Radial Engineering JDI Duplex

Radial Engineering JDI Duplex

Two-channel Jensen-equipped passive DI. A stereo version of the Radial JDI. Ideal for recording, broadcast, and live sound where extreme dynamics such as those produced by digital sampling devices and keyboards are encountered. Completely passive, the JD4 employs two Jensen® JT-DBE transformers for 100% isolation and low phase distortion.

Radial JDI Passive Direct Box

  • Jensen Transformer equipped passive DI
  • 100% isolation eliminates ground loops
  • Virtually zero phase & harmonic distortion
  • Exceptional signal handling
  • High common mode noise rejection
The Radial JDI is considered by many to be the world’s finest direct box. It is a passive DI that employs a Jensen isolation transformer for optimum audio performance offering outstanding linearity at all frequencies, combined with extraordinary level handling without introducing distortion. Being completely passive, the plug & play JDI does not require any powering and has the unique advantage of eliminating 60-cycle ground loops. This makes the JDI a definite favorite with informed audio engineers. The JDI is great for guitars, bass, and keyboards and is particularly adept with high-output, broadband instruments such as active instruments and digital keyboards. The JDI may be rack-mounted using the J-Rak and is also available in a stereo (2-channel) version called the JDI Duplex and the 6-channel rack-mountable JD6.

Radial JDI Options

  • Radial JDI Duplex:  A two channel version of the JDI for stereo keyboards, or bass rigs. All the same features as the JDI in a single box. Extremely rugged construction, virtually zero phase distortion, the Duplex is the perfect choice for live stages where eliminating noise is essential
  • Radial JD6 Six channel DI:  The Radial JD6 is a 6 channel direct box that has been optimized for stage use - Six of the legendary Radial JDI channels are mounted into a single rack space for maximum density. Channels 1 and 2 offer a ‘Swiss Army’ extended feature set for computer sound cards, keyboards, and studio interface while channels 3 to 6 are optimized for stage.

Radial J-Rack Direct Box rack-mount adaptor

The J-Rack is a unique direct box shelf and 19” rack adaptor that allows up to 8 JDI or J48 direct boxes to be rack-mounted in 2 rack spaces. This unique adaptor allows improved inventory management for sound companies allowing DI inventory to be used as stand-alone or rack mount depending on tour needs.

JDI Development & Evolution

Direct boxes have the primary function of interfacing musical instruments with sound systems by converting unbalanced high impedance signals to 600 Ohm low impedance balanced mic levels. Proper impedance matching significantly improves the audio performance while balancing allows long cable runs without adding appreciable noise.
Passive direct boxes employ a transformer to isolate the input (primary) from the output (secondary) using only electro-magnetic induction to transfer the signal. The term ‘passive’ refers to the fact that these direct boxes do not require power such as a battery or 48V phantom to make them work. Because isolation transformers have the unique ability to bridge the signal without a ‘hard’ electrical junction, DC noise and stray ground currents are eliminated, thus 60-cycle hum caused by ground loops are removed.
Although passive direct boxes have been around for years, the liberal use of inexpensive, low quality transformers have the not so obvious consequence of compromising the sound. This is caused by the use of low-permeability steel in the transformer core which has the adverse effect of causing group-delay (phase distortion) and a lower saturation point (harmonic distortion), which of course are most prevalent at lower frequencies.

With the advent of better sound systems and recording techniques, the demand for quality grew. This spawned the development of active direct boxes as a means to improve the performance of these lesser passive DI’s without adding cost. The active DI herald in a new generation of improved sonic performance but alas, did so at the expense of isolation.

Evolution of the Radial JDI

The Radial JDI was introduced in 1996 as means to reintroduce the advantages of isolation while raising the bar for sonic performance. The use of a high quality Jensen® DBE Transformer is critical and is easily justified when one considers the hundreds of thousands of dollars spent on the sound system in the quest for great sound.
The 1st version of the JDI was a simple device that was made using an extruded aluminum shell and an inner ‘drawer’ or frame that contained the PC board and electronics. A dual recessed control panel was developed to protect the more fragile toggle switches. This was quickly upgraded to the JDI Mk2 with the introduction of the ‘merge’ function in
1997. This handy feature has become a staple of the Radial direct box feature set ever since.
In 2001, the JDI was given a major face-lift and redesigned using our innovative bookend enclosure. The bookend design provided several major advantages: The I-beam construction increased the inner frame strength thus reducing potential for outside stress; we were able to employ higher duty cycle push switches instead of the more fragile toggles; and manufacturing time was reduced from 45 minutes per unit down to under 5 minutes, thus reducing cost.
The Radial JDI Mk3 employs the same audio signal path as the original, yet is both more durable and more affordable.

JDI Features & Functions

The Radial JDI is a passive direct box that employs the world’s finest Jensen JT-DBE transformer as its main drive engine. The Jensen DBE transformer employs a laminated nickel core that has the distinct advantage of low-permeability. Permeability (see 'Transformer Distortion' side bar) is a term that describes the metals ability to become magnetized and demagnetized without retaining any magnetic memory. When one considers that full bandwidth audio is made up of a series of fundamentals and harmonics that range from 20Hz to 20kHz all of which ‘move’ in tandem, disrupting their relationship is not a desired effect! This underscores the importance of a very high quality transformer and our decision to use a Jensen.

  • Welded I-beam construction makes it impossible to torque the PC board. This virtually eliminates cold solder joints - the No.1 cause of system failure in the field.
  • An innovative ‘slide-in-place’ XLR trace makes it possible to use I-Beam girder construction.
  • The JDI uses a Jensen transformer for distortion-free performance. The Jensen features a nickel core for best signal transfer and minimal loading.
  • A military-grade PC board is employed for maximum wear and tear in abusive environments. Welded steel standoffs ensure the PC board will not shift.
The Radial JDI is characterized by a distinctive book-end design. This has the advantage of protecting the switches against damage from abuse by providing a protective zone on both input and output panels. A heavy duty baked enamel finish resists scratching while a full-bottom no-slip rubberized pad stops the JDI from moving around and ads both mechanical and electrical isolation.
Inside, the JDI features a compound, welded ‘I beam’ construction that protects the sensitive electronics from outside stress. This makes it impossible to torque the PC board, assuring years of trouble-free performance. The internal skeletal frame is locked together with 4 hex screws.
The input panel features a ¼” instrument input and traditional thru-put. An innovative ‘merge’ function allows the thru-put to be converted to a second input to allow stereo to mono summing. This is often used to interface stereo sources such as keyboards or CD players in live shows where audio channels may be not always be plentiful. The JDI also sports a -15dB pad to interface with higher output levels.
  • A -15dB pad allows super hot inputs such as that from a CD player to be connected and balanced for long runs.
  • The Merge function turns the input and thru into a passive mixer to sum stereo feeds to mono at the XLR output. This saves valuable inputs on the mixer!
  • The input connects to the instrument.
  • The thru-put is connected to the instrument amplifier. The JDI has a high input impedance to reduce loading.

JDI output panel

  • Our book-end design creates a protective zone around the switches & connectors for best protection!
  • A heavy-duty baked enamel finish for excellent durability and great looks for years! 100% 14-gauge steel construction.
  • The XLR balanced mic level output provides 100% isolation for runs to 1000 feet without appreciable noise.
  • A polarity reverse switch flips the polarity from the AES standard Pin-2 to Pin-3 on the XLR and is used when combining sounds that may be out of phase.
  • The ground-lift disconnects the ground path to the transformer at the XLR output. This is used when a ground path is made at the input side such as with a keyboard.
  • The parallel speaker interface option introduces a second -30dB pad with band-pass filter to emulate a 12” guitar speaker.
  • A no-slip full bottom pad reduces mechanical vibration, improves electrical isolation and keeps the JDI from moving around on busy stages!

The output panel features balanced 600 Ohm XLR connector with pin-2 hot following the AES specification. A polarity reverse switch toggles pin-2 and pin-3 to allow the JDI to interface with older non-AES equipment and for creative tinkering. The ground lift switch disconnects the XLR pin-1 ground from the JDI for 100% input-to-output isolation.
Finally, a second -30dB pad may be engaged that, when used in tandem with the -15dB input pad, allows the JDI to be connected in parallel with a speaker. This, for instance, allows the engineer to ‘tap’ the sound coming from a guitar amplifier speaker ‘post distortion’ thus adding another creative dimension. This unique function incorporates a speaker emulation filter circuit that rolls off highs and lows to more closely approximate the sound of a 12” driver.


As with all Radial direct box, the Radial J48 is designed for use in abusive environments such as on fast-paced stages of concert touring. An innovative book-end design provides a protective zone around the switches and connectors that makes it practically impossible to damage these components.
The inner skeletal frame features a unique 14-gauge welded I-beam construction that makes it impossible to torque the PC board. Stress on the printed circuit board can introduce intermittent solder joints, which by all accounts is the primary cause of direct box failure.
An innovative full-bottom no-slip rubberized pad works double duty by assuring the J48 won’t slip and slide around when placed on an amplifier. The protective pad also ads an element of electrical shock resistance by providing a mechanical shield from guitar amp handles that are often directly coupled to the amp chassis.

Using the JDI

Being completely passive, the Radial JDI is ‘plug and play’ easy to use. Simply connect the instrument to the input, the amplifier to the thru-put and the mixer to the output and you are ready.
The JDI input features a high 10kOhm input impedance for minimal loading. This makes the JDI ideal for interface with all instruments. The 600-Ohm mic level output allows the JDI to be used with typical snake systems that incorporate bridging transformers (splitters) without fear of overloading them.
As a passive device, the JDI is particularly ideal for use with high output devices such as active or self-powered instruments such as acoustic and bass guitars with built-in battery powered preamps and keyboards. These devices tend to overload most active direct boxes making them sound thin and edgy. The JDI is capable of exceptionally high signal handling without distortion.
The real benefit of isolation is immediately realized when connecting two pieces of gear together that are electrically powered systems. These typically employ different reference to ground voltage schemes which will often result in what is commonly known as ground loops or an annoying 60 cycle hum. The JDI’s exceptionally high common mode noise rejection completely eliminates this problem. This of course has the benefit of making stage set-ups quick and effortless.


The Radial JDI is Jensen Transformer equipped, high quality passive direct box that has the advantage of providing complete isolation for reduced noise. The exceptionally low phase distortion, almost perfect linear response and low harmonic distortion make the JDI a superb choice for critical recording or live performance. The rugged construction allows the JDI to be used in the most abusive environments without concern for damage. The Radial JDI Mk3 is ideal for acoustic guitars, bass, and keyboards.


User Guide

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