DI Boxes

DI Boxes

For "Direct Injection, or Input", Box.

What are they ? | Why use them ? | Connections | Different sorts of DIs

What are they ?

A DI box serves two purposes :

First of all, it lowers the output impedance of a device whose output impedance would be too high to plug directly into another device. Typically, a guitar equipped with a pick-up that you'd like to plug directly into a mixer input.

Then, but that's really secondary, it balances the signal, which can then be carried over long distances, without any risks of hum induced in the cable.

Some boxes, when they have a power supply (battery or mains), also manage to amplify the signal, but that's not the initial purpose of a DI box.

Why and when should we use a DI Box ?

There are several reasons :

To plug into a mixer input

Instruments, like an electric bass or guitar, as well as synthesizers, expanders and samplers, have an output impedance that is way too high for a mixer's mic or line input.

If you're stubborn and plug directly into the mixer, the instrument will not be well defined in the lower spectrum, which is a bit sad for most of the instruments we've just listed above ! In some cases, the instrument will even lack in the upper frequencies, the transients will be dull, etc...

To carry the signal over long distances

Especially for concert sound, but also in the studio, the instrument is rarely a few feet away from the mixer. Moreover, these environments are hostile, not because of the rapper's pit-bull, but because of all the Radio Frequency Interference in there.

Once the signal is balanced, it can be carried safely to the place where it will be used (mixer, tape recorder, etc...).

Beware of the so called "line outputs" of synthesizers/samplers/expanders. Since they are generally on a 1/4" Jack, you'd be easily drawn to plugging directly into the line input of the mixer (most of the time, 1/4" Jack also, on home-studio mixers).

Wrong : very few manufacturers offer a true line out impedance on their machines, like those found on professional tape recorders. On a Synclavier, a Kurzweïl K-2000, an expensive EMU or a Fairlight, which are studio tools and eventually rich musicians' instruments, the line outputs are really line outputs, often on an XLR connector, by the way.

For everything else : DI or crummy sound ; your choice.

Example :

Ten years ago, at CREAR, I showed students the difference between the left channel of a Roland D50 plugged directly into the line input of a Soundtracs Saphyre mixer, and the right channel passing through a DI Box before going into a mic input. They discovered an entire octave in the lower spectrum on the right channel, with a lot more edge on waveforms with a lot of high harmonic content.

Since then, manufacturers have worked a bit on the subject, but they still don't offer true line outs, in my opinion.

Connections

Not much to say, other than the fact that it's completely normal to have an XLR connector on the output of a DI box even though the input is almost always on a 1/4" Jack

- "But then : Where should I plug my synthesizer now that I've gone through the DI Box and I end up with an XLR ? My mixer has 1/4" Jacks on the line inputs !!!"

No panic ! I've told you it's normal ! Plug into a mic input which is, almost always, on an XLR connector. If your mixer isn't like that, change it ! Now !! Out you go !!!

- "But the output level of my synthesizer is a line level. It's written on the back, see : line out ! Why am I going to plug that into a mic input ? I'm going to kill my mixer doing that ?!?"

A synthesizer has a high level output, compared to an electric bass (beware of some basses with an active output circuitry) or guitar (now that's always true) but that has nothing to do with the impedance problem we're trying to solve here with the DI Box. Mic inputs on a mixer are designed to accept a large range of voltage levels, from a few millivolts typical of a Beyer M160 microphone or an electric guitar, to a few volts, typical of a Neuman U-87 in front of a snare drum (careful, not to try on any style of maniacal music !), a bass or a synth.

Now you must be careful with these levels. If you're bringing a rather weak signal from your DI Box (guitar, fingered bass) into the mixer's mic input, no problemo. If you're bringing a rather high level signal (synth, massacred bass), you might overload the mic input, that's why you'll often find a button next to the gain pot called PAD (or ATT, or -30), that you'd better activate to reduce the basic amplification (generally 30 dB) of the mic pre-amp.

If your mixer's mic input doesn't have this button, either it's a weird mixer (change it), either it's just that the gain pot sweeps the entire gain range, which lacks precision, but the manufacturer saves a few cents per channel strip !!!

Some DI Boxes feature an output pad, that you can use to take care of the problem before you get to the mixer's input. Typically, the switch will have three positions :

0
Normal level output

-10
Output reduced by 10 dB

-20
Output reduced by 20 dB

It's up to you if you want to use it, knowing that you should avoid any level reduction if you're going to re-amplify at the mic input !

Différent types of DI

There are numerous types and brands of DI Boxes (a lot are even home-made), all of which can be sorted in two categories :

DIs that use a transformer, called PASSIVE

A very simple circuitry, which needs no power supply, since the mere ratio between the transformer's primary and secondary windings manage both the impedance adaptation and the balancing of the signal.

Their advantage is that they can be used in both directions, so you can also raise the output impedance. Such as when you wish to reinject a guitar track from a tape recorder into a guitar amp, in the normal input (instrument) of the latter.

DIs that use op-amps, called ACTIVE

These contain several electronic circuits, including op-amps, that need a power supply, that's why these DIs are called ACTIVE.

They only work in one direction : high impedance (instrument) to low impedance (professional equipment inputs : mixer, tape recorder, etc...).

Vacuum tube DIs, called... Vacuum Tube DIs !

-"But you said there were two categories"

These inevitably active DIs contain additional circuitry, based on vacuum tube preamp technology, supposed to "warm-up" the signal going through the box. I'd add faders, an EQ section, a routeing system and...... Darn ! I just reinvented the mixer.

Just a gadget, but nonetheless, interesting if the name of the game is to add "color" to the signal.

Go back to :

Tips for

Bass | Guitars | Keyboards

	First of all, it lowers the output impedance of a device whose output impedance would be too high to plug directly into another device. Typically, a guitar equipped with a pick-up that you'd like to plug directly into a mixer input.
	Then, but that's really secondary, it balances the signal, which can then be carried over long distances, without any risks of hum induced in the cable.

0	Normal level output
-10	Output reduced by 10 dB
-20	Output reduced by 20 dB

	DIs that use a transformer, called PASSIVE A very simple circuitry, which needs no power supply, since the mere ratio between the transformer's primary and secondary windings manage both the impedance adaptation and the balancing of the signal. Their advantage is that they can be used in both directions, so you can also raise the output impedance. Such as when you wish to reinject a guitar track from a tape recorder into a guitar amp, in the normal input (instrument) of the latter.
	DIs that use op-amps, called ACTIVE These contain several electronic circuits, including op-amps, that need a power supply, that's why these DIs are called ACTIVE. They only work in one direction : high impedance (instrument) to low impedance (professional equipment inputs : mixer, tape recorder, etc...).
	Vacuum tube DIs, called... Vacuum Tube DIs ! -"But you said there were two categories" These inevitably active DIs contain additional circuitry, based on vacuum tube preamp technology, supposed to "warm-up" the signal going through the box. I'd add faders, an EQ section, a routeing system and...... Darn ! I just reinvented the mixer. Just a gadget, but nonetheless, interesting if the name of the game is to add "color" to the signal.