Going Direct

Nov 1, 2001 12:00 PM, By Scott Wilkinson

           
 

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.

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.