The D.I. Box (direct injection box) (fig. 1) is a component which serves to adapt the level of voltage and impedance of an outputting signal from an audio device (generally musical instrument, unbalanced signal) and entering into another having characteristics of different circuit, such as to optimize the correct load transfer without sending distortion in the input circuit and correctly pick up the right voltage level so that it can work at will without the introduction of background noise and distortions on the signal.
In the audio environment D.I. Box is mainly an analog component (for which only used for the connection of analog audio equipment), although to date as we will see are born some devices with similar structure to that of D.I. Box but for the conversion and voltage adaptation from analog to digital or vice versa, then A / D and D / A as real audio interfaces which we’ll explain in other articles. In this regard, also a D.I. Box Analog can be considered an analog converter, including a form of voltage / impedance to another and with the essential purpose of transforming an unbalanced signal at its input into a balanced signal at the output, so as to allow the system to be transported signal for longer distances and can be interfaced to the microphone signal level with pre-amplifiers.
It can also be useful to eliminate the signal noise as there is the presence of a transformer (for the passive ones), which does not allow the crossing of DC disturbances.
The effectiveness of D.I. Box is the elimination (sometimes only attenuation, depends on the quality of the transformer and circuit used, as well as shape of the input noise-distortion) of noise coming from the source (and therefore for the input signal to the DI Box, while for disorders generated after the output stage of the DI Box could be other problems as a little shielded cable or unbalanced, a problem on the grounding of the input circuit to which the output of the DI box is connected and much more).
The tools that generally require a D.I. Box are:
1. Any instrument in which at its output we have an unbalanced signal to enter a balanced circuit (pre-amplifier microphone or line) and / or to travel more than 10 meters.
2. Any instrument in which at its output we have a line level signal to be sent to a microphone preamplifier (which accepts microphone signals).
3. Instruments for which, especially when recording, it is necessary to have different sound quality as well as the use of various microphones and microphone techniques.
4. Instruments that unbalanced or balanced output generate audible noise in the circuit.
As mentioned in the article Analog Audio Cable II in which explains the operation of the unbalanced lines that are qualitatively better than those balanced, if there is the possibility to connect within 10 meters an outgoing line unbalanced (that does not generate noise) in a unbalanced input, the D.I. Box is always better not to use it to not introduce additional noise and delays in the signal path caused by D.I. Box itself.
The D.I. Box may have different technical characteristics which determine the use and quality, there are basically two types of D.I. Box, PASSIVE and Active (we’ll see how they work in detail in the second part of this topic ).
They occur most often as a square or rectangular shape hardware (fig. 1) with smaller size or a little larger than a hand, in some cases it is also possible to find D.I. Box multichannel rack (fig. 2).
Number of inputs and outputs: Generally in the market are D.I. mono (one input and one output, fig. 1), D.I. stereo (two inputs and two outputs fig. 4), D.I. multi-channel (multiple inputs and as many outputs, Fig. 2).
There is no D.I. Box better than the other, let’s say that as we will see the passive is good for a certain type of situation while the one active for another.
In addition there are D.I. Box active better than those passive and D.I. Passive boxes better than those active, so it is good to know the technical information of the product and even better to hear the opinion of a competent technician that already those D.I. Box has used and tried them.
For examples and considerations that we will say an equivalent level of quality for both D.I. Box.
To the input circuit we always have a Jack TS connection used to receive unbalanced signals such as the output from the electro-amplified instruments (fig. 1).
The ideal voltage level to be sent to the input of D.I. Box Passive is always to line (or 0.775V 1,223v) (as that output from electro-acoustic guitars, keyboard and digital pianos, drum machines, electric active low), while for the D.I. Box Active can be either the one line that even the instrumental one (electric guitar and electrical passive bass guitars).
Other D.I. Box in some cases include the addition of other input formats such as the XLR (figure 3) with the primary objective of eliminating signal noise and interference on a balanced or unbalanced XLR line, always for line level signals generally for OF Passive Box and / or Instrumental or Microphone for D.I. Active Box (When connected to a balanced signal this is unbalanced at the input and re-balanced at the output).
Others also expect to add connections for picking up signals always line level but on PIN connectors for D.I. Box Passive or even consumer signal level for D.I. Box Active (fig. 4), which signal may be in some cases mixed with that in jack input and / or xlr. In addition, always in figure 4 can be found ts jack combo inputs – xlr.
n.b. When the input line is unbalanced on Jack TS it is good not to insert Jack TRS to not accumulate or introduce along the interference line. Combo connections provide Jack TRS input to accept even balanced signals on Jack.
Depending on the type of D.I. Box especially in the most professional we can find also:
One or more PAD attenuation (fig. 3 and 4) in the event that the input level is too high and can bring distortion in both the D.I. Box that the pre-amplifier to which the output of D.I. Box is connected. They generally have an attenuation value of – 20 dB – 30 dB – 40 dB.
Earth Lift or Ground for the removal of mass from the output XLR signal in case of background noise and interference.
Link or Direct Out or Thru Earth or Ground Lift for the removal of mass from the output XLR signal in case of background noise and interference.
Link or Thru or Direct Out where you can extract the copy of the input signal to be sent to a preamplifier device (guitar or bass preamplifier) or to a recording device to have a copy of the signal without D.I. Box and one with D.I. Box, or send it within the D.I. Box a further signal to be mixed with the input signal. OF. Mono box will present 1 link (figure 6), D.I. Stereo boxes will feature 2 links (figure 7), D.I. Multichannel boxes have a link for each channel (figure 5).
N.B. In D.I. Box quality link is often applied to an inductor, or it uses the same transformer for D.I. Passive or amplifier D.I. Activate that shows the signal at the input level, when split it you get a loss of 6 dB (which happens in D.I. poorer Box).
Other D.I. Box (Fig. 6) provide for the MERGE function or SUM which has the purpose of mixing the signal present in the input to that present in the link or thru or direct out in such a way as to have at the output a single mono signal (useful, for example, to mix a stereo signal into a mono output in order to save channels in the audio mixer), this feature also provides for the cancellation of any phase opposition caused by the mixing of stereo signals that normally would go to create.
n.b It is not recommended to send two signals to be mixed (one in the input and one in the link or thru) in D.I. Boxes that do not provide Merge or Sum functions because the input circuitry is different and would generate more than antiphase and interference transmissions between the two signals.
n.b. If you are using the D.I. Box just to pick up the unbalanced signal in the input and send the copy of the signal in the Thru to a pre-amplifier, please do not click the MERGE button since in some cases the return of the signal copy on the Thru and others will not work Cases would be mixed to the input signal in input any noise and feedback interference present in the Thru connection.
Other D.I. Box, exclusively those stereo and multichannel have a SPLIT call option (fig. 7) which has the purpose of sending an input (generally Left or 1) into a copy to all or pair of outputs Link, Thru, Direct Out (in such a way that to use the DI Box as a signal splitter).
Still others (Fig. 8), have a low-pass filter, HI – CUT to attenuate in equalization any excess emphases of the signal in the high frequencies.
Is also possible to find instead of HI-CUT filter (fig. 10), SPEAKER that instead of being a low-pass filter is a bandpass filter, useful for sending output a range of well-defined frequencies (generally from medium-low to medium-high) thus eliminating any background noise in high frequency and presence low-frequency resonance.
n.b. Regarding the equalizer filters (considering the use of quality filters), if there is a filter in a D.I. Box or microphone it is always good to activate the filter from these which generally represent the first stage of the audio signal path instead of external devices and farther into the audio path stream (eg filters in audio mixers), allowing a Wider signal dynamics along the signal path up to the input stage such as a preamplifier, as well as less work stress for the same pre-amplifier and audio mixer.
Some D.I. Box (Figure 9) are suitable for power level signal picking, usually the output from Electric Guitar and Electric Bass Amplifiers to clear the signal before mixing and allow it to travel long distances. Generally used in the studio to record the amplifier sound without microphone resumption or with a sound mix coming out of the pre-mix and microphone recording.
These D.I. Box generally accept at the input of an unbalanced power line on Jack ts to 50 watt or 100 watt or 300 watt, they generally have a 100 Hz to 20 kHz linear frequency response (ideal for electric guitar also acting as a high pass filter) with The possibility (as in the example in Figure 9) to click on BASS EXT to extend the frequency response from 20 Hz to 20 Khz when using an electric bass.
At the output stage the D.I. Box always have a male XLR connection from which the balanced signal can be sent to a mic input at long distances may be withdrawn (so microphone signal) (fig. 5 – fig. 7 – fig. 10).
Some D.I. Box also have the REV 180° or 180° button (fig. 10) which allows to reverse the polarity of the signal in the event that this output has a delay such as to arrive in phase opposition, for example, a copy thereof to be mixed together.
Example when it mixes together an electric bass whose signal is taken from the D.I. Box + microphone, it can be useful to try to reverse the polarity of the output signal from the D.I. Box and feel if the mixed sound has an effective increase in the loudness (in phase) or create effects of hole and cancellation at low frequency (in antiphase).
n.b. To date, especially if in the process of recording, it is preferred to record the signals without the introduction of electrical or electronic components such as phase inversion, so as to obtain the highest possible quality, then in the process of editing put everything in phase (the sample), much more precise, quality and effective that done with a simple phase inversion, that most of the time solves the problem to 40% not ever getting a full phase correlation.
The Hi-CUT filters and SPEAKER or Band Pass can be found for the input or output circuit according to the constructive method.
Even the Earth Lift or Ground may be for the Input or Output Circuit they are usually present for the output circuit as the signal is balanced and then the mass can be eliminated from the signal but in the presence of a Balanced connection can also be found for the mass elimination input circuit before the imbalance imposed by the input circuit of the DI Box.
As for the Active D.I. Box as we will see need an external or battery power to operate the internal amplifiers capable of picking up the signal at the input and balancing it at the output, have a generally red LED that serves to signal the presence of this voltage and Then the proper functioning of DI Active Box (Fig. 11). This LED is generally designated + 48 v or Phantom Power (Figures 7 and 11), as the required voltage for proper operation is +48 V DC, which is usually sent by the audio mixer For each input microphone channel, Figure 13), or from external Phantom Power distributors on a Rack, or even directly from the Active and Digital Stage Boxes.
Phantom power is taken from pin 3 of the XLR balanced output of D.I. Box.
Not all Active D.I. Box have instead the possibility of housing the battery as an alternative to the Phantom Power (fig. 12), this is because the battery power is generally taken from a 9 V battery (fig. 14) which guarantees is the operation of D.I. Box but not compliance with its maximum performance and technical features.
Fig. 13 (Example of the present button in analog mixer)
n.b. If, despite sending this power, the LED does not work, it may simply be a malfunctioning led problem, and therefore the audio signal should circulate and feel, or if there is no audio signal, it may be a problem in sending this Power supply that may be a defect of the XLR cable on the pin 3 (return conduit in which the Phantom Power goes too) (disconnected or unbalanced cable) so it is useful to try to change the cable or even in extreme cases use a 9 V battery, or if it is powered on an external battery, you may have a low battery, so try changing the battery.
Some D.I. Box can also accept Phantom Power with different standard from the one most commonly used to + 48 V, for example Phantom Power to + 12 V or + 24 V.
n.b. Avoid using Phantom Power + external power supply to avoid sending a surge that might damage the circuitry of the D.I. Box itself.
External Power Devices and Adapters
Some manufacturers also provide among their products some distributors (generally on racks) of phantom power (+ 48V but sometimes also different voltages), these devices allow and are useful for powering, for example, condenser microphones and D.I. Active Box (put both the link) which exactly require this kind of power to be able to work but that this may not be present to be sent by other means available (for example, it is usually sent by analog or digital audio mixers Active audio or digital splitter to which active microphones and DI boxes are connected. It is also important that these devices are as transparent as possible when passing the audio signal and correctly perform the work for which they are designed and built (send phantom power along the audio line steadily and steadily to avoid creating problems with the connected devices and alterations of the audio response of the device itself). These devices require external power supply from which to draw and process the voltage level you need to send. They can be single-channel (fig. 15) or multichannel (fig. 16) even on a rack (fig. 17).
Other devices instead can adjust a level of tension (Phantom Power) at the input, usually at + 48 V but also other values at one or more different voltage values at the output. In order to allow the use of different devices (generally widespread for condenser microphones which as we shall see may require different voltages to supply their circuitry) which require different Phantom Power values when the generator is one and a certain Voltage level.
Notes and Other Features
Those views are just the features that are most in D.I. Box, some manufacturers may have more variations and different components used for the management of audio signal input and output.
Some manufacturers insert a D.I. Box in equipment such as Microphone Pre-Amplifiers, Stage Box, Audio Mixer, in order to create a more complete product, but generally do not have the quality that can be compared to the best D.I Box external modules .
In figure 18 an example of a microphone pre-amplifier with a built-in D.I. Box module, named Instrument (front) as high impedance input for low passive instrument signal such as electric bass.
Another important feature that should have the D.I. Box both active and passive is to be impermeable especially for those used in live environments, and then subject to all bad weather (water, sand, dust) and that especially active ones have no defects in the control of Phantom Power, then constructed with an optimum circuit that does not create the power dissipation on the metal casing of the D.I. Box which can cause problems on both the audio signal that the person in use.
Here other key features of D.I. Box that we will see in detail in Part II of this article.
A D.I. Box of excellent quality it is necessary to have a more linear frequency response possible in the audible audio band (20 Hz – 20 kHz).
Generally variations of 0.5 dB – 1 dB.
A D.I. Box of excellent quality must have a higher signal-to-noise ratio possible.
Generally from 100 dB up.
A D.I. Box of excellent quality it is necessary to ensure the most possible phase coherence of the audio signal, for which along its transformation from unbalanced to balanced not create inversion and / or phase shifts on the audio signal that may lead to easy alteration of the frequency response and a reduction in the overall dynamics.
Generally 8 ° – 10 ° in the low frequency and 0 ° – 1 ° in medium and high frequency.
A D.I. Box of excellent quality is needed that would allow the highest values of the audio signal before distortion.
Generally, from a minimum of + 10 dBu up.
The THD determines the percentage value of the distortions introduced by D.I. Box on the audio signal and the lower this value, the more dynamic and quality will have the D.I. Box
Generally over 0.05% – 0.01%.