DI stands for direct injection, they are also known as: DI's, Direct input box, Direct box and Hum isolators. There are two main types, passive and active. Both do the same job, but the active type can add gain to the system. The main purpose of a DI Box is to:
1. Match input and output impedance's so correct levels can be achieved.
2. Allow for long cable lengths to be used, by turning unbalanced signals to balanced signals. Which reduces interference from other equipment.
3. Isolate individual pieces of equipment from each other to reduce unwanted noise and hum.
Passive DI Boxes
Passive DI's are handy as they do not need any form of power supply. So, for example, a guitar can be plugged straight into the box with an unbalanced jack lead and connected to a mixer microphone input, via a balanced XLR lead, with no other connections are required. The long part of the lead can be balanced, reducing noise and hum and matching the mixer input correctly. Passives DI's use an internal transformer to match levels between equipment. For guitar/line level connections to a microphone input, the transformer ratio would be about 10:1. If you are connecting an unbalanced line level signal to a balanced line level connection then a transformer ratio of 1:1 would be better suited. A disadvantage of passive DI boxes is they cannot improve the input signal, so if it is too low nothing can be done. Also, they need to be designed with the very best transformers to make sure they have the best frequency response, these transformers can be quite expensive.
Active DI Boxes
An active DI needs some form of power, whether it be an AC/DC power supply, battery or power from the mixers phantom power supply. Since they have power other features can be added into their design, so a pre-amplifier circuit could be added to boost very low-level signals for example. Electronic compensation can improve frequency response with the active designs, but inputs can easily be over-driven more easily, compared to the passive versions. The clear disadvantage of using active boxes is that they need power. If they use phantom power from a mixer and your mixer has phantom power, then this is a fairly painless and elegant solution. If you are using batteries on the other hand, there is always a chance they will run out when the equipment is in use. Separate power supplies are an option, but they do add yet more cables to the stage or studio environment, so this may be something you want to weigh up.
The most popular types of DI box are what could be call the “guitar type”. This is a fairly simple device with one transformer with a ratio of about 10:1, they have a unbalanced ¼” jack input and a parallel through connection. The through connection is connected straight to the ¼” jack input. This is for connection to an on-stage guitar amplifier for monitoring, offering some kind of foldback to the performer. The output connection is normally a XLR socket, this would then connect straight to a mixers balanced microphone input and on through to the main PA or front of house system. A ground lift switch is normally included to help reduce hum problems and you may also find a pad switch, maybe 20 or 30db to lower the signal when using the device with tablets, CD players or laptops. These types are often moderately priced, although there are expensive types available, often preferred for studio recording.
Another type is designed for stereo line level signals. These are sometimes called hum isolators. They have two transformers inside with a ratio of 1:1 These can have unbalanced input and output connections, connections can be 3.5mm jack, RCA/Phono and ¼” jack. These devices isolate individual pieces of equipment, so help remove unwanted ground loop and hum noise from laptop power supply for example. These types can sometimes have balanced XLR outputs as well, giving you the option to run long lengths of cable with little chance of noise interference.
As you can start to see, there are many types of DI boxes and hum isolators. If you have to work in different venues and recording environments it would be best to have different types in your kit ready to use.
Technical blurb associated with DI Boxes and isolators:
Unbalanced Signal and Cables:
These are signals sent with one hot wire+ and a ground, so use two cables or conductors. The ground wraps around the inner conductor which helps suppress external electrical noise from lights, transformers and radio transmissions. Unfortunately, this cable also works as a type of antenna, picking up unwanted noise so should not be used for long distance connections. Keep unbalanced cables no longer than around 10 metres and less if there is a lot of electrical equipment around. The connections associated with unbalanced cables are normally RCA/Phono , ¼” or 3.5mm jack. The connections can often be stereo, say from a tablet via a 3.5mm connection, a hum isolator or dual DI is required here.
Balanced:
Is a signal sent with a hot +, cold – and ground, so 3 cables or conductors. The ground wraps around the two inner conductors. Balanced cables greatly reduce unwanted noise. This is done by carrying the same signal down the inner cables, but one is inverted, any external noise will effect both internal cables, but is cancelled out at the receiving equipment end with only the balance signal being amplified. Normally connections are via XLR or stereo ¼” jack plugs. Balanced cables are the best way to interconnect equipment and instruments, they significantly reduce noise and allow the use of long cables with little trouble. This is not intended as a definitive description of how balanced cables work, since impedance also has an impact on the noise reduction effect. There are plenty of technical texts on the subject if you are really interested.
Ground Lift Switch:
This is a switch that isolates the ground between the input and output. Hum noise can be created from the amplifier connected to the through socket or via the ac power from a keyboard, laptop or similar device. Hum is created from ac current flowing down the shield cable which transmits to the inner signal conductor. This is called a ground loop lifting the ground switch totally isolates the input and output ground connections.
PAD Switch:
A pad switch reduces the input signal, helps match input levels and stops overloading of the mixer input circuits. There can be one of several gain reduction options, usually noted in dB. As an example, a 20dB gain reduction would mean reducing the signal voltage by a factor of 10.
Link/Through Socket:
The link through socket is connected in parallel with the input socket. This is often used to connect directly to an on-stage monitor or guitar amplifier, whilst sending the isolated signal to the main mixer and front of house system.
Polarity Reverse:
This reverses the polarity of pin 2 and 3 on one of the DI boxes XLR connections. This can sometimes help prevent feedback, get stereo connections back in phase or be useful as a studio effect.
Pre-amps:
These are found in active DI boxes, they are a circuit which amplifiers the input signal before it is then balanced and sent to a mixer or amplifier.
Transformers:
Are the heart of most DI boxes. They isolate the input and output connections and often act as an impedance converter, changing high impedance circuits to low. There is normally a ratio of 10-20:1 between input and output. Transformers found in hum isolators do not provide any impedance matching, they have a ratio of 1:1, so whatever is plugged in is copied to the output connection.
Phantom Power:
Phantom power often provides power to the internal circuits of Active DI boxes. Normally this type of DI uses a battery as well, but when phantom power is detected the battery becomes inactive.
Multiple Channel:
Multiple channel or dual DI boxes, are just two DI boxes in one. These are handy to carry a stereo signal from a laptop, tablet or other music source. They cut done on extra unwanted equipment on stage, one instead of two boxes. They sometimes have a mono/stereo switch which allows one signal to be sent to both outputs.
Impedance in DI:
Any equipment that generates a voltage, has an output impedance and any equipment that receives a voltage has an input impedance. Impedance is measured in Ohms, and is a reactive component of alternating voltage, capacitance and induction. You should always try and match output impedance of the source to the input impedance of the next device. By doing this there is an almost perfect signal transfer. Mismatched connections are not efficient and may affect frequency response or have other unwanted effects.
Capacitance in Cables:
Capacitance occurs in all cables and this can create an unwanted low pass filter when cables are long. The longer the cables the more the internal capacitance increases, and high impedance unbalanced cables make this effect more pronounced. Balancing and reducing the cable matching impedance with DI box is another excellent reason for their use.
