Welcome to the next installment of "Tech Talk." This monthly feature
will hopefully help you understand about certain sound and musical
gadgets that everyone sees but are just not sure how they operate in
the real world.
This month we will be talking about
direct boxes, commonly referred do as "DI's."
Passive Direct Box
The most common DI box is the Passive Direct Box. This unit is often
used to connect Guitars, Keyboards and other electronics that have a
line level out from the instrument. Often the line level voltage is
between .5 volts to 3 volts (Some DI's units can handle an input signal
of +8dB). As a passive unit, the signal is as good as the transformer
that is built within it. An important feature of many good quality DI
boxes is ground lifting. Since there is no universal standard for audio
equipment and instruments, grounding problems often occur (Perhaps the
new ISO9000 standard may help.... but let's see what happens in the
next few years.) Many DI boxes are able to isolate grounding problems
between various items of equipment. Generally, by going through a DI,
you loss from 3 to 6dB of singal.
Active Direct Box
The second most common DI box is the Active
Direct Box. These units either work from a battery or phantom power
from a mixer. An active DI box can handle higher signal levels and put
out a higher signal level. Furthermore, the frequency response is often
better too. When you are performing in a room that has low
reverberation and good performance qualities, it is better to use the
active DI box. Also if you plan to use a digital signal in
reinforcement or recording, use the active DI box. Generally, a DI has
0dB signal loss.
Active Direct Box with Preamp
A new type of Direct Box may have a built in preamp that works off the
Phamton power of a mixer. I haven't had a chance to test one, but the
are supposed to boost a singal level up to 10dB. When I learn more
about them, I will post the info here.
Speaker Director Box
The less common DI box is called the
Speaker Director Box. A speaker director is used when the only signal
output available is from an amplifier. Many older 16mm film projectors
use a 10 watt tube amplifier for driving a 10 watt speaker. A tube
amplifier should always have a nominal load of 4 ohms or higher on the
output or the amplifier will burn itself out. A good speaker director
will present to the amplifier a proper load and convert the signal to
mic levels to either a 150 or 600 ohms. You should never take a signal
from an amplifier direct into a mixer. You will either fry the channel
or the power supply in your mixer.
Remember, all good direct boxes have ground lift switches
and there are a few units that have an automatic grounding system. Make
sure that your direct box has this feature.
Line Matching Transformer
Another common method of connecting low level electronics to a sound
system is by using a line matching transformer. The transformer is
usually mounted in a barrel type connector with a 1/4 inch-tip sleeve
connector at one end and an XLR three pin connector on the other end.
The whole unit is often about 4 inches long. There are only a few
manufacturers of these products and they seem to work.
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.
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.
Til next time,
Roger
For custom guitars and basses, check out
Fortner Guitars.
