Review: Radial’s HDI High Definition Studio DI Is a Unique Tone-Shaping Tool for Guitars

I found the HDI to have a robust sound and a very useful range of colors that should enhance any guitar pickup.

Acoustic guitar pickups offer convenience when recording, but often produce thin, sterile tones. To address the issue, Radial, known for making high-quality direct boxes for stage use, has introduced the HDI High Definition Studio DI, a unique tone-shaping tool for guitars as well as other instruments. 

The HDI is most at home in the studio, with a solidly built, AC-powered, two-space half-rack design. The active DI has a clean front panel that features a VU meter; Level, Color, and Presence controls; switches for high pass filter; input impedance selection; and an optical compressor. A power switch and 1/4-inch input jack, as well as a 1/4-inch through output jack and ground lift, complete the front-panel controls. The back panel has XLR outputs for both mic and line level—capable of delivering up to +25dBu—as well as a 1/4-inch output and additional input jacks.



The key to the HDI’s sound is its two parallel signal paths, one clean, the other heavily colored. Both paths incorporate a high-quality Jensen transformer, but the clean path uses negative feedback to produce a very linear response, while the colored path emphasizes the saturation potential of the transformer and adds a tube-like tone. The Color knob controls the balance between the two paths. 

The clean path supports an optical compressor with two compression thresholds. The Presence control adds a high frequency boost, while engaging the high-pass filter switch rolls off lows below 40Hz or 100Hz. You can also choose between an input impedance of 200K or 2M ohms.



I found the HDI to have a robust sound and a very useful range of colors that should enhance any guitar pickup. The Color control makes it possible to dial in anything from a clean but warm tone to crunch-style distortion. When triggered with more aggressive strumming, the optical compressor pleasantly smooths out the sound of the clean path. The Presence control can add just a touch of clarity to the sound, or a strong shimmer that would make a rhythm acoustic guitar pop in a mix. Together, these controls—which interact to some degree—offer a rich set of tonal options to beef up the sound of a pickup. While the HDI produces excellent tone with any setting, I leaned toward the middle range of the Color control, exploring subtle changes that leveraged both signal paths.

The HDI’s form factor might be optimized for the studio, but it would be a welcome addition to a stage rig as well, and its multiple outputs provide flexible routing and interesting options for live sound. With a street price of $799.99, the HDI is an impressive, professional-quality piece of gear for the money. 

This article originally appeared in the November/December 2020 issue of Acoustic Guitar magazine.

Doug Young
Doug Young

Doug Young is a fingerstyle instrumental guitarist, writer, and recording engineer. He is the author of Acoustic Guitar Amplification Essentials.

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