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.
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
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.
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.
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.
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.
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)
Passive DI Boxes (For Computers)
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.