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Mojo Rising: Apply Those Direct Boxes Correctly…
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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.


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