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Transcript of Direct Boxes
- Justin Note that most microphones already use balanced XLR outputs and can go in to the interface directly. So, why do I need something like that? Most instruments use an unbalanced connection Most recording interfaces and mixing boards use balanced connections An unbalanced signal uses a single wire to send a voltage difference representing the information being sent. In our case, an audio signal. A balanced signal, on the other hand, uses a pair of wires to send the same information as opposite voltage differences simultaneously. At the other end, these signals are combined to re-create the original information. All lines are susceptible to noise in transmission. On an unbalanced signal, there's no way to recover from this. With a balanced line, the noise affects both signal lines equally. However, since the signal is in opposite polarity, these differences cancel out once the balanced signal is decoded. What's the difference between balanced and unbalanced signals anyway? This is where the Direct Box comes in A direct box uses a common circuit known as a "balun" to translate an unbalanced signal to a balanced signal, or vice versa. In a music studio, it's usually a case of translating an unbalanced instrument signal to a balanced input signal. This is the direct box that I use in my own home studio: a Radial ProD2. This is effectively two passive direct boxes in a single enclosure. I'm a solo musician and my interface has two inputs, so it's a perfect match for what I need to do. But wait a minute, my audio interface already has direct instrument inputs! Do I still need a direct box? Strictly speaking: no, you don't. Your instruments will plug directly into the interface, and you'll be able to record them. You can get by without a direct box in most instances in a recording studio, especially with quality preamps and interfaces. However, There's one key feature that many direct boxes have that's still important, particularly when recording multiple streams at once (such as stereo channels from a synthesizer). Ground Lift This feature breaks the ground circuit between the balanced and unbalanced parts of the signal chain. On many direct boxes, this is activated by a small switch. Due to their noise-canceling capabilities, balanced wires are significantly better for longer cable runs. This isn't usually a problem in a small home studio, but larger studios and live venues absolutely must use balanced signal wires for the majority of wiring or else suffer from unmanageable amount of noise. But even in a home studio, you don't want to get in the habit of using unbalanced instrument cables any longer than you really need to, and direct boxes are a good way to combat that. Why would I want to break the circuit? Let's say that you've got two electric instruments plugged in to the wall and also plugged into the interface simultaneously. Depending on how the wires are run, you might have just set up a ground loop. Active or Passive? So far we've described what's called a Passive direct box: all it does is passively translate the signal coming in to a signal going out, doing nothing to the level or character or the sound. But there's another flavor out there: Active direct boxes. These are handy for allowing you to boost a signal before transmitting it longer distances over a balanced cable. They're a bit more complicated and expensive than passive boxes, so for home studio use, a passive direct box or two should suffice for most cases. An active direct box is, quite simply, a direct box and a preamp in a single enclosure. Since there is an amplifier involved, active direct boxes do require power, either from an external source or via phantom power over the balanced line. + ? A ground loop allows electricity to flow through the ground wire as if it were its own circuit. With regular alternating current (known as mains power), this can lead to a particular kind of interference known as "mains hum" or "ground loop hum". Mains hum has a characteristic frequency graph, shown here on a frequency analyzer. A big peak shows up at the base frequency of the alternating current: 60Hz (US, shown) and 50Hz (UK, Europe) are common. There are lots of very messy harmonics of this fundamental frequency that spread across the whole spectrum, making it very difficult to filter out of a recording after the fact. If this were the only unwanted portion, we could easily filter this out, but that's not the case at all. Very, very messy. So it's best to cut it off before it ever enters the signal path. There are lots of very messy harmonics of this fundamental frequency that spread across the whole spectrum, making it very difficult to filter out of a recording after the fact. By breaking the ground loop by using the ground lift feature of a direct box or audio interface, you can avoid getting mains hum in the first place. So while it's not strictly necessary, a direct box is a very handy device to have around the studio. Now you know how they work, what different types are available, and what they're good for. Happy recording! (Addendum...) Direct Boxes