A DI box is a tool that is utilized to directly inject a high impedance (high electrical resistance) unbalanced (no electrical ground) signal into a system that is utilizing microphones, which have a balanced low impedance signal. The DI takes the unbalanced high impedance signal and converts it to a low impedance balanced signal to be able to transmit it over longer distances without any signal degradation and at that point plug directly into a console. As with microphones, there are some DI’s that are better than others and some that come with fancy bells and whistles, buttons and knobs, that others do not.

When you first do a Google search or look at Amazon or Sweetwater, you’ll be faced with a significant price range (though not as significant as a microphone price range) and a few potentially unfamiliar terms. DI boxes usually can be purchased for anywhere from $20 to $300 and are either Passive DI’s or Active DI’s. In a way, a passive DI is similar to a dynamic microphone. It is designed to be used with an instrument which does not require phantom power. It is different from microphones however, because that instrument already has a power source (keyboards, guitars, etc.). This type of instrument would be known as an Active instrument, whereas the Active DI’s (similar in ways to condenser microphones) require phantom power to operate and are designed to be used on instruments that do not have an external power source (any “passive” instrument, but most often bass guitars).

The sonic difference between DI’s isn’t as significant as the sonic difference between microphones. The type of material the DI is made of is going to affect how the instrument sounds, and usually speaking the more expensive ones will sound much better, but on a basic level they operate in a very similar fashion to each other. When you purchase a DI, no matter which model you purchase, that DI will have a high impedance (high Z) in, a ground lift (GND/LFT), and a low impedance (low Z) out. The high impedance in, as we said earlier, will be the instrument cable from your guitar, keyboard, or other instrument, and the low impedance out is the XLR cable (the cable microphones use) which is designed to transport the signal over a longer distance without negatively impacting the sound. At that point it can plug directly into a console or snake. The ground/lift switch/button is designed to lift the electrical ground. When you are hearing a “buzz” or “hum,” this is because you have two separate grounds competing with each other: the ground from the instrument/DI and the building’s electrical ground. The ground/lift switch “reduces noise from ground loops by lifting the audio ground (pin-1), letting the source and destination derive their respective grounds from the buildings’ power system.  " 

As you start getting to higher quality DI’s some additional functions become available, and they perform the standard functions better. First, the transformers that are inside the DI’s typically improve in quality. In lower costing DI’s such as the infamous Direct One DB-01 or the Whirlwind Imp Direct Box, the transformers are made with steel. However, with the Radial’s JDI (not the Pro DI), their transformers (manufactured by Jensen Transformers Inc.) are made out of nickel. Nickel transformers tend to be more desirable than steel transformers because, “As you increase the input [signal], the transformer acts like a natural limiter or compressor, and, depending on its quality, yields a warm tone that is often referred to as ‘vintage sounding’.  ” Additionally, “Nickel tends to be preferred because it renders much more natural tone with less odd-order harmonic distortion.  "

I believe Radial puts it very well on their article Direct Boxes: The Good, The Bad, and The Ugly, “There are many types of DI boxes to choose from, and quality can vary significantly. While a weekend-warrior musician can make compromises, a professional audio engineer cannot. The pro will look for both sound quality and durability, because if the DI box fails, and suddenly there’s no bass sound, for example, the show comes to a screeching stop.  " That being said, here are a few gear choices if you are looking to purchase some DI’s for your weekend needs. Again, these are my suggestions based off of my experience and what I have observed being used in churches and other venues both small and large.