What is the Difference Between a Passive and an Active DI Box?

by ReverbLxnd in Mixing

What is a passive direct box? What is an active direct box? What is the difference between a passive and an active DI box in pro audio work?

In the what is a DI Box we saw what DI boxes are, how they work, and why you need them in your music production setup. Have a look at that if you haven’t already.

As a continuation of that discussion, in this article, we are going to look at the differences between passive and active DI boxes.

We’ll see how they are different in terms of construction, functionality, and how you would use them in music production and pro audio work.

Let’s jump right in.

The Differences Between Active and Passive DI Boxes

A good DI box should indicate (in lettering) whether it’s an active or a passive DI box. That should take the guesswork away right off the bat.

If it doesn’t say that (maybe it just says “Direct Box” without an indication whether it’s active or passive), this is where you have to get a bit more creative.

With a bit of research — going through the manual and specifications, searching on the web, and so on, you should be able to turn up this information.

Failing this, you need to examine the DI box itself a bit more closely.

Let’s begin by looking at the design differences between active and passive DI boxes.

Most active DI boxes can be Identified by at least one of three features—

  1. LEDs
  2. A ON/OFF switch
  3. A battery compartment

Difference #1 — Active DI Boxes Have an LED Light

If you look at an industry-standard DI box such as the BSS Audio AR-133 Active D.I. Box, it has an LED, and an ON/OFF switch.

When you flick the switch on, the LED glows and then it begins to flash. This LED will glow steadily if the DI is being powered with phantom power, otherwise, it will keep flashing to indicate the direct box is running on the internal battery.

The presence of an LED light is a sure sign that the DI box is of the active type.

Difference #1 — Active DI Boxes Have an ON/OFF Switch

The second thing that will allow you to identify an active DI box is the presence of an ON/OFF switch.

Passive DI boxes, do not have ON/OFF switches and are always functional. Active DI boxes on the other hand need to be switched (powered up).

In the case of some DI boxes, the ON/OFF switch might be integrated into the input connector. That means, when you plug into the DI box, the circuit within it switches on.

But usually, if you can see an ON/OFF power switch, that’s a sure sign it’s an active DI box.

Difference #2 — Active DI Boxes Need a Power Source

The third thing that will allow you to identify an active DI box is the presence of a battery door.

An active DI box uses an active circuit. That means it needs a source of power, this can be an internal battery, external phantom power, or the case of some tube DI boxes, a dedicated external power supply.

A passive DI box, on the other hand, uses a transformer. It does not require any external power.

The BSS Audio AR-133 Active D.I. Box has a very obvious battery door, which has a thumbwheel for accessing the battery.

If you use active DI boxes, perhaps it’s a good idea to put a label on them indicating when the battery was changed, and to change the batteries out at least once a year.

Some active DI boxes, however, do not have any provision for battery power. Some manufacturers opt for a phantom power the only circuit.

The reason for this is battery powering presents some challenges and limitations such as the maximum input signal that the DI box can handle. An example of this the Radial J48 Active D.I. Box.

Going with phantom power only raises the maximum input signal that the DI box can have.

Despite this, having a battery capability in a DI box can be very useful. You might encounter devices that do not have phantom power, either because the models are so old or you cannot physically access the device to switch on phantom power.

In such cases, having a battery power supply can be invaluable.

Another thing about having a battery supply is that it allows you to use a DI box in unconventional ways where phantom power may not be available, such as using a DI box with an XLR input connector as a line balancer.

Many passive direct boxes are extremely basic with probably only a single switch.

A perfect example of this is the Whirlwind Passive Direct Box. There’s only one switch used to lift the ground, a pair of Jack connectors on one end, and a single XLR connector on the other.

If you see a really simple Direct box, no LEDs, very few switches if any, chances are, it’s a passive DI box.

Should I use an Active or Passive DI Box?

Now that we know the differences between passive and active direct boxes, let’s look at the application.

Should you use an active or passive DI box?

As a direct result of the design and technology that is used in DI boxes, two things determine your choice—

  1. Input Impedance
  2. Input Signal Levels

These are the two things that influence our choice of an active or passive direct box.

#1 — Passive DI Boxes Have a Much Lower Input Impedance than Active DI Boxes

The first thing we need to consider is that passive direct boxes have a much lower input impedance than active boxes.

Impedance is resistance as applied to an alternating current such as a sound wave.

Active DI boxes usually have input impedance between 1-10M Ω. On the other hand, passive DI boxes have an input impedance an order of magnitude lower than that of active direct boxes.

This is simply because an active circuit can be designed to offer a much higher input impedance than a passive transformer.

#2 — Passive DI Boxes are Better Suited or High Input Signal Levels

The second thing is that passive direct boxes are much better suited for handling very high signal input levels such as those produced by active acoustic guitars, bass guitars, and keyboards.

This is because a transformer will tend to saturate rather than distort. Once active circuits reach clipping or overload points begin to sound quite ugly.

As a rule of thumb—

If your source is active, you choose a passive DI box, if your source is passive, you choose an active DI box.

An active source would be active bass guitars, as well as anything that is mains powered such as a keyboard, a sampler, an electric guitar, etc.

A passive source, on the other hand, would be an instrument that is not mains powered and that does not have any onboard electronics. There won’t be any battery or any electronic controls or any form of a preamp, such as is the case is passive jazz or precision bass.

If you plug an instrument such a passive guitar to a battery-powered effects pedal, it becomes an active source.

Similarly, line-level outputs of any kind such as those from amplifiers, or mixing consoles, are all active sources.

Why Use Passive DI Boxes with Active Sources

The main reason for this is mainly level control.

Most of the time, we will be connecting our DI box to a mic input on our mixing console or audio interface.

Because we are using mic input, it is important to present that input with something as close to a mic level signal as possible. A mic level signal will be between .01-0.1 volts. A line-level signal, on the other hand, will be ~ 1 volt.

When you plug an active source to a passive DI, it drops the level to near mic level

If you plug an active guitar to an active DI box, for instance, you’ll have a very very hot signal. This can easily overload certain mic preamps. You’ll turn the gain control down, and still have level issues.

That is the main reason why you use a passive DI box with active instruments.

Now that we understand why we use a passive DI box with active sources, what about the reverse?

Why Use Active DI Boxes with Passive Sources

The main reason for this is to avoid the loss of high frequencies.

If you plug a passive guitar into a passive DI box, for instance, you’ll sometimes experience high-frequency loss because of the input impedance mismatch.

It is generally recommended that for passive instruments, you have an input impedance of ~ 1M Ω

In Conclusion

Using an active DI box with a passive source or a passive DI box with an active source, we end up with a manageable signal level.

The level of an active DI box with a passive source and the level of a passive DI box with an active source will be the same.

This helps to streamline your setup and keep your levels as uniform as possible.

You should now have a good idea of the differences between passive and active DI boxes and when and which use case calls for each.

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That’s it for this article.

ReverbLxnd

I've been a musician and brought in my stuff for mixing and mastering, I've been my own producer where I wrote, recorded, mixed and sold my own stuff. Now, I'm *mostly* an audio engineer, where I only record and mix for clients. I'm currently based in Berlin, Germany, where I operate ReverbLand out of. Got a question? DM me on Instagram or Twitter @reverblxnd everywhere, or shoot me an email reverblxnd@reverbland.com. I'd love to hear from you.

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