Active vs. Passive DI’s

A primer on active & passive DI’s from Radial’s website. I really like the JDI and J48 but really hadn’t spent much time studying the differences between them.

Q: What is the difference between Active & Passive?
A: This is somewhat similar to comparing dynamic mics to condensers. Active DI’s have a built-in pre-amp that requires power to run while passive DI’s use a transformer to convert the signal. Both ‘transform’ or convert the instrument’s high-impedance output to a low impedance balanced microphone level. This allows long cable runs without adding unwanted noise and will improve the instrument’s sound quality. The J48 uses phantom power to derive its power source.

Q: Why do people use active direct boxes?
A: Active direct boxes tend to have more ‘reach’ in that they can capture harmonics and dynamics that can be lost with cheap passive direct boxes with steel core transformers. They are also less prone to loading the instrument, which can change the instrument’s tone

Q: Can you explain loading?
A: Years ago, most bass guitars used regular output pick-ups such as those found on Fender basses. Musicians found that connecting the bass ‘thru’ a direct box and then to their amplifier caused the sound to change. This was caused by the added load of the direct box driving the signal to the mixer (and cable) that could be as far as 200 feet away. This would reduce the level going to the amplifier.

Q: Why is loading no longer the main concern?
A: Back in the 1970’s and 1980’s, acoustic guitars did not have the sophisticated pick-ups and built-in pre-amps that are standard today. Electric basses now incorporate high-output pick-ups or have active electronics built in. Keyboards inherently have high output levels. These high output levels are so powerful; loading is no longer the concern. This ‘fix’ has in fact caused a new problem; input distortion or saturation.

Q: What makes the Radial J48 so special?
A: Let’s begin by understanding how active DI’s work: Active direct boxes are in fact signal preamplifiers. They boost the instrument’s signal to a manageable level. This means that active direct boxes require power to run. As such, they must either get their power from batteries or phantom power from the console.

When using batteries, for the direct box to work properly, the batteries must be fresh. As soon as the power is low, the direct box will distort. This is why engineers hate batteries, and prefer phantom power. But phantom has limitations…

Phantom power was originally developed to supply low-current condenser microphones. Back then, no one ever figured that we would have to manage the high dynamic levels of today’s active instruments. We recently tested a Takamine acoustic guitar with built-in pre-amp and found that when pushed to the max, the output peaked at 7 volts. Considering that most DI’s can barely manage 2 to 3 volts, its no wonder guitars often sound harsh in a PA system. Active basses push the DI’s further due to their powerful low frequency content and keyboards (especially digital pianos) are even more demanding.

The Radial J48™ was specifically designed to solve this problem by boosting the internal rail voltage so that these instruments would not be able to overload the input. The J48™ can be hit with as much as 10 volts and still sound great! This is called headroom.

Q: Why do people use passive direct boxes?
A: Passive direct boxes are often chosen as they are ‘plug & play’ easy to use. When equipped with a high quality transformer, they can handle exceptionally high signal levels without harmonic distortion. A good one like the Radial JDI™ will process the signal without introducing artifacts such as phase distortion. This is achieved because the Jensen Transformer we use is extremely well made.