Use of Direct Injection (DI) box
DI (DIRECT INJECTION) BOX
It takes unbalanced inputs, and coverts them to a balanced output. In addition to this, many DI boxes offer the ability to take an instrument level input (such as a guitar or bass) and match the impedance to an XLR mic input, so that it can be easily plugged into a mixing desks having XLR input.
The DI Box (the DI stands for Direct Injection) is a small size rugged unit usually kept on the floor of the stage, innocuous looking , but one of the most useful gadgetrs to have in your possession, in both live and studio situations.
Conversion into Balanced Signals
A DI Box converts unbalanced signals to balanced signals. Unbalanced connections transmit a signal via a single connection, with a ground. These are susceptible to background noise and interference, particularly when run over long distances. Unbalanced connections include mono jacks and RCA connections.
Balanced audio connections have a ground, live and return. This results in the cable rejecting interference as the signal is carried. Typical balanced connections are via 3-pin XLR cables (such as microphone connections) or balanced stereo jacks.
Many musical instruments and bits of music gear have only unbalanced outputs (keyboards, guitars, basses etc). To connect these to a mixer with the lowest noise levels, using a DI box is the perfect fix. It takes unbalanced inputs, and coverts them to a balanced output.
In addition to this, many DI boxes offer the ability to take an instrument level input (such as a guitar or bass) and match the impedance to an XLR mic input, so that it can be easily plugged into a mixing desk.
Types of DI Box
The simplest and most common type of DI Box is the passive DI. This is generally used to match line level or instrument level sources such like guitars and keyboards, and matches them to the mic inputs.
Often, these also have a ground lift switch (this can sometimes reduce unwanted hums and noise). The drawback of passive DI Boxes is that they reduce the signal level a little (typically from 3-6dB), and can lose a little tone with low-level signals, and sound thin and distorted when overloaded Active DI Boxes have active circuitry, which means that they require power. This is usually supplied via phantom power or an internal battery.
Whereas passive DIs use transformers to change signal impedance, the active DI uses powered amps. This negates the problems that occur with passive DIs at the limits of their dynamic range. However, when the signal is too hot, active DI boxes can distort in an unpleasant way. They don’t have the problem of disappearing low-end like passives do, though. This is all dependent on power levels- as a battery drains, performance will get worse. Plus, phantom power is not always 48V, which again can cause problems with performance.
When to use a DI Box?
With PA System: Though many pro stage pianos feature balanced outputs, many don’t, and neither do synths and keyboards typically. DI boxes will ensure a noise free connection to the PA. You may also be connecting an acoustic guitar to a PA- A DI Box is perfect for this. If an amp fails at a gig, this can also be a quick (though not ideal) fix. Many stage boxes only feature balanced XLR connections. DIs will enable you to easily connect your gear with the highest quality.
Matching impedance during recording
Plugging something with instrument level output, like a guitar, straight into a line level input can result in a very ‘dead’ lifeless sound. Matching the impedance with a DI Box will ensure that the signal is recorded at its best.
Many producers like to record bass through a DI. Nile Rogers famously likes to record his guitar dry through one, and processing later.
Many passive DI box models can also be used to re-amp signals. This means that the can work in reverse, taking a line-level signal and converting it to an instrument level signal. Producers commonly employ this to take a dry, recorded guitar part, and run it through an amp later, before re-recording. By doing this, it means that the guitar tone can be completely changed after recording. Alternatively, different tones can be created with different amps and compared or mixed. Cost of a passive DI box is generally less than that of Active DI box. This gadget is not very expensive and can be purchased in the range of INR 5,000 and 20,000.
by Col. VD Singh (Retd.)