GOING DIRECT: The proper use of direct input boxes
Have you ever
tried to patch a tape deck or a synthesizer from the platform, through a
microphone line to the mixer? Did it buzz? Did it hum? Did it distort the
input channel? If you only adapted the 1/4 inch jack to an xlr plug it
probably did hum, buzz or distort. It is not you, it is the equipment. The reason it did is that electronics which are powered by different AC
circuits do not always get along. A keyboard on stage plugged into a socket
with the piano lamp will require a direct input (DI) box to isolate the
keyboard from the mixing console.
A DI box
contains either a transformer or a circuit to isolate one voltage from
another. The most common DI boxes are passive devices that use transformers.
They are available from music stores, sound contractors and mail order
houses. The cost is quite modest from $35 to $100 dollars. Active DI boxes
use a powered circuit to achieve the same results. These are less common and
are mostly used in studios. They range in cost from $150 to $300 dollars.
You may have
seen a transformer device that looks like a long XLR connector with a phone
plug on one end. You may have seen these transformers used to connect a
keyboard to a mic jack. If you have ever seen this type of interconnecting
work, forget it, (or remove it) and wipe the image from your memory. It is
an error. It is among the most common mistakes in audio today. Those cute,
little, inexpensive transformers have one purpose: convert a low impedance
microphone signal into a high impedance microphone signal (or vice versa).
They simply do not have any other purpose.
The DI box
has a specific purpose. It connects devices to the microphone inputs on a
mixing console. It provides a balanced signal from an unbalanced line. It
isolates electrical devices. A good DI box will allow you to lift the
grounding connection from the signal to eliminate ground loops, some use a
switch while others provide a separate input jack. More elaborate units
provide for the different levels of signals from various sources. (i.e.: an
acoustic guitar is a very low level signal, a digital piano is a vary high
level signal.) Even the least expensive DI's will have two 1/4 inch jacks.
One is to feed the signal into the device. The other is to provide an
unaffected output to feed to a stage amp. This allows a guitar player to
feed to his own amp while still feeding the PA system.
allow tape decks, keyboards, guitar pickups, etc. to be connected to a
mixing console by way of platform microphone lines. This is how it is done.
phone plug from the source device to the direct input box
Attach a microphone cable with XLR connections to a platform input.
The platform input should connect to the console as a microphone.
Set the input gain and go to it.
The next time you need to patch that instrument into the
sound system make sure that you are going direct.
© 1998 Jordan Audio Consultants