Contrary to popular belief, a direct box is not a device that comes right out and says what it really thinks of your playing. Also known as a “DI” (for direct inject or direct input), its main function is to convert the unbalanced high-impedance output of an instrument to balanced low impedance, which in turn allows longer cable runs without adding noise. At their most basic, DIs are equipped with at least one input and two outputs; one being a thru-put for an instrument amplifier, and the other for input to a PA system. The output of a DI is mic level, which allows it to be connected to the microphone inputs of a mixer. Another useful function of the direct box is its ability to eliminate AC hum that stems from ground loops. Direct boxes divide themselves into two families, Clan Active and Clan Passive (which cannot die).
A short history of the direct box (DI)
Back in the golden age of music, men and Elvises roamed the midrange together in peace and all DIs were passive. First appearing in the 1970s, as venues became larger and touring equipment became more powerful and complex, the direct box served to isolate a musician’s stage amplifier from the PA system (eliminating hum). They allowed longer cable runs by lowering impedance and balancing the signal. Back then, a direct box was basically “a transformer in a can” and mainly built on the fly. The active direct box didn’t appear until the ’80s, after it was noticed that the passive DIs were having a negative effect on the sound of certain instruments, most notably the single-coil Fender bass. Active DIs operate either by battery or phantom power. Both active and passive types are in use today, and each has its advantages depending on the application.
What is an active DI?
Active direct boxes are in fact preamplifiers, whose primary function is to provide higher input impedance than that of a transformer to drive instruments with low-level signals. For example, if you were using a passive DI to send an old Fender bass to the console, its single-coil pickup had to drive amp, transformer, mic-splitter transformer, and a few hundred feet of cable. The result would be a lack of punch and clarity due to the excessive loading of the pickup. This is not to say that all passive DIs sound bad. Remember, application plays a large role. Another reason for sound quality issues with passive DIs in the past was that cheap transformers were being used to keep prices down. Unfortunately, poorly designed transformers can cause ringing and smearing of the upper frequencies, or a reduction of sound quality in general. Active DIs were created as a means of improving sound quality while keeping costs down. They did so by providing enough gain to drive the aforementioned Fender bass without adversely loading the instrument’s pickup. The tradeoff with active devices is the loss of ground isolation.
Active DIs can take their power from batteries, which is less desirable to professional touring FOH (Front of House) engineers, or phantom power coming off the mixer (more desirable). Active DIs, being much like preamps, also have features including pads, ground-lift switches, high-pass-filters, polarity reverse, and equalization switches.
What is a passive DI?
As we stated earlier, a direct box is basically a transformer. It acts as a bridge between input and output by virtue of electromagnetic induction. In essence the signal is passed through a shared magnetic field. Since there’s no hard wiring, the signal is electronically isolated from input to output. As a result, 60-cycle hum from ground loops and DC noise are eliminated. The word “passive” means that the unit does not require power in order to operate. Another advantage of transformers is that they don’t distort the way active circuits do when they are driven into overload. With active circuits, it’s all or nothing. Once overdriven, they abruptly go from clean to completely distorted and harsh, whereas transformers saturate, thus presenting a smooth transition that mimics the behavior of a limiter. On the downside, transformers aren’t particularly good at handling low-output instruments, such as old electric guitars, basses, and vintage keyboards, since there is no power to drive a weak signal. On the upside, along with hum-eliminating abilities, passive DIs are good at handling high-output instruments (anything with a battery-powered preamp, for example).
Some specialty active DIs have added features such as the BBE DI100x, which features a Sonic Maximizer circuit, and the Radial J48, which has a mono summing switch (that may not sound like a big deal, but it can really come in handy when you have limited console inputs and a rack of stereo keyboards shows up at the gig).
|Rolls Mic Splitter MS20|
A Passive Mic Splitter, as its name implies, is a DI that splits a signal from a low-impedance source, such as a microphone, to two outputs. This enables you to feed two preamps from one source. Applications include onstage monitoring for musicians, live recording, videotaping and/or recording lectures or religious services, or simply for driving long microphone lines feeding an unbalanced input.
|Radial ISO Twin|
An Isolation Transformer is a DI that uses a floating transformer-isolated output (see Isolated Line Output below) to eliminate ground loops between equipment connected with unbalanced lines. The floating transformer provides the noise-reduction benefits of balanced lines. Applications include connecting the unbalanced output of a mixer that must drive long unbalanced cables to power amplifier inputs, and connecting line-level sound modules and samplers, etc. to live sound or recording mixers.
|Radial ProAV1 Single-Channel Direct Box|
Connecting an iPod/Laptop To A Mono Mixer
Using iPods and Laptops for backing tracks has become increasingly popular for live performance. There are DIs that are built specifically to address the problems that can occur when connecting an unbalanced stereo source to a mono mixer’s input channels. These problems include noise, signal loss and permanent damage from phantom power. To run mono (especially for long distances), you need a DI that will sum the stereo outputs to balanced mono and provide transformer isolation to protect your equipment from damage by phantom power. One such device that handles all of this beautifully is the Radial ProAV1, a passive direct box with 1/4", RCA, and 1/8" stereo connectors (see Audio Cable Buying Guide) for instruments, iPods, and computers that are summed to mono. Never, we repeat, never try to combine two signals into one with a “Y” cable or use a single mono cable. Here’s why. Outputs are low impedance and always must be connected to a high impedance input. If you tie the two outputs together (with a “Y” cable or mono cable), each output will try to drive the other, which can force them beyond the safe current limit and possibly into short circuit. At minimum, you will experience a severe loss of signal. Worst case; you can damage your iPod or laptop.
Choosing a DI
Choose a DI for the sound you like!
Like preamps or anything else that makes a sound, when it comes to choosing a DI, there are no hard and fast rules. Much like the Pirate’s Code it’s more like guidelines. Just as certain combinations of mic and preamp offer different sound qualities (not good or bad, just different), the same is true of DIs. In general, a passive DI will offer a “rounder” tone, while an active DI will provide more high-frequency content—much like the sonic distinction between tube and solid-state preamp design. We also recommend getting on the online audio gear forums to see what live sound and studio professionals are using. Another good source of information are the customer reviews of DIs posted on our website.
The law of opposites
In general, passive DIs are the choice for active sources and active DIs are preferred for passive sources. There are some exceptions to the rule. Some active sources can work very well with active DIs, particularly those designed with higher voltage-handling ability. There are also DIs that can operate as either active or passive, which is quite a handy thing for the studio or live sound pro who may not know what instruments will be showing up on any given day.
Active uses and things to consider
For direct recording in the studio, you’ll most likely want the sound of an active DI. Today, there are both tube and solid-state active direct boxes designed for studio use. Basically any instruments that do not have a form of internal power are suitable fare for an active DI. Examples would be acoustic instruments with piezo electric pickups, classic basses, and old keyboards such as the Wurlitzer and Fender Rhodes electric pianos.
For acoustic-electric guitars and electric basses, active DIs can add high-frequency brilliance, which is certainly a plus for slapping styles of bass and bringing out attack information to aid tonal definition. However, the dramatic transients produced by aggressive rhythm guitar playing and “slapping” and “popping” on bass may cause an active DI to distort. If you’re using a high-output acoustic-electric or bass, look for a DI with the ability to handle high voltages such as the Radial J48.
|A Designs Audio REDDI|
If you’re playing a classic bass and wondering, where’s the beef? then an active DI is the answer. Today, there are active direct boxes that are specifically made with bass in mind such as the A Designs REDDI, Summit TD100, and the Avalon U5. (They all work extremely well on guitars, synths, and electric pianos too.) There are also units that straddle the DI and preamp categories designed specifically for electric and acoustic guitar (see Instrument-Specific Preamps in the Preamp Buying Guide).
Some advice for bands setting up a PA system for gigging and aspiring FOH engineers: Certain active DIs are AC powered. Not a problem in the studio, but it might make them a little complicated for live performance use since you’ll have to make sure there’s AC power near the stage setup. Also, plugging an active DI into a nearby wall or power source can cause ground loops. Most, if not all, active DIs have a ground lift switch to solve that problem; however, you might find yourself switching between loud noise and less noise. From an FOH engineer’s point of view, a DI that uses phantom power might be a better choice.
As with preamps, look for an active DI with high-quality transformers (the same holds true for passive DIs) and a wide dynamic range (see preamp sidebar “So High It Hertz”). In live performance a wide-bandwidth DI will preserve the harmonic content of your instrument that lives above 20kHz. Along with enhanced sound quality, there is acoustic information in the upper frequency range that is a component of your instrument’s attack. And, as we stated earlier, poorly designed transformers can wreak havoc on the upper frequencies. The benefit of quality transformers and wide bandwidth is better definition and spatial localization, which allows your instrument to carve out its space in the mix.
Whirlwind IMP 2
If you’re using an instrument with a built-in battery-powered preamp, such as an acoustic-electric guitar or bass, a passive DI will do the trick. Also, modern electronic keyboards and CD players are quite capable of producing output levels sufficient to overdrive an active DI. As we mentioned earlier, there are active DIs that can handle the high voltage requirements of these instruments and devices; however, a passive DI with a quality transformer is less expensive and won’t require batteries or an AC line, or take up a phantom-powered input in your console.
For noise-related problem solving in live performances when there’s little time to troubleshoot noise from ground loops, etc., it’s good to have a few passive DIs on hand.
TIP: If you’re using a passive DI to send an output from an electric guitar amplifier to a console, do not sit the DI on top of your amp! The magnetic fields produced by the amp’s power transformers will defeat the DI’ s electronic isolation properties by interfering with its transformer. Noise and possible phase and frequency shift will be the result.
Features to look for
At its most basic, a passive DI only requires an instrument input, balanced XLR output, and a ground lift switch (and a good transformer, of course).
Pad — attenuates (reduces) the voltage or power level of a signal by a fixed amount, thus allowing very hot signals (such as that of a CD player) to be connected without overload, and balanced for long cable runs.
Thru-put — allows you to connect to an instrument amplifier along with the console.
Polarity reverse — used when combining sounds that may be out of phase.
Ground-lift — a switch found on DIs as well as many other pro audio products that separates the signal ground from the unit’s chassis ground. For example, when a ground path is made at the input side of the DI (from a keyboard for example), the ground-lift switch disconnects the ground path at the output (XLR) side.
Isolated line outputs — without going into too much technical detail, this is a floating-transformer-isolated feed. “Floating” means that the output is not connected to ground or another output, which provides maximum electrical isolation. The transformer-isolated feed provides common-mode noise rejection by fooling the circuit into thinking that it’s balanced.
Tricks of the trade
As any experienced FOH engineer will tell you, the fastest and easiest way to eliminate ground loop hum is to put a transformer in-between the offending units. Passive DIs such as the Whirlwind IMP-2 are quite affordable and do the job more than adequately.
A common use of the DI for an impressive sound in the studio is to combine a miked signal and a DI signal into two channels and blend them. The trick is to use phase reverse on the DI. If your DI or console does not have a phase reversal switch, you can wire an XLR cable to be out of phase. (Just swap pins 2 and 3 on one end of the cable and put some red tape on it so you don’t use it elsewhere.) If you’re recording to a DAW, just invert one of the channels (you can find this command in the Audio menu of most DAW software).
If you’re using an unbalanced line-out from your console to feed a house speaker system and experience loud 60-cycle hum, put a passive DI with isolated outputs (floating transformer) and a ground-lift switch such as the Pro Co DB-1 in-between your console’s unbalanced output cable and the house system’s input cable. If you still hear noise, flipping the ground-lift switch should solve the problem.