J48™ Using & Applications

The following page describes using the J48 Stereo's basic functionality. For complete details, please download the full manual by clicking the icon at left.

Making connections

As with all audio gear, always ensure audio system levels are turned down or equipment turned off before making connections. This will avoid plug-in or turn-on transients from damaging more sensitive components such as tweeters.

The Radial J48 Stereo is an active direct box that derives its power from the 48 volt phantom supply from the mixing console or mic preamp. You simply pug in and it will quietly go to work. There are two channels on the J48. These are identical, and feature a ¼" INPUT, a ¼" THRU-put and a balanced XLR out.

Connect your source instrument to the input and the balanced XLR out to the PA system or recorder. Unbalanced cables are much more susceptible to noise than their balanced counterparts. Keeping unbalanced cables under 8 meters (25') in length is good practice while balanced cables can easily extend 100 meters (300'). The balanced output of the J48 Stereo is mic level, meaning that it should be connected to the mic input of a mixing console or mic preamp. Once connected, make sure phantom power is activated. The red LED on the J48 will illuminate to let you know phantom power is present.

The THRU connector output provides the means for connecting the signal to a stage amp or personal monitor system. Simply connect the THRU to your amp or monitor.

Using the J48 stereo with an acoustic guitar and stage amp
Using the J48 Stereo with a keyboard, mixer and powered speaker
Click images to enlarge

Using the -15dB PAD

Inside the J48 is a unique switching power supply that is designed to provide plenty of headroom even when pushed very hard. There are however instances when the output from and instrument can be extreme. To prevent distortion, the J48 Stereo has a -15dB input pad that reduces the input sensitivity. Examples could be a very high output digital piano, active bass guitar or maybe the overly aggressive output from a DJ mixer. If you hear distortion, simply depress the PAD.

Using the Ground LIFT

An active direct box is in essence a unity gain amplifier that requires power to work. In the early days, active direct boxes typically employed a battery for power. Today, phantom power from the console has become the most common the power source. This 48 volt supply uses the pin-1 (ground) and pin-2 (hot) on the XLR cable to provide DC power to the direct box while audio is sent in the opposite direction over the same cable.

A common problem occurs when connecting a stage amp to the direct box whereby a so called ground loop can present itself. This is often caused by differing DC offsets or voltage differentials that can introduce hum and buzz in the sound system. The first line of defence is to lift the audio ground - Pin-1 on the XLR. With most direct boxes, when you lift the ground you also disconnect the power. This then requires using a battery or external supply to provide the power – options that are not desirable. With the J48's unique circuit design, instead of lifting the ground at the XLR, it is done in the power supply. This eliminates the noise problem without losing power. (smart!) If you hear noise, simply depress the ground lift switch.

Using various instruments

The J48 Stereo is active and will work with almost any instrument. This means that it has a built-in preamp or buffer circuit that drives the signal. In other words, if you have a passive source such as an old Fender bass, the J48 presents a very effective interface. Many sound engineers also prefer the added gain of an active direct box over the passive counterpart on active instruments such as with acoustic guitars and vintage keyboards that may not have the optimal output.

Click on images to enlarge
Fender bass to DI to amp and PA
Acoustic guitar with active electronics to DI to PA
Upright bass to dedicated preamp to DI to PA
Stereo keyboard to DI to PA

Using the high-pass filter

A very handy feature that is built into the J48 Stereo is the high pass filter. When activated, it gently rolls off low frequencies below 80Hz and to help eliminate resonance and feedback. This handy feature is particularly effective at cleaning up the low registers, making it easier to mix.

The other benefit with eliminating bass is that you actually increase the available dynamics. Low frequencies and their huge waves require more power than their high frequency counterparts. Reducing the low frequency content results in more available headroom.

Click to enlarge

Using the polarity reverse switch

The RIGHT channel on the J48 Stereo is equipped with a polarity reverse switch that toggles Pin-2 and Pin-3 on the XLR. This handy feature can be used in several ways:

  1. Stereo imaging

    In some instances, particularly when recording – changing the relative phase between the left and right channels can produce very pleasing results. Similarly, when using two sources such as a combining a magnetic pickup with a piezo on an acoustic guitar, changing the relative phase between them can sometimes produce a more natural sound.

  2. Eliminating Feedback

    Resonant feedback on a live stage is often produced when sound sources such as the PA and stage amp combine to create a hot-spot at certain frequencies. This can cause an acoustic instrument to vibrate, resulting in low frequency feedback. This can often be minimized by reversing the polarity at the XLR , electronically moving the problem out of the way. Without having to resort to excessive (tone altering) equalization.

  3. Adapting to non-AES standards

    When interfacing with older vintage audio equipment, the polarity may be reversed whereby pin-3 may be hot. Depressing the polarity switch lets you match the phase if needed.