I think there is probably more confusion than clarity when it comes to the world of direct (or DI) boxes. What are they for? Do I need one? What does it do? Over the years I have seen all kinds of answers and a lot of misinformation. 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.
Fiction: Keyboard/Interface outputs are high impedance outputs and need a DI
Nope. Not so much. All modern active keyboard instrument and interface outputs are low impedance and could be directly connected to a modern mic input without a problem. Modern microphone amplifier inputs are at least 10k Ω and should not load down an instrument output.
This myth is perpetuated by the idea that an “instrument” output needs to have a DI in order to connect it to a mixer. This has always been true for instruments with high impedance outputs. These are passive guitar pickups and classic electro-mechanical keyboards with wound coil pickups. These sources may have a source impedance of 10k to 1 million ohms and DO need a DI between them and the board or else the mixer will put a tone sucking load in the input. Not nice.
Kind of true: a DI helps to prevent interference on long runs to the mixer
This is true if your keyboard has an unbalanced output (check your manual!). If your outputs are balanced and you are using a 3 conductor cable to run to the board (XLR or TRS) then the DI is not going to help you. Most professional interfaces have balanced outputs and do not need a DI to balance the signal.
It can also be argued that the signal in an active instrument output is SO loud that any interference picked up on a long run would be very small compared to it and can be ignored. It’s probably true but why take chances? A DI will turn your unbalanced synth output into a balanced signal with the best possible interference rejection.
True-ish: Prevent overloading your mic input
Historically mixers featured separate inputs for mic and line level signals. The output of an active pickup or keyboard is much louder (higher voltage) than a mic input and would need to be attenuated to prevent clipping the mic preamp. Many of today’s mixers tolerate line level signals at the mic input without complaint.
Unexpected benefit – Lower noise floor
Every instrument source has a baseline level of noise – even today’s modern instruments! An important part of managing noise is to run your output level as high as is practical. If we were to run to the board directly we would likely have to run our output levels low to prevent clipping the mic input. A DI will drop the strength of the signal and the noise equally so we can run our output hot and not overload the mic input.
Possible benefit – Protect your keyboard or interface from phantom power at the board
Some microphones require voltage from the mixer to operate. This is called phantom power (because it comes down the mic cable) and is typically 48 volts. Usually there is not a lot of current involved but if the board were to suffer a short or voltage spike your keyboard outputs would see that electricity and it could damage your synth (theoretically). Passive DI boxes effectively decouple your keyboard from the circuit and block phantom power.
Most modern outputs are designed to ignore or tolerate phantom power but it may be a risk you are not willing to take. Talk to your sound guy and see if there is phantom power at your input and choose your DI accordingly.
So do I need a DI?
In many cases – no! Sometimes – Yes! Lets take a couple of scenarios.
Case 1: Keyboard on stage with short 20-30 foot run to digital mixer input rack or mixing board
If the keyboard output is quiet and the mixer input can handle the signal without clipping then you will see little benefit from using a DI. The exception would be if phantom power is turned on at the mixer input in which case we may want to use a DI to block that power from hitting the keyboard outputs. Over such a short run it does not matter if the signal is balanced or not – noise is not likely to be a problem.
Case 2: Keyboard on stage to mixer inputs far far away
If your keyboard outputs are balanced then you probably don’t need a DI. If they are unbalanced then the DI will give you additional noise protection over that long run by balancing the signal.
Case 3: You can’t run your outputs higher than “2” because of clipping at the board
First I would say to check with the FOH guy to see if they can pad (reduce) the signal at the board. If not then a DI will help by dropping your output voltage (remember voltage = volume) to something the board can handle better. Then you can run your synth at a volume with a better signal to noise ratio and the FOH can be happy about having an input that’s not clipping.
Case 4: Connecting a consumer output to the mixer
If we are connecting a laptop output or other consumer type output to a mixer then a DI is a good idea. These outputs were not designed to have to deal with possible phantom power from the board and are universally unbalanced connections. Play it safe here.
I hope that I have added some clarity to the voodoo around DI boxes and what they are for. Please let me know if I wasn’t clear on anything in the comments below. To be fair I should note that, although they are not necessary, I usually use a DI on my outputs anyway – I guess old habits die hard. Thanks for reading!