupert Neve Design's RNDI. Made by audio master Rupert Neve, this DI Box is a Class A, discrete design that offers full harmonic depth on any source that you run through it.

There are countless options to choose from. Do some research and find a DI that offers the features you need at the price point you want.

How to Use a DI
To convert your unbalanced HI-Z instrument to a balanced low-z signal, simply plug an instrument into the unbalanced input on the DI box via 1/4” TS cable, and the balanced XLR output connects to a preamp.

Guitar > Unbalanced 1/4” HI-Z DI Input

Balanced XLR DI Output > Recording Interface Input

Alternatively, you may want to record the sound of the guitar cabinet, not just the guitar itself. Most guitar cabs feature a “Thru” jack, which can be connected to DI box. Just be sure to set the DI to “speaker/amp” instead of “instrument.”

Speaker Cab Thru Output > Unbalanced 1/4” DI Input (in speaker mode)

Balanced XLR DI Output > Recording Interface Input

If you want to split the signal, to record the direct signal of a guitar while simultaneously capturing the sound of the performance through the amp for instance, use the Thru Output to send a duplicate signal to the amp.

Guitar > Unbalanced 1/4” HI-Z DI Input

Thru DI Output > Unbalanced 1/4” HI-Z Guitar Amp Input

Balanced XLR DI Output > Recording Interface Input

Author’s Note: When tracking direct and amp’d guitar signals, you may need to reverse the polarity of the DI box to keep the signals in phase.

Some DI Boxes, such as the Radial J-Series, allow you to use both the 1/4” Input and Thru Output as Stereo Inputs when in Stereo or “Merge” mode. This allows you to convert a stereo 1/“4 outputs to a mono XLR output.

Keyboard Out Left > Unbalanced DI Input

Keyboard Out Right > “Thru” Input (in Thru Mode)

Balanced XLR Output > Recording Interface Input

Finally, after setting up your signal chain, it’s important to check for grounding issues. Isolate the DI track and listen carefully for a 60Hz hum or buzz caused by a "ground loop”. Hopefully, you won’t hear anything, but if you do hear a buzz, try flipping the ground/lift switch to remove it.

How to Re-amp a Guitar
Re-amping a guitar may seem confusing because the signal keeps switching from unbalanced to balanced, or HI-Z to line level, but it’s actually pretty straightforward.

First, record the direct guitar into your DAW like you normally would:

Guitar > Unbalanced 1/4” HI-Z DI Input > Balanced XLR DI Output > Recording Interface Input

Then, use a DI box equipped to convert the line-level signal from the DAW back into an unbalanced HI-Z signal for the amp to process.

Then, simply connect the re-amping box into the amp of your choice using an unbalanced 1/4” TS cable, press play on the DAW, and record the amp using mics as if the guitarist were performing live.

DAW Output > Re-Amp Box line-level input

Re-Amp Box Unbalanced HI-Z Output > Guitar Amp Input

Hear It First-Hand
Now that you know everything there is to know about DIs, the only thing left to do is go out there and use them! Find a DI that brings out the best in your bass or guitar, pick up a proper re-amping kit and get rid of all that nasty noise.

The only way to understand why engineers swear by certain DI boxes is to try them and hear the differences for yourself. So get out there and record some guitar!

If you're interested in learning more about DI Boxes or want to figure out the right set-up for your studio or live setting, please contact one of our Audio Consultants via email or by phone at 888.653.1184.

DI Box Set-Up Images c/o Radial Engineering

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