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Going Direct

November 1, 2001
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The Jensen Iso-Max stereo DI box uses two JT-DB-E 12:1 step-down transformers, which are the round, metal objects in this open view.

The Jensen Iso-Max stereo DI box uses two JT-DB-E 12:1 step-down transformers, which are the round, metal objects in this open view.

When I first started playing around with electronic music equipment as a child, I tried connecting the output of an electric guitar directly to a line input on a mixer. I thought I didn't need a guitar amp because the mixer, power amp, and monitor speakers would do its job instead; however, that didn't quite work out. When I played the guitar, it sounded completely dead and low in volume. “How can I correct this?” I wondered.

The output from a DI box can be sent over long cables with much better noise rejection than is possible with a high-impedance, unbalanced signal from a guitar or synth. But that is not the only application for direct boxes.

To understand how DI boxes work, you need to grasp the basics of impedance, decibels, and levels. If those terms are unfamiliar, see “Square One: The Shocking Truth” in the June 2001 EM and “Square One: Decibels Demystified” in the July and August 2001 issues.

I didn't know it at the time, but I had two relatively simple options. If I really had wanted to use the mixer's line-level input, I could have plugged the guitar in to an instrument preamp and connected the preamp's output to the mixer's input. But a simpler and cheaper alternative would have been to use a direct-injection box (also called a DI or direct box). Such a device converts an unbalanced, line- or instrument-level, high-impedance signal from an electric or electronic musical instrument to a balanced, microphone-level, low-impedance signal that can be connected directly to a microphone input on a mixer or mic preamp (see Fig. 1).

TRANSFORMERS

FIG. 1: The Whirlwind IMP2 DI box is about as simple as it gets. The two 1⁄4-inch input jacks are connected to the ­primary of a 10:1 step-down transformer, and the secondary is connected to an XLR output jack. Notice the ground-lift switch. The unused input can be used as a thru jack.

FIG. 1: The Whirlwind IMP2 DI box is about as simple as it gets. The two 1⁄4-inch input jacks are connected to the ­primary of a 10:1 step-down transformer, and the secondary is connected to an XLR output jack. Notice the ground-lift switch. The unused input can be used as a thru jack.

The basic component of a simple DI box is a step-down transformer. All transformers consist of two or more long thin 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 of the transformer, and the other pair is the output. The input coil is called the primary, and the output coil is called the secondary.

When you send an electrical signal through the primary coil, it creates a magnetic field around the coil. That field 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.

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. If the primary and secondary coils have the same number of windings, the component is called a 1:1 transformer. That type of transformer is used in noise-reducing signal-isolation boxes.

In general, the primary and secondary coils are wound concentrically around the core (for example, if the secondary coil is wound first, the primary coil is wound around it). In addition, the primary and secondary coils are often separated by copper foil called a Faraday shield, which helps reject radio-frequency interference (RFI) between the coils. As the number of turns in the coils increases (that is, as the length of the wire increases), the transformer exhibits greater level-handling capability as well as lower distortion, but it also has less high-frequency response. The primaries of step-down transformers used in DI boxes typically include thousands of turns.

The relationship between the input and output signals is determined by the ratio of the number of turns in the primary and secondary coils. The change in signal level is directly proportional to the turns ratio. For example, if the turns ratio of a step-down transformer is 10:1 (that is, the primary has ten times as many windings as the secondary), the signal level drops by a factor of 10; if the signal level into that transformer is — 10 dBV (a typical guitar or line level), the output level is — 30 dBV (a typical mic level).

The change in impedance is proportional to the square of the turns ratio. For example, if the turns ratio of a transformer is 10:1, the impedance changes by a factor of 100. However, a transformer has no intrinsic impedance; instead, its impedances are determined by the impedances of the devices connected to it. Specifically, the output impedance of the source device (say, a guitar) is modified by the square of the turns ratio to calculate the transformer's output impedance. Likewise, the input (load) impedance of the destination device (say, a mic preamp) is modified by the square of the turns ratio to calculate the transformer's input impedance.

For example, the normal input impedance of a mic preamp is 3 kilo-ohms (kΩ). If the input to that preamp is connected to the output of a 10:1 step-down transformer, the transformer's input impedance becomes 300 kΩ. If you were to connect the same transformer to a mic preamp with an input impedance of 1.5 kΩ, the input impedance of the transformer would be 150 kΩ.

Most audio transformers have a metal core constructed from thin E-shaped laminations. The core provides a magnetic path to couple the primary and secondary coils (that is, it facilitates the transfer of magnetic energy between coils). Without that core, the transformer would have no low-frequency response below about 10 kHz.

In high-quality audio transformers, the core material is an 80 percent nickel alloy (commonly called Mu-Metal), which provides the best low-frequency response and lowest possible distortion. Lower-cost transformers use a 50 percent nickel alloy, and steel is used in the cheapest transformers. Steel is also used in high-power transformers, such as guitar-amp output transformers, because of its higher level-handling characteristics.

FIG. 2: The Jensen JT-DB-E transformer''s frequency ­response is 3 dB down at 0.6 Hz and 100 kHz and very flat in the range of 20 Hz to 20 kHz.

FIG. 2: The Jensen JT-DB-E transformer''s frequency ­response is 3 dB down at 0.6 Hz and 100 kHz and very flat in the range of 20 Hz to 20 kHz.

FOR EXAMPLE

To illustrate the previous concepts, consider the Jensen JT-DB-E, a 12:1 step-down transformer used in many DI boxes. It has a low-frequency response that is 3 dB down at 0.6 Hz as well as a high-frequency response that is 3 dB down at 100 kHz (see Fig. 2). (Frequency response is often specified with 3 dB down points, which are the frequencies at which the device's response is 3 dB below the nominal level.)

Why would you need a transformer with a 0.6 Hz to 100 kHz frequency response? For one thing, the transformer is basically transparent (which means it exhibits a very flat frequency response) in the audio range. It also ensures a flat phase response (the phase relationship between different frequencies remains constant) in the audio range (see Fig. 3).

FIG. 3: The phase response of the Jensen JT-DB-E transformer is very flat in the range of 20 Hz to 20 kHz, deviating only 1 ­degree in the low end.

FIG. 3: The phase response of the Jensen JT-DB-E transformer is very flat in the range of 20 Hz to 20 kHz, deviating only 1 ­degree in the low end.

Phase response affects the fidelity of the output waveform and the relative delay of different harmonics (which is also known as time alignment). Poor phase response results in a lack of localization, clarity, and imaging. Maintaining a flat phase response from 20 Hz to 20 kHz requires a flat frequency response between at least 0.8 Hz and 50 kHz and higher if possible. A high-frequency response as high as 100 kHz is fine for audio requirements; 200 kHz is state of the art.

The JT-DB-E's usual input impedance is 200 to 400 kΩ, depending on the preamp's input impedance. That's fine for a synth, but it might seem a bit low for a guitar. However, if the DI box's input impedance is in that range, it helps roll off the excessive brightness that often occurs when a guitar is connected directly to a mixer through the DI box. The transformer's output impedance depends on the source device, but its typical value is 150Ω, which is ideal for a mic-preamp input.

OTHER COMPONENTS

Some DI boxes, such as the Whirlwind HotBox, do not use transformers. Instead, they use active electronics for level and impedance matching. (Active electronics include semiconductor components, such as integrated circuits, whereas transformers are a type of passive component, like resistors and capacitors. Active direct boxes require power from a wall outlet or battery, and passive transformer — based DI boxes need no power at all.) The HotBox has an input impedance of 10 MΩ, which improves the frequency response at both ends of the spectrum.

In addition to a transformer or active electronics, many DI boxes include a ground-lift switch, which helps eliminate ground loops in certain equipment. A wide variety of good and bad techniques exists for designing and manufacturing audio equipment, and ground-lift switches can help compensate for bad designs.

If you run a guitar directly into a mixer through a DI box, it can sound bright. As a result, DI boxes often include a switchable lowpass filter that simulates the rolloff of a guitar-amp-and-speaker combination. The speakers in a guitar cabinet generally don't have much response above 3 to 5 kHz, so the filter normally rolls off above that frequency range.

Some DI boxes, such as the Whirlwind Director, include an input attenuator that allows you to connect the speaker output from a guitar amp to the mic input on a mixer through the DI box. That lets you include the compression and distortion components of the amplifier in the signal that goes to the mixer.

APPLICATIONS

DI boxes are helpful in several ways beyond letting you connect a guitar to a mixer's mic input. For example, many home-studio owners have lots of synths that eat up the line inputs on their mixer. However, the mixer might have plenty of mic inputs available. Using DI boxes, you can bring the signals from extra synths into the mixer using the open mic inputs.

One of the most common applications for DI boxes is to connect equipment with high-impedance outputs (such as synths) to a mixer's low-impedance inputs using long cables (such as snakes or studio tie lines). Cables that must run a considerable distance should always be balanced and low-impedance to minimize signal loss, induced noise, and grounding problems.

If you were to run a long cable (say, 100 feet) from a guitar to an amp, it would completely load the guitar; you'd lose high-frequency response and add noise. However, if you connect the guitar to a nearby DI box with a short instrument cable, you can then run a 100-foot mic cable to a mic preamp near the guitar amp. The mic preamp's output is then connected to the input of the amp.

Instead of using a mic preamp in such a situation, you could use a less expensive matching transformer, which is usually a 1:10 step-up transformer used to connect a low-impedance mic to a high-impedance input, such as a guitar amp. Matching transformers are normally housed in barrel-type cases with a female XLR connector on one end and an unbalanced ¼-inch plug on the other.

Such a transformer works best if it “sees” a few hundred kilo-ohms as a load, which means a typical 10 kΩ line input does not provide enough input impedance, but a 1 MΩ guitar-amp input is fine. Using the “square of the turns ratio” rule in reverse (remember, this is a step-up transformer), a 10 kΩ load on the transformer's output would present a 100Ω input impedance to the signal from the DI box, which is far too low. However, a 1 MΩ load would present a 10 kΩ input impedance to the signal from the DI box, which is sufficient.

Unfortunately, most matching transformers are not very high fidelity. Because the transformer must fit within a barrel-type housing, it isn't large enough to produce good low-frequency response. In addition, those transformers exhibit lots of phase distortion in the lower midrange and bass ranges. They're okay in noncritical applications, but you wouldn't want to use them in the studio.

DI boxes are one of the unsung heroes of electronic music. They can help improve your sound in many ways, but many people don't understand how they work or how to use them. Hopefully, you can now appreciate the important role that DI boxes play onstage and in the studio and begin to use them in your setup.


Scott Wilkinson recently purchased a DI box for his wife's Martin guitar with a built-in pickup and preamp.

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