December 2012 Issue
Theory and Practice

Passive Aggressive: Direct Box Styles, Explained

Fig. 1: A basic passive direct box using a Jensen JT-DB-E transformer, with color-coded hookup wires.  Although this has a ground lift switch, other passive DIs may include pad/attenuator or polarity  reverse switches and a second 1/4-inch “thru” jack wired in parallel to the input. Image courtesy of Jensen Transformers.A lot of people take DI boxes for granted. After all, what’s the big deal? You plug them in, and they work. If not, you rummage through the utility drawer until you find one that does work, and off you go. Most people never give any thought about whether or not they are using the best DI for a given application. Of course, we’re here to help.

Your Crash/Refresher Course

A DI has several functions, one of which is converting the high-impedance unbalanced signal from a guitar, bass or other instrument into a low-impedance, balanced signal.

Electric guitars and basses produce unbalanced signals intended for connection into (duh…) guitar and bass amplifiers. These amps accept the signals without any trouble, but the line inputs of an audio mixer are basically incompatible with them — the impedance of a line input is far too low for a guitar or bass. As a result, the pickups work too hard, resulting in an increase of noise, loss of signal level and degraded frequency response. Additionally, unbalanced signals (particularly those from guitar and bass) can’t travel very far before they succumb to noise from RFI and electromagnetic interference, so we recommend keeping guitar cables shorter than 20 feet. Once a signal is balanced, it can travel safely over the long cable runs necessary to get it to the FOH and monitor desks. Finally, the DI changes the level of the instrument, knocking it down to microphone level, while at the same time providing a safe way to avoid ground loops and hum.

The Direct Approach

DIs come in two basic flavors: active and passive. Neither design approach is necessarily “better” than the other. The obvious difference is that a passive DI does not require a source of power, as it employs a transformer to perform impedance matching and balancing, as shown in Fig. 1. This transformer is the most important component in a passive DI. A good transformer is expensive, but provides several benefits.

According to Peter Janis of Radial Engineering Ltd., “transformers create a magnetic bridge that allows audio to pass while blocking DC. This makes passive direct boxes very helpful at eliminating noise due to audio ground contamination from varying DC offsets and reference voltages between the instrument and the mixer or preamp that can cause buzz or hum. This is manifested in the form of ground loops due to power distribution issues. Another benefit stems from the sound of a transformer. Unlike active circuits that go from say 0.5 percent distortion to 100 percent distortion once you overload the input and exceed the rail voltage (a.k.a., clip the input), transformers do not distort the same way — they saturate. This creates a natural compression that we humans find pleasing, and is often referred to as ‘vintage’ sounding.”

Getting Active

An active DI is, by nature, a form of preamplifier. Active DIs typically employ electronic circuitry to achieve impedance matching, and may or may not use a transformer at the output. As the name implies, active circuitry requires power to do its job, with the vast majority deriving power from either a 9 VDC battery or +48-volt phantom power from a mixing console. Given a choice, it’s probably better to avoid batteries because they’re environmentally unfriendly and can fail at the worst possible moment. It’s a definite advantage when an active DI can operate from phantom power, as phantom power is available from mic inputs on most mixing consoles, though a small percentage of DIs use wall-wart power supplies, or (in the case of many tube designs) source power from AC mains. The input to an active DI does not distort gracefully or saturate as does a transformer, so in ye olden days, there was concern of high-output instruments overloading the input of an active DI, causing distortion. This is no longer the case.

Which Do I Choose?

Active circuitry allows the DI designer to create an extremely high input impedance (thus making the pickups in passive basses and guitars very happy) or implement various forms of audio “massage” such as frequency contouring.

“For low-output instruments, such as an older Jazz Bass, even a small bit of loading can cause a bass player to notice a minute drop in level,” Janis explains. “For these artists, an active direct box tends to be a better solution. This also applies to some vintage instruments like a Fender Rhodes piano. The active DI has a built-in buffer or unity gain amplifier that delivers a stronger signal than its passive counterpart. In the past, high-output instruments would overload the typical active DI. In recent years, new switching power supply technology has enabled us to increase the signal handling to a point where this is no longer a problem. We often associate active direct boxes to condenser mics and passive DIs to dynamics. You can use either in many cases, but if you want more reach — say, on a violin — the condenser tends to be a better choice, just as an active DI will produce more air from an acoustic guitar. Active DIs also enable us to control various parameters that are simply impossible using a passive circuit.”

Generally, passive direct boxes are often preferred on active sources, such as keyboards, drum machines, etc. Conversely, an active direct box often works best with instruments without onboard electronics or active pickups, such as vintage basses, guitars, mandolins, etc. In either case, reducing the number of additional gain stages in the signal path tends to reduce noise.

However, “the lines blur with acoustic guitars that have built-in pickups,” says Janis. “Folks are comfortable with active direct boxes, and since we can now handle higher signal levels, this seems to work well. When it comes to piezo transducers, active is the only choice. These finicky devices sound best when they see a very high input impedance. Our newest DI has a 10 Meg-Ohm (10,000,000) impedance for this very reason. This smoothes out the peaks, eliminates the squawk and extends the frequency response, making it much more pleasing for orchestral instruments such as upright bass, violin or banjo.”

Certain features are shared by active and passive DIs: a 1/4-inch “thru” jack for routing the instrument to an amplifier, a ground lift switch to safely eliminate hum due to ground loops, and a pad switch so that speaker-level signals can be patched into the DI. Less common features include a polarity reverse switch that swaps pins 2 and 3 of the XLR output (this may be helpful in dealing with phase issues) and an LED to indicate when phantom power is present.

The author wishes to acknowledge Peter Janis at Radial Engineering for his assistance in preparing this article —ed.

 


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