If there's one thing every pro audio setup has, it's equipment - and probably lots of it. That means a lot of different components to connect together in order to get your audio from source to speaker or recorded track. The trick with pro audio gear is that it's not always as simple as just running a cord; different devices communicate in different ways and getting the best result depends on giving each piece the exact sort of signal it's expecting. This is where direct boxes come into the picture: they're compact signal processors that handle the "translation" between audio source and destination, making sure the music is captured perfectly right from the beginning.
The most common use for direct boxes is to connect an instrument like an electric guitar or bass (or a microphone) to your mixing console. However, there are plenty of others as well, so it's important to know what you need your direct box to do before you get too caught up in your search. That way, you can narrow down the options to just the models that fit your needs.
For instance, if you have a big recording setup and you need to send a guitar signal to all kinds of components at once, you're looking for something like the Radial Engineering JD7 Injector Guitar Signal Distribution System. This rack-mounted box can be used for driving up to seven amplifiers at once during live gigs, or for channeling your signal to different pedals onstage or in the studio.
Direct boxes aren't just for guitarists, either. Check out the Radial Engineering J33 RIAA Turntable Preamp Direct Box for instance: it's got an internal preamp that lets you play vinyl LPs through pro-grade mic preamps or mixing consoles. Whether you're a DJ hitting the booth or simply an audiophile looking for a high-quality way to archive your vinyl collection, this box will make a great addition to your setup.
The larger the pro audio configuration you're working with, the more direct boxes you're likely to need. Think of these signal processors as the middle-men between all of your other gear: they take care of the little things, like adapting connector types, inserting phantom power and combining or splitting signals, so you can get everything hooked up the way you want it.