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.


This graph shows the JDI's remarkable performance, exhibiting virtually flat frequency response well beyond the entire audio range.

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.