Remember that each example made and advice given is referred to D.I. Box having the same quality, while as we know in a real context there are Passive D.I. Box better than Active ones and vice versa independently of the use made of it.
Passive D.I. Box
The D.I. Box Passive can be as simple center-tapped transformers (for the balancing of the unbalanced signal) of which the transformation ratio is 1: 1 (fig. 1) so it does not attenuate the signal level at its input but has the only and simple purpose of balancing an unbalanced signal and eliminate possible interferences and noises present along the line and for this reason often called adapter.
This type of D.I. Box is generally used to send unbalanced line signals on Jack to balanced line inputs on xlr, as it does not match the level of voltage and impedance.
It is important, however, that in order to optimize the load transfer, the impedance of the output signal from the instrument is at least 6 to 10 times lower than that of the input of this type of D.I. Passive Box. These adapters have an impedance generally ranging from 200 Ω to 600 Ω depending also on the frequency you going to consider in which low frequencies generally have lower impedances and high frequencies have higher impedance. From these impedance values, it is understood that they are not very suitable for balancing unbalanced signals at the instrumental or microphone level (passive electric bass output, electric guitar, dynamic microphone or condenser), which is referred to as KOhm and hundreds of ohms.
The other type of Passive D.I. boxes are those with a center-tapped transformer with a transformation ratio of x : 1 (figure 2), which have the purpose of eliminating interference along the line also to adapt the impedance and voltage between two devices I / O. These D.I. Box are suitable, for example, to connect unbalanced line signals to balanced microphone inputs, and in some cases, instrumental and microphone signals to microphone inputs, as well as to reduce any interference and background noise.
The Passive D.I. Box is the ideal tool to balance the unbalanced line signals (0.775 V or 1.223 V) to be sent to balanced microphone inputs on XLR.
The Passive D.I. Box is composed essentially of the center-tapped transformer that determines all the signal quality parameters, so it is important that the transformer used is of the highest quality, since the main defects of a transformer are the introduction of background noise (so Low dynamic value), frequency response changes and signal phase. On the other hand, good signal balance is provided, transferring the highest dynamic variations, the possibility of connecting several transformers (even D.I. Box) in series without the introduction of noises due to impedance between devices, if not the noise produced by the transformer itself.
Quality transformers have against the fact of being very expensive, as well as they worn more quickly than the amplifiers.
Even for this type of D.I. Box it is important that the input impedance is at least 6 – 10 times the output impedance of the instrument from which it picks up the signal.
Below is a hypothetical circuit D.I. Box Passive (fig. 3).
Some features that must have a quality Passive D.I. Box:
The input impedance of an excellent Passive D.I. Box generally varies from 100 to 150 Khz, sometimes even less, and it is a correct impedance to pick up line signals (if it was too high would place too much resistance on the circuit and a greater dissipation into heat signal with dynamic loss, frequency, etc..).
The output impedance generally has values from 150 Ω to 600 Ω and even less, impedances adapted to connect the output of D.I. Box with the most common input impedances of the microphone pre-amplifier (which in turn must have an input impedance of at least 6 – 10 times the output of D.I. Box).
The transformation ratio of Passive D.I. Boxes are from 10: 1 and more (generally the higher the input impedance and the greater is the transformation ratio).
The frequency response of a good quality Passive D.I. Box is about 1 – 2 dB and less variation in the band from 20 Hz to 20 Khz.
An excellent Passive D.I. Box must have a minimum dynamic of at least 110-115 dB RMS.
n.b. It will also be necessary to have a pre-amplifier, mixer, power end which allows to pick up and offer the same dynamic output, in order to allow the signal flow to continue its path up to the diffusion stage without compression and a No use of DI Passive Box with such high dynamics.
The more D.I. Box will bear a high signal peak and the more quality will be, generally an excellent Passive D.I. Box must have a value of Max. Input +10 dBu up. On average, Passive D.I. Boxes allow to accept voltage values greater than the Active D.I. Boxes, as the transformer allows to accumulate more energy before generating values of distortion compared to the amplifier.
The distortions values in the audible band (20 Hz – 20 kHz) must be less than 0.05% up to the highest of the input signal values.
It’s important that the phase in the audio band from 20 Hz to 20 Khz does not exceed 10 ° variation in high frequency and 1° of variation in low frequency.
D.I. Box Active
The D.I. Box Active (fig. 4) presents a signal balancing circuit and impedance adaptation using no more transformers but BJT or FET amplifiers of class A (some manufacturers also produce a mix of active system and transformer, usually the active for balancing and adapting the output impedance, transformer to achieve high input impedance, suitable for the purposes we are going to see), which allow to obtain a better frequency response linearity and less distortion and background noise compared to the transformers. The main defect of Active D.I. Box is the limited dynamic for which it is necessary the use of good quality amplifiers (expensive) to reach the dynamic levels of the transformers.
In addition to this there is to say that amplifiers connected in series tend to create eddy currents along the circuit and to increase the levels of distortion and background noise, for this the D.I. Box Active are not very suitable for the withdrawal of line level signals but only for those instrumental (electric bass, electric guitar) as the instruments from which the signal out at line level always have a pre-amplifier, which in addition to the D.I. Box would generate additional background noise and instability of the answer.
The signal balancing is that Servo balanced.
As we have seen it is necessary to send phantom power to + 48V or in some cases also other values (or even via 9 V battery) to power the amplifiers that are part of servo balanced and to operate the Active D.I. Box.
Other models also involve the use of an external supply, generally 9 Vdc (fig. 5).
The D.I. Box Active are not suitable for the reduction of any background noise, as they present amplifier circuits. Sometimes it might just be the circuit of D.I. Box to create noise (so always use Passive D.I. Box to eliminate or reduce any noise).
Below is a hypothetical circuit Active D.I. Box (fig. 6) (from Radial Engineering Ltd.).
Active D.I. Box on rack (multi D.I.) (Fig. 7) have both the ability to withdraw the Phantom Power to + 48 V through the XLR cable (not all), but also to be able to be connected to an external power supply network and manage the signal through this line (in this case better use external power supply) (all).
The Ground or Earth Lift to detach the mass from the output balanced circuit may not be sufficient to eliminate any disturbances, as in servo balancing the amplifier circuit has a mass which loads noise and interference and which cannot be detached. So, in case of Hums it is always better to place a Passive D.I. Box.
n.b. Some Active D.I. Box have a small transformer in the output circuit which has the main task to adjust the impedance of the output circuit and to eliminate any interference.
Some features that must have quality Active D.I. Box:
The D.I. Box Active as seen, are more suitable for the withdrawal of instrumental signal and microphone and for this they need a high input impedance (at least from 6 to 10 times higher than that of the device from which the signal out).
The input impedance generally used on Active D.I. Boxes are from 200 KΩ to 2 MΩ.
In output we always have an impedance standard that goes like for Passive D.I. Box from 40 Ω to 600 Ω.
By being the servo balanced circuit and therefore not using the transformer, the transformer ratio does not exist, but the adaptation of impedance through an active balancing lead to a circuit with signal ratios more or less equal to that of Passive D.I. Box 10: 1.
Even for the Active D.I. Boxes the frequency response should be about 1 – 2 dB and less variation in the band from 20 Hz to 20 Khz.
The Active D.I. Boxes generally allow to have dynamic lower than those of Passive D.I. Box, around 100-105 dB RMS.
Also for this parameter as for Passive D.I. Boxes, those Active must have at least + 10 dBu bearable signal before generating distortions over the allowed limit.
The value of the distortions in the audible band (20 Hz – 20 kHz) must be less than or equal to 0.005% up to the highest of the input signal values.
It’s also important for Active D.I. Boxes that the phase in the audio band from 20 Hz to 20 Khz does not exceed 10 ° of variation in high frequency and of 1 ° variation in low frequency.
Some devices similar to the D.I. Box for wiring style, construction and size but with different functionalities are the Reamps.
The Reamp is a device which serves for picking up a balanced line generally at line level signal and allowing for its output an unbalanced signal at the microphone or instrument level to be sent to, for example, guitar and bass amplifiers.
Even the Reamp can be Passives (fig. 8), and Active (fig. 9).
Generally it is used for picking up a recorded signal (for example the sound of pure non-amplified electric guitar, or a sampled guitar) (if line level use passive Reamp, if microphone level, consumer or instrumental use active Reamp).
This signal is then picked up at the output and sent to one or more amplifiers (for example by electric guitar) in order to have the pure sound recorded or sampled with the amplifier timbre (Hence Reamp), so you can perform microphone recording at will without the need to present the musician or that this feels the same sound across multiple amplifiers to hear which can play better.
By having a pre-recorded pure signal, it is possible at any time to send the sound to any amplifier and record even without the presence of the musician or in parallel to have different sounds simultaneously.
Below is an example (from Radial Engineering Ltd) (fig. 10)
The pure signal of a recorded electric guitar is sent to different amplifiers have different reference timbre.
The Reamp can be used generally for the withdrawal of balanced signals line and the connection of this signal to unbalanced inputs at mic or instrumental level.
The input is always a balanced XLR and if it is an Active Reamp it requires sending the phantom power, some Reamp have their own external power supply.
Often there is also the lift to raise the mass in case of disturbances a button for the phase inversion.
At the output we always have one or more unbalanced TS Jacks from which it is possible pick up the signal to be sent to external sources such as electric guitar and electric bass amplifiers.
It can also found a Mute button to change the Reamp signal for example to altenate the sounds of different Reamp while leaving only the reference ones.
You can find different equalization filters so as to send to your amplifier the only band of useful frequencies.
Generally, there is also the presence of a pre-amplifier (both for passive and active reamps) with the aim to adjust the signal intensity to be sent to the output (such as an electric guitar or electric bass for the control of the volume).