June 12, 2007
Have you ever wondered why a dynamic microphone can be plugged into a 100-foot long cable and still sound great, but your guitar tone starts to crap out after just 20 feet? The difference is in the signal.
Your guitar is sending a high-impedance unbalanced signal out of its 1/4” jack. This tiny voltage is having to fight its way down the line against a ton of electrical resistance just to be heard.
Meanwhile, the mic signal is sent twice – it consists of two mirror image signals happily flowing down a low-impedance cable. This “balanced” signal also has the benefit of built-in noise cancellation when the signal reaches its destination.
Could this unfair fight be the origin of longstanding conflicts between frontmen and guitarists? It’s doubtful. Nevertheless, axemen need to know there is a secret weapon – one that will turn high impedance into low, and transform an unbalanced signal into a balanced one. It’s called a direct box, and it’s one of the most misunderstood pieces of gear in the guitar player’s arsenal.
In its most basic form, the direct box (or DI box, for “direct insertion”) converts a high-impedance unbalanced instrument signal to a microphone-level balanced signal. This is accomplished through one of two methods: either a transformer is utilized to isolate the signal and change the voltage, or a preamp cranks up and processes the signal.
The transformer method requires no external power; such boxes are called passive DIs. The good thing about passive DIs is they don’t have issues with 60- cycle hum picked up from ground loops. However, their performance is completely dependent upon the quality of the transformer in the box – and good transformers are expensive.
There is another problem with them as well: loading. Many passive DI boxes have too low of an input impedance for guitar. Usually a guitar without a built-in preamp wants to see a 1 meg input impedance, so I recommend looking for a direct box with input impedance at least that high. If the input impedance is too low, you could lose sustain, hear low-end distortion, and lose highs. How can you tell if you have a problem? Plug it in and listen! Your ears will tell you how well your tone is getting through.
In addition to their tone-affecting low impedances, some passive direct boxes are designed to work better with line level signals, not the tiny voltages generated by guitar pickups. For these reasons, I often recommend that guitarists consider the active direct box.
Active DI boxes use power to preamplify the signal. This voltage comes from a battery, an electrical outlet, or phantom power sent from a mixer. Batteries mean one less strand of cable spaghetti on the stage, but be forewarned: if your Duracell isn’t fresh, the direct box could introduce distortion (and we aren’t talking about the good kind!).
There are many manufacturers of active direct boxes out there, including Whirlwind (whirlwindusa.com) and Radial Engineering (radialeng.com). Each offers an array of useful features (see sidebar for a list of some common direct box features).
At your next gig or session, don’t rely on the sound guy who drops a box on the stage and says, “Plug into this.” While a good direct box may not have the same sex appeal as the latest digital delay or vintage tube amp, it is an essential piece of gear for any guitarist’s gig bag. Be kind to your tone – be sure you have a good one ready to go!
Direct Boxes can offer lots of arcane-seeming features, including:
Ron Daniel Ron Daniel is a professional guitarist, engineer, and producer. He holds a degree in Electronics Engineering Technology, and is currently a Sales Engineer for Sweetwater Sound in Fort Wayne, Indiana. He can be reached at firstname.lastname@example.org or at 800-222- 4700 x1233