Mystery Magicians RBDI Tube Instrument Preamp and Re-Amper Direct Box

The LBDI (Lunch Box DI) and RBDI (Rack Box DI) are fully tube based, Class A direct boxes and instrument preamps that deliver classic tube tone for tracking, reamping, and tone shaping. Both share the same all tube circuit, giving natural harmonic richness, fast transient response, and ample headroom. Unlike starved plated designs, they run the 12AU7 tubes at a robust 320V, ensuring full Class A performance.

The LBDI comes in a portable “lunch box” chassis with a robust chromed handle, making it perfect for gigging musicians who want something easy to carry and set on top of an amp. The RBDI is a slim 1U rack unit that fits neatly with your studio or stage gear. Both units are international ready, with auto line detecting power supplies for plug and play operation anywhere in the world.

The Hi Z input handles passive and active pickups. The switchable HIGH/LOW Gain front end sets the sensitivity by tapping either the full first dual triode stage (high gain) or the second half (low gain). Input and Output Gain controls let you drive the tube stages from clean headroom to rich, controlled saturation.

A true bypassable three band tube EQ gives precise tonal control. Bass can be cut or boosted by 15dB, middle by 6dB, and treble by 10dB. The Bright switch is a position dependent preset tied directly to the Input Gain control. At lower gain settings, where human hearing naturally loses high frequency perception, the Bright circuit boosts high frequencies and gently rolls off the low end to maintain clarity and presence. As input gain is increased, the Bright effect progressively tapers off, preventing harshness and keeping the tone balanced at higher levels. Using the Bright switch with the EQ lets you simultaneously boost and cut certain frequencies, somewhat enabling the Pultec (boost while cut) trick and a wide range of tonal options.

The dedicated reamp input accepts line level signals, allowing prerecorded tracks or stems to run through the tube circuitry for tonal enhancement and harmonic enrichment. Flat, sterile, or poorly captured stems can be recolored, equalized, and reshaped using the tube gain stage. The result is added harmonic density, controlled dynamics, and tonal depth that is difficult to achieve with plugins alone. The RBDI adds a front panel REAMP/NORMAL switch, letting you quickly flip between the rear XLR Reamp Input and the front Hi Z Input, making it easy to switch sources on the fly.

With up to 53 dB of maximum gain, the unit provides sufficient clean gain to properly drive high impedance microphones with 1/4 inch outputs, such as harmonica microphones, without the need for an external booster or transformer. This allows the LBDI to function effectively as a dedicated harmonica preamp.

Designed for guitar, bass, synths, keys, and other Hi Z sources, the LBDI and RBDI combine DI, preamp, tone shaping, and reamp capabilities in a single all tube package. They deliver hands on control, authentic Class A tube character, and flexible tonal options.

Other features include:


• Phase switch for phase reversal


• Ground lift switch to eliminate ground loops


• Front Thru connection for tuners, amps, or parallel processing


• -20 dB balanced output pad to safely manage interface levels while pushing the tube stages hard

Premium construction specs:


• Hand wound output transformers built in house


• Phillips ECG 12AU7 NOS tubes


• WIMA, Panasonic, and TRW audio grade capacitors


• Metal film resistors


• Japanese TOCOS sealed potentiometers for smooth, long lasting control


• Omron audio grade relays for all signal switching


• Custom designed and CNC machined anodized aluminum knobs


• Dual tone hammertone powder coated steel chassis

Electrical Specifications:

Input Impedance:

• HI Z input impedance: 1MΩ

• Reamp XLR input impedance: 100KΩ balanced (Pin 2 hot, Pin 3 cold)

Gain:

• High Gain setting maximum gain: 53dBu (Input and Output knobs at maximum)

• Low Gain setting maximum gain: 32dBu (Input and Output knobs at maximum)

• Reamp input maximum gain: 10dBu (Output knob at maximum)

Output:

• Line Output impedance: 150Ω balanced (Pin 2 hot, Pin 3 cold)

• Recommended load impedance: 10KΩ or higher

• Minimum recommended load impedance: 150Ω

Frequency Response:

• Frequency response: 5Hz to 50kHz ±0.5dBu

Noise Performance:

Signal to noise ratio, Reamp input:

• –101.71dB below 12.19dBV reference at 1% THD+Noise

Signal to noise ratio, Hi Z input:

• –83.5dB below 11.15dBV reference at 1% THD+Noise

Distortion:

• Maximum output level at 1% THD+Noise: +22dBu (Input and Output knobs at optimum position)

• THD plus Noise at +4 dBu: 0.02 percent, second harmonic dominant

Power Requirements:

• AC input voltage: 86VAC to 260VAC

• Mains frequency: 50 to 60Hz