Lets take a look at the DI and see what it can do for us:
What’s In a Name?
The direct box is named for the function it performs – a very sensible idea! The primary function of a direct box is to take a variety of non-microphone signals “direct” to the mixing console microphone input. The earliest direct boxes relied on a transformer to both reduce the input voltage (volume) to something the board could handle and to make the output impedance compatible with a mic input. Today direct boxes come in many flavors but can be broken down into passive versions that still use a transformer and do not require any external power, and active versions that use an electronic circuit to convert the signal and need some sort of power either from the mixer or a battery. A good direct box can do several things for us – let’s take a look.
High Impedance to Low Impedance
Impedance is the resistance of an alternating current circuit and is measured in Ohms (Ω) like normal resistance. In audio we want outputs to be lower in impedance than inputs because this effectively prevents any significant current from moving and doesn’t load down the audio output.
Mixer inputs are universally high in impedance (10k Ω or so) and almost any microphone or instrument output can be directly connected to them with the exception of electromagnetic pickups which have an output resistance of 10k Ω to a million Ohms – as high as or much higher than the input impedance at the mixer. The direct box converts this very high output impedance to a much lower one by using a transformer which effectively “moves” the signal over into a new circuit with a much lower output resistance.
Decouple Ground Paths
A direct box can also interrupt the ground path to the board preventing ground loop hum from entering the audio circuit. Ground loops occur when the local ground for the mixer has a different voltage than equipment elsewhere in the performance space. When those pieces of equipment are wired together the mixer sees this voltage difference as audio signal and the result is hum.
If we can interrupt the audio circuit the hum will be prevented. In an audio circuit the signal wire is actually the only wire that has to be present, so almost all direct boxes have a switch that will break the ground and prevent a continuous ground between the mixer and your equipment.
Convert to a balanced signal
A balanced audio signal is one where the audio has been duplicated into both a normal in-phase version and a reverse-phase version and sent along the signal cable. Any noise that enters the cable will be cancelled when the reversed-phase signal is flipped back into phase at the other end. This allows for long cable runs with less noise pickup and is preferred for low-amplitude sources like dynamic microphone outputs where the noise may be as loud as the signal itself.
A direct box transformer has a positive and negative taps that perform this out-of-phase conversion so that an un-balanced input will be converted to a balanced output.
Separating Truth from Fiction – Dun Dun Dunnnnn!
So we have been over the basic functions of any direct box so lets look at some myths and truths regarding their use. We are going to start from the premise that the best signal would be straight out of your instrument and into the board, so if we are going to put another piece of equipment in between it needs to be doing something for us.