When the sound tech needs to connect an instrument-level source (such as a bass guitar or keyboard) to the PA system, they will typically reach for a direct injection box (DI box). At first thought, this might seem counter-intuitive, since most mixers have 1/4" inputs and enough gain to match a wide range of signal levels. So, what are DI boxes, and why are they used?
Don't Lose Your High Frequencies
Instrument-level outputs are high impedance, which is fine for short cable runs on stage. If you tried to run long cables back to your mixer, the high impedance output coupled with the cable’s capacitance creates a low pass filter effect. You are certainly familiar with the way a tone control rolls off your high frequency response. Cable capacitance does the same thing to your tone, robbing your sound of sparkle and transience, leaving it muddy and indistinct.
DI boxes produce a low impedance signal by converting the high impedance output of your instrument with a special transformer. This allows you to use short instrument cables on stage, and low impedance cables for the mixer run, avoiding high frequency loss. It is worth mentioning that it is also advantageous to select the best quality low-capacitance instrument cables that you can find. The difference in price over the life of the cable is only a few dollars per year. Better cables are more reliable too!
Less RF and EM Pickup
Another great thing happens when you use a DI box. Instrument cables are unbalanced, so they have less resistance to outside signal noise caused by RF and EM interference. Because the low impedance output of a DI box is balanced, it rejects a lot more RF and EM interference. The lower the signal-to-noise ratio of your input signal to the mixer, the better your instrument will sound. The lower the cumulative signal-to-noise ratios of all channels, the better your band will sound.
A Firewall for Electricity?
Since DI boxes use a transformer to reduce signal impedance, they also offer galvanic isolation, meaning there is no direct connection across the DI box. Instead, the input signal runs through the primary transformer coil and the output is connected to the secondary coil. When there is current fluctuation (signal) in the primary coil, it induces current in the secondary coil. Why does this matter? Since the two sides of the transformer are isolated from one another, you have isolation between the stage and the mixer, reducing ground-loop problems and alleviating a potential electrical safety hazard in the event of a fault.
Compatible with Stage Snakes and Available Inputs
Most PA snakes offer balanced mic inputs but lack unbalanced ones. DI boxes convert your unbalanced outputs to low impedance mic outs that are compatible with your PA snake, saving you time and long cable runs through high impedance cables.
The Carvin Audio FDR60 Direct Box includes a ground lift
A Sound Investment
A good quality DI box, such as the Carvin Audio FDR60, will come in handy time and again over many years. They are incredibly reliable and usually built to be very sturdy. Avoid the temptation to choose the cheapest available DI box. Transformer quality does affect your signal quality greatly. Features like ground lift switches make setting up easier as well. Like most utility tools, a DI box isn't very glamorous. But having great sound is. Do you use a DI box? Let us know in the comments.
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