Time to talk DI boxes and answer all the related questions I get. For example, what’s the difference between an active direct box and a passive one? For that matter, is there a difference in audio quality between a $40 passive DI and a $200 passive DI?
And then there’s the big one, “do I even NEED a DI box?”
Let’s start with understanding WHY you need a DI box.
Why A DI Box Is Needed!
First, the Needs of a Mixing Console
A mono mixer channel is designed to take in a mono line (not stereo) that’s balanced, as the input. A balanced line means the incoming cable will have three wires:
- Pin 1: Ground
- Pin 2: Hot (+)
- Pin 3: Cold (-)
Regarding the hot and cold wires, the Cold wire carries the same information as the hot except it has the polarity reversed. If you viewed the signal on the hot compared to the cold, it would look like the cold is an inverted copy of the hot. This method of sending two complimentary signals is called differential signaling. The benefit of these two signals is they aid in the elimination of interference.

Hot and Cold of a Balanced Signal
Phase versus Polarity
Don't confuse polarity with phase. Phase is dependent on time where polarity doesn't have a time component. If a sound wave has its polarity reversed, it's like seeing a mirror image, as in the above image. A wave with a peak at +5 which had the polarity reversed would now have a trough at -5. The wave is inverted
Something that was out of phase would be copy of the sound wave that appears at a delayed time. This is what happens when two microphones pick up the same sound source, like a drum, but at different distances from the drum and therefore the microphones pick them up at different times. Something that's 180-degrees out of phase would appear as an inverted wave but it's a result of a time delay - it's not inverted at the same time, the time delay just makes it appear so.
The words phase and polarity are often confused. Remember phase has to do with time and polarity has to do with electrical signals.
How Interference is Eliminated
The balanced line comes into the mixer and what happens?
Here come a little math. What happens when we sum the two signals from the above image? If the max/min was +5 and -5, conventional math says +5 + (-5) = 0. However, it doesn’t work like that with balanced lines.
The Standard Method for Calculation
The output voltage is calculated by subtracting the voltage on the two wires (pins 2 and 3).
- Pin 2 – Pin 3 = voltage
Let’s substitute values such as we used a moment ago:
- (+5 V ) – (-5 V ) = voltage
If you recall basic mathematical processes, two negatives equal a positive:
- (+5 V ) + 5 V = voltage
- 10 V = voltage
Noise Cancellation
Now comes the fun part.
There are methods to shield the hot and cold wiring to prevent as much interference as possible from getting into the hot/cold wires. However, if it does enter in, let’s look at what happens.
The interference would occur somewhere during the cable run which means it’s going to appear the same on both wires. See the red lines on the chart below. Imagine those are an instance of interference picked up along the way.

Interference in red.
Now we apply the same calculation of Pin 2 – Pin 3. And let’s say that interference is at +3.
- (+3) – (+3) = voltage
- 3 – 3 = voltage
- 0 = voltage
Thus, the interference is eliminated.
It’s also important to know a mono balanced line can run for several hundred feet. Note that a mixer channel can take in either a line level or a mic level signal. For some mixers, a switch determines the input while others reserve the XLR jack for mic level and the TRS jack for line level, and for others, well so much of it depends on how the manufacturer designed it to work so read your manual if you’re not sure.
The Problem – What Instruments Send Out
But what are most instruments on stage sending out? Mostly*, unbalanced lines in which you don’t get the benefits of noise cancellation that comes with a balanced cable. This means the electric guitar line that’s mono unbalanced is only good to send a signal around 20 feet or so. Some will say they can run longer but the longer you go, the more you risk signal loss (and tone loss) and possibly interference – especially if the cable is run parallel to a power cable such as for their pedal board.
If you can run the guitar cable longer to have a cleaner stage, say 30 feet, and avoid interference and tone loss, then go for it.
*I said “mostly” because some gear, including high-end keyboards, have balanced outputs so there’s no need for a DI. These outputs can be via TRS cables, so not the XLR you’re used to seeing for balanced cables.
Also, anything that’s sending out a stereo line, like a computer, isn’t sending out a balanced line even though it has three wires. In this case, they are the left signal, the right signal, and the ground.
Therefore, something is needed to convert those unbalanced cables to balanced ones. See where this is headed?
How Do We Get The Signal From the Stage to the Booth?
On the stage, there are the microphones, instruments, and in some cases computers or smart devices like iPads. The microphones are designed to send out a mono balanced signal so they can be plugged directly into a jack on the stage. No problems there.
It’s the other audio sources that require something special. And that something special is a DI box.
The DI Box
No matter what you call it, a DI box, a direct box, a direct input box, or simply a “DI,” this little box turns the signal of what you have on stage into the type of signal the mixer is expecting.
Before, I mentioned the mixer needs a mono balanced mic-level (or balanced line-level) line and while that’s true, it needs one more thing. The mixer wants that signal to be a low impedance signal.
Now about that word impedance. Any electrical system will have a combination of resistors, capacitors, and inductors connected in circuits, as well as more active components like transistors. All of those resistors, capacitors, and inductors impact how much current can run through the system. Thus, the sum of all resistors and the like is considered the impedance level.
In the case of balanced cables, both the hot and cold will be of equal impedance.
Hi-Z and Lo-Z Impedance
“Z” is the common abbreviation for impedance. If you’ve ever seen the term Lo-Z, this is where it comes in. That vocal microphone is sending a low impedance, low voltage that’s called a Lo-Z. In comparison, some instruments (not all) send out a signal that’s Hi-Z, meaning it’s high impedance, high voltage. Do be aware that “high and low voltages” are relative terms.
The Transformation
Here’s where the DI box comes into play. It takes the incoming unbalanced line and converts it to a balanced Lo-Z line. The incoming line can be either Hi-Z or Lo-Z, depending on the source. For example, a passive guitar pickup can have a low impedance level while an active guitar pickup might be at Hi-Z – but it can vary even within these.
This means that unbalanced electric guitar cable that’s only good for twenty feet, can be plugged into a DI box which then allows the cable to run hundreds of feet to the mixer.
The DI Innards – In Geek Speak
The basic component of a DI box 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 moves 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.
That’s what the DI box is doing!
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.
BEWARE!
Transformers are NOT all the same. They can negatively affect the sound because of how they are made. The great ones will not. Transformers like Lundahl and Jensen are top of the line. In fact, those Radial DI boxes will all – as far as I know – contain Jensen transformers.
Active DI Boxes and Passive DI Boxes – The Differences
Passive DI Units
A passive (non-powered) live audio DI box commonly consists of an audio transformer used as a “balun.” Balun means balanced to unbalanced. Don’t let the name fool you. In the case of audio DI boxes, baluns convert between high-impedance unbalanced (input) and low impedance balanced lines (output). Yes, it’s kinda the opposite of what the name “balun” implies.
This means the passive unit is designed to do one thing; convert the signal from one type (Hi-Z) to another (Lo-Z). Some inputs, as stated above, can be Lo-Z unbalanced, in which case the DI doesn’t alter that property of the signal.
Such passive DI boxes won’t do much else though they might have a ground lift switch to remove hum from the line – hum is usually an indicator of a ground loop problem in your system or with a piece of equipment. More on that in a moment.
Cheap passive DI units can create natural hum in the line because of the low quality of the transformer, as previously mentioned. Additionally, they can even negatively affect the signal such as to take away the crispness of a sound. Is it noticeable? Yes, no, maybe. I’ll cover that later in this article.
The Ground Lift Switch
Ground loop problems can be caused by small voltage differences that normally exist between the ‘grounds’ at the send and receive ends of a cable. Yes, the ground will normally carry some amount of low level of voltage.
Imagine an electrical socket on stage and an electrical socket in the sound booth. Now image they are on different electrical circuits. This means they can carry different voltage levels along their ground. This isn't a problem UNLESS audio components are used and connected.
Let's say the mixer is on electrical circuit A and the electric guitar is plugged into a pedal board that's on electrical circuit B. When the output line from the pedal board is connected to the stage jack and the audio cable is connected to the audio mixer, hum is heard in the system because you've connected the two grounds with different voltage levels.
When this is the case, adding a ground isolator in the signal path is one way eliminate such system ground loop problems. It will use a transformer to step up or step down the voltage of the incoming signal to match the other.
Another method, via a DI box, is to use the ground lift switch to "lift" the ground to break the loop. This is a standard practice AS LONG AS the equipment on both ends is properly grounded - when the power cords have the third prong that's used for a ground.
The WRONG METHOD for hum elimination is to yank out the third prong on a power cable. This method DEFEATS the safety feature of the ground. DON'T DO IT!
Ground loop problems can also occur because of damaged or loose components within a piece of equipment, such as a grounding wire working loose within an electric guitar or piece of rack equipment.
Any time unexpected noise is heard, it's best to investigate and find out the reason. It might be as simple as different grounding voltages or a piece of broken (UNSAFE) equipment that needs fixed ASAP.
The Problem with Low Frequency Instruments and Passive DI Boxes
Look at the two common low-frequency producing instruments, the piano (keyboard) and the bass. Yes, the lowest piano note competes with the bass at a whopping 27.5Hz. The problem is low frequencies, those under 100Hz, can overload passive DI boxes. In such cases, active DI boxes are best.
Later in this article, you’ll find a link to listen to a bass guitar run through different DI boxes – active and passive.
Active DI units
An active DI is a robust piece of equipment meant to not only convert the signal into a balanced one but to also alter the sound in some way. But let’s step back and look at the words Active and Passive.
Have you ever heard of active and passive speakers? What’s the difference? The difference is that active speakers are powered. In the same way, an active DI box requires power. They can be powered via a standard power outlet, batteries, or even phantom power.
A note on the phantom power; they cannot pass the phantom power on, such as with a condenser microphone that requires phantom power. The reason is that phantom power can only be passed on a balanced cable.
Cheaper units offering both options (battery and phantom) may perform far better on fresh batteries than on phantom power, or vice versa, so it’s important to test a prospective purchase in the mode in which it will be used.
So why the power requirement?
An active DI box contains a preamplifier.
Active DI Benefits
These units can provide gain adjustments, EQ controls, mono and stereo inputs, tone shaping, just about whatever you can imagine. They can even offer a passthrough connector as a second output. This means it outputs the colored sound to the balanced cable but then passes an unaltered copy of the incoming signal to an output jack. This is usually called a bypass.
A 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 usually some slight change in the signal. The extent of change and how noticeable it is can vary widely from one brand to another.
Which is Better, Active DI Boxes or Passive DI Boxes?
Active, no, passive, no…wait…it depends.
The best tool for the job is the tool that does what you need. For example, there are a number of acoustic guitar active DI boxes that provide tone controls. I’ve used them and love them. But I wouldn’t use one if I needed to only convert an unbalanced keyboard output to a balanced line. That would be overkill. If I needed to get a bass guitar into my system and I want to give it some oomph, I’d grab a SansAmp active DI box.
My standard rule is that if all that’s needed is conversion, passive is the way to go except in the case of passive guitar pickups which require active DI boxes.
Other Types of DI Boxes
Active and passive are the two primary types. There are sub-types such as multimedia, digital, re-amp, and amp DIs. It’s all about the type of signal they take in and what they do with it – or what they need to do with it. For example, the Radial ProAV1 converts different types of stereo inputs, including RCA and 3.5mm, over to a mono balanced output. It’s considered a multimedia DI but at heart, it’s a passive DI.
Are Expensive DI Boxes Worth the Cost?
How much of an audio quality difference regarding DI boxes can be heard in a live environment?
I hate this question. The reason is that I beleive we should get the best audio quality from all of the audio components because every little bit matters while at the same time, how much of these little things matter when it’s live as compared to listening in isolation with headphones?
Truthfully, given the modern amazingly clear audio systems in place, every little bit matters even when it’s live.
But don’t just listen to me on this. Check out what I’ve heard from hundreds of audio engineers when I asked them about this very question of ‘Does DI quality matter.’
Their resounding answer: YES, DI QUALITY MATTERS!
When comparing the low-priced to the high-priced passive DI boxes, there is a difference in the quality of the sound, namely a loss of quality in the mid-range frequencies. A blind test showed this to be accurate. Many of us listened to the audio samples and it was clear that low-budget DI boxes were giving a low-budget sound and it’s enough to want to avoid them.
When it came to comparing the mid-priced to the high-priced passive DI boxes, that was a little tougher and I found, through testing, it depended on the quality of speakers or headphones I was using to listen to the samples.
I don't have a link to that DI shootout (darn if I can't find it) but Sweetwater did one this year using a bass guitar and both passive and active DI boxes.
I will say this, considering the length of time a DI box will be used on your stage - and I mean how long you’ll own it, doesn’t it make sense to pay more to get one that’s going to sound the best? The Radial ProDI is $100 compared to a $30 DI box. Take that $70 difference over ten years and that’s nothing.
Recommendations
There are a number of direct box manufacturers and they aren’t all producing the same quality of products. As I said before, given how long you’ll use it, it’s best to get the best. Of course, that’s not always possible. Therefore, consider these suggestions for common needs.
You’ll note the absence of certain big-name manufacturers. If you like the sound you get from your DI, great, use it. However, I do suggest you borrow one of the below and swap to see if you get a better sound. If not, so be it. Just don’t assume that what you have is giving you the best sound. In audio, improvements often come in small increments.
Passive DI Boxes (For Instruments)
- Radial ProDI / ProD2 (LR outs)
- Whirlwind Imp 2
Passive DI Boxes (For Computers)
- Radial ProAV1/ ProAV2 (LR outs)
- Whirlwind pcDI
Active DI Boxes (For Digital Usage)
Active DI Boxes (For Guitar)
Active DI Boxes (For Bass)
The Next Step
If you’re ready to purchase new equipment and want to be positive the new gear is right for your situation and will last for years, check out our complete guide to selecting audio gear. It even comes with an in-depth 7-page worksheet to help you along the way.
Great explanation! I appreciate the detail! Thanks for including the science.
Another rule of thumb I was taught
If your instrument is passive (ie doesn’t have a battery / power) use a active di box.
If your instrument is actively powered. (has a battery / power) use a passive.
Hi There
Thanks for the info post. This has clarified many issues I have experienced. in the past.
I will forward this to my colleague and fellow deejays
Great article Chris!
I plan on sharing this with my team. I have been gradually replacing cheep DI’s with higher quality. We use piezoelectric pickups for our violins which are VERY HiZ. Radial makes a DI specifically for that (Radial PZDI), worth the investment. I truly appreciate the level of effort and time you put into equipping the church sound tech community.
Thanks, Stephen!
Excellent article, Chris. A friend (who is a pro sound engineer) and I were chatting just yesterday about how frustrating we find it that people spend thousands on expensive instruments, run them through great mixers into a top quality front of house…and yet don’t want to fork out more than $30 for a DI box! (And also try and get away with buying cheap cables)
I’m now the proud owner of five Radial DI’s (J48, JDI JPC, Pro48 and ProD2) which, along with a couple of BSS AR-133’s, cover me for all eventualities. And if i’m ever stuck down at church without any of my gear and forced to use the likes of a Behringer DI100, I can definitely hear the difference!
Hi Chris,
nice article (of course).
Question / comment – I read somewhere (a while back) that the general rule of thumb for instrument DIs was if the Instrument is ‘active’ (e.g. acoustic with battery) then use a Passive DI, if the Instrument was ‘passive’ (e.g. ‘traditional pickups in an electric) use an Active DI.
Is that a fair recommendation, or is it ‘BS’ ;) ?
I have an active DI (Radial SB-4) for my acoustic, mainly because it does some shaping of the signal and removes (most of) the ‘honk’ from the Martin Thinline piezo under-bridge pickup.
Yes, that’s true – an update I made to the article as well.
Chris,
I recall reading (probably in your book or site…cuz that’s where most of my learning comes from)that a mixer is looking for “Line” level.
This latest article seems to say it is looking for “Mic” level.
Are they the same?
And do most keyboards produce line level?
One more: A keyboard connected with only balanced cables might not need a DI?
Depending on the mixer, they can take both. If you’re plugging into the XLR plug, it’s a mic level. Some also have line level. For keyboards, it’s usually if they have XLR plugs then they are balanced but check the manual.
Our Yamaha CP300 has twin XLR balanced outputs.
XLR inputs on a mixer can receive mic level and line level on the XLR input. Keyboards and some direct boxes like an Avalon U5 can give you line level or close to it. Electric Guitars and Keyboard signals will almost always be hotter than mic levels. Having an XLR cable does NOT automatically make it mic level as Chris suggests. Read my other comment for important corrections to this article. Mixers are designed to handle a broad range of signals coming in on the XLR input. Many even offer pads to allow for hotter line level signals.
I’ve updated the article.
We’ve used EMO DIs for decades as they are bomb-proof (almost musicians’ feet-proof) and have the unusual ability to take a power amp in & convert it to LoZ balanced out. Only had to use that once with a power mixer & active speakers but it worked.
Thank you! A wonderful article explaining something I’ve used for years but only understood the most basic usage. Do you have any advice on knowing when it is time to replace a DI?
The only time I’ve ever had to replace a DI was when it stopped working. If your DI box is 30 years old, then I’d replace it just for audio quality improvement.
When comparing DI boxes for computer use, what is the reasoning for choosing a DI instead of a USB Audio Interface? Something like the Focusrite Scarlet 2i2
The 2i2 is more of a mini-mixer. If you’re only passing audio through, such as using a computer to play an audio file to to pull the audio from a video feed then go the DI route. If you’re doing something like a podcast where you want ot have control over what’s passing through then go the 2i2.
The interface does something different, well slightly, than a DI box. A DI is intended, generally, to go into a mixer. As Chris pointed out a mixer is expecting certain signals at input. A computer, in most cases doesn’t have any facility to plug in audio signals, of any flavor. So in the old days we used to have to open up the box and plug in an audio card or sound card – a PCB that plugged into a slot inside the computer and had audio inputs on it (commonly RCA stereo and 3.5mm mini stereo). More expensive sound cards had breakout boxes with more and professional input jacks.
Back to the Scarlet, these boxes are typical today since USB is far more convenient than opening up the computer. Look at them as external sound cards. They have balanced mic and either balanced or unbalanced line level inputs (depending on the price/quality), so you may need to still use a DI box into your sound card. I’m not certain on the specific Scarlet specs, but Focusrite make very high quality units. It goes without saying, but I’ll say it anyway, that anything with a mic (XLR) input on these USB boxes has a mic preamp built in, and obviously the mic pre’s in a Focusrite are going to be better than those in a cheap Amazon brand. Hope that helps some. Maybe Chris can confirm or expand upon this.
Chris, We started using Radial Direct boxes several years ago because of issues that we were having with cheap Direct boxes. Problems went away to the point that we forget that the Direct boxes are even in the signal path.
Niiiice
I have to agree with David. I was – for a time – using the supplied DI100’s, and it was “ok”. But I decided I wanted something “more”, and as I mentioned above, bought a Radial StageBug-4. The difference was like chalk and cheese! There was more low, there was more mid, there was more sizzle on the top. The guitar gained a clarify that was not there before in FoH or Foldback. I knew it was all there, because my acoustic amp was showing it to me – but it was all lost somewhere. that somewhere was the DI100.
I have used a direct box for years…started with the “cheap” Whirlwind box (cost me $30 at GC) and even got a second one for use with my synth. After a while I started spending significantly more on my acoustics and thought that I should proportionally be spending a bit more on replicating the sound I was hearing naturally out of the acoustic. I started with a small bump to a radial and wasn’t hearing much of a difference and THEN I came into some funds (from selling some unused electric gear) and decided to take the plunge into the Avalon U5!!! I thought this was going to be the last DI I EVER bought…it was not…in our small church with volunteer sound crew the difference was barely noticeable for almost $500! So I sold it and tried the FireEye RedEye DI and nothing, finally I just went back to the trusty LR Baggs. Fast forward to now almost 5-6 years later I am playing with a full band, in a larger church, with a MUCH better sound system and dedicated sound engineers and I have gone completely wireless (IEM, guitar, vocals)…the tonal difference is so insignificant that I’ve stopped stressing and the added benefits of being free without wires is SO worth it. No my question is: can we do anything from the board side, like add a DI between the receiver and the board, to improve the tone of the guitar?
At that point, your tone shaping is better upstream such as using an LR baggs or other active DI on stage, or doing normal mixing work to improve it. I like as much as possible on the stage. What you might do is pull out your guitar and gear into the room so you hear directly from the house speakers and then tweak your active DI or other gear so you’re hearing it as the room hears it, not via the monitors.
still not quite clear on what I should do to retain the wireless and improve sound quality?
I do not want to go back to the wired setup…so do I maybe use an active direct box out of the receiver on the back and plug that straight into the board? The receivers are in a rack by the board so getting the receiver on stage is not an option (unless I purchase my own and I really would prefer not to do that).
I own both the Avalon and the Red DI and I doubt I would ever use them live. I like them both on Bass in the studio. If you are looking to improve your acoustic guitar sound I highly recommend the LR Baggs Session Acoustic DI. It will give you a great starting point with just about any acoustic guitar. Then experiment with EQ in your front of house mixer. Try a cutting around 1 kHz and don’t be afraid to make it a deep cut if it helps. Also don’t be afraid to try rolling off the treble which is usually shelved around 12 kHz. These moves tend to make most guitars sound more natural. Of course, you still have to watch the low mids and a cut between 200 Hz and 400 HZ is often helpful but mids and low end on acoustic often needs to be shaped to get it right in live applications.