The Radial JDI as it was introduced circa 1996

The JDI as it was introduced circa 1996.

Primary winding transfers prevent DC noise from passing through

The primary winding transfers the audio signal to the secondary via the inductive magnetic bridge. Only AC (audio) can pass, DC noise cannot cross the magnetic bridge.

Problem: Ground loop creates DC hum and buzz

Problem: ground loop creates
DC hum and buzz.

Solved: transformer isolation eliminates DC noise

Solved: Transformer isolation disconnects the 'hard wire' electrical input from the output while allowing the audio signal to pass. Ground loops are eliminated.

Radial President Peter Janis with bass player extraordinaire Mark Egan

Radial President Peter Janis with bass player extraordinaire Mark Egan (Joan Osborne, Sting, Pat Metheny, Larry Coryell, Gil Evans).

Radial JDI MkII

Radial JDI MkII.

JDI™ Development

"It all started back in 1981, when I was working in a music store in Edmonton, Alberta." recounts Radial president Peter Janis… "The store's service technician also happened to be one of the top bass players in the city. He not only could fix anything, but he was incredibly musical. I remember the day when he ordered a Jensen transformer and when it arrived, it was as if he had uncovered a gold amulet from the lost city of Atlantis! A few days later, he came into the shop with his hand-made direct box and exclaimed that he had finally been able to hear his bass the way the Gods had intended. I heard what he heard and became a fan of transformers from that moment on. Fast forward a dozen years later… Radial Engineering Ltd., was manufacturing custom cables and snakes for various PA companies and we noticed that many were building their own DI boxes using left over aluminum extrusion or steel boxes. These folks understood the benefit of a high performance transformer but could not buy an off-the-shelf solution. The original Radial JDI was born in 1996, updated in 2001 and further refined in subsequent years."

Building a Better Mousetrap
The process is simple: If you intend to build a better mousetrap, you have to start by understanding what makes it work and where things can be improved. Striving for continued improvement is in fact the very foundation for each Radial product we have ever developed.

A direct box has the primary function of interfacing a musical instrument with the PA system by converting the unbalanced high-impedance instrument signal to a low-impedance balanced mic level signal. Proper impedance matching significantly improves the audio performance, while balancing the signal enables long cable runs without adding appreciable noise. Passive direct boxes employ a transformer to electrically isolate the input from the output using 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 power 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 is virtually eradicated.


Image of how a transformer blocks DC noise

DC noise from electromagnetic interference (EMI), radio frequencies (RFI) and ground loops are inducted into the audio signal. The transformer blocks noise because only AC (alternating current) can pass through the magnetic bridge.

Although passive direct boxes have been around for years, the liberal use of inexpensive, low quality carbon or silicone steel core transformers has put into question the quality of a passive direct box versus their active counterparts. Problems with low cost transformers show up in the form of excessive loading, limited frequency response, phase shift, noise and distortion of all types. Cheap transformers sound bad. Good transformers are difficult and expensive to produce.

Buliding a better engine

The JDI's engine is a custom ordered transformer that incorporates a series of very specific manufacturing processes: For instance, the inner core that creates the magnetic bridge between the primary and secondary windings is critical to the transformer's performance. The nickel itself is tested for permeability and the laminations are carefully hand-stacked while paying attention to the magnetic properties to ensure the audio signal is transferred without group delay, phase shift or premature saturation. To certify uniformity, the ultra-fine insulated copper wire is coiled onto the bobbin using digitally controlled winding machines set to a very high tolerance. This not only ensures the exact number of turns is maintained, but the winders are programmed to scatter the wire in a proprietary fashion in order to achieve the desired sonic results. An internal copper Faraday electrostatic shield is carefully inserted between the primary and secondary to eliminate capacitive coupling between the coils. The transformer is then housed inside a MuMETAL® can where the magnetic bridge is protected from outside electro-magnetic fields. The transformer is then mounted on a double-sided military spec PC board and affixed inside a rugged 14 gauge enclosure. The steel shell serves an important role: protecting the inner workings from outside magnetic pollution. Few realize that aluminum - which is used by many other manufacturers - does not provide effective shielding. In fact aluminum is often used to build antennas! The relative permeability of aluminum is rated at 1 while steel is 4000, and MuMETAL® 40,000! The steel outer shell and internal MuMETAL® combine to give you maximum protection. Keep in mind that DI boxes are often placed right on top of bass amplifiers where huge power transformers are generating powerful magnetic fields. Power cables, lighting and dimmers also emit tremendous noise, If left unchecked, these will pollute the magnetic core and cause the unprotected transformer to distort.

Optimizing the audio signal

Way back in the 1970s, the first passive direct boxes employed common 600 broadcast line-level transformers. Broadcasters adopted the 600 standard due to their proliferation in telephone systems and this remains a standard that we employ today. Although these low impedance transformers work well for + 4dB to +20dB line level signals, low level sources such as magnetic pickups that produce -40dB to -20dB signals suffer due to loading. In other words, when the wrong transformer is placed in between the instrument, stage amp and PA system, too much of the signal is sent to the PA and the tone on stage suffers. For a passive DI box to work well, it must have a high input impedance. The JDI is designed with a super-high 130,000 input impedance which virtually eliminates the loading problems of the early 600 transformer.

Another trick that is designed into the Radial JDI is the extended frequency response. In audio systems, a greater degree of phase distortion occurs at the cut off points or where the frequency response shifts abruptly. By extending the frequency response beyond the usual 20Hz~20kHz audio range, we essentially move the problem out of the way. The JDI does not introduce phase anomalies that would otherwise become apparent within the listening range.

A good passive direct box must also be able to handle extreme dynamics without choking. Passive direct boxes do not distort like their active counterparts: they saturate. The JDI is truly amazing in that it naturally rounds out the dynamics like a very mild limiter. This effect is often referred to as sounding 'vintage'. When you consider most older audio devices are equipped with audio transformers, you can understand the connection. This makes the JDI particularly effective when used with high output devices such as digital pianos, active basses and electronic percussion. Ultimately, a world-class DI must deliver the sound of the instrument without noise, distortion or artifact of any kind.

Optimizing the performance

When you look at the JDI, there are a number of features that make it unique: The first thing you notice is the book-end design. Introduced in 2001, the book-end creates protective zones around the switches and connectors. DI boxes are often tossed on the ground, stacked and kicked. With this in mind, you will come to appreciate the extra protection when other DI boxes fail before an important show. And this protection is not merely cosmetic. Pick up a Radial DI box and you will immediately be struck by the weight. We use heavy, 14 gauge steel construction throughout. Inside, a unique I-beam frame houses the sensitive PC board and electronics. There is nothing stronger than an I-Beam. This is why they are used to build bridges and roof spans on commercial buildings. This ensures the PC board will not torque which could lead to solder joints going cold and premature failure. Inside, the switches are steel encased for durability and the connectors are made from glass-filled nylon to provide isolation. Nickel silver contacts will not tarnish and provide excellent signal transfer.

Standard features such as input, thru and output are of course included. The JDI goes a step further by adding a merge function that passively mixes the left and right outputs from a source such as a keyboard to mono. This saves valuable channels on the PA and eliminates the extra work of having to reprogram the keyboard or computer sound file.

An important feature built into the JDI is the 180º polarity reverse switch. This serves two functions: First, when combining two sources such as a direct feed from a bass and a microphone, reversing the polarity of one of the signals can often improve the sound by better aligning the stereo sources. This is most noticeable in the studio but also applies when attempting to align a loud stage amp with the PA system in a club setting. The second has to do with managing resonant feedback on stage when using the JDI with an acoustic instrument. Depending on where you are standing on stage, the sound from the PA system can interact with the room acoustics where certain frequencies will either sum together when in phase or be cancelled out when out of phase. Reversing the polarity on the JDI can often help by electronically moving the problem out of the way. And because you are not applying radical notch filters to the signal, you are not degrading the tone of the instrument.

As icing on the cake, the JDI is equipped with a second speaker level pad that lets you capture the sound from the high level output from the amplifier and speaker cabinet. When activated, this introduces a bandpass filter that rolls off the bass and treble to better replicate a 12" loudspeaker. This can be used to capture the distorted sound of guitar amp, the grind from a bass amp or even the distortion from an old tube Hammond organ or Leslie cabinet.

Over the years, the Radial JDI has received tremendous accolades from audio magazines around the globe, has won countless comparison studies and has become the single most specified passive direct box for concert touring on the globe today. Artists who use the Radial JDI include bassists Tony Levin and Nathan East, keyboard players Chic Corea and Tom Coster and specified on tours by audio engineers such as Robert Scovill (Tom Petty), Derek Williams (Bonnie Raitt) and Cub Colby (Phil Collins).

It is available in multiple formats such as the stereo Duplex and the JD6 six channel rack. The JDI can also be fitted inside the J-Rak and J-Clamp.

The Radial JDI is a passive direct box that combines a custom specified nickel laminated audio transformer with a proprietary passive circuit to perform the impedance conversion and balancing to deliver a mic-level signal. Analysis reveals an exceptionally linear response from 10Hz to 40kHz with virtually zero phase shift at all frequencies. The excellent performance is further substantiated with a Fourier transform that produces desirable even order harmonic generation and a warm Bessel curve when pushed hard. With its heavy-duty steel shell and internal I-beam construction, the JDI is designed to work dependably for years in even the harshest of touring environments. Ideal for keyboards, bass and acoustic instruments of all kinds the Radial JDI™ is quite simply the world's finest passive direct box.