A DI unit, DI box, Direct Box or simply DI is an electronic device that connects a high impedance line level signal that has an unbalanced output (a.ka. a piece of equipment) to a low impedance mic level balanced input, usually via XLR connector. DIs are frequently used to connect an electric guitar, electronic piano, or electric bass to a mixing console’s microphone input.
The DI performs level matching, balancing, and either active buffering or passive impedance bridging to minimize noise, distortion, and ground loops. DIs do not perform impedance matching.
DI (pronounced dee EYE, not "DIE" as in "die feedback, die!") is variously claimed to stand for direct input, direct injection or direct interface. DI units are used with professional and semi-professional PA systems and sound recording studios.
The basic component of a DI unit is a step-down transformer. Transformers consist of two or more wires that wind many times around a metal core. The ends of each coil of wire protrude from the windings; one pair of ends is the input, and the other pair is the output. The input coil is called the primary, and the output coil is called the secondary.
When an electrical signal through the primary coil, it creates a magnetic field around the coil. The field then induces an analogous signal in the secondary coil, which appears at the output leads. If the primary has more windings than the secondary, it is called a step-down transformer because the signal level and impedance are lower at the output than they are at the input.
If the secondary has more windings than the primary, it is called a step-up transformer because the signal level and impedance are higher at the output; however, the power does not increase with respect to the input. Step-up transformers are used at the input stage of mic preamps and adapters to connect a microphone to a line-level or guitar-amp input.
Types of DI’s; active DI units and passive DI units.
Passive DI Units
A passive DI unit typically consists of an audio transformer used as a balun. Typical turns ratio is about 500:1, to match a nominal 50 kΩ signal source such as the magnetic pickup of an electric guitar to a 100 Ω input.
Less commonly, a passive DI unit may consist of a resistive load, with or without capacitor coupling. Such units are best suited to outputs designed for headphones or loudspeakers.
Cheaper passive DI units are more susceptible to hum. Passive units also tend to be less versatile than active. However, batteries are not required, they are simple to use, and the better units are extremely reliable.
Some models have no settings, while others can have a ground lift switch (to avoid ground loop problems) and a pad switch (to accommodate different source levels). Some passive DI units also have a filter switch for coloring the sound.
Active DI units
An active DI unit contains a preamplifier. Active DI units can provide gain and are more complex yet versatile than passive units.
Active DI units require a power source, via batteries or a standard AC outlet connection, and may contain the option for phantom power use. Cheaper units offering both options may perform far better on fresh batteries than on phantom power, or vice versa, so it is important to test a prospective purchase in the mode in which it will be used.
Most active DI units provide switches to enhance versatility. These include gain or level adjustment, ground lift, power source selection, and mono / stereo mode. Ground lift switches often (perhaps unintentionally) disconnect phantom power.
A passthrough connector is a second output, sometimes simply connected to the input connector, that delivers the input signal unchanged, to allow the DI unit to be inserted into a signal path without interrupting it. This is essential in many applications. Passthrough is common on active compared to passive DI units. Passthrough is commonly referred to as a bypass.
True bypass occurs when the signal goes straight from the input jack to the output jack with no circuitry involved and no loading of the source impedance. False-bypass or simply ‘bypass’ occurs when the signal is routed through the device circuitry with no intentional change to the signal. However, due to the nature of electrical designs there is almost always some slight change in the signal. The extent of change and how noticeable it may be can vary widely from unit to unit.
Typical Applications
Direct boxes are typically used in instances of instruments or other devices that only contain an unbalanced 1/4" output which needs to be connected to an XLR input.
Headphone outputs
A DI box can be used to receive a signal from any headphone jack, such as those on personal stereo systems or keyboards. If the signal is to be connected to a single input then a mixing facility is required in the DI unit. If stereo is required, then either two DI units or a single stereo unit can be used. The jack cannot normally be used for headphones as well.
Suitable units:
- Active.
- Passive resistive load.
Acoustic or electric instruments
DIs can be used on instruments with electronic circuitry and pick ups that do not contain an XLR balanced output. An example of this application would be an electric keyboard that needs to be connected to a mixer board, either directly or through a snake. Another example would be an acoustic guitar with pickups or an electric guitar or bass guitar that would be mixed through a mixing console into a main or monitor mix.
Electric keyboards
For best results use the line output(s), unless the keyboard has built-in balanced outputs (some high end units only) which are essentially built-in DI units and should give the best results of all. If monitor amplifiers are also to be driven directly from the keyboard, the DI unit must have a passthrough connector. Alternatively, take a signal from the amplifier instead, see below.
Suitable units:
- Active.
- Passive balun type.
Electric guitar
A DI can be used to take a line in from an electric guitar. When dealing with electric guitars and electric guitar amplifiers, better results will often (not always) be obtained by instead using a microphone in front of the loudspeaker. This is because the tone of the guitar is often shaped by the amplifier and speaker used in the setup. A DI in the chain before the speaker or amplifier will often result in a loss of fullness or pleasant tone. Using a microphone eliminates hum from ground loops which are often troublesome when using DI units with mains-powered amplifiers. But a microphone will of course pick up background noise which a DI connection will not, and will most often be more susceptible to feedback in live situations.
If an electric guitar is to be connected to a DI and an amplifier is to be connected as well, then the DI unit must have a passthrough connector. Alternatively take a signal from the amplifier, see below. For players using effects (including distortion) built in to their amplifiers, this is the only option, otherwise the contribution of these effects will be lost.
If a passthrough is used, normally the DI unit is between any effects units and the amplifier for the same reason.
Suitable units:
- Active.
- Passive balun type.
Electric bass guitar/Acoustic Guitar
When dealing with electric bass or acoustic guitar, a DI is most often preferrable to using a microphone on an amplifier. This is because these instruments are often valued in a mix for being clean. The signal path from the instrument should go into the DI unit and should then pass through to any sort of instrument amplifier. Often any amp used in this setup would be for monitoring purposes only, with the major component of the sound coming from the balanced send of the DI. The DI should be chosen with the specifications of the individual instrument in mind. Often the best possible tone is achieved by one stage of preamplification. Following this idea, an active instrument, which means that the instrument has a preamplifier inside of it, should utilize a passive DI unit, while a passive instrument, meaning there is no preamplifier inside, should utilize an active DI.
Instrument amplifiers
Some high-end instrument amplifiers contain built-in DI units. Most of these work as well as or better than any external unit, as they are well matched to the signal, but caution should be used with these as they often are not transformer isolated.
Better results will often (not always) be obtained by instead using a microphone in front of the loudspeaker. This is especially true of electric guitar. Using a microphone eliminates hum from ground loops which are often troublesome when using DI units with mains-powered amplifiers. But a microphone will of course pick up background noise which a DI connection will not, and will most often be more susceptible to feedback in live situations.
Suitable units:
- Active:
From line or slave output.
From loudspeaker output (parallel to loudspeakers).
From effects loop (may need passthrough).
- Passive resistive load type:
From loudspeaker output (parallel to loudspeakers, and check impedance and power handling capacity of the DI!).
- Passive balun type:
From line or slave output.
Tips and Tricks
One of the most common applications for DI boxes is to connect equipment with high-impedance outputs (such as synths) to a mixer’s low-impedance inputs using long cables. If you were to run a long cable (say, 100 feet) from a guitar to an amp, it would completely load the guitar; you’d lose high-frequency response and add noise. However, if you connect the guitar to a nearby DI box with a short instrument cable, you can then run a 100-foot mic cable to a mic preamp near the guitar amp. The mic preamp’s output is then connected to the input of the amp.
Brands: ART, Audix, BBE, Behringer, Boss, BSS Audio, DOD, Horizon, Live Wire Solutions, Nady, Pro Co, Radial,* Rolls, Sabine, SMPro Audio, Summit Audio, Tapco, TL Audio, UltraSound, Whirlwind
Keith Woodbridge says
July 11, 2019 at 1:11 pmNice article in general.
In the second paragraph, you state “DIs do not perform impedance matching”. In the “Passive DI section you state “Typical turns ratio is about 500:1, to match a nominal 50 kΩ signal source such as the magnetic pickup of an electric guitar to a 100 Ω input.” This is a bit of a contradiction… I agree with the second statement, not the first.
I like your “Tips and Tricks”, I’ve often used a passive box in the reverse direction to get a high impedance output signal from the board to the stage. Using passive DI’s with Line level signals is a little tricky because at a high level, a cheap direct box transformer may saturate and distort the sound.
I’ve outfitted a couple passive DI boxes with stereo 1/8″ jacks in parallel with the 1/4″ jack for easily connecting laptops or other remote audio devices to the board in mono (thus using only 1 board channel). DON’T TIE THE LEFT & RIGHT INPUTS TOGETHER (Like stereo to mono adapters do). This shorts the Left and Right outputs together which isn’t always a good condition for the connected device. Instead, insert a 50 to 100 ohm resistor between each signal and the common hot wire on the 1/4″ jack to act as a mini “L-R mixer”. You also need to isolate the 1/8″ jack from the case or you may render your ground-lift switch ineffective.
TommyO says
January 11, 2019 at 5:29 pmI tried to boost my accordion players gain by running him into an active DI box and got nothing,
but when I hit the grounding button it wailed out feedback so I am nervous now about hooking this active Behringer back up. Should I test it agin with a 1/4 inch guitar cable into the PA first and leave that grounding button alone??
Chris Huff says
January 12, 2019 at 9:02 pmThat’s a tough one. I’m assuming the accordion has a passive pickup – no on-board battery. As long as that’s the case then the active DI box with gain control could help. Make sure the active DI either has phantom power engaged or has some other power supply that’s used. Make sure you don’t have the PAD engaged on that channel. The fact you get a nasty sound when you don’t have the ground lifted is concerning. I’d need more details about the DI.
AJ says
September 18, 2019 at 12:30 amIt actually sounds like it worked great. Hook it up again but make sure the inputs on your mixer are all the way down. After hooking up, slowly bring up the input level on your mixer until it’s loud enough.
Saravanan says
November 12, 2019 at 3:04 amThanks for this article. It was good learning for as a newbie. Have been playing the acoustic guitar for sometime now. Am planning to play around with some pick ups, DIs and Amps. Your write has been very enlightening. Thanks
r says
October 18, 2018 at 2:03 pmIf you accidentally plug a line level signal into a passive direct box will you damage
the transformer or anything else. It was an electric guitar plugged into a mixer amp then into the passive direct box. What would the signs be if there were any damage to the direct box.
thanks r
Chris Huff says
October 18, 2018 at 2:55 pmAs long as the sound coming out still sounds good, you’re fine. But if it doesn’t produce sound or sounds distorted, it’s junked.
Al Brady says
April 10, 2018 at 6:42 amHi, Chris,
Thank you for your timely and valued response.
Another thing I meant to ask (but, forgot) –
A) is there a volume difference between using Active or Passive DI boxes?
B) is there a volume difference between using or not using a DI box?
Regards,
Al Brady
Chris Huff says
April 10, 2018 at 9:56 amThere could be a signal strength different between passive and active. Some active DI boxes have gain controls so you could boost the signal level, or decrease it. As far as with or without a DI box, that’s not really an option. The instrument cable can only run 10-15 feet and after that you get signal degradation and interference because it’s an unbalanced signal. The DI converts it to a balanced signal, among doing other things, so interference is rejected becaues of the cabling make up or it’s eliminated at the mixer when the mixer compares the two copies of the signal that’s sent to it – where one is a mirror copy of the other. Also, it’s a stronger signal.
Al Brady says
April 10, 2018 at 1:51 pmOnce again, thank you for your invaluable response.
I’m going to go for an Active DI Box.
Much obliged,
Al Brady.
Al Brady says
April 7, 2018 at 11:42 amHi, Chris,
I’m playing a standard Fender Strat through 2 Boss pedals (1 Digital Delay and 1 Distortion).
I want to output to a PA amp (with an XLR cable about 3 meters long) – to save me from having to
carry a guitar amp as well as a PA amp.
Should I use an Active or a Passive DI Box?
Regards, Al Brady.
Chris Huff says
April 9, 2018 at 2:25 pmShould be fine going passive. Active would only be if you wanted to add more processing within a DI.
Raymond Underwood says
April 4, 2018 at 9:43 pmI have the radial D2 passive D I , but I don’t normally hook my keyboards up stereo , I just usually use thr left mono input, I have two keyboards can I run both keyboards one from the left input and one from the right input into the radial direct box, or do I have to hook the keyboard Up In Stereo
Chris Huff says
April 4, 2018 at 10:17 pmThe D2 is made for stereo conversion from instrument level to mic level signals. In our case, if the keyboard’s left output also serves as a “mono out” then you could use a regular passive DI. Or, you can do as you are doing and use the one DI box to work for two mono signals. If the house speakers work in stereo then use stereo, otherwise, keep doing what you’re doing.
Ed Masters says
December 7, 2017 at 3:58 pmI am running 2 keyboards with 1/4″ outs straight into a JBL Prx812 powered speaker.
With the volume controls on the power speaker turned all the way up, I am still lacking the Spl that I need., but get a nice clean signal from the powered speaker into the main mixing console via the XLR out from the powered speaker. Now then, to get more power/punch that I need, I can press in the Mic button for my input source on the powered speaker and then I get the sound and power I need..unlike simply using the input selector for instrument input .. Problem now tho, at the mixing console that once was really clean..is now dirty and hums thru the p.a. Should I use a d.i. from the powered speaker in-line to the mixing console to make things better? I really need the increased volume I get by selecting mic input instead of normal instrument input on the powered speaker. Thanks for any thought suggestions.
Chris Huff says
December 9, 2017 at 9:49 amWhy are you running a signal from the powered speaker to the house? If you’re using it for a monitor, here is what I would do.
1. Turn up the volume level on the keyboard. I’ve seen this impact what’s being sent.
2. Sum the left/right outputs of the keyboard and then split that.
3. Send one feed into an XLR to go to the mixer.
4. Send the other feed to the powered speaker (monitor).
Arthur says
December 5, 2016 at 3:15 amI am mainly a bass player who plays guitar, piano, and recently organ as well. Since the instruments I am dealing with is both active and passive, I wish to have a DI that can fit them all. I realize that I’d probably better off buying 2 kinds of DIs, but for practicality purposes, I would want something that can solve both issues. So, what are the options for a hybrid (active/passive) DI in the market now?
Ben says
September 13, 2016 at 2:52 amYou begin the article by stating that a direct box connects a “high impedance line level signal” to a low impedance mic level balanced input, but it is my understanding that a direct box most commonly takes an instrument level signal and then converts that signal into a mic level signal which is then in turn brought up to line level by a preamp. The explanation of the step-down transformer makes sense because a direct box takes an instrument signal and weakens it into a mic signal (albeit a balanced signal capable of longer runs of cable). I’m a bit confused when you write that a direct box takes a “line level signal” as opposed to an instrument level signal. Could you clarify this for me? I guess I’m just confused why one would begin with a line level signal then attenuate it and then run it through a preamp all just to return back to line level where it all started!?!?
I have a Yamaha MO8 keyboard, and I have always connected it straight into my audio interface’s analog line inputs because Yamaha claims that the analog outs are line level (yet they don’t say whether they are balanced or not — I’m assuming they are unbalanced outputs and as such I use a t/s guitar cable to connect it). My question to you is whether you think I could reap some manner of benefit from using, say, a Radial Pro D2 to convert the low impedance unbalanced line level signal that the keyboard outputs into a mic signal and then running that into one of my preamps? I understand I could employ a preamp for coloration purposes. Also my cable runs are under 15 feet. Sorry for the lengthy post and thank you sir for providing this informative and interesting article. Genuinely interested in your elucidation.
Chris Huff says
October 20, 2016 at 12:59 pmWhile you could do that, it’s the length of the cable that matters. The impedance level makes a difference. The higher the impedance, the harder it is for the signal to travel long distances. That’s why it’s converted to a low impedance mic-level signal for that 100 or 200 foot run from the stage to the sound booth.
Joshua says
July 7, 2016 at 12:45 amis it wise to connect a active di to an active instrument, since in the article you don’t specified it.
thnks in advanced
Chris Huff says
July 8, 2016 at 9:35 amCan you or should you? If you have an active instrument, let’s say you’re sending out the signal from a guitar amp to a stage box. You’d usually run it through a passive DI because you’ve already got an amped signal and one in which the coloring is defined by the guitar, the guitar pedals, and the amp. So there’s no reason to use an active DI.
steve says
January 3, 2016 at 1:36 amIt help me understand more uses of a DI,thanks
Stephen Iddison says
August 16, 2013 at 2:57 amPs Thankyou.
Stephen Iddison says
August 16, 2013 at 2:56 amDear Chris, I am running a Vestax VCI-300mk2 DJ Controller and wish to use Active speakers. On my unit are two unbalanced 1/4 jack outputs whilst on my speaker I have the option of XLR/1/4 JACK input. I need to run 6 meter cable and as a result may suffer some humming. Would a good DI box solve the problem here and if so what would you recommend that is not to expensive please.
AJ says
September 18, 2019 at 12:28 amAbsolutely! Short 1/4″ cable into your d i, then long balanced (xlr) cable into your speakers. Your noise problems should be over.
Luis Mierles says
July 2, 2012 at 8:23 amVery usefull with technical language but easy to understand really a great help to understand the Di issue…
doucebag says
March 16, 2012 at 12:53 amRight on hear ya
nimmtheory says
March 16, 2012 at 12:49 amTried that idea, it does not work so easy breesy
Chris says
March 16, 2012 at 8:04 amWhich part isn’t so easy?
nimmtheory says
December 2, 2011 at 3:23 pmI have tried simply turning the volume down on my active bass guitar and the input signal is not distorted by the designed for passive preamp. The problem is that I can still play a passive system bass wide open with higher amplifier output volumes. The active bass at lowered input volume will sound cleaner, but the output volume at the speaker is lower. I set the active bass at several settings and a passive instrument will still out perform it’s modern active brother through this tube amplifier, even when using lower gain preamp tubes. I must assume the the design of the amp prevents effective usage of active instruments. I do like the idea of the line signal reducer which will help keep vintage gear from being damaged accidentally.