DI Box explained: what they do and when to use one

A DI Box, short for direct injection box, is a really useful piece of equipment in audio recording. But for many, DI boxes pose a bit of a mystery. DI boxes are used to convert an unbalanced, high impedance, instrument or line level signal to a balanced, low impedance, mic level signal. But what does that mean and why would you need to do that? In this article, we’ll break down the DI box. We’ll look at the numerous purposes that a DI box serves. We’ll also define the occasions that you’ll need to use a DI box.

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Why do you need to use a DI box:

First of all, it’s important to note that different devices (mics, guitars, keyboards etc.) putout different types of signals with regards to their signal level and their output impedance. So when you connect a device to an audio interface or mixing desk, it’s important to connect it to an input which is designed to receive that specific type of signal.

With modern interfaces, it’s increasingly common to have dedicated inputs for ‘Mic’, ‘Instrument’ and ‘Line’ level devices. Having these three types of inputs available will allow you to connect just about any device to your interface. The ‘Mic’ input is there to receive the signal from a microphone. The ‘Instrument’ input is there to receive the signal from things like electric guitars, electric basses or electro-acoustic guitars. The ‘Line’ input is designed to receive signals from things like electric pianos and drum machines.

But what do you do if you do not have access to the type of input that you need? For instance, when you want to connect an instrument like an electric guitar to your interface, but no instrument input is available.

This is where a DI box comes in…

What does a DI box do:

Whilst interfaces and mixing desks do not always provide you with all three types of input, almost all will provide you with mic inputs. These will usually come in the form of an XLR input. This input is designed to receive a low impedance, mic level signal. As previously mentioned, a DI box converts signals to low impedance, mic level signals. As such, you can use a DI box to convert the high impedance, instrument level signal produced by your electric guitar to a low impedance, mic level signal. So now, the signal from your electric guitar can be connected to your mic input. You can also connect any other devices which would not normally be suitable for connection to a low impedance, mic level input (such as an electric piano for example) to a DI box to make them suitable for connection to a mic input.

Being able to convert a signal to a low impedance, mic level signal is really handy. That’s because finding yourself in a situation where only mic inputs are available to you is not uncommon. When you play a live gig, the sound engineer will often use a stage-box. This is used in order to send signals from the stage to the mixing desk. In this instance, you will usually only have access to low impedance, mic level inputs on the stage box in the form of XLR inputs. The same is often true in a studio setting. In this case, a stage-box may be used to send signals from the live room to the control room. Once again, it is likely that you will only have access to low impedance, mic level inputs.

Use a DI box to convert an unbalanced signal to a balanced signal:

In addition to converting impedance and signal levels, a DI box carries out another highly valuable job. It converts unbalanced signals to balanced ones. In audio, we have both balanced and unbalanced signals. Balanced signals reject interference, humming, buzzing etc. whereas unbalanced signals do not. As such, there will be instances where it is sensible to use a DI box to convert an unbalanced signal to a balanced one. This is of particular importance when transmitting signals over long distances. That’s because longer cable lengths are more susceptible to interference and/or signal degradation. Connections of just 4 to 6 meters are considered the maximum safe length for unbalanced signals. Whereas balanced signals can be sent over hundreds of meters without the risk of interference, noise or signal degradation.

How do I connect my DI box:

DI boxes are very straight forward to use. You simply connect the output of the device that you want to process through the DI box to the DI box’s input. You then connect the output of the DI box to your mixing desk, audio interface, mic preamp or where ever else it is that you want to send your signal to.

So now that we know what it is that DI boxes do and how to use them, let’s look at the different types of DI box. DI boxes come in two main types. They are ‘active’ and ‘passive’.

Active DI boxes:

Unlike passive models, active DI boxes require power to function. That’s because unlike passive models, they contain a pre-amplifier. Depending on the model, active DI boxes usually source their power from batteries, a dedicated power supply or 48v phantom power. If you opt to power one with batteries, then be sure to have some spares on hand. It stands to reason that performance can deteriorate as the batteries become weaker.

Because active DI boxes contain a pre-amplifier, an active DI box will generally have a stronger output than that of a passive model. For this reason, active DI boxes are often most useful when working with passive instruments such as electric guitars or any other instruments that don’t have their own preamp. Passive instruments generally output a weaker signal than active ones. So they benefit from the extra amplification provided by an active DI box. That said, many active models also have more headroom than passive ones. As such, they can usually accommodate active instruments comfortably as well. An active DI box will usually come at a higher price point than a passive one.

Passive DI boxes:

Unlike active DI boxes, passive DI boxes do not require power to function. That’s because passive DI boxes do not contain a preamp. Because of this, they are often best suited to use with active instruments which have their own preamp and which in turn have a stronger output than passive instruments. In addition, passive DI boxes are generally cheaper to purchase than active ones.

In addition to the availability of these two different types of DI box, different models offer various additional features to help you manage the signal going into and coming out of your DI box.

Ground Lift on a DI box:

DI boxes can be susceptible to humming or buzzing noises caused by ground loops. That’s why on many models, you’ll find a ground lift switch. The ground lift switch allows you to break the ground loop to reduce or eliminate the humming or buzzing noise.

Pad on a DI box:

Some models of DI box include a pad switch. This allows you to attenuate the level of the signal that is going into the DI box. This is useful when working with active instruments which may potentially have a high output level which could overload the DI box. The pad usually allows you to attenuate the incoming signal by a predetermined amount i.e. -10dB, -20dB, -40dB etc.

Polarity flip on a DI box:

Some DI boxes allow you to flip the phase of the signal coming out of the DI box. This is useful when you are capturing both a direct signal and a microphone recording of one sound source. Common applications of this include recording an acoustic guitar with a mic whilst also recording a direct signal from the guitar’s pick-up via a DI box.

Multichannel DI boxes:

DI boxes come in many forms. Whilst single channel and two channel DI boxes are generally the most common, bigger options such as eight channel rack mountable designs are also available.

Throughput on a DI box:

Many DI boxes offer you an additional output known as a ‘throughput’. This allows you to take two signals out of the DI box. One signal will be the processed signal in the form of the low impedance, balanced, mic level signal. The other (the throughput) will be an unprocessed version of the signal. The signal leaves the throughput at the same signal level, impedance and balance as it entered the DI box at.

Sound engineers often utilize this in live sound for bass guitars. The engineer takes the processed signal from the DI box and connects it to a mic level input on the mixing desk. This signal then goes to the front of house. Meanwhile, the sound engineer will route the unprocessed signal from the throughput to a bass amp which is used for onstage monitoring.

In audio recording, the throughput is often used when recording an electric guitar to send the processed version of the signal to your interface to capture a clean signal. Meanwhile, the unprocessed signal is sent to a guitar amp which is recorded with a mic. Having both signals gives you the option to use the clean signal with an amp simulator in your DAW in addition to, or instead of, using the recording of the amp. Alternatively, you could also re-amp the clean signal. This is the process of sending the clean signal back out of your DAW and routing it to a guitar amp. This is useful if you want to experiment with different guitar amps later.

Conclusion

So as you can see, DI boxes are hugely versatile items which play a vital role in both audio recording and in live sound settings. Do you use a DI box in your home studio? If so, what do you use it for?