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Monday, November 09, 2009

Road Test Underway: Switchcraft SC800 Instrument Direct (DI) Box

Veteran audio professional Mark Frink provides perspective and an overview of a recent entry in the passive direct box field, the SC800 (Part 1 of 3)

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.

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.

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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.

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.

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A passive DI doesn’t require phantom power, further simplifying their operation.

Active DIs can be a good choice for matching the lower signals provided by passive instrument pickups.

However, many acoustic guitars and electric basses have on-board active electronics (“battery inside?”), making a passive DI a good match for their higher output levels.

Other applications include electronic keyboards, and various “Prosumer” line level playback devices with unbalanced outputs.

The connectors and switches on each end – which of course are all Switchcraft - are on anodized cerulean blue panels, recessed to protect their “short handle” pad and ground-lift switches.

One end has the usual pair of quarter-inch TS jacks for signal input and through-patching, with a 20 dB pad switch between. The other end has a male XLR and a pin 1 lift switch, plus a second quarter-inch through jack. The SC800’s rugged industrial design is an instant classic.

The specifications match the Jensen transformer inside, like the common mode rejection listed as greater than 100 dB at 60. Input impedance is greater than 150,000 Ohms at 1,000 Hz and output impedance is less than 170 Ohm.

Stated phase response is less than 3 degrees at 20 Hz, less than 1 degree at 1,000 Hz and less than 16 degrees at 20,000 Hz. With a +4 dBu input (1.23 volts RMS), Total Harmonic Distortion (THD) is less than 0.05 percent at 20 Hz and less than 0.006 percent at 1,000 Hz.

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It has a maximum input level of +21 dBu, or 8.7 volts, which is where THD at 20 Hz rises to 1 percent. With the 20 dB pad engaged it will take a +41 dBu signal, enough for most small guitar amps if there’s also a speaker or resistive load attached.

Stated frequency response is 10 Hz to 40 kHz, plus or minus 0.3 dB, and stated voltage gain is minus 23 dB, or minus 43 dB with the 20 dB pad engaged.

Finally, the SC800 is hand-built in Chicago, Illinois.

Next I’ll take it to a few shows and try it on stage with a variety of audio inputs…

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Mark Frink is a long-time audio professional and is also associate editor of Live Sound International magazine.

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