Radial Engineering > Radial Engineering J48

Radial Engineering J48

An active direct box that has been specifically developed to address the problem of limited current and resulting dynamic range associated with 48V phantom power. A unique switch-mode supply steps up the internal rails to allow an input signal of up to 10V without any noticeable distortion.
  • Active 48-Volt phantom powered direct box
  • Minimal power draw with high rail voltage
  • Will not choke when driven hard
  • Low inter-modulation distortion

The Radial J48 is a great sounding active direct box that has been optimized to produce maximum headroom while working within the limited current available from standard 48V phantom power. By employing a unique switching power supply, the J48 is capable of peak transients to 9 volts without choking. The result is lower harmonic distortion, half the inter-modulation distortion and significantly improved dynamics over other DIs.
Features include a -15dB pad, 180º polarity reverse, a unique merge function to mix two signals to mono, an 80Hz high-pass rumble filter, and ground lift. Like all Radial products, the J48 is built to handle the abuse on the road and features a unique welded I-beam construction for years of trouble free performance.

J48 Development & Evolution

Over the past quarter century, sound systems have evolved as have the instruments that are now used. The listener has also evolved. CD quality sound is no longer a luxury, but expected. Only now are we beginning to hear and appreciate the advantages of a good direct box. But defining a good direct box is not just a matter of ease or use. A good DI must provide a wide distortion-free frequency response and the dynamic range to handle today’s powerful instruments. The Radial J48 is an active direct box that has been specifically designed to work around the limited current that is available with 48-Volt phantom power, and do so while delivering the pure and natural sound of the instrument. At Radial, we call it the “Un-Sound”™.
When direct boxes first came about, these were passive devices. They employed a transformer to convert the high impedance instrument level to a low impedance balanced line. Although transformers presented a formidable advantage in that they isolated the source and the destination, good passive DIs were expensive. Knowing that a more affordable DI would sell more readily, manufacturers introduced lower priced passive DIs using inexpensive steel core transformers. These cost much less but the trade off was objectionable sound quality. Inexpensive steel core transformers introduce problems such as group delay (phase distortion) and core saturation (harmonic distortion), which are most prevalent in the lower frequencies.
Another major setback with low quality transformers was an effect called loading. For instance when using a Fender Precision Bass (the typical bass of the day), the bass guitar output would connect to the DI which would then split the signal to the bass amplifier and then drive 200 feet of cable to the mixer. Since these passive DI’s did not boost the signal in any way, the lowly single coil pick-up would have to drive both the amp and mixer. The musicians complained that the sound was thin because the guitar pickups were being ‘loaded down’ with too much work.
The active direct box came about as a solution. Although active DIs no longer provided 100% isolation of the source and the destination (thus opening the door to ground loops) they had two distinct advantages: Active DIs could boost the signal without loading down the pick-up and they could be made for less money. Powering the DI came from batteries and eventually, from the 48V phantom power that was intended to supply condenser microphones.
Here in lies the problem: The standard specification for Phantom power is 48-volts with a mere 5 milliamps of current. This works perfectly fine with the intended application of powering the capacitive plates on a condenser microphone. But with a direct box, the problem is managing the dynamic headroom of today’s powerful active instruments. This relatively new problem has emerged with the advent of active guitars and bases and their built-in electronics.
It must be understood that for the most part, the ‘working model’ for the typical active DI is based around the Fender Precision Bass. Powering is derived from 48V phantom power, as phantom is both convenient and universal. A traditional passive bass such as a Precision, when played hard will develop between 2 and 3 volts peak. In this scenario, phantom power is sufficient to handle these transients. The problem is that today’s active instruments with their 9-Volt batteries and built-in pre-amps will easily produce anywhere from 5 to 8 volts of peak energy.
Those old direct box designs were never intended for this type of input level and the result is predictable: Overload the input and you get distortion. They choke. Bass guitars sound muddy and lack punch while acoustic guitars sound scratchy and have no body. Digital keyboards and in particular piano samplers produce huge harmonics and extreme dynamics, which unfortunately get lost in the mix.
The excessive instrument level saturates the direct box. The DI’s power supply is unable to manage the input level, and this causes overload distortion. The sound is compressed due to square wave distortion and the end result is lack of definition and natural dynamics.
The Radial J48 is designed specifically to solve the problem of 48V phantom’s limited current by increasing the internal rail voltage and signal handling capacity of the direct box so that it can handle these extreme transients without choking.
The J48 incorporates a digital switching supply; much the same as today’s lightweight high-current power amplifiers. One can ‘hit’ the J48 with as much as 9-Volts and it will cruise right along without a whimper. It took 3 years to develop the J48, but was worth the wait!

J48 Features & Functions

The single most important development goal of the Radial J48 was increasing the headroom without compromising sound quality. To this end, every possible effort was made to deploy the limited energy resources of phantom power to the audio signal path.
To begin, phantom power is converted using a super-efficient digital switch-mode power supply that steps up the internal rail voltage to an impressive 10 volts. This allows one to hit the J48 with as much as 9-Volts without choking! To further increase available horsepower, all non-essential devices such a power-hungry LEDs are turned off when the J48 is in use. A typical LED requires almost as much current as does the complete audio circuit!

  • Welded I-beam construction makes it impossible to torque the PC board. This virtually eliminates cold solder joints - the No.1 cause of system failure in the field.
  • An innovative ‘slide-in-place’ XLR trace makes it possible to use I-Beam girder construction.
  • A digital switching power supply steps-up the internal rail voltage to 9-Volts. The J48 will not choke like other DI’s when hit with large transients.
  • A military-grade PC board is employed for maximum wear and tear in abusive environments. Welded steel standoffs ensure the PC board will not shift.
Open up the Radial J48 and you will see huge, over-sized capacitors that provide ample current storage to assure clean, unobstructed power for the signal. With added headroom comes improved performance in all areas: Less than half the phase distortion of competing DIs, significantly lower harmonic distortion and single-digit inter-modulation distortion are all realized. Paying attention to these basic details has the net result of better sound.
But great sound is not the only innovation: The Radial J48 introduces several new functions that improve functionality and open the doors for greater creativity. The input panel features the usual ¼” instrument input and thru-put connections. An innovative Merge function converts the thru-put into a second input to sum stereo sources to mono such as stereo keyboards or CD players when mixer input channels may be of short supply. A -15dB pad may also be inserted when confronted with abnormally high signals.
To reduce low-end rumble and run-away acoustic resonance, an 80Hz high-pass roll-off filter is provided. As with any power system whether it is acoustic or electrical in nature, bass always contains significantly more ‘RMS power’ than high frequencies. This is why low-frequency drivers require anywhere from 10 to 20 times more power in PA system designs than high-frequency tweeters. By gently removing unnecessary lows, the J48’s dynamic handling is increased by a full 3dB or twice the power handling. This innovative feature both reduces mud and increases headroom at the same time!
The filter switch also performs a second function. Depressing the switch will momentarily illuminate the front panel LED to advise if 48V phantom power is present. The LED does not stay illuminated as it would draw energy away from the audio circuit and reduce headroom.
  • A -15dB pad allows super hot inputs such as that from a CD player to be connected and balanced for long runs.
  • Momentary 48V Check LED will flash once when the 80Hz filter is depressed to tell you phantom power is present.
  • The Merge function turns the input & thru into ‘left & right’ inputs to mono mix at the XLR output. This handy function saves valuable inputs on the mixer!
  • The input is connected to the instrument.
  • The thru-put is connected to the instrument amplifier. The JDI has a high input impedance to reduce loading.
  • Our book-end design creates a protective zone around the switches & connectors for best protection!
  • A heavy-duty baked enamel finish for excellent durability and great looks for years! 100% 14-gauge steel construction.
  • The 600 Ohm balanced XLR out is mic level to properly match concert splitters - will drive 1000 feet without appreciable noise.
  • A polarity reverse switch flips the polarity from the AES standard Pin-2 to Pin-3 on the XLR and is used when combining sounds that may be out of phase.
  • An 80Hz low-cut (high-pass) filter gently reduces excessive bass for more headroom and clarity. Eliminates run-away resonant feedback from acoustics!
  • The ground-lift disconnects the ground path to the transformer at the XLR output. This is used when a ground path is made at the input side such as with a keyboard.
  • A no-slip full bottom pad reduces mechanical vibration, improves electrical isolation and keeps the JDI from moving around on busy stages!

The output panel contains the balanced 3-pin XLR wired ‘pin-2 hot’ following the AES standard along with a polarity reverse switch that toggles pin-2 to pin-3 ‘hot’. This allows the engineer to match older equipment and ads a dimension of creative freedom. A ground lift switch disconnects the XLR pin-1 thus grounding the J48 to the input side.


As with all Radial direct boxes, the Radial J48 is designed for use in abusive environments such as on fast-paced stages of concert touring. An innovative book-end design provides a protective zone around the switches and connectors that makes it practically impossible to damage these components.
The inner skeletal frame features a unique 14-gauge welded I-beam construction that makes it impossible to torque the PC board. Stress on the printed circuit board can introduce intermittent solder joints, which by all accounts is the primary cause of direct box failure.
An innovative full-bottom no-slip rubberized pad works double duty by assuring the J48 won’t slip and slide around when placed on an amplifier. The protective pad also ads an element of electrical shock resistance by providing a mechanical shield from guitar amp handles that are often directly coupled to the amp chassis.

Using the JDV

The Radial J48 is a pleasure to use. It is ‘plug & play’ easy and is perfectly adept for studio as well as live concert sound. Connect the instrument to the ¼” input, the instrument amplifier to the ¼” output and the mixer to the XLR. Lift the ground if you encounter hum or buzz in the system.
The polarity reverse can sometimes help when looking for a ‘sweat spot’ and is always worth trying. In the studio, when combining a mic’ed signal with a direct feed, reversing the polarity can be huge!
Using the 80Hz roll-off filter is generally recommended for acoustic guitars as this will reduce the boxy sound that is often associated. It will also lessen run-away resonant feedback that can creep-up, especially as systems are being run loud. Depressing the filter switch will momentarily provide ‘48V power status’ which of course is a good idea before turning up sound system volume levels.
Another innovative function on the J48 is the merge switch. This resistive mixer allows a stereo source such as a stereo keyboard to me merged or mixed mono at the XLR output. This handy feature makes life easy for the engineer when confronted with more instruments than available channels. For the keyboard player, reprogramming stereo-panned outputs is not required. Just plug the stereo source in, hit the merge and you are ready to go.
As an added bonus, the Radial J48 and the Radial JDI can be mounted into a 19” rack device called the J-rack. This ‘inventory management device’ allows the same DI inventory to be allocated to individual instruments or be rack-mounted for keyboard set-ups as needs arise. There’s nothing more frustrating than having to waste a rack-mount DI when a single unit is required and nothing more irritating and unprofessional than a pile of DIs messing up a stage.


The Radial J48 is a great sounding active direct box that is specifically designed to work with today’s powerful active instruments. It is unique in that it provides a real-world solution to the limited headroom caused by inadequate current from 48V phantom powering. The expanded headroom has the direct advantage of increased dynamic range and the bi-product of reduced harmonic, phase and inter-modulation distortions. Plug and play easy to use, the Radial J48 is ideally suited for studio, broadcast and stage use where high quality sound is required.


User Manual


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