Active models utilize electronic circuitry that requires power, provided via a battery or more commonly via phantom power from a mixing console.
Active DIs are much like preamplifiers and can also offer features like ground-lift switches, high-pass filters, mono summing, polarity switches, and equalization circuits. Solid-state electronic designs are common in live audio production, but there are some tube DI models that see some usage on stages with the thought being that the tube circuitry adds a bit of “warmth” to the signal.
A common rule of thumb when I started out many years ago was to use a passive unit for high-output instruments like electronic keyboards or guitars with powered pickups, and use an active unit for low-output instruments. I still basically follow this but it’s not an absolute rule any more because many active DIs now have input pads to reduce hot signals while many modern passive units work quite well with low-power signals.
There are several manufacturers that make both active and passive units, with Countryman, Radial and Whirlwind popular choices (for good reason). My own preference for bass is an active DI that has a pad switch so I can reduce the input signal to the box if the performer has active pickups on the instrument.
A common and effective technique is to mike the bass cab in addition to getting a DI feed so that the two can be blended into a solid but natural tone. For players that use foot pedal effects in their signal chain, I’ve found that both a DI and a mic on the amp is a great way to go. With just the mic, it can tend to be effects heavy, and with just the DI, there’s none of the intended effects.
Around In Circles
With keyboards, I prefer passive DIs. If the player has multiple units, I may deploy a rack-mounted DI that places four devices in 1RU. Most manufacturers make multi-channel units, and they help cut down on stage clutter. The exception to my “DI on keys” approach is a Hammond B-3 with Leslie speaker or other rotating speaker instrument.
While there are many different models of Leslie speakers, past and present, the most common one is the model 122. It’s in a wooden cabinet that has slots around the front and sides to let the sound out, but most players remove the backs of the upper and lower rotator sections to let even more out.
Usually I deploy three mics on a Leslie cabinet – two condensers for the top rotor and a larger diaphragm dynamic for the bass rotor. The dynamic resides about 12 inches away from the bass rotator, with the condensers for the top at opposite corners at the rear. Wide panning of the top mics can deliver a rich, full sound. However, if console channel space is at a premium, a single mic can be used on the top horns, placed about 12 inches away from the rotors.
There are some modern versions that consist of real rotating horns for the tops, but use a stationary speaker(s) for the lows, with digital effects to recreate the bass chorale and tremolo sounds. In these situations, I deploy two mics for the real rotating horns as with a 122 Leslie, but then position a large diaphragm dynamic as I would on any bass speaker.