The DI or Direct Injection Box - Lab Report Example

The basic function of DI was to enable the sound engineers to lay long lines of three-core balanced cables between musicians and the engineer, who might be seated at the far end of the venue. In order to send him proper and noiseless signals, one would require balanced lines with high impedance. But the electric guitar, bass guitar, electric keyboard, or drum machine outputs are all unbalanced and have low impedance outputs of around 50-kilo ohms. In order to convert it into impedances of around 3-kilo ohms, DI boxes are employed. They basically consist of an impedance-matching transformer.

A transformer consists of two coils – the primary and the secondary. The number of turns in each actually determines whether it is a step-up, a step-down, or just an isolating transformer. Having a transformer also serves another very important purpose – isolation between the input and output signals. This helps to keep the expensive musical instruments out of the way of damage by the intrusion of unwanted electrical impulses such as accidentally switched-on phantom power in the mixing console. In DI boxes, however, a step-down transformer is used. The number of turns in the primary coil is more than in the secondary coil. Therefore a lesser amount of current is induced in the secondary coil, resulting in a mic-level output (around -30dBV), from a line-level input (around -10dBV). The change in the signal level is directly proportional to the turn ratio in the primary and secondary coils. Output transformers in DI boxes typically employ a 10:1 winding ratio.

            Another important feature is the ground lift switch, which helps to eliminate unwanted hum or any other type of noise that might be induced into the system by the interference of more than one ground loop, resulting from more than one grounding loop employed in a single system. The ground lift breaks the ground loops and prevents the reproduction of low-frequency hum and interference (usually about 60 Hz or 50 Hz, depending on the AC power cycle of a particular country).

            The lop side of using such a passive DI box is usually a little loss of signal (about 3 to 6 dB) strength and signal quality (a high-frequency roll-off and phasing problems) that sometimes may result from a cheap output transformer. However, these little difficulties are easily got over by employing an active DI box that uses either the phantom power from the mixing console it is connected to or uses batteries or main power adaptors to drive the unit. They basically consist of a pre-amplifier with an adjustable input sensitivity to match the source signal. These units sometimes don’t employ a step-down transformer in the output and thus they do not provide the physical isolation between the incoming and the outgoing signal. They do their job by employing complicated and active electronic components such as ICs and chips. The active DI boxes can also provide other features such as basic filtering circuits that can be switched in or out, input attenuators by different selectable pads, and sometimes polarity reverses to compensate for a faulty cable or connector and sometimes an output gain control to match the input sensitivity of any mixing console.

            The DI box, though playing a small part in the audio chain is not to be overlooked. They are to be selected very carefully and only after considering their full specification and the application should the choice be made.