Today we’re going to be taking a closer look at DI (direct injection) boxes and the benefits they offer. But, first, if you’re not sure what a DI box does:
A DI box converts an unbalanced high impedance (Hi-Z) instrument-level signal into a balanced low impedance (Low-Z) line-level signal. This allows you to plug a high impedance instrument e.g. a guitar into a low impedance source e.g. a mixing desk, which will typically utilize mic level XLR inputs, as opposed to ¼” instrument cable inputs.
Anything to do with audio electronics can get technical fairly quickly, so in the following article, we’re going to attempt to keep things simple and just focus on the information you need if you are deciding whether you would benefit from owning a DI.
Why Is A balanced Signal Important?
For acoustic-electric guitars, the main advantage of using a DI is being able to run an XLR cable between a DI box and a mixing board. The guitar has a 1/4″ input jack that takes an instrument cable that can then run directly into the DI, which provides an XLR output from the DI to the source.
XLR cables, as opposed to ¼” instrument cables are balanced, meaning they offer protection against outside interference, making them ideal when used in a recording environment.
They can also run long distances e.g. up to 200 feet without interference, making them ideal for live performance. The longer a 1/4″ instrument cable, the more prone it is to interference.
This is because, unlike an instrument cable which features a signal wire and ground wire only, a balanced cable has an additional pin for a second signal wire, along with the ground wire.
As a result, XLR cables are less prone to interference because the polarity of the two signal wires is reversed along the length of the XLR cable. Negative becomes positive, and positive becomes negative, essentially canceling the other out along the length of the cable which would otherwise be susceptible to interference from other power sources e.g. lights and audio equipment.
The polarity of the negative wire is reversed at the end of the cable resulting in both signal wires being in phase.
Additionally, because unlike an unbalanced cable two signals wires are utilized, there is also an increase in signal flow resulting in an increase in volume, compared to an unbalanced cable.
XLR cables also offer the added benefit of being able to carry phantom power e.g. 48V power from the equipment you are plugging into which will power a condenser microphone or in some cases an active DI (more on this shortly), as both require a power source.
What is Impedance?
If you have an acoustic-electric guitar, you most likely have a pre-amp integrated into the guitar. Usually on the side or just inside the soundhole depending on the pickup system used.
Passive guitar pickups e.g. pickups that do not utilize a power source (9V battery in most cases) produce an AC current. An AC current simply means the current alternates constantly between positive and negative, as you might expect with a guitar pickup as the strings vibrate e.g. change direction between positive and negative.
Impedance refers to the level of resistance there is to an alternating current (AC), measured in Ohms (Ω).
Ohmage doesn’t refer to signal strength, it’s an indicator of how much resistance the current has to work against e.g. the resistance to the current through the coils of a pickup for example.
In this capacity, a DI is useful when used with a bass guitar. The guitar’s pickups produce a weak, high impedance alternating current (AC). All passive pickups (pickups that do not utilize a power source e.g. a 9V battery) are high-impedance.
If you happen to plug a high impedance instrument directly into a low impedance board or recording console, depending on the efficiency of the current flow, there will be insufficient resistance to the signal flow which can result in problems occurring such as distortion, hum, or a loss of high-end frequencies.
Essentially when there is an “impedance mismatch” such as this, you are introducing inefficiency to the transfer of current and some of that current will be reflected back to the source, meaning maximum current is not delivered.
Alternatively, if plugging a low impedance source into a high impedance source the opposite occurs resulting in signal drop.
Do I need a DI Box If My guitar already has a preamp?
In the case of acoustic-electric guitars, if your guitar requires a battery (you can read more about that here) your guitar has a built-in pre-amp (as pictured above).
A pre-amp’s job is to take the weak signal produced from a passive pickup and boost it to line level, making it suitable for a low impedance source. But that signal remains unbalanced and prone to interference, so a DI is still extremely useful, despite the presence of a preamp.
Alternatively, if playing an instrument like a bass or electric guitar the signal from the pickup is not boosted by an onboard pre-amp, so integrating a DI into your signal chain will match the impedance between the instrument and source, which for reasons we have already described above is optimal.
In the case of electric guitars, a guitar amplifier accepts a Hi-Z signal so a DI box isn’t essential, but if you want to split your signal (more on this shortly) between an amplifier and mixing console, or are running a bass guitar directly into a mixing board a DI is required to match the impedance between the two.
What Else Can A DI Box Do?
As mentioned above, another use for DI boxes is to split the signal from your guitar.
This can be useful, for example when playing live and wanting to send the signal to the mixing desk along with using an amplifier on stage.
In a recording environment, this allows you to send a completely dry signal to your audio interface, along with a signal from a microphone (if micing up your acoustic guitar or amplifier) allowing you to blend the two and add effects to the dry signal.
Many also come with a ground-lift switch.
A ground lift switch essentially eliminates ground loops. A ground loop occurs when two devices are connected, which creates a circuit, and there is more than one path for the current to flow between ground wires, resulting in additional noise being introduced, mostly in the form of an audible hum.
The Difference between Active and Passive DI Box
Ok, so if you have decided that you need a DI box, you may have had a look around and noticed they come in two distinct options, passive or active.
Passive means there is no external power source required, just like a passive guitar pickup. An active DI, like an active guitar pickup, requires a power source usually in the form of 48v phantom power drawn directly from the equipment being interfaced with.
Because an active DI has additional power it can include a preamp to further boost the signal and can offer additional features e.g. EQ.
That’s not to say active is better than passive.
Passive DI boxes, for example, don’t introduce additional voltage so are less chance of introducing additional noise, and as a result, they are also less inclined to alter the sound being produced and tend to sound more natural.
The choice really comes down to what sounds the best for your particular application.
If you have been playing acoustic guitar for a while and are ready to start recording your own creations or begin playing live a DI box will come in extremely handy and is an accessory I’d recommend, but like all guitar accessories, there is a lot to choose from out there. Keep in mind, that if you are merely looking to balance your signal you really don’t need anything too elaborate, so it shouldn’t cost you an arm or a leg.