Why Buy An Active DI

An active DI box can cost a bit more, but they have big advantages.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

The beat-up device up there with a missing knob is one of my well-loved passive DI boxes. It’s sounded plenty decent on a number of sources, solved at least its own share of ground-loop issues, and has never had any problems (beyond losing its attenuator knob).

Passive DI boxes are very handy creatures. They solve connectivity problems with almost no fuss at all, and the well-designed models are highly resistant to both stupidity and malice. My guess is that, about 80% of the time, they’re a perfectly decent choice.

The thing is, though, that active DI boxes let you cover the full 100% at all times. They’re also cheap enough now that there’s really no reason not to go active (if you’re starting from scratch).

The “cheap enough” bit is pretty self-explanatory. Head on over to your favorite music-gear retailer – Sweetwater, PSSL, Zzounds, whoever – and find their direct box category. Sort by ascending price, and you’re almost sure to find active units before you leave the $30 price point. (Some of the really cheap units are junky, but to be fair, I own two Behringer DI800 units that have never let me down…and at $120 a pop, their per-channel cost is $15.)

What isn’t so self-explanatory is why passive units don’t quite cover 100% of the direct-input situations you’ll encounter. There’s a bit of science involved.

A Few 10s of kOhms Is Usually Enough

Modern audio is all about voltage transfer. Voltage transfer is all about connecting an output device to an input device with an impedance (opposition to current flow) that is high when compared to the output circuit.

Okay, that sounds like gobbledygook. An analogy would be helpful.

Think of a bunch of cars on the freeway. Traffic is flowing nicely. Everybody’s just flying along without a care in the world. This is low impedance. There’s very little opposition to traffic flow.

Now, we construct an exit to the freeway. The exit leads to a one-lane road. The one-lane road, in comparison to the freeway, is a high-impedance device. Fewer cars can flow down that one lane road, and as a result, the freeway has no trouble keeping the little road supplied with cars.

This condition, when applied to electrical connections, is called “bridging impedance.” An output device with low impedance is like a freeway, and an input device with a comparatively high (10x or more) impedance is like a one-lane road. For audio types, we’re not concerned with preserving the amount of electrical flow, so much as we’re concerned with preserving electrical force (voltage). Bridging impedance lets us do that.

Most passive DI boxes have an input impedance that’s in the range of several tens of thousands of Ohms. Some can even be in the 100,000 Ohm range. Connect a device with an output impedance of a few thousand Ohms or less, and – no problem! A lot of devices are perfectly suited to interacting with a passive DI, because a lot of the gear and instruments that get connected are active units. Keyboard outputs are low-impedance creatures. Guitar-processors have low-impedance outputs.

Heck, a lot of acoustic-electric guitar outputs are low impedance. The actual pickup might be anything under the sun, but quite often you’ll find some sort of preamp sitting between the pickup and the output jack.

In a lot of cases, you can even get away with connecting a bass or electric guitar with passive pickups to a passive DI. It’s not theoretically ideal, but it usually sounds fine.

This covers the “80% of the time” thing. The 20% comes in when you encounter an instrument with a very high impedance pickup, and no preamp. Plug one of those into a passive DI, and…yuck.

Easy As Pie-zo. (Yeah, That Was A Cheesy Pun…)

The ur-example of the high-impedance pickup is the piezo. Piezo pickups are neat because they’re small, put in direct contact with the instrument (which makes them resistant to external noises, insofar as the instrument resists those noises), affordable, and simple.

The problem with piezos is that they are passive devices with a very high output impedance – so high that getting into impedance bridging territory requires millions of Ohms or more.

So, you plug one of the little darlings into a passive DI, and what happens?

First, you probably get a weak signal out of the pickup. Poor impedance bridging means poor voltage transfer, and voltage transfer is how we ensure good signals in the world of pro-audio.

Second, the instrument probably sounds terrible.

Why?

A piezo pickup (when connected to another audio device and viewed as a set of electrical building blocks) is a capacitor, inductor, and load resistor in series, with a capacitor connected in parallel before the load resistor.

What all of that means is that passive EQ is happening – the capacitor, inductor, and load form a classic resonant circuit. The capacitor and inductor in series allow a range of frequencies through, and the parallel capacitance acts as an additional low-pass filter. (Whether or not this low-pass is significant after the capacitor-inductor bandpass is a whole other issue.)

The issue with passive filter circuits is that everything has an effect on everything else. If the load impedance is adequately high, then we get a nicely damped, wideband filter that sounds natural. If the load impedance is too low, however, the filter gets narrow and odd sounding. This effect can become so pronounced that string instruments start to sound like horns(!)

The obvious fix, then, is to connect the piezo pickup to a very-high impedance device. An easy way to do this is to use an active DI box.

The Buffer Zone

Active DI boxes solve the piezo impedance problem because they can employ buffer amplifiers. The great thing about a buffer amplifier is that its input impedance is very, very high (millions or even billions of Ohms). It also does this in a very small package. You could probably construct a passive DI box with an input impedance in the millions of Ohms, but the size and weight of the thing (not to mention the cost) would be really off-putting.

The downside of using a buffer amplifier is that it requires a power supply. This means batteries, or engaging phantom power from the console. In practical reality, though, this downside is almost negligible. Almost any modern console that’s capable of mixing a full band will have phantom available, and a battery in a DI box will probably last for tens (if not a hundred or so) hours.

So – all of this is just a very long way of saying, “Buy active DI boxes.” They’re pretty much guaranteed to work with any kind of instrument output you encounter, and they can be powered by any half-decent console or mic pre. They remove any need for guesswork, and they can even have nifty extras like signal boosters and guitar cab emulations.

Passive direct boxes are the right choice most of the time, but a reliable, full-featured, active DI is the right choice all the time.

No contest.