Ten years ago, in the process of putting seven active direct boxes (DIs) - including the original Radial JDV - to a critical measurement and listening test, I discovered what has gone on to become an industry standard, Radial’s JDI. passive DI.
You know, the green one with a genuine Jensen transformer inside.
Now there’s another classic passive direct box, the Switchcraft SC800.
Over the years, Jensen transformers have established themselves as a reference benchmark.
Other manufactured Jensen DIs include the Whirlwind Direct-JT and D.W. Fearn PDB passive direct box.
Both employ the Jensen JT-DB-EPC, which is a PC-mount version of their classic JT-TB-E.
Many an old-timer earned his DI badge building a JT-DB-E into a bud box to make a homemade direct box: the way they were all made before the 1980s.
Not surprisingly, all these excellent passive DIs exhibit similar specs and sound, leaving most comparisons to design, features and packaging.
Jensen JT-DB-EPC transformer.
The SC800 passive direct box is built from a black anodized, laser-etched rectangular aluminum extrusion, with countersunk #2 Phillips screws fastening the recessed end panels.
At 2 inches high by 3.5 inches wide. it’s similar in shape to many familiar DIs.
Its 4-inch length is shorter than most others by about an inch, and there are two strips of rubber on the bottom to act as isolation feet. I like that there’s enough room to tape a business card on one side with clear packing tape to ID it when lost or misplaced. The SC800 looks sharp and professional.
A passive direct box simply converts the outputs of high-impedance unbalanced line level audio devices or musical instrument pick-ups by means of a transformer to match balanced XLR low-impedance professional mic pre-amps and mixers.
Front panel view of the SC800. (Click to enlarge)
By eliminating the need to use a microphone, they also remove troubles of microphone choice and placement, as well as leakage into the mic from nearby sound sources.
A passive DI doesn’t require phantom power, further simplifying their operation.