What does a DI box do?

Question

I'm not new to recording music but I've almost always used a microphone to gather the sound.

I have heard people referring to a "DI box" when plugging an instrument (guitar/bass) directly into a recording/mixer input.

I have tried a DI box but not knowing what it's supposed to do, I couldn't really hear that it was doing anything.

I realise the signal could go via effects etc as well, with amp simulators etc, but my question is this :

1. Is this how a DI box is generally used ? Instrument -> effects -> DI box -> recording device ?
2. What does a DI box actually do to the signal ?
3. What is the resulting effect?
4. Especially this : What would I listen for in the recorded sound to know that the DI box is doing something ?

Answer 1

Very simply: a DI (direct input) box is a utility, not an effect. It lets you run the input of an instrument directly into, say, the mixing board. This is sometimes necessary for the grounding/balance reasons user15196 mentions. Other times, it's desirable for aesthetic reasons.

For example, I've seen bass sometimes recorded directly: bass -> di box -> mixer, rather than bass -> amp -> microphone -> mixer. Personally, I usually prefer to record both and mix the two signals to taste, which requires:

bass -> di box -------> mixer (channel 1)
           |
           v
          amp -> mic -> mixer (channel 2)

The path from the DI box to the amp is the "bypass".

Also, di boxes sometimes have inputs/outputs that boost or dampen (pad) the signal, which might be necessary if the signal you're recording is, for example, too hot (loud).

To answer your question #4: "What would I listen for in the recorded sound to know that the DI box is doing something?" Answer: you are able to hear/record the signal directly (without the use of a microphone) in your mixer/recorder cleanly, and the levels/volumes are appropriate.

Lastly, DI boxes sometimes have phantom power. So if you have a condenser microphone, but your mixer doesn't provide phantom power, you can instead plug it into a DI box that can be used to power the microphone.

Answer 2

A DI box breaks the galvanic connection. It's most important use is breaking ground loops, and also for changing unbalanced signals to balanced (the latter transfering better over long connections).

If your effect boxes are running on battery and are not mounted on metal and go to an instrument amp (rather than a mixer) next, the DI box will not do much in its main function (it might still change impendances and levels, partly depending on whether it is an active or passive box).

If you are connecting devices that are intended to be connected, and there is no separate signal path, you won't need a DI box as a rule.

The most common "definitely needed" case is going from one amp to another, or from amp into mixer. This will usually even be the case when using a preamp output from one amp, for example in order to get the amp effects (like a coil reverb) into the mixer via a direct path. Since you connect two grounded amps in that case, the DI box will save you from getting ground loop hum.

At any rate, it is a good idea to have a few spare DI boxes in your toolbox and play with them. That way you develop a feeling for when throwing one in might make a difference, and when not.

Answer 3

A DI box makes an instrument look like a microphone electrically, so it may be plugged into a mic input on a mixer or recording interface, using a long cable if needed. There will also probably be a ground-lift switch, which can cure hum problems when the instrument uses mains power. But a DI box is a bit more than just an isolating transformer.