DI box, passive. What are they for?

[godd2]

Hello,
I have a number of DI boxes and I am really not sure how they work or what for. I do not do anything with guitars or whatever. I work at a high school and do stuff with dvd players being connected to the theater sound system. besides the normal microphone with the xlr plugs. is this something these could be used for? connect a dvd player to the xlr imput of the established sound system?

I am just lost really what they are for.

thanx

 

[mbenonis]

DI boxes provide isolation and impedance transformation. The isolation is important, because it can prevent ground loops from getting into the sound system from your DVD player, iPod, computer, etc. The second part is impedance transformation. The output impedance of the line level device is fairly high compared to the output impedance of a Low-Z microphone, so the transformer makes it look lower to the sound console. This means that less current will be drawn by the console. At the same time, it makes the console's input impedance look higher, hence drawing less current from the line level device.

DI Boxes have a feature called a ground lift, which completely separates the two devices from DC coupling.

 

[godd2]

thank you for the explaination but i am really stupid with sound..so what does this all mean.what can i use them for specifically...

 

[mixmaster]

Hello,
I have a number of DI boxes and I am really not sure how they work or what for. I do not do anything with guitars or whatever. I work at a high school and do stuff with dvd players being connected to the theater sound system. besides the normal microphone with the xlr plugs. is this something these could be used for? connect a dvd player to the xlr imput of the established sound system?

I am just lost really what they are for.

thanx

Yes, you could use your DI box to interface your DVD player with your console. You probably would need an adapter of some kind to go from the RCA outputs of the DVD player to the 1/4" inputs of your current DI box. Or, any number of manufactures make DI boxes with RCA an mini plug inputs specifically for that application. (whirlwind PCDI comes to mind offhand, Rapco has similar)
Otherwise put the DI box between any instrument with 1/4" output and your board to satisfy the assorted electrical issues. If your DI box has a 1/4" output in addition to the xlr, it can be put between a guitar/bass/keyboard and the instrument's amplifier also, giving you a send point to the console.

 

[blalew]

Most inputs on a sound console are balanced XLR (think 'high quality circuit'). Most instruments have an unbalanced 1/4" output (think 'low quality circuit'). A direct box or DI provides a way to interface the two.

Hint: minimize the length of the instrument cable (10' preferrably, no more than 20') going from the instrument to the DI. The mic/XLR cable coming from the DI can be as long as practicable (100s of feet with good quality cable).

Does that help?

-Blanton

 

[bishopthomas]

An unbalanced line (like the audio from your DVD player, or a guitar cable) is very receptive to noise, which is made worse by longer cable runs. A balanced cable (like for a microphone) can be run long distances with much greater noise rejection. A DI (direct injection, FYI) box converts the unbalanced signal to balanced, allowing the longer runs without the noise issues.

If you were to open up a basic passive DI you would see that there are connectors and a transformer mounted to a box. That's all it is in it's basic form. Most have a ground lift switch for getting rid of 60 Hz hum, and some have pads so that you can attenuate the signal.

It is NOT an isolation transformer, as mbenonis suggests. The only "isolation" it provides is by being able to not pass the ground signal to the rest of the system. If you need an iso tranny use one of those (they are a different thing entirely).

 

[blackisthenewblack]

Think of it this way, in a typical 1/4" jack, you have two cables, a positive and a negative. Signal only goes one way down one cable. Because of this, it is receptive to interference (which is why you can sometimes pick up radio stations through monitors). This would be an unbalanced cable. A balanced cable is a cable with three cables inside. the purpose of this is so that any interfernce that comes down the one line is cancelled out due to negative interference on another cable. therefore, no interference.

 

[mbenonis]

It is NOT an isolation transformer, as mbenonis suggests. The only "isolation" it provides is by being able to not pass the ground signal to the rest of the system. If you need an iso tranny use one of those (they are a different thing entirely).

It is an isolation transformer, in that it magnetically couples the signal and does not pass the DC component. Perhaps what you are thinking of are 1:1 transformers meant to isolate two line level signals.

BTW, by default the ground on the unbalanced side is tied to ground on the balanced side. The Ground Lift switch eliminates this connection.

 

[wolf825]

You can also check out our Control Booth Wiki page here for a definition and explanation of how a basic connection would work using a DI...Direct Box.

-w

 

[TimmyP1955]

The output impedance of the line level device is fairly high compared to the output impedance of a Low-Z microphone, so the transformer makes it look lower to the sound console.

The output impedance of most line level devices in between 50 Ohms and 150 Ohms - the same as most mics. The console neither knows nor cares. All that matters to the console is the signal level it's being hit with.

This means that less current will be drawn by the console. At the same time, it makes the console's input impedance look higher, hence drawing less current from the line level device.

Not a concern. The input impedance of most mic inputs is 1500 Ohms or more. 'Tis a rare line level device that cannot comfortably drive such a load.