JDX™ Reactor™ Development

Not long ago, we received an email from a staff music writer and producer for a major broadcaster. He said: "Since I got the Radial JDX, I have not mic'd an amplifier in over a year. I get great tone, it is amazingly consistent and it lets me get the job done quickly." If you think about it, that's exactly what a good tool should do: make it easier to get the job done.

The original Radial JDX was designed for live sound. Our intent was to deliver a good sounding 'photograph' of what one could expect when placing a Shure SM57 in front of a Marshall 4x12 cabinet. And when we say photograph, what we mean is that the sound of a guitar on stage is more like a video with 30 plus frames per second while a microphone or the JDX is more of a single frame or static photo. Think about it… as you move around the stage, the sound will change depending on where you are standing. This is because each frequency within the complex waveform that creates your tone has a different wavelength and depending on where you are standing these frequencies will either amplify each other or cancel each other out. When you move the sound changes as it arrives at your ears. With a static source such as a microphone or a JDX, the sound no longer changes like a movie… it becomes a snapshot in time.

Comparing the JDX to a Microphone
When mic'ing an amp, the position of the microphone is critical: engineers know that by moving the mic by as little as 2.5cm (1"), the tone can change dramatically. Where the microphone and JDX differ is that the microphone's sound is subject to change depending on where it is placed on the speaker cone and how far away it is from the grill cloth. The mic introduces variability. The JDX is pre-set. In other words it has 'one' sound derived from many listening tests.

Is the JDX perfect? It sounds good, but this in no way implies that it is perfect… it is merely a snapshot of a sound just like the microphone. Only with the JDX, you get the same sound each time you use it. This, in fact is the advantage… consistency!

Creating the Sound
There are in fact two processes involved in creating the JDX sound. The first is capturing the 'feel' of the amp using a reactive load. The second, filtering the sound so that it ends up delivering convincing results.

Early guitar amp direct boxes would merely tap a signal off the amp or speaker cabinet. Devices like the Red Box merely filter the sound while others employ various techniques to try to replicate the effect of the loudspeaker. One example is Palmer which uses a light bulb inside their box to create a load.

When developing the JDX we asked ourselves a simple question: "Why try to emulate the effect of a loudspeaker using a light-bulb? Light bulbs do not work like a loudspeaker; they only absorb energy while a loudspeaker also generates a back electro-magnetic impulse. Instead… why not simply capture the effect?" This is how we came up with the idea of using a reactive load.

To understand a reactive load, one merely needs to take a direct feed from the back of a guitar amp and listen to it. The sound is OK, but it pales in comparison to the sound of the combined guitar amp head and cabinet. This is because speaker cabinet plays a huge part in delivering the tone. A term used in amplifier design is damping factor. The damping factor is basically the amp's ability to control the loudspeakers excursion. When the speaker is fully extended out and pushes back to its resting position, the loudspeaker generates a back electro-magnetic impulse. By positioning the JDX in between the head and the cabinet, the JDX captures both the sound of the head and the back electromagnetic impulse via a transformer.

But this is not enough. The typical 12" loudspeaker in a guitar amp concentrates the sound in the mid range. There is no tweeter. The speaker cabinet actually extends the frequency response down into the bass region to create the full sound. To make this work, the guitar amp tone is optimized to sound good through a speaker cabinet. So when you take this signal 'direct' from the amp and run it through your studio monitors or a PA system, it usually sounds like a 'swarm of bees'. Ugly, buzzy… unusable. To solve the problem, we incorporate a sophisticated multi-stage filter that is designed to replicate the tone of a 4x12 half stack. It is the combination of a reactive load and advanced filtering that creates the JDX sound.

Today, the JDX is used by all kinds of bands including Slayer, Dimu Borgir, Racal Flats and many others - mostly on guitars. But recently, bands such as Chickenfoot (Michael Anthony ex-Van Halen) have been using the JDX on bass. And it sounds amazing. The low-mid clarity is astonishing which seems to translate well without the usual mud that makes bass notes indistinguishable. This got us thinking… how can we improve upon the JDX?

And with so many top artists now enjoying the sound of the JDX, we felt we should take advantage of our great sounding cabinet emulation filters to provide the engineer with the option to record direct without having to necessarily use an amp each time a guitar track is needed. With the JDX 500, you can now connect your guitar to a distortion pedal and then go direct into the JDX before feeding the recording system. You now have the perfect guitar interface to help speed music production for film and other time-pressure environments.

For greater tonal control, we then added a high frequency roll-off filter that would enable the engineer to tame overly bright amps like a Marshall. By smoothing out the high end, you will find that the tone will often sit better in the mix. We then equipped the JDX with a ground lift switch to help eliminate hum & buzz caused by ground loops and a 180º polarity reverse to help phase align the JDX if it is being used in conjunction with a microphone.

For even more fun, try combining the JDX with a Phazer. Just be careful… this can be addictive!