Direct Boxes – What They Do, Why They’re Needed, and Why Price Matters

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:

1. Pin 1: Ground
2. Pin 2: Hot (+)
3. 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.

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.

 

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 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.

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)

• Radial USB-Pro
• Peavey USB Audio Interface

Active DI Boxes (For Guitar)

• Radial J48
• Countryman Type 85

Active DI Boxes (For Bass)

• SansAmp