Objective differences between Passive D.I. Box and Active D.I. Box (from Radial Engineering Ltd.)
The representations that follow are references of 2 D.I. Box of the highest quality.
Passive D.I. Box (Radial JDI)
Activate D.I. Box (Radial J48)
Dynamic and Frequency response:
The Passive D.I. Boxes dominate the aspect of Dynamics, but only in the case of Central Power Transformers high quality and cost-effective, and only for the low and medium frequency band, while the Active D.I. Boxes having a more linear response and less distortion are more efficient.
The Active D.I. Box generally has a slight loss of linearity in low frequencies (typical servo balanced problems) due to the use of various amplifier components that tend to create induced and phase-out currents that alter the response above all in this band portion.
n.b. Active D.I. Box has a lower background sound than a D.I. Passive box but its dynamics is lower because the maximum tolerable voltage value is much lower.
Below is a representation of the frequency response of Passive D.I. Box (fig. 1) vs. Active D.I. Box (fig. 2).
The level of harmonic distortion of Passive D.I. Boxes (fig. 3) is higher than those of Active D.I. Boxes (fig. 4).
In these graphs taken from the site of the Radial Engineering Ltd. the level of harmonic distortion of Passive D.I. Box seems lower than that of D.I. Box Active, actually the signal level sent for the measure of the Passive D.I. Box is – 10 dBu while that of Active D.I. Box is – 5 dBu so much higher, since – 10 dBu the level of harmonic distortion would have been impossible to detect via the graphic as it is too low, plus the measure was made taking into consideration also the background noise that in Passive D.I. Box with its high-level central power transformer is lower than that of Active D.I. Box. In fact, in the technical characteristics we have a THD value of 0.002% for the Active D.I. Box and 0.05% for the Passive D.I. Box.
From the two graphs below D.I. Box Passive (fig. 5), D.I. Box Active (fig. 6), note as a Passive D.I. Box generally has a higher phase shift in the high frequencies precisely where it is unless the linearity of the frequency response. For Active D.I. Box instead the majorr phase shift is generally on the low frequencies precisely in the area where most suffer from linearity in the frequency response.
The transformer by its nature is bidirectional so unless you have a passive D.I. Box that in the output circuit present a precise protection circuit against induced currents it is possible to use such Passive D.I. Box both in a direction signal from unbalanced to balanced that another signal from balanced to unbalanced signal (and therefore the output becomes the input and the input becomes the output).
The Active D.I. Box having instead an amplifier do not allow this bidirectionality.
Examples of musical instruments which generally use D.I. Box :
Electroacoustic Guitar: (fig. 7)
Keyboard and Digital Pianos: (fig. 8)
Electric Bass Passive or Active: (fig. 9)
Electric Guitar: (fig. 10)
Electronics Drum: (fig. 11)
Strings or electro-amplified woods: (fig. 12, electric violin)
As for the outputs of Computer audio, notebooks, smart phones, CD players, mp3 players the ideal connection is always to connect their output to a consumer input – 10 dBV (most of the time it is found in the form of RCA connection ), but in case of having to bring long distances to them then it is good insert a Passive D.I. Box (better if with the presence of a consumer input to – 10 dBV) due to the low-impedance input and the possibility to reduce any background noise that would occur to place in series with the output amplifier of the consumer line and the input amplifier of any Active D.I. Box.
A D.I. Box Passive can also be used for the withdrawal of a balanced line signal on XLR or TRS or unbalanced TS depending on the type of input that has the D.I. Box to eliminate or mitigate any disturbances along the line, if balanced and D.I. Box allows, always to eliminate the disturbance you can also try to insert the Passive D.I. Box and then raise the mass as this is possible in the passive balanced circuits or active servo-balanced. Or to introduce a possible attenuator more (the pad present in DI Box) in the event that the signal needs to be adapted to a consumer input without having to change the master fader position of audio mixer that as we will see is recommended for quality issues to maintain its position at 0 dB.
Examples of application: (reference examples from Radial Engineering ltd.)
Generally, the signal of the electric bass is taken directly from the pre-amplifier that presents most of the time already a balanced output, it can happen, however, that output (especially for non-professional pre-amplifiers) generating noise or is of poor quality compared with Professional D.I. Box, so it is a good idea to take the output signal from the bottom and connect the input of an Active D.I. Box (if passive bass), Passive D.I. Box (if active electric bass), so that it can properly pick up the appropriate voltage level (in case of noises always try with a Passive D.I. Box), then take the output link or direct out or thru of D.I. Box and use that to connect with the pre-amplifier (the signal travels increasingly unbalanced to jack TS).
The balanced output can then be withdrawn for sending the signal of the electric bass on long cable paths towards playback devices, processing, recording.
At level of studio but also in some live contexts, tends to use the electric bass sound in D.I. Box plus output microphone from the speaker to obtain a parallel sound that gives dynamic sensations and depth as well as a higher brilliance in that as the speaker of the electric bass very often, and especially if it is composed only of a woofer (less instead for multiway systems with woofer and tweeter) it tends to have a poor response in high frequency, whereby it recovers from the sound in D.I. Box so that unlike from the electric bass speaker it appears as a more compressed sound.
The microphone in the bass diffuser is used less in the live environment as a microphone in more means even more indents and less dynamics for the mix.
For classical guitar is generally used a Passive D.I. Box as the instrument in the output presents a line level pre-amplifier in which Active D.I Box that presents a circuit amplifier may introduce alterations in the frequency response and an increase of the background noise, in addition to the fact especially if poor quality of not being able to transfer to balancing the whole dynamic of the instrument especially for fast transients (high frequency), which for electroacoustic guitar is high.
The principle is the same as seen for the electric bass, to the input of D.I. Box the output signal from the guitar and from the output balanced is taken the signal to be sent to the various remote instrumentation. In case there is also a speaker with pre-amplifier that the guitarist uses as a monitor or to obtain an additional sound is provided to send to the pre-amplifier the output link or direct out or thru and eventually resumes its sound with a microphone.
The keyboards are generally mono / stereo outputs, if we pick up the mono signal it is useful a mono D.I. Box Active or Passive, while for using of a stereo signal it is useful to use Stereo D.I. Box Active or Passive.
The keyboards have a pre-amplifier output at the line level but the signal has a much more reduced dynamic range compared to that of the electro-acoustic guitars and for this in the case of not generating any noise at times can be a more qualitative Active D.I. Box compared to a Passive one, generates a higher brilliance and clarity of the instrument.
The output signal from the keyboard is connected to the input of D.I. Box and the balanced output of the D.I. Box must be sent to an external device present a distance longer than 10 m.
With some D.I. Box as those that have merge functionality can pick up the stereo signal to send L in the Input and R in the link or direct out or thru of a Mono D.I. box and clicking merge its possible to mix stereo signal to mono without introducing particular phases to be picked up at the balanced output.
In some situations it is also possible to withdraw, for example, the output signal from an electric guitar, then with low voltage values, to be sent to an Active D.I. Box in which the balanced output is sent to a mixer or processor or recorder and then its return is sent to the input of an Active or Passive Reamper depending on the level of the signal that arrives (Passive if arrives a line signal, Active if arrive a consumer signal ) with a balanced input and the task of unbalancing a balanced signal reducing any disturbances before the withdrawal of the signal to be sent to the preamplifier for guitar.
It is used for example in the recording studio, more than when it is necessary to extract the pure sound of the electric guitar together with that amplified by a clean recording performed previously.
The same thing can also do with the electric bass in figure 5:
The pre-amplifier output line is sent to the input of D.I. Box while from Thru, Link, Direct Out we take the signal to be sent to the input of the speaker. The balanced output on XLR of the D.I. Box we’ll connect it to our mixing devices, recording, etc… Useful for mixing and / or record the pure sound of the output instrument from the pre-amplifier with that picked up by the microphone.
Other types of D.I. Box
Bal to Unbal D.I. Box
Some D.I. Box are specially built to transform and control by eliminating interference from the ground and by converting an analogue audio signal (+ 4 dBu) into a consumable audio signal (- 10 dBV), (figure 6) or vice versa (figure 7).
75 Ohm – 110 Ohm
Another type is the 75 ohm to 110 ohm impedance adapter (as can be the interfacing BNC digital lines – S/PDIF and AES/EBU), adaptation realized through the use of passive transformers, as shown in figure 8.
You can find both in one direction or another, with XLR for 110 ohm male or female.
Digital D.I. Box
Dante D.I. Box
In 2016 the first digital D.I. Box were introduced that allow to obtain by trough an analogue input signal a digital output signal for the target protocol. Currently the only protocol used in a digital D.I. Box is to DANTE (fig. 9), because of its ability to handle different digital signal of network addresses for the creation of star-based networks. Other protocols have however still numerous blocks from being able to exploit this type of solution due to the requirement of point to point destinations or daesy chain (we will see when we talk about digital audio the detailed functioning of each commercially available commercial audio protocol, some notes are Found in articles Stage Box – Splitter and Adder).
AES / EBU digital D.I. Box
Another type of D.I. Box is that AES/EBU in which the input is analog and the output is on digital standard AES3, even in this case the quality of the A / D converter is crucial.
Some digital D.I. Box enable the transfer on Ethernet cable both the analog signal that digital AES3.
The audio signal is carried on 3-pole XLR connector on a balanced line for the analog or digital input, while the AES3 output is over CAT5 / 6 Ethernet cable (fig. 10).
MIDI D.I. Box
Another type of D.I. Box can be the MIDI, able to accept MIDI signals IN – OUT – THRU to manage digital interfaces via MIDI protocol and obtain at the output an analog audio signal generally on balanced XLR (fig. 11).
Bluetooth D.I. Box
Another innovative D.I. Box introduced in 2016 is the Bluetooth D.I. Box (fig. 12).
Which allows through a remote control with the ability to send and receive RF signals via Bluetooth (smartphones, tablet, notebook with a dedicated application), to manage signal levels and dsp varied toward the output connected to it. Currently the only commercial version available is the remote control of the analog signal line via Bluetooth, for example to control the signal level to power amplifiers or line inputs (fig. 13) or to interface and manage the signal level from musical instruments with Bluetooth technology (fig. 14).
Another type of D.I. Digital box is a classic audio interface that can be connected to the computer via a USB connection and outputs an analog stereo line-level signal to XLR connection as shown in figure 15.
Some productors of professional D.I.Box:
- Art Pro
- AMS Rupert Neve
- Audio Tools
- Klark Teknik
- LA Audio
- Little Labs
Some productors of Phantom Power generator:
Below some video on the D.I. Box: