Radial J48 Active Direct Box User Guide

Introduction

Congratulations on your purchase of the Radial J48 direct box. The J48 is a unique direct box designed to handle the new generation of battery-equipped acoustic guitars and basses that often overload typical direct boxes and cause distortion. The J48 achieves this without the use of batteries.

The J48's internal digital switching power supply converts 48V phantom power to produce a higher internal rail voltage, allowing it to handle transients of up to 9 volts without choking. This results in more dynamic headroom and less square wave distortion. Every aspect of the J48 design is optimized for maximum headroom while introducing minimal phase, harmonic, and intermodulation distortion.

Additionally, the Radial J48 features a transformer that isolates 48V phantom power, helping to eliminate ground loops and remove system noise. This ensures that the signal remains pure and unaltered, preserving the natural tone of the instrument.

We recommend reading this manual before using your Radial J48 to maximize its potential. For further details and FAQ updates, please visit www.radialeng.com. Comments, questions, or suggestions can be sent to info@radialeng.com.

Input and Output Panel Feature Set

The J48 features a comprehensive set of controls and connections:

• 1. -15dB Pad Switch: A provisional -15dB pad reduces the input signal to the circuit, ensuring a clean and distortion-free signal.
• 2. 48V Check LED: Used in conjunction with the low-cut filter switch to check for the presence of phantom power.
• 3. Merge Switch: A unique 'merge' function that mixes the input and thru-put connectors into a pair of left-right inputs with a mono output at the XLR.
• 4. 1/4" Input and Thru-put Jacks: These are used to connect the instrument to the direct box and the direct box to the instrument amplifier.
• 5. Protective Bookend Cover: An innovative cover that provides protection to switches and connectors against damage during hectic stage set-ups.
• 6. XLR Male Output: This connector is wired to the AES standard (pin-1 ground, pin-2 hot) and outputs a 600 Ohm mic level signal.
• 7. Polarity Reverse 180° Switch: Toggles the XLR pin-2 and pin-3 to interface with older equipment or to avoid phase cancellation when combining multiple sources (e.g., mic and DI).
• 8. Low-Cut Filter Switch (-6dB down at 80Hz): Gently rolls off bass frequencies to reduce rumble and acoustic resonance. It also serves as a dual-function switch to check for 48V phantom power.
• 9. Ground-Lift Switch: Breaks the ground connection between the input and output to eliminate hum and noise caused by ground loops.
• 10. Full Bottom No-Slip Pad: Provides mechanical isolation to reduce slipping and electrical isolation from amplifier frames and handles.

Internal Components

Key internal features contributing to the J48's performance and durability:

• 11. Welded I-Beam Construction: 14-gauge steel welded I-beam construction ensures maximum durability and a lifetime of trouble-free performance.
• 12. Silent, High Cycle Switches: Super high repetition switches withstand heavy use. The 'make before break' design eliminates pops and clicks when toggling switch positions.
• 13. Double-Sided Circuit Board: Military-grade double-sided PC board provides maximum durability and resistance against part failure.
• 14. Power Supply Transformer: The transformer in the power supply helps eliminate ground loops by isolating the power coming in from the XLR's 48V phantom feed.
• 15. Digital DC to DC Switching Power Supply: Steps up phantom power to produce a higher internal rail voltage, providing more headroom and resulting in less distortion and a more natural sound.
• 16. Oversized Capacitors: Oversized capacitors contribute to maximum headroom.
• 17. Baked-on Enamel Finish: Provides a durable and attractive finish suitable for stage or studio use.

The J48 Quick Start

This section provides quick instructions for users familiar with direct boxes. For new users, reading the full manual is recommended.

Caution: Always ensure power is off or audio levels are set to zero when connecting or disconnecting audio devices to prevent noise or speaker damage, especially with phantom-powered equipment.

The Radial J48 is an active direct box requiring standard 48V phantom power from a console or pre-amp. Ensure phantom power is turned on. You can check for phantom power by depressing the low-cut filter switch; the LED on the front panel should illuminate briefly if phantom power is present.

Set all switches to the 'out' position. Plug your instrument into the INPUT jack. Connect the THRU jack to your guitar amplifier or keyboard mixer. Connect the XLR OUTPUT to the PA or recording mixing console.

With the PAD set to 0dB (outward position), slowly turn up the input level at the mixer. The J48 can handle most instrument levels. For exceptionally high signal levels (e.g., digital pianos, high-output basses), engage the -15dB pad to reduce input sensitivity for a clean signal. If distortion is heard, engage the pad. The low-cut filter (-6dB at 80Hz) reduces low-frequency mud and acoustic resonance, also increasing dynamic range when engaged.

The J48's transformer isolates the input and output electrical connection, eliminating buzz and hum from ground loops. For persistent 60-cycle hum or noise, a different ground potential might be the cause; depressing the ground lift switch can resolve this.

The J48 also features a polarity reverse switch, which toggles XLR pin-2 and pin-3. This is useful for interfacing with older equipment or aligning phase when combining a mic'd and direct signal from the same instrument.

You are now ready to use the J48, renowned for its musicality.

Direct Box Basics

What is a direct box anyway? The term 'direct box' (or DI) comes from 'direct insert box'. It means inserting a signal directly from the source to a mix point or recorder. Radial direct boxes are designed to reproduce the most accurate representation of the source signal.

Direct boxes perform key functions: impedance matching and signal balancing. They transform a high-impedance instrument signal (like from a guitar or keyboard) into a low-impedance balanced microphone-level signal suitable for a sound system. Balanced signals inherently cancel noise and allow for very long cable runs, making them standard in professional audio.

Diagram Description: A diagram labeled "Typical DI box setup" illustrates a guitar connected to the J48. The J48's THRU output is shown connected to a guitar amplifier. The J48's XLR output is connected to a mixing console and a tape machine, demonstrating how the signal is split and processed.

Getting to Know Your Radial J48

Connection Procedure: Typically, the instrument plugs into the IN jack, the musician's amplifier connects to the THRU jack, and the mixer or recorder connects via the XLR OUTPUT. The XLR output captures the signal before it reaches the musician's amplifier, allowing the mixing engineer to obtain the purest source signal, which may differ from what sounds best on stage.

Sending an unaltered signal to the front-of-house (FOH) mix position enables the engineer to achieve the desired sound with minimal distortion. This is a key reason for Radial direct boxes' popularity, as they do not alter the sound before it reaches the mix position.

High to Low Impedance Conversion and Balancing: The Radial J48 features a 220k Ohm input impedance for instruments. As the signal passes through the J48 to the XLR output, it is converted to a balanced 600 Ohm low-impedance signal. This enables long cable runs without introducing noise. The J48's output is mic level, allowing it to be used alongside microphone signals without cross-talk or saturation issues.

Phantom Powering: The J48 is an active direct box employing a buffer circuit. It derives its power from the 48V phantom power supply found on most mixers and pre-amps. Phantom power is a 48V DC supply transmitted over the XLR cable, originally designed for condenser microphones. Unlike early direct boxes that used batteries (which could distort when low), phantom power provides a stable, remote power source.

Dynamic Headroom

Early active direct boxes were designed for instruments like passive Fender Basses, which produced low output voltages (1-2 peak volts). Modern instruments, especially those with active pre-amps or high-output pickups, can generate voltages as high as 7 volts, leading to overload in typical direct boxes.

The Radial J48 addresses this by using a digital switching power supply to raise its internal rail voltage from a typical 3 volts to 9 volts, similar to high-output power amplifiers. This increased headroom allows the J48 to handle transients without overloading, preventing square wave distortion. Larger internal capacitors further reduce harmonic and inter-modulation distortion, resulting in a less stressed and more musical sound.

Buzz and Hum Caused by Ground Loops: System noise, particularly buzz and hum, is often caused by ground loops. These occur when two ground paths with different voltage potentials interact, such as when connecting a keyboard powered by AC to a mixer. While lifting the ground can help, it's not always sufficient. The Radial J48's isolation transformer in the power supply helps eliminate most of this noise. In severe cases, engaging the ground lift switch can break the internal ground path between the input and output.

Plug and Play Easy: The J48 is designed to be sophisticated yet easy to use. Understanding its design allows users to better utilize its features.

The J48 Features and Functions

1/4" Input and Thru-put Connectors: The J48 features two 1/4" jacks. The input jack accepts signals from instruments like basses, guitars, and keyboards. The left 1/4" jack is the primary instrument input, while the right 1/4" jack serves as a parallel 'thru' connector, feeding the instrument signal directly to the musician's on-stage amplifier.

Diagram Description (Fig. A): Illustrates the J48 used with an on-stage instrument amplifier. The instrument connects to the J48's input. The J48's THRU output connects to the amplifier, and its XLR output connects to the PA system. This setup allows the musician to manage their stage sound while the audio engineer processes the signal for the venue.

For instruments not requiring on-stage amplification (e.g., acoustic guitars), the J48 can be connected directly to the PA system using only the balanced XLR output (Fig. B). This sends the signal to stage snakes, monitor mixers, and the main PA, similar to microphone signals. In this scenario, the thru jack is not used.

Diagram Description (Fig. B): Shows the J48 used with an acoustic instrument connected directly to an audio system. The instrument connects to the J48's input, and the XLR output goes to the mixing console.

High Input Impedance: The J48 presents a high input impedance to the instrument, minimizing pickup loading. This ensures that low-output instruments sound as intended when connected to an amplifier. The J48 taps only a small signal, sending it through a buffer to the PA system. While loading is less of an issue with modern high-output instruments, overload distortion due to limited dynamic range remains a concern.

Input Distortion: Modern instruments with active pre-amplifiers can output high signal levels (up to 7 volts), which can distort the input of many active direct boxes, causing harsh square wave distortion. Even with phantom power, the limited current available in some direct boxes prevents them from producing sufficient rail voltage to handle these signals. This can result in instruments sounding 'thin' or 'scratchy'. The J48 is designed to handle these high signal levels without overloading, preventing distortion that cannot be fixed in the mix.

Input Distortion and Using the -15dB Pad: For devices with extremely high output levels that might overdrive the J48's input circuit, the -15dB pad can be engaged. This reduces input sensitivity for a clean, distortion-free signal. Devices like digital pianos, samplers, electronic drums, CD players, or headphone outputs from MP3 players may benefit from using the pad.

The Merge Switch: The J48 features a unique 'merge' function that combines the input and thru-put signals into a mono mix at the XLR output. This is achieved via a resistive mixer circuit. It's useful for summing stereo sources like keyboards or CD players into a mono signal, saving mixer channels. To use this feature, connect two instruments with similar output levels to the IN and THRU jacks, and depress the MERGE switch.

The Ground Lift Switch

Proper grounding is essential for safety and noise reduction in sound systems. When the Radial J48 acts as an interface between two grounded AC-powered devices (like a keyboard and a mixer), differences in ground reference potentials can cause 60-cycle hum or system buzz, known as a ground loop.

The J48's isolation transformer helps eliminate most of this noise. In severe cases, engaging the ground lift switch breaks the internal ground path between the input and output. It's important to note that if you are connecting directly (without using the thru-put), the ground switch should remain in the grounded (out) position. If using the thru-put to your personal amplifier, both your amp and the mixer will have ground paths, and engaging the ground lift switch may be necessary.

Diagram Description: Two diagrams illustrate ground loop issues. "The Problem" shows a signal cable and AC power cable creating a ground loop between connected equipment and an electrical utility. "The Cure" shows an audio transformer (DI box) introduced at the signal cable, effectively eliminating the loop.

The Balanced XLR Output

The J48 features a mic-level, balanced output jack using a standard XLR male connector. It is wired to the AES standard (pin-1 ground, pin-2 hot, pin-3 cold), which is common in modern audio equipment. This output connects to the mic-level input of a mixing console or pre-amplifier and is ideal for concert snake systems and splitters, allowing signals to be directed to multiple destinations like recording, monitoring, broadcast, and front-of-house mixes.

Phantom Powering: The J48 draws power from the 48V phantom power supply found on mixers and pre-amplifier channels. This system is safe for dynamic microphones and other connected equipment.

The 180° Polarity Reverse Switch: This switch toggles the XLR pin-2 and pin-3 to be the 'hot' or positive signal. This is beneficial for interfacing with older consoles that may have pin-3 hot, or for resolving phase cancellation issues when combining a mic'd and direct signal from the same instrument. Experimenting with this switch can also provide different tonal options.

Mechanical Attributes

Radial products are known for their durability. The J48 is constructed with a heavy-duty welded I-beam steel frame housing the PC board, circuit, and connectors. The double-sided PC board has components soldered twice for enhanced reliability, making it virtually impossible to torque the board once installed.

The 'book-end' construction features recessed front and back panels to protect switches and connectors from damage during transport or stage use. The baked enamel finish ensures years of performance and appearance. A full bottom no-slip pad provides stability on stages, preventing the DI from falling off amplifiers or sliding when stacked. This pad also acts as an electrical isolator, minimizing shock hazards, especially with older guitar amplifiers.

J-Rak and J-Clamp Mounting Kits: The J-Rak is a rack shelf/chassis allowing up to eight Radial JDI's, J48's, or similar J-Class products to be mounted in a 2U rack space. Units can be front or rear mounted. The J-Clamp is a mounting adapter for permanently mounting Radial J-Class products in various locations like podiums, under tables, or inside amplifier racks.

Image Description: Images show the Radial J-Rak, a rackmount chassis, and the Radial J-Clamp, a versatile mounting adapter.

Frequently Asked Questions

What is the difference between active & passive? Active DIs have a built-in buffer requiring power, while passive DIs use a transformer. Both convert high-impedance instrument signals to low-impedance balanced mic-level signals for long cable runs without noise. Passive boxes offer isolation, which can reduce ground hum.

Which is better - active or passive? Both have their applications. Passive DIs are often simpler to use as they don't require power, while active DIs offer more 'reach'. Passive boxes are generally suited for keyboards, while active boxes are preferred for acoustic guitars and basses.

Can you explain loading? In the past, low-output instruments connected via passive DIs to an amplifier and mixer could alter the sound due to added load. Active DIs like the Radial J48 buffer or pre-amplify the signal, preventing loading issues.

What makes the Radial J48 so special? Active DIs function as preamplifiers, boosting instrument signals. They require power, either from batteries or phantom power. Batteries can distort when low, making phantom power preferable. Phantom power (48V, low current) was designed for condenser mics. High-output instruments (up to 7V) can overload typical DIs (which manage around 3V). The J48 is specifically designed to increase its internal rail voltage to handle these high-output instruments without overloading the input.

How does Radial increase the rail voltage? The J48 uses a custom DC-to-DC transformer coupled to a timing chip to raise the internal rail voltage, enabling it to handle dynamics effectively.

What is phantom power? Phantom power is a 48V DC supply from a mixer, originally intended for condenser microphones. The J48 converts this phantom power using an internal switching power supply to handle higher transients without distortion.

Why does the LED not stay on? Power is prioritized for maximum sound quality. LEDs require significant current, which is diverted to the audio circuit.

What happens if I am not getting any sound? Check that phantom power is enabled on the console and verify all cable connections. Cables are a common cause of no sound.

Is a flat frequency response better than a colored response? While microphones offer 'color' or 'flavor', direct boxes are generally expected to be faithful to the source. The J48 aims to transport the original signal as faithfully as possible, allowing engineers to make artistic choices. The J48 direct box is described as natural and uncolored.

If my J48 ever fails, what do I do? Contact your dealer or Radial for a return authorization number. It is recommended to have your dealer check the unit and cables first, as cables are often the source of problems.

Block Diagram and Specifications

Diagram Description: A block diagram illustrates the signal flow within the J48. It shows the INPUT and THRU connections feeding into a MERGE function, followed by PAD and LO-CUT filters. The signal then passes through an ACTIVE BALANCED DRIVER and PHASE REVERSE stage before reaching the XLR OUT. The diagram also indicates the PSU TEST, D.C. CONVERTER, GND LIFT, and SWITCHING PSU components.

Radial Engineering 3 Year Transferable Warranty

Radial Engineering Ltd. warrants this product to be free from defects in material and workmanship for a period of three (3) years from the original date of purchase. Radial will repair or replace, at its option, any defective component(s) free of charge, excluding normal wear and tear on finishes.

To make a warranty claim, the product must be returned prepaid in its original shipping container (or equivalent) to Radial or an authorized Radial repair center. A copy of the original invoice showing the date of purchase and dealer name must accompany the return. This warranty does not apply if the product has been damaged due to abuse, misuse, misapplication, accident, or unauthorized service or modification.

There are no expressed warranties beyond those described. Radial is not responsible for any special, incidental, or consequential damages arising from the use of this product. This warranty grants specific legal rights, and you may have other rights that vary by location.

FCC Compliance: This device complies with section 15 of the FCC Rules. Operation is subject to two conditions: it must not cause harmful interference, and it must accept any interference received.

Company Information:

Radial Engineering Ltd.
#1165 - 1845 Kingsway Ave, Port Coquitlam BC V3C 1S9
Tel: 604-942-1001 • Fax: 604-942-1010 • Email: info@radialeng.com

J48 User Guide Part #: R800-9616-00 / 05-2021. Copyright 2021. All rights reserved. Features and specifications are subject to change without notice.