The lowly direct box. In terms of status in pro audio, its profile ranks right above power cables. Most direct boxes are qualified by one criteria — whether they pass signal or not. Direct boxes (or DIs) are tossed in drawers or on shelves as soon as the gig is over and never discussed unless they break.

A direct box (commonly referred to as a DI, which stands for Direct Input) serves one fundamental purpose — allowing a musical instrument with a 1/4″ output to plug into the XLR input of the sound console. But there’s more to a DI than just a 1/4″ in, an XLR out, and a rugged chassis. What’s that, you ask? The sound. I can sense your skepticism already. “How much difference is there between the sound of different direct boxes? Do you think anyone can hear a difference?” Both good questions. And ones we decided to put to the test.

The Goal

The goal was to see if we could actually hear a difference between direct boxes. When I mentioned this comparison to my coworkers, some of them said, “I’ve never done any A/B listening of DIs,” even though they had compared lots of other components in their recording or live signal chains. It’s interesting how even industry professionals often overlook DIs. Given the wide range of prices among direct boxes, we wanted to figure out if we could “hear the money.” Plus we wanted to give you the chance to hear the differences for yourself.

The Process

Here’s a list of questions that came up when we first started discussing this comparison.

What instrument should we use?

We considered bass or guitar — instruments that routinely use DIs. Ultimately, we decided on bass guitar because bass players live and die by the sound of their DIs, unlike guitarists who’d rather run through an amp for their sound.

What bass should we use?

That answer was easy. We went with the standard for electric basses — the Fender Precision. We chose a Fender American Professional Precision Bass with passive pickups and roundwound strings. To hear how each DI would handle the hotter output of active pickups, we chose the Ernie Ball Music Man StingRay 5 H, which not only sounds great but is also very purple. And then Dave Martin, our resident studio bassist who performed on these tracks, wanted to hear how his vintage P Bass with flatwounds would sound. How could I say no? So we included that too.

Should we audition them soloed or in a track?

Listening to the bass soloed would be incredibly informative, but how many people listen to a bass all by itself? (Insert your favorite bass solo joke here.) In a track would be much more real world. So we decided to use Don Carr’s track titled Centipede Burst that was recorded here at Sweetwater Studios by engineer Nathan Heironimus featuring Don on guitar, Phil Naish on keyboards, and Nick D’Virgilio on drums.

Should we re-amp the signal so that all the performances are exactly the same?

While some suggested a re-amped bass signal for repeatability, that would undermine what we wanted to hear — that critical interaction of the output of the bass and the input of the DI. I felt strongly that removing the coupling of “bass output to DI” by using a re-amp box would be less meaningful. So I decided to use a separate performance played live for each pass. Since our bass-playing robot was on vacation, we had to settle for using a human bass player.

How should we set up and calibrate all the DIs?

Once we decided which DIs to include, we needed to make sure the levels of each performance were matched. We used a 1kHz calibration tone sourced from Pro Tools HDX through a Radial X-Amp re-amp box patched to a 1/4″ cable as our source. The output from each DI was patched into a Millennia HV-35 preamp, and I adjusted the gain to yield -17.9dBFS in Pro Tools going through the Focusrite RedNet interface using an Antelope Audio Isochrone Trinity word clock. We also checked the polarity of each DI with the Galaxy Audio Cricket to make sure that the tip of the 1/4″ cable was connected to Pin 2 on the output XLR. (Not all were.) Thanks to Shawn Dealey for help in setting up our testing procedure.

Does polarity matter on a DI?

Anyone who knows me knows that I’m a real stickler about checking polarity when I do comparisons, for two reasons.

I want to make sure the comparisons are fair, and all the DIs (or other gear) need to have uniform polarity from the input to the output.
Polarity on a direct box can be critically important, especially when it comes to low frequencies.
For instance, say you have a bass player onstage with his bass amp, and you want to also run the bass through the house PA for control. So you add a DI. But if the polarity of the amp’s speakers and the signal coming from the house PA speakers is opposite, then the bass amp speaker may be pushing (forward excursion) while the house system speakers are pulling (rear excursion). The net result is that certain low frequencies will cancel out. And that’s not good. The solution is reversing the polarity of the DI output or flipping the bass polarity at the console.

What about the DIs that have gain and don’t need a preamp?

In this shootout, I included three DIs that include preamps, so they didn’t need an external preamp — the A-Designs REDDI, the Avalon Design U5, and the BAE 1073DMP. Not allowing them to use their own gain stage seemed an unfair representation of their sound, so those three bypassed the Millennia preamp and went directly into Pro Tools.

The Direct Boxes
Behringer DI400P
Go to the Behringer DI400P product page

Passive DI
Thru output
Ground lift
Radial JDI
Go to the Radial JDI product page

Passive DI
Thru output
Ground lift
Pad
Polarity reverse
Radial ProDI
Go to the Radial ProDI product page

Passive DI
Thru output
Ground lift
Pad
Whirlwind IMP 2
Go to the Whirlwind IMP 2 product page

Passive DI
Thru output
Ground lift
A-Designs REDDI
Go to the A-Designs REDDI product page

Active DI
Thru output
Ground lift
16dB gain
Avalon Design U5
Go to the Avalon Design U5 product page

Active DI
Thru output
Ground lift
30dB gain
BAE 1073DMP
Go to the BAE 1073DMP product page

Active DI
Thru output
Polarity reverse
71dB gain
Behringer DI100
Go to the Behringer DI100 product page

Active DI
Thru output
Ground lift
Pad
Radial Pro48
Go to the Radial Pro48 product page

Active DI
Thru output
Ground lift
Pad
Rupert Neve Designs RNDI
Go to the Rupert Neve Designs RNDI product page

Active DI
Thru output
Ground lift
Padded speaker input
Telefunken
TDA-1
Go to the TelefunkenTDA1DI product page

Active DI
Thru output
Ground lift
Pad

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