# Sound QotD: Direct Boxes



## mbenonis (Jan 29, 2009)

Generally speaking, why is it necessary to use a Direct Box when plugging a guitar into a console? Both devices have 1/4" connectors, don't they?

There are multiple correct answers to this question.


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## waynehoskins (Jan 29, 2009)

So that when a cable breaks and we have to do a field repair, we can use one of the connectors of a direct box that's in the mike box to hold the connector while we repair the cable.

Or to keep the door to the venue from locking while we run out to the truck to get something important we forgot.

(I'm overqualified to answer this one, so I'll watch it)


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## giwiens (Jan 29, 2009)

There are many reasons for using a DI box for plugging guitars, keyboards and even laptops into and audio system. Here are the reasons I have for doing it.

Reason One: change the unbalanced signal to a balanced one. I just feel more comfortable having a balanced signal coming into the mixing board. Eliminate the chance of signal noise. Also lifts any ground difference problems between a guitar amplifier out and the sound board.

Reason Two: It can give you 'proof' that it is not your cables that are at fault when the guitar player gets crappy sound from their beat-up cable after you plug something else into the DI box and it works great.

Reason Three: When I've sometimes needed to run a monitor mix from a seperate sound board. The DI box can split the signal quite nicely if it is set-up to do so.

Those are my three basic reasons.


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## TimmyP1955 (Jan 29, 2009)

If the instrument has a passive pickup (no active preamp), the 1/4" line input on the mixer will load the pickup down, owing to the input's comparatively low impedance (usually between 10K and 20k). If the pickup is a Piezo, this will result in a very thin and nasty sound.

Going into a DI (which converts the interface to a balanced line) as close to the instrument as possible will insure that noise reduction is maximized.

A DI is designed to feed a console mic input. Owing to the very low (commonly 2k) impedance of a console mic input, the use of very long cables (such as 100' or more of snake to FOH) will result in much less high frequency rolloff than running a long line directly from the instrument to the mixer.


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## museav (Jan 30, 2009)

mbenonis said:


> Generally speaking, why is it necessary to use a Direct Box when plugging a guitar into a console?


Because the sound tech told you to, that should be good enough reason, do not question them!


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## renegadeblack (Jan 31, 2009)

Because it changes XLR to 1/4 inch easily


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## Chuckiemhs (Feb 5, 2009)

In an installation where the architect only provided XLR Balanced inputs for the wall jacks, a D.I. Box is the best way to convert from 1/4" M - XLR M for plugging into the system.

The other options include an XLR-1/4" etc. adapter (shudder), or a franken-cable (1/4" F - XLR M, anyone?) put together by some desperate SE making spare parts work for him on performance day.


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## Dionysus (Feb 9, 2009)

The other reason for changing between 1/4" and XLR is not just because of balance, but also impedance. If you want to go a long way with a weak 1/4" signal... Well good luck. The DI (some more than others) will help you get a good clean, strong signal back at the console.


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## bhallerm (Feb 10, 2009)

DI's have more to do with impedance than actually converting anything. Audio consoles like two levels: mic, and line. Guitars, keyboards and the like are in this wonderful nebulous world of instrument level. When you look at it from a voltage standpoint it's almost half way between a mic and a line level. Passive DI's will make the signal useable by the previously mentioned XLR input. The board will see it more as a mic level signal. Active DI's can do either that, or boost it to a line level signal. If it was just about changing a plug, a couple radio shack adapters could do it. Not that it isn't tried everyday. 

BJH


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## mbenonis (Feb 10, 2009)

Mmmm, I think we're getting voltages and impedances mixed up here. 

Here's the answer, at least how I think about it. The output impedance of a guitar is very high, on the order of 1000's of ohms. The mic input impedance of a mic console isn't much higher (probably no more than 10 kOhms, likely less). In this situation, the console is said to "load" the guitar - that is, draw more power from it than it should. The Direct Box presents the guitar with a load impedance on the order of 100's of kOhms, so that it doesn't load it down. It then presents the board with a source impedance of a hundred or so ohms, so that the board doesn't load the direct box. In this way, we maximize voltage transfer and minimize power transfer.

A side note, which might be useful to some. If impedances are matched (that is, output impedance equals input impedance), then we obtain the maximum possible power transfer. If they are very unbalanced (that is, source << load impedance), then we maximize voltage and minimize power transfer. We could also do it the other way (load << source), but then we get like zero volts across our load, which isn't terribly useful.

That make any sense to anyone?


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## museav (Feb 15, 2009)

A DI potentially has several functions. Presenting the proper load to many devices is certainly one of them. However, DIs also step down the nominally line level signals to a mic level output, can provide isolation and convert unbalanced inputs like guitar outputs to a balanced signal. So for the guitar example, a DI essentially provides the proper loads to the instrument and console, converts the nominal line level signal to mic level and adapts the unbalanced, 1/4" TS guitar signal to a balanced, XLR output in order to interface the guitar to a standard mic input. The DI also provides isolation of the instrument from the system and many DIs offer ground lift switches (which lift the signal ground, not the power ground) to potentially address ground loops. Some DIs even provide stereo to mono summing, not typically used for the guitar example but good for keyboards, etc.

The input impedance of the DI box is also sometimes a reason for selecting between either an active or passive DI. Some electronics, such as passive guitar and bass pickups, like to see a very high input impedance and active DIs often have input impedances several times that of most passive DI boxes, so you often see active DIs used for instruments with passive pickups. Active DI boxes are also often better for driving very long runs. On the flip side, passive DI boxes can sometimes handle greater input levels and transformer style passive DIs sound more 'musical' when overdriven as you are saturating the transformer rather than clipping a circuit.

However, there can be great differences in transformer based passive DI boxes and where quality is important, especially at low frequencies, the added cost of a high quality Jensen or Lundahl transformer based product is usually a good investment.


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