In short, then, a direct box is a great idea when you need to plug a passive instrument into a mixer or microphone preamp. It will shield your signal from unwanted noise while preserving the detail in your tone—especially the high frequencies—and allowing more volume.
Fender even offers the Micro DI pedal. Many working musicians and sound professionals consider a couple hundred dollars a worthwhile investment for a good direct box. Skip to main content. Tech Talk. An unbalanced cable is the most commonly found on stage and is your typical guitar cable with a wide variety of uses. As you can see in the picture below, there's a black marking on the cable which is your 'Sleeve' along with the 'Tip' itself which match the two wires inside, a signal and a ground that transport your sound.
A typical, unbalanced instrument cable with 'Tip' and 'Sleeve'. As you can see in the picture below this balanced cable has two black markings underneath the tip, because a balanced cable has three wires to transport your signal, two signal wires called hot and cold and one ground.
This is the same for XLR cables, the only difference being the connector itself having a different format. A balanced instrument cable with 'Tip', 'Ring' and 'Sleeve'. So aside from the look, what's the difference? An unbalanced cable, because it only has one cable carrying the signal plus your ground wire, is more susceptible to noise because the cable acts as an antenna for interference. The ground wire does a decent job of noise reduction but only to a certain extent, which is why you'll be hard pushed to find an unbalanced cable longer than 20ft, as the noise level associated would become too much.
A balanced cable does the neat trick of cancelling any noise thanks to the hot and cold signal cables. Both send a signal, but one has its polarity reversed. Now bear with me here, if you have two signals, one with reverse polarity and one normal, what you actually get is silence because each signal cancels the other out see image below.
This is called Phase Inversion and is a common pitfall of recording in stereo. We can't work with silence so the audio gear receiving the signal at the end of the chain flips the reversed signal back to its original orientation.
The noise itself doesn't have its polarity reversed as it travels along the cable but is subject to the flip at the end of the chain, resulting in a strong, noise-free signal reaching your audio device. This may seem confusing, but for the purposes of this article, it's enough to know that balanced cables give you noise-free operation.
So after that brief sojourn into electronics, let's look at the DI Box itself. At its core the DI box is actually quite a simple device, it's what you can do with it that opens up a world of possibility. The essential function of a DI box is to convert a high-Z or high impedance signal to a low-Z or low impedance mic signal, but why would we want to do this? Passive pickups on a guitar emit a high-Z signal, whereas gear that receives this signal such as mic preamps and mixing desks require a low-Z signal.
The signals need to be at low-Z for recording and live purposes, thus the DI box ensures you get the right level signal for recording and the PA system. The DI Box also allows you to split your signal, sending 'Wet' and 'Dry' signals to different places, which is why it's such a useful bit of studio and stage gear. On stage this allows you to send the balanced output to the desk whilst the unprocessed signal carries onto your amplifier.
In the studio, you can record both effected and non-effected signals, which means you have the original signal available for re-amping or adding extra texture. This is purposely done to allow DI boxes to happily live along side microphone signals in a snake system without cross-talk due to one signal being louder than the other.
The other advantage of having a mic level output is enjoyed during sound check. If one channel of a snake or mixer is not working correctly, you can quickly change channels around without having to worry if one is at a different level than the other.
A common feature that is found on the direct box is a THRU or a throughput connector. As shown here, the DI box is actually splitting the signal between the on-stage amp and the PA system. In order to avoid a perceptible drop in level in the stage amp, active direct boxes are generally preferred when using a passive instrument like a Fender bass. Active DIs employ a buffering circuit or unity gain amplifier that boosts the signal going to the balanced XLR output so that very little draw is taken from the source.
For higher output devices that have built in buffers such as electronic keyboards or active basses, a passive direct box is usually selected. Passive DIs employ a transformer to do the work. Back in the early days low impedance ohm transformers would be used which basically split the signal coming from the instrument causing it to loose punch. This effect is known as loading. Today, Radial DI boxes employ transformers with , ohm input impedance, thus greatly reducing any of the loading effect that was once a concern.
Because active DI boxes employ a buffer, they tend to have more reach. This makes them a good choice when used with lower output instruments or instruments that benefit from extra sparkle. Most engineers prefer to use an active DI on acoustic guitar and bass.
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