Mojo Rising: Apply Those Direct Boxes Correctly…

February 05, 2013, by Gary Gand

di boxes

For those who missed my first “newbie” column, here’s a re-cap. I’m the guy you love to hate, a musician almost from birth who is also a long-time professional sound engineer (35 years and counting).

My goal is to inform and educate the less-experienced sound people that I work with on a regular basis from my spot at stage right in front of my guitar amp, behind my 36-pound pedal board, in my seven-piece blues, rock, soul – and whatever else I feel like playing – band.

Younger folks have great gear and brains full of code. But when it comes to the basic gremlins that have dogged us since the “stoned age,” some things, sadly, haven’t changed. So let’s talk about one of them: direct boxes.

The term “DI” (for direct input or injection) was applied to the first units to migrate across the pond from England in the mid 1970s. They were homemade things in ugly metal enclosures (sometimes in coffee cans before it was called “recycling”) that contained a large transformer usually pirated from a telephone, background music loudspeaker or various common home appliances.

They allowed us to “directly inject” the sound of a bass guitar into a snake to the console. A true breakthrough game changer! (And for some reason, they were mostly painted red.)

Now, of course, direct boxes are common place and come with all kinds of features like 15 dB input pads, ground lifts, feed through jacks, stereo inputs with linking capability, onboard loudspeaker emulation, and even active “innards” that require battery or phantom power for instruments with low outputs. (And color choices abound.)

Blow It Up
The most troubling aspect of direct boxes: the first thing that must be known is whether a direct box has a transformer. If not, I don’t want to see it on my stage.

Use it to prop up the house console, or as a drink coaster, or whatever – but it’s not going to cut it for anything to do with audio when I’m in the house. Transformers block DC (direct current), which can be death to loudspeaker drivers.

In my early days of working with name bands, I had the misfortune of blowing up an expensive speaker cabinet belonging to a keyboardist who went on to play with Michael Jackson. I was bragging to him about this new active direct box that I’d just acquired – how it was super sensitive and had frequency response into dog hearing range, and that this was because it didn’t have a transformer.

Further, this wonder box could be powered from the 48-volt phantom power on the console like an expensive studio mic, so there would be no battery failure during the middle of his solo.

To make a long story short, right after I plugged it in and flipped on the phantom power, a DC current spike went up the input cable into the mixer output (which was also not transformer protected) and right into the 18-inch driver in his loudspeaker cabinet. Poof – no more 18.

Now, how did this happen? Because there was no transformer to block DC in the direct box, and even though we’re talking about outputs to inputs (backward signal flow), that’s what occurred.

Keep in mind, though, this was a rare occurrence, and it was a long time ago, so the design may not have been as “battle tested” as it should be. In addition, there are now thousands of active DIs on stages around the world working seamlessly. Many modern high-quality active DI boxes include a transformer on the output after the active circuitry. That’s a good thing. So check it out before buying or using a DI to make sure we’re all protected.

Keep Making Sound
Another peeve is sound folks who hook up DI boxes to our stage equipment; I like them to be plugged into the line out of the bass and keyboard amps. The goal is to receive the signal right from the instrument for a cleaner sound with no amp EQ, but the problem is that if the DI fails, most of the time the signal won’t get to the amp. And no sound = bad.

By using an amp output, if the DI has a problem, the amp still makes sound and the audience doesn’t know anything is wrong, nor does the band. They just keep playing as if nothing has happened. The DI can be swapped out, and perhaps the cable replaced (whatever it takes), but the show isn’t affected
and it’s likely no one is even aware that there’s a problem.

This approach also prevents the bass player from unplugging his/her instrument from the DI, causing a big pop through the PA.

Further, I tend to shy away from stereo direct boxes, unless the band has their own. In my experience, this is the DI that’s usually at the bottom of the work-trunk drawer, and the last to come out. Therefore, the odds are pretty good that the user does not understand exactly how it works.

I’ve found that many stereo direct boxes include some kind of summing switch to take the two inputs into one output or to sum the inputs into mono to both outputs. When it gets put on a bass guitar by someone who doesn’t understand how to use it, the bass sounds terrible – a sure way to wreck even the most basic sound of any band.

This happened to me at a recent festival, so I pulled out my own $49 transformered no-frills direct box and a 1/4-inch cable and handed it to the stage tech to use. It worked fine, of course, except he neglected to get it back to me during the harried stage change.

I also have a horror story involving a stereo direct box and an overloading DJ mixer blowing up all of the compression drivers in a large club PA on a Saturday night with a sold-out audience on hand – just before the headliner went on. As a result, my take is to “know thy stuff ” when it comes to deployment of stereo direct boxes.

So let’s all get our DI box mojo together. And if not, at least have the courtesy to return my stuff to me at the end of my set.

Gary’s DI Choices

(click to enlarge)

Radial JDI. The most versatile DI box around, it’s a passive unit with a Jensen transformer for top fidelity. It also introduces a magnetic bridge that passes signal while rejecting stray DC voltage. Includes -15 dB input pad, ground lift, and 180-degree polarity reverse.

A secondary circuit is added for direct interfacing with high-output devices such as guitar or bass amp loudspeakers, plus a merge function allows stereo sources such as keyboards to be summed mono. (www.radialeng.com)

(click to enlarge)

Whirldwind Director. A passive box using the company’s TRHL-M transformer with metal shield. It includes a ground lift switch to help eliminate hum and buzz, a -30 dB pad for selecting between line/instrument or speaker level input signals, and a switchable high-cut filter to eliminate amplifier noise (hiss).

I also carry a Whirlwind IMP 2 passive transformer DI with ground lift switch, a basic “no frills, get it done” unit that fits easily in my guitar case. (www.whirlwindusa.com)

(click to enlarge)

Countryman Type 85. Includes an active FET op amp front end with transformer output. Gives increased fidelity due to low loading to amp or instrument from active FET input, but has full DC protection on output from transformer, unlike some other active DI boxes. Requires phantom power from console or internal 9-volt battery.

If you’re looking to use an active DI, this is a strong selection. Plus, the indestructible case can double as a wheel chock for a truck. (www.countryman.com)

Gary Gand is president of Gand Concert Sound in Glenview, IL. GCS has been on the forefront of large-scale audio since the 1970s and are known in some circles as the “NEXO guys.”



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Mojo Rising: Apply Those Direct Boxes Correctly…
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