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.
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).
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
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
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.
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: 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.
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
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.
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
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.