Radial HDI High Definition Studio Direct Box

Introduction

Thank you for purchasing the Radial HDI, a unique and powerful direct box and studio tool designed to create amazing audio for recording or live performance. This manual will help you familiarize yourself with its features and get the most out of the device. For additional resources and frequently asked questions, please visit www.radialeng.com.

Overview

A note from the HDI design engineer W.C. "Hutch" Hutchison:

The HDI Direct Box stems from the experiences of a live sound and recording engineer, aiming to provide the signal processing needed to make a DI sit well in a mix, or to emulate the sound of a great old tube amp. It was designed to be cleaner and more transparent than traditional DIs, blurring the line between instrument and hardware, becoming an extension of the instrument itself. The goal was to offer appealing tonal colors and characteristics, inspiring musicians and engineers alike. Simplicity of use was also key, allowing users to achieve great sounds quickly, with "instant gratification" being a primary feature.

The HDI combines the functionality of a direct box with the musicality of a guitar amplifier, offering a unique sonic landscape for studio recordings and live performances.

Front Panel Features

The front panel of the HDI features several controls and inputs:

• 1. IN/THRU: Standard 1/4" TS input for mono instruments (top jack). This input overrides the rear-panel 1/4" input when used. The THRU (bottom jack) is a 1/4" TS unbuffered passthrough to feed a stage amplifier.
• 2. HI-Z: Adjusts input impedance at the 1/4" inputs (200k Ohms or 2Meg Ohms) for tonal options with passive pickups or piezo transducers.
• 3. LEVEL: Input gain control. Higher settings drive the signal into the Opto compressor and Color control's distortion channel for increased saturation.
• 4. OPTO: A 3-position switch for an internal opto compressor circuit, providing smooth dynamic control on the clean channel. The LED illuminates when gain reduction is active. This control only affects the clean channel of the Color knob.
• 5. COLOR: Blends between an ultra-clean DI channel (counter-clockwise) and a unique distortion channel with transformer saturation and amp-like distortion (clockwise). This control interacts with the Level control, acting as a 'drive' control.
• 6. HPF: A 3-position switch for High-Pass Filter settings, affecting only signals processed through the Color control's distortion channel. Options include bypass, 40Hz rolloff, and 100Hz rolloff (-6dB/octave).
• 7. PRESENCE: Emphasizes high and high-mid frequencies for added bite and presence in a mix. Flat when fully counter-clockwise.
• 8. LIFT: Ground Lift switch. Disconnects the HDI audio ground from the chassis ground to eliminate hum and buzz from ground loops.
• 9. POWER: On/off switch. The Output Meter LED illuminates when powered on.
• 10. OUTPUT METER: A wide-range peak meter indicating the signal level at the Line-Level XLR output.

Rear Panel Features

The rear panel provides various input and output connections:

• 11. POWER INPUT: IEC power supply connection. Ensure the front panel POWER switch is off before connecting.
• 12. 15dB PAD: Reduces the Line-Level XLR output signal by 15dB to prevent clipping interface inputs, preserving front panel control relationships.
• 13. LINE-LEVEL OUTPUT: Jensen transformer-equipped balanced XLR output for direct connection to line-level inputs of consoles or interfaces.
• 14. MIC-LEVEL OUTPUT: Balanced low-level XLR output for stage snakes or mic inputs, featuring a Jensen transformer for isolation and mic-level signal.
• 15. PROCESSED OUTPUT: Unbalanced 1/4" TS output carrying the tone-shaped signal, suitable for stage amplifiers or amp profilers.
• 16. SYNTH INPUT: Mono 3.5mm input for modular synthesizers, with lower sensitivity for hotter synth outputs. Overrides 1/4" inputs.
• 17. INPUT: Standard 1/4" TS jack for mono instruments, mirroring the front panel input.

Making Connections

Before connecting, turn down audio system levels to prevent transient damage. Power the HDI using the rear-panel IEC input and the front-panel POWER switch. The Output Meter LED will illuminate when the unit is on.

Connecting a Mono Instrument

Use the front or rear 1/4" TS inputs for instruments like bass or acoustic guitar. The rear input is useful for rack-mounted setups with patch bays. The front input takes priority. A Thru Output (1/4" TS) is provided to send a clean signal to a stage amplifier.

Diagram description: An instrument (e.g., guitar) connects via 1/4" cable to the HDI's input. The HDI's Thru output connects via 1/4" cable to an amplifier. The HDI's XLR output connects via XLR cable to a mixing console or audio interface.

Connecting a Mono Synth

Use the rear 3.5mm mono Synth Input for modular synthesizers. This input offers lower sensitivity for hotter synth signals and allows processing through the HDI's distortion and transformer saturation. It overrides the 1/4" inputs. It can also be used for hot line-level signals from a DAC.

Diagram description: A modular synthesizer connects via 3.5mm cable to the HDI's Synth Input. The HDI's XLR output connects via XLR cable to a mixing console or audio interface.

Using the Outputs

The HDI offers dual XLR outputs: a mic-level output and a line-level output (up to +25dBu). Both are transformer-isolated with Jensen transformers and can be used simultaneously. The line-level output connects directly to recording consoles or interfaces, bypassing preamplifiers. The mic-level output connects to stage snakes or mic inputs. The 1/4" Processed Output is an unbalanced TS connection that carries the tone-shaped signal, suitable for stage amplifiers or amp modelers, and can be used alongside the XLR and Thru outputs.

Diagram description: The HDI's Line-Level XLR output and Mic-Level XLR output are shown. Another diagram shows the HDI's 1/4" Processed Output connecting to a guitar amplifier or amp modeler.

Using the Front Panel Controls

Start with all controls at their lowest position (counter-clockwise) and switches set to their lowest setting.

• HI-Z: Adjusts impedance for tonal flexibility with passive pickups.
• LEVEL: Controls input gain and drives the Color control for saturation.
• OPTO: Engages an opto compressor on the clean channel for dynamic control. Three positions offer bypass, mild compression, and more aggressive compression.
• COLOR: Blends between clean and saturated/distorted tones. Acts as a drive control for distortion.
• HPF: Filters low frequencies on the distortion channel to improve mix clarity. Options: bypass, 40Hz, 100Hz.
• PRESENCE: Boosts high and high-mid frequencies for clarity and cut.
• LIFT: Engages a ground lift to eliminate hum and buzz.

Diagram description: A visual representation of the front panel controls (Level, Color, Presence, Hi-Z, Opto, HPF, Lift) with initial settings suggested as all knobs down and switches at lowest position.

Under the Hood: The Color Control

In its clean mode (counter-clockwise), the Color control uses negative feedback for a transparent sound. As it turns clockwise, the Jensen transformer saturates, adding even-order harmonics, particularly in low frequencies. The distortion channel employs a multi-stage topology similar to tube guitar amplifiers, featuring asymmetrical clipping for natural-sounding grit and distortion.

Additional Applications

• Overdrive a Mic Preamp: Feed the HDI's line-level output into a mic preamp. Experiment with preamp levels and consider adding a high-pass filter (e.g., 8kHz) or guitar cabinet impulse response after the preamp.
• Using Distortion Pedals: Place distortion pedals before the HDI for unique tones or connect the 1/4" Processed Output to pedals that then feed a guitar amplifier.
• Wet/Dry Setup: Use the 1/4" Thru Output for a dry signal to a stage amplifier and connect an echo pedal to the 1/4" Processed Output, feeding that signal to a second amplifier for a wet/dry mix.
• Using the EXTC-SA: Combine the HDI with the Radial EXTC-SA to simultaneously record two channels: one with pedals via the EXTC-SA, and one with the HDI's tone. The EXTC-SA allows direct connection of effects loops and line-level I/O. This enables blending tracks or phase shifting for unique sonic combinations.

Diagram descriptions: Various connection diagrams illustrate these applications, showing signal flow from instruments through the HDI and other gear to amplifiers or recording interfaces.

Rack Mounting the HDI

The HDI can be mounted in a standard 19" equipment rack using the included adaptors. It occupies 2 rack units (2U). Mounting can be left or right-aligned. Two HDI units can be mounted side-by-side in a single 2U space using an additional connector bracket.

Steps for mounting a single HDI:

1. Remove three screws from each side of the front panel (six total).
2. Remove all four rubber feet from the HDI.
3. Align the long and short rack ears on either side of the HDI, using a metal spacer between each piece.
4. Attach the rack ears to the sides of the HDI using the four provided large screws.

Steps for mounting two HDI units side-by-side:

1. Remove three screws from the outer side of each HDI (six total).
2. Remove two screws from the top and two from the bottom of each HDI.
3. Remove all eight rubber feet.
4. Use eight small screws to secure the two HDI units together using the large rectangular braces on the top and bottom.
5. Align two short rack ears on the outer side of each HDI, with a metal spacer between each piece.
6. Attach the rack ears to the sides of the HDI using four large screws per unit.

Specifications

*Specifications are subject to change without notice.

Warranty

Radial Engineering Ltd. provides a 3-year transferable warranty against defects in material and workmanship. To claim warranty service, the product must be returned prepaid with proof of purchase (original invoice). The warranty does not cover damage from abuse, misuse, accident, or unauthorized modification. Radial is not liable for special, incidental, or consequential damages.

California Proposition 65 Warning: This product contains chemicals known to the State of California to cause cancer, birth defects, or other reproductive harm. Handle with care and consult local regulations for disposal.

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