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Most churches are constantly pursuing higher-quality live sound. With this in mind, we were eager to learn more about a new stereo version of Radial Engineering's J48 active DI, widely used in pro touring, studio and other professional applications.
DI Basics
DI boxes provide three basic functions. First, they convert unbalanced signals from sources such as instrument pick¬ups and electronic instruments into balanced signals that can travel longer distances without induced interference or signal degradation. Second, they help with impedance-matching, especially from high-impedance sources like passive guitar pickups being fed into low-impedance mic inputs on the mixing console. Third, while performing the above electronic functions, DIs act as an interface to change from one connector type to another, typically from 1/4-inch to XLR.
DI boxes are made in both passive and active formats and each has its primary uses and advantages. Basically, a passive design requires no external power to function and its internal audio-quality transformer performs the conversion functions. An active DI requires power from a phantom power source and/or from a battery; electronic circuitry is used for the signal balancing and impedance matching functions. Passive, and some active DIs (such as the J48), have an audio-quality transformer within the circuitry and will break a ground loop so the hum won't enter the audio system.
Active DIs have built-in preamp or buffer circuitry to drive the signal–useful for adding gain to low-level passive instrument pickups and similar sources. Though the rule is flexible, audio engineers typically use a passive DI with an active source, and an active DI with a passive source.
Features of the J48 Stereo DI
"The J48 does its job so well that it’s wonderfully unnoticeable."
Gary Parks
Reviewer
The Radial J48 Stereo is an attractive and ruggedly built unit with a 14-gauge steel chassis and weight of 1.5 pounds. Details such as the metalwork and a very durable finish are not overlooked. A rubber pad is securely attached to the bottom, isolating it from the stage and making it non-slip. The connectors and switches are labeled on both the top panel for basic orientation and on the side panels to positively identify each connection or function. The lettering is fine and readable with enough contrast to be read in low light.
While some manufacturers produce active DIs that are battery-powered, Radial chose not to use batteries. There are certain potential limitations that battery power poses to the maximum signal level the DI can handle, as well as the fact that batteries deliver less voltage as they're used up and can run out of juice in the middle of a show. The DI is virtually plug-and-play, deriving its power from the console's 48-V phantom power via the XLR connection. A green LED on the input panel shows when it is powered and active.
In situations where the switchable ground lift must be used to break a ground loop, Radial uses an innovative circuit that maintains the phantom power while eliminating the hum.
The J48 Stereo is about the same size as many monaural DIs, yet provides two channels. The channels function independently, each featuring a 1/4-inch input with a 220,000 Ohm impedance and a balanced, 600-ohm XLR output. The inputs have a 1/4-inch loop-through jack, so the instrument output can be sent to both the house console and to amplifiers, monitors or other on-stage devices. To compensate for input signals that are too high in level, a 15-dB pad that affects both channels can be engaged. A high-pass filter switch with an 80-Hz knee can be activated to eliminate lower frequency stage noise from the signal. A recessed polarity reversal switch that affects only the right channel is also included.
Stereo Advantages
With the J48 Stereo DI, a performer can route a pair of acoustic guitars, a mandolin and violin, an acoustic and electric bass, or some other combination of instruments through one device, while completely isolating one from the other. A keyboard or other two-channel device can also be routed to the console. This capability saves room and eliminates confusion on stage.
In testing the channel separation, I found no discernable crosstalk between the two channels. I put an acoustic guitar in each input channel and sent out the XLR connectors to two mixer channels. Muting the mixer channel for the guitar connected to the left channel and auditioning through headphones, I cranked up the mixer trim and gain for the other channel while playing that guitar. Not a bit of bleed-through. Reversing the process, I had the same result.
Other Observations
The J48 Stereo DI is very transparent with an uncolored frequency response and no detectable distortion (0.002 percent at -5 dBu). And it's quiet, with a noise floor of -99 dB below 0 dBu. Working with it so far, the J48 does its job so well that it's wonderfully unnoticeable.
The Bottom Line
The quality of the switches, connec¬tors, electronic components and circuit boards directly affect how well the DI performs its functions, how long it lasts and its immunity to induced noise. In most cases you get what you pay for, and since the cost of even a relatively expen¬sive DI is inconsequential compared to the price of a good instrument or mixing console, investing in quality is wise.
The J48 Stereo Active DI from Radial Engineering is a professional quality audio device, both in appearance and performance. The stereo capability in a DI with the same approximate footprint as a single-channel unit saves space and makes it more convenient for a multi-instrumentalist to plug in. It's well-labeled, easy to power and use, has the necessary switches to meet a variety of applications and is audibly neutral. And it's durable enough to last for many years. The J48 Stereo DI has a list price of $330.
