The following article first appeared in the January 2011 edition of Live Sound International.
What type of direct box works best for bass guitar? The answer is easy: it depends. In fact, more than anything else, it depends on the type of bass that the DI is going to be used with.
When it comes to signal flow, there are two types of bass guitars: passive and active. The first electric basses, i.e., the original Fender Precision, were passive, and in fact still are today.
They employed magnetic pickups to generate the signal – as the string moves in and out of the magnetic field, a low-level alternating current is generated.
The signal from the bass travels through the cable to the amplifier, which in turn increases the voltage level so that it is sufficiently powerful to drive another electromagnetic device: a loudspeaker. In essence, the signal is amplified by a series of buffers that work together to increase the voltage and/or current as needed.
For years this worked well, until bands like the Beatles messed everything up! The problem was that the fans at those concerts were so loud that the bass amp was unable to produce enough ‘thump’ to overtake the screaming. The solution: send the bass guitar signal through the PA system.
Eureka! The amazing direct box was born. The first direct boxes were basically hand-made black boxes that had transformers inside.
These passive devices would tap a signal off the bass and split it so that part of the sound would go to the bass amp on stage, and the rest of it would go to the PA system some 50 to 100 feet away.
As the PA systems got larger, so did the performance venues (or vice versa). Eventually, things escalated to the point where concerts moved to arenas and stadiums.
And bass players complained because they noticed that when their bass was connected to all of the long cable runs in these larger systems, the sound changed. It was not as beefy, and there was no more thud.
This shouldn’t have come as a surprise—if you take the signal from a magnetic pickup and ask it to drive hundreds of feet of cable in addition to the bass amp on stage, the level will be weaker. And it will not sound the same. This effect is known today as “loading.”
The solution: buffer the bass signal. In other words, incorporate a small amplifier inside the direct box so that 99 percent of the signal is directed to the bass amp and 1 percent is split off to drive the PA. And thus the active DI box was born! Ye old Fender P-Bass was happy—the thud had returned.
So for the next bunch of years, everything worked just ducky, until one day, some guy decided to put a 9-volt battery inside the bass and buffer the signal.
Now all of the sudden, instead of the bass producing around 1 volt, the battery powered preamp inside the bass was kicking out 5 to 7 volts.
Then the CEO of the Acme Bass Company had a revelation: “We can do even better—let’s put in a second battery!”
A modern 6-string bass could now deliver a whopping 18 volts of mayhem, and bass players rejoiced. They could overload the front end of “ye old SVT” and finally out-blast that pesky lead guitarist and his lowly Marshall!
All good, except for one problem: that 18-volt output now overloads the direct box, resulting in a distorted, muddy, no-punch sound in the PA system. Or, if you prefer, it just plain sounds bad.
The solution? Dust off the old passive direct box, connect it up and bingo, great tone – the thud is back.
Here’s the deal. Early active direct boxes were powered by batteries and in fact, some still are. But the problem with batteries is that they go dead… usually right in the middle of the second set.
So some years ago, DI manufacturers started to use phantom power as a means to supply the needed voltage and current to the active DI box (buffering amplifier).
But phantom power, invented by Dr. Neumann as a means to supply a polarizing voltage to his condenser microphones, was never intended to be a power source for an amplifier. And without current, you do not get headroom.
Think of a bass playing through a miniature guitar amp – turn it up, and it distorts like crazy. DI boxes do exactly the same. Without headroom, high-output bass signals will cause the buffering amplifier in the DI to distort.
But remember, back then, basses were all passive for the most part, so they worked fine with regular phantom power as the buffers only had to process 1 to 3 volts. The advent of active basses with their huge output levels changed the rules.
So the rule of thumb is that for a high-output bass that already has a built-in buffer, a passive direct box will likely do a great job—the bass will produce the drive. On the other hand, for a low-output passive bass, an active DI will leave the bass sound unaffected while generating the drive for the PA system.
Keep in mind that the sound quality of DI boxes depends on the circuit design and parts that are being used. Better designs focus on eliminating all types of “bad” distortion such as harmonic, phase and inter-modulation distortion. These designs are then categorized into two groups.
Some direct boxes are designed to transfer the signal without artifact or distortion so that the original sound of the bass is delivered as purely and naturally as possible, while others, such as tube DI boxes like the Radial Firefly™, tend to be designed to “color” the sound with “good” distortion to create new bass tones and exciting textures. Both are useful, depending on the desired outcome.