APPRECIATING THE DIRECT BOX

In Uncategorized by tfwm

One of the common complaints we hear on our HOW-TO Church Sound Workshop™ tours is that stage volumes are out of control. Of course, anytime you use electric instruments in a sanctuary originally built for acoustic instruments and voices you are bound to have problems. Nonetheless, there are several ways to corral runaway stage levels. One way is to use stage amplifiers as instrument monitors at low levels, and feed the electric bass, keyboards and, sometimes, electric guitars signals directly to the mixing console via a Direct Box.

The versatile DI (Direct Instrument or Direct Inject Box) is a very helpful tool for your worship sound system. The most common type is called a Passive DI, which includes an isolation transformer and ground-lift switch. The primary function of a DI Box is to convert the unbalanced 1/4-inch, 2-conductor phone plug output on your keyboard, bass or guitar into a low-impedance (low-Z), balanced (3-conductor) XLR jack, which then can be plugged directly into your signal snake or console without any fidelity loss or hum problems. In more basic terms, the DI is a convenient interface device that allows connecting musical instruments with 1/4-phone plugs directly into a signal snake or console with XLR mic connectors. Once converted into a balanced signal, you can send the signal hundreds of feet to the console in the back of the room.

Never try to use a 1/4-inch-phone-to-XLR adapter plug (without a transformer) since the hi-impedance (hi-Z) output from the passive pickup in your guitar will lose most of its high frequencies on a 100-foot trip down a snake. Even the 1/4-inch, unbalanced (low-Z) output of your keyboard can pick up hum down a long microphone snake since there’s unequal impedance feeding the input stage of the console. Note that any unbalanced (2-conductor) cables will tend to pick up hum from every electrical appliance in the room including light dimmers and ballasts, so convert to balanced signal as quickly as possible. The balanced input circuit (XLR Input) in the mixing board effectively nulls out these stray signals if the correct cable type (3-conductor twisted pair) and termination (XLR low-Z) are used.

The Whirlwind IMP-2, shown here, is a good example of a Passive DI. Costing around $50 and available at most music stores, it will save you time and energy, eliminating ground loops and making your audio system sound as good as possible.

In addition to the Instrument/Line Level In and Mic Level Out jacks on a DI, there’s a second 1/4-inch jack on the input of the box that functions to loop-through your signal to a stage amplifier. It is not used to add the stereo outputs of a keyboard or CD player into a mono signal. Distortion or oscillation can result from hooking two outputs together without a summing circuit of some kind.

A direct box usually includes a ground-lift switch, which will assist in breaking ground-loops in the system. A ground loop can occur when two pieces of gear are plugged into different electrical outlets. For example, a keyboard might be plugged into an AC outlet on the stage and the mixing board plugged into an outlet in the back of the room. Any small difference in the ground voltages between these two outlets will cause a current to travel down the shield of the signal cable connecting them, most commonly the signal snake. This makes for a nasty hum though the whole sound system. The ground-lift switch will open up this connection and eliminate the hum in all but the worst situations. Never try to stop this hum by cutting off the ground lug (third pin) on the power cord of an instrument or amplifier. That can lead to electrocution.

Another function of the DI box is to block phantom power at your mixing board from getting into any instruments. For instance, phantom power operates by sending 48 Volts D.C. back up the snake to power a condenser microphone. If, however, you plug in a keyboard or acoustic guitar directly to the microphone snake with only a 1/4-inch-phone-to-XLR adapter plug, that same 48 volts will feed back into the output stage of the musical gear. Most gear only has 25-volt blocking capacitors on the output, so after a while that 48-volts can cause catastrophic failure resulting in smoke, a dead keyboard and an expensive repair. Always use a DI box to plug any stage instrument with 1/4-inch phone plug outputs into XLR inputs that can have phantom power. FYI: Don’t use a 1/4-inch-phone-plug-to-XLR cable adapter in place of a DI box. It might work, but a 1/4-inch- mono- phone-plug-to-XLR cable will short out pins 1 and 3 in the XLR side, which in turn will short out any phantom power enabled on that mic channel. At worst, you’ll shut down the phantom power for that whole section of the console, knocking out all condenser mics.

If you have any questions about this article, or if you would like to see other topics covered, then please contact fitsandstarts@comcast.net