The original JD4 rackmount DI.


Sidebar: Bandwidth response
                and distortion

A direct box is very similar to a microphone in that it must be able to handle all frequencies equally well. Many folks believe that since DIs are ‘electronic’ or hard-wired, they will simply work. This is a fallacy.

 A good direct box must be engineered to perform well. During the development process, the electronic designer must make decisions on parts and costs based on who the intended client is and the level of performance required. For instance, the home recording enthusiast does not require the same durability or sonic performance as the professional.

 In audio systems, the trouble is in the bass. Consider these facts: Elephants are large – mic are small, 15” speakers require upwards to 20 times more power than the high-frequency counterpart, stand outside a night-club and all you hear is bass thump. To make matters worse, when two signals are ‘out of phase’, it is the bass that suffers.

This is no different at the circuit design level. When one uses large transformers and over-sized capacitors, bass handling generally improves. For instance when comparing the Radial JDI or JD6 to others, most companies do not even specify their performance at 20Hz. Why? Because it is not easy to make a device work correctly throughout the 20Hz-20kHz bandwidth. Our phase distortion at 20Hz is a mere 4º off while our nearest competitor is close to 20º out of phase. Phase distortion causes lows to sound muddy and the mid-band to loose definition while the shifted highs, which by the way provide the brain with localization, only cause us to get confused.

 Of course there is linear distortion or the ability for the device to transfer all frequencies in a uniform manner. Most direct boxes roll-off the lows and highs while the JD6 is designed to work from 10Hz to 40kHz, thus ensuring the 20Hz to 20kHz is perfectly linear.

 Finally, harmonic distortion, usually specified at 1kHz, does not tell you anything about how well the device will work when subjected to hugely dynamic and complex piano samples. At 20Hz, the Radial JD6 shines in at 0.06% distortion or 1/5th that of our nearest rival. This gives the pro confidence that low frequencies will be handled gracefully which oddly enough makes mixing easier.
 

JD•6 Development and Evolution

The Radial JD6 circuit design stems from the original JDI single channel direct box, which was developed by Radial in association with the Jensen® transformer engineering team. Over the years, we received many requests from touring sound engineers for a rack-mount high-density version of the JDI for keyboards and electronic drum machines. This resulted in our developing the Radial JD4 (see review) which became standard equipment with bands as diverse as Journey, Styx, Garbage, Nine Inch Nails, kd lang, and Linken Park among others.

The irrefutable advantage of transformer isolating the source instrument from the destination mixer weighs in as ‘top priority’ for touring engineers by providing exceptional common mode noise rejection, sometimes referred to as ground-loop prevention against noise, hum and system buzz.

The Radial JD4 heralded in several new and innovative features, some of which had never been seen on a DI before including:
- Merge function to convert the input and thru-put to a mono mix.
- High frequency filter for interfacing with vintage equipment.
- Polarity reversal on each channel for phase matching the inputs   when combining with pre-AES standard gear.
- Front and rear jacks with front panel priority for quick on-the-fly   system reconfiguration.

With the introduction of the new Radial JD6, all of these features have been retained while adding several new innovations. It is important to note that these innovations have come from discussions with ‘real world’ engineers and keyboard technicians that design and configure concert-touring systems. In developing the JD6 we have also tried to incorporate features that are important for the ever-increasing integration of computers and video as part of a total system.

Notable changes include six channels in the same single rack space, two of which have ‘Swiss Army Knife’ functionality to provide the engineer with greater hook-up flexibility. These include front panel priority switching jacks, selectable RCA jacks, merge function, high-frequency roll-off filter, along with standard features shared across all channels such as -15dB pad, ground lift and 180º polarity reverse.

The Radial JD6 sets the bar a step higher by introducing a series of internal ground switches that allow each channel to either float (factory default) or be grounded to the chassis. For system engineers that prefer to ground devices from a single point, a Channel-1 chassis ground switch is provided. This switch is recessed to prevent accidental use and is actuated using a ‘tweaker’ (like a pen). We have even added a rear-panel ground lug, should a rack ground be preferred.

Best of all, in designing the JD6, we have been able to significantly reduce our manufacturing assembly time and metal costs over what they were with the JD4 to a point where we can now offer two extra Jensen Transformer equipped channels at around the same price as the old JD4! And what could be better than six channels for the price of four, all in the same physical size?

 
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