Direct boxes are something that we use every day in recording, yet take for granted because of their simplicity. Here’s an excerpt from my Recording Engineer’s Handbook that looks at the ins and outs of this useful recording tool.
“Direct Injection (DI or “going direct”) of a signal means that a microphone is bypassed, and the instrument (always electric or electrified) is plugged directly into the console or recording device. This was originally done to cut down on the number of mics (and therefore the leakage) used in a tracking session with a lot of instruments playing simultaneously. However, a DI is now used because it either makes the instrument sound better (like in the case of electric keyboards) or is just easier and faster.
Why can’t you just plug your guitar or keyboard directly into the mic preamp without the direct box? Most preamps now have a separate input dedicated for instruments, but there was a time when that wasn’t the case and plugging an electric guitar (for instance) into an XLR mic input would cause an impedance mismatch that would change the frequency response of the instrument (although it wouldn’t hurt anything), usually causing the high frequencies to drop off and therefore make the instrument sound dull.
Advantages of Direct Injection
There are a number of reasons to use direct injection when recording:
- Direct Box transformers provide ground isolation and allow long cable runs from high impedance sources like guitars and keyboards without excessive bandwidth loss.
- The extremely high impedance of the DI insures a perfect match with almost every kind of pickup to provide a warmer, more natural sound.
- The length of cable can be extended up to 50 feet without signal degradation.
Direct Box Types
There are two basic types of direct boxes; active (which can provide gain to the audio signal and therefore needs electronics requiring either battery or AC power), or passive (which provides no gain and doesn’t require power). Which is better? Once again, there are good and poor examples of each. Generally speaking, the more you pay the higher quality they are.
An active DI sometimes has enough gain to be able to actually replace the mic amp and connect directly to your DAW.
An excellent passive DI can be built around the fine Jensen transformer specially designed for the task (www.jensen-transformers.com for do-it-yourself instructions) but you can buy basically the same thing from Radial Engineering in their JDI direct box (see the figure on the left). Also, most modern mic pres now come with a separate DI input on a 1/4” guitar jack.
Direct Box Setup
Not much setup is required to use a direct box. For the most part, you just plug the instrument in and play. About the only thing that you might have to set is the gain on an active box (which is usually only a switch that provides a 10 dB boost or so) or the ground switch. Most DI’s have a ground switch to reduce hum in the event of a ground loop between the instrument and the DI. Set it to the quietest position.”
You can read more from The Recording Engineer’s Handbook and my other books on the excerpt section of bobbyowsinski.com.