| Here is a very nice discussion of the active versus passive DI issues (the J48 and JDI are top of the heap in each of these categories IMO), posted by Genz Benz engineer Agedhorse (in a recent thread discussing the general issues of active versus passive DI's.
Either can work very nicely. If you have a passive bass, in general, the active J48 has the edge. If you have an active bass (i.e., a bass with either active pickups and/or an on-board preamp), in general a passive DI like the JDI has some advantages.
*************
Here's a few rule of thumb tidbits that affect DI use and how to get better performance from such...
1. Generally, a passive bass will benefit from an active DI. The reason is the output impedance of a passive pickup is fairly highy (anywhere frfom 2k to 10k) and when combined with the typical input impedance of a passive DI (from 10k to 50k) can result in loading down of the pickups. The practical results of this are lower output from the instrument, and since a transformer is not a pure resistive load (there's a lot of inductance) this will interact with the pickups leading to a different frequency response. An active DI has a much higher input impedance which results in less interaction (for better or for worse). A passive DI works by transforming the impedance, along with this transform the voltage steps down and the current steps up in accordance to Ohm's law.
2. Still, a passive DI amy work with a pasive bass juust fine and MAY even product a "better" sound for some players in some situations. Definately worth a try before dismissing it outright.
3. An active bass (most) have low output impedances, and also have the "same" active electronics driving the output that an active DI has. An active DI typically has active electronics driving the primary winding of a passive transformer, often the same transformer that's in a passive DI. So an active DI will usually drive a passive DI with no problems.
4. An active bass with an active DI will work ok BUT you pay a penalty in noise floor because you have an added stage of electronics in line and the high impedance input is going to be a little noisier. There's also greater potential for RFI noise. Generally not my favorite way to go, but as I am famous for saying... "it depends".
5. An active DI that's designed into a quality amp may be one of two different topologies. Both types are designed knowing the internal grounding scheme of their associated amps, and generally work quite well driving long lines with a minimum loss of fidelity.
6. The best isolation is with a passive DI. All phantom powered active DI's, by the nature of being powered by the console, suffer a bit from isolation. Fully isolating means that phantom power is lost. The exception to this is the battery powered option of the Countryman 85, which is one reason this DI is a favorite (if that's your flavor of course).
__________________
Engineer: Genz Benz | 
