What direct boxes (DIs) do and what are the different types. By Al Keltz
Direct boxes are often referred to as "DI" boxes. This stands for "Direct Injection" as their main purpose is to convert unbalanced and/or high impedance instrument signals into a format suitable for direct connection to a mixing console's mic input - without the use of a microphone. In 1981, Whirlwind developed and produced the first commercially available Direct Box in the audio industry. (See www.whirlwindusa.com/bbstor.html .)
A "DI" serves several basic functions:
* It converts a high impedance signal to a low impedance signal (although it will also accept a low impedance signal from a preamp, keyboard, active pickup or other electronic device).
* It converts an unbalanced signal to balanced.
* It reduces a strong instrument or line level signal (and sometimes even speaker level signal) to a mic level signal suitable for connection to the mic input of a mixing console.
* It isolates electronic equipment on stage from the mixing console, which can help eliminate interference and noise caused by electrical interaction or ground loops. It also blocks phantom power sent from the mixer so it cannot interact with the the device connected to the DI's input.
There are two types of Direct Boxes, passive and active.
The simplest form of a DI consists of a box containing a transformer. (A high quality transformer is critical for preserving the frequency characteristics of the signal.)
The input connection is usually made via an unbalanced 1/4" guitar type tip/sleeve jack or RCA phono jack and there are usually two jacks wired in parallel. These dual jacks allow an instrument to be connected into the DI and then out again in parallel. This could be used, for example, in a bass guitar setup where the bass is fed first to the DI and then back out to a bass guitar amplifier located on stage for monitoring. (See picture below).
(Some DIs also have two independent sections for use with stereo sources.)
Although DI transformers are often referred to as having an intrinsic impedance, they actually reflect the impedance connected to the output back to the source and vice-versa. In a DI, the turns ratio of the transformer will usually be about 11.5:1, primary to secondary. This results in an impedance ratio of about 133:1. It's this step down characteristic of the transformer that reduces the voltage level of the line signal to mic level.
At the same time, the the impedance connected to the DI's output is reflected back to the input and magnified 133 times. This high impedance presents a proper load for various sources whether from a high impedance guitar pickup or low impedance output from a keyboard, preamplifier or other active device. This helps preserve the frequency response of the input signal.
The secondary (output side) of the transformer converts the signal to balanced and lowers the impedance making it suitable for transmitting long distances to a mixing console.
The output connection is made via a male 3 pin XLR and the ground connection from the input jack's shield to pin 1 of the XLR is made through a switch on the DI. This switch provides the ability to break the ground connection between the input and the output.
WHY A GROUND LIFT?
A guitar with passive pickups and its cable shield get a ground connection from the mixing board through the DI's ground lift switch. However, if that instrument is connected through the DI to an amplifier, it also introduces a second grounding point at the amplifier. Slight differences in the actual resistance to ground can exist between the amplifier and the mixer and this can cause AC current to flow in the shield. This is called a ground loop and the 60 Hz signal "riding" on the shield radiates into the center conductor causing hum. Similarly, in the case of AC powered keyboards or preamps, a ground connection through the DI can also set up a ground loop.
As a rule of thumb, the ground switch should be left in the “ground lifted” position. This helps avoid any nastiness that can occur if you were to set the DI on an amp with a hot chassis or had equipment that was accidentally wired with a "hot" ground. Only place the switch in the grounded position if it's needed because of connection to a source that requires the ground, like a passive guitar or bass."
Examples of passive DI boxes are the Whirlwind DIRECT-JT, IMP2®, DIRECTOR®, DIRECT2, pcDI, EDB1, DIRECT4, and snake / DI combo, the MultiSnake. All of these units (with the exception of the EDB1) are hand made in our Rochester NY factory.
The DIRECT-JT, Whirlwind's top-of-the-line passive direct box that is equipped with the Jensen T-DB-EPC transformer - world reknown for wide frequency response and superior noise rejection characteristics. This unit features a "COMBINE" pushbutton to safely sum stereo inputs to a single channel, accepts instrument, line or speaker level input and includes a 15dB pad.
A "REVERSE / IN PHASE" button reverses the polarity from pin 2 hot to pin 3 hot for correcting non-standard equipment and mis-wired cables or creating special effects and a Ground Lift helps eliminate hum and buzz.
The IMP2 is a basic, yet rugged and high performance DI. It contains a high quality transformer (Whirlwind's TRHL) for transparent response, parallel input and through jacks and a ground lift switch.
The DIRECTOR is Whirlwind's premium passive DI.
In addition to the features found on the IMP 2, the Director features a special metal shield around the transformer (TRHL-M), a low pass filter switch that helps eliminate hiss from noisy sources and a 30dB pad switch that allows it to be connected directly to speaker level signals without overloading.
It is also available in a 4-channel rack mount version, the MultiDirector.
The DIRECT2 is a dual direct box that combines features found on two of our Director DIs in one package. It's perfect for stereo keyboards, computer sound cards, acoustic guitar preamps, etc. The two sections are also totally isolated from one another to prevent ground interaction.
The pcDI is the first dual direct box designed to interface your unbalanced stereo line sources with professional balanced low impedance equipment. This unit contains two completely separate DIs with color coded RCA phono type input and through jacks and color coded XLR outputs. Each section is totally isolated from the other, features a ground lift switch to help eliminate hum and a 20dB pad switch for connecting to "hot" signals. Perfect for interfacing with CD players, iPODs® / MP3 players, computer sound cards, tape decks, etc.
The EDB1 is an economy direct box featuring rugged construction while maintaining exceptional sound quality. Features parallel input and through jacks, a pad switch for connection to "hot" signals and speaker outputs, a ground lift switch and all metal construction for superior shielding. (Imported)
The DIRECT4 combines features found on four of our DIRECTOR® direct boxes in one unit. Perfect for converting unbalanced signals from multiple instruments, keys, acoustic guitar preamps, CD and tape players, computer sound cards, etc.
MultiSnakes are perfect for organizing acoustic instruments, keyboards and vocal frontlines. We've combined our Director's premium direct boxes with a Medusa snake into a single small stagebox measuring 9.5" (with handle) X 5.25" X 2.5".
* Acoustic and bluegrass bands
* Stereo keyboard rigs
* DJs and hip-hop mixing
The same conversion that occurs with a passive DI can also be accomplished with an active electronic circuit. One advantage of using an active circuit is that it can be fine-tuned to produce a wider frequency response than can be achieved with an entirely passive DI. However, active DIs require a power source - phantom power from the mixer or internal batteries. Also, when converting impedance passively with a transformer, the impedance and signal reduction required for the output dictates that the impedance ratio, primary to secondary be about 133:1. Although this results in a fairly high impedance at the input of a passive DI, the frequency response, especially when connecting high-impedance passive pickups, would be even better preserved if the input impedance were even higher. Active DIs accomplish this with a very high input impedance of a million Ohms or more. For example, the HotBox , Whirlwind's reference DI: Its input impedance is 1 million Ohms. This ultra high impedance preserves the upper harmonics from all signals, especially passive guitar pickups. Also, the frequency response of the HotBox is extended from 20Hz to 200kHz, ±3dB. The HotBox also features a pad switch for connection to speaker level signals. It operates with (2) 9V batteries or phantom power and is available in 4-channel rack mount version, the HotBoxQuad, which operates on 110VAC.
No matter what the application, Whirlwind DIs provide inDIspensable and virtually indestructible solutions to your interface needs.