08 April 2013

Home Studio: Testing Radial J48 DI


Note: This entry was already published in my old Spanish version of this blog, around November/2010. I am just revisiting it here.
The Unit comes in a solid cardboard box, with same instructions you can directly download from the page of Radial Engineering, together with a triptic listing their products.

This Unit seems to be built as a tank and, it's very well finished (as usually in Radial products).

On the front side, a push button named "15dB PAD!", a bulb that reads "48V check", a push button called "Merge" and, an instrument input jack (Input) and a T output jack (Thru).

On rear side, a push button named "Low Cut", one more named "Ground", still one more called "Rev 180º" and, a balanced XLR Output (Output).


Very important Warning:
This Unit needs to be connected to an XLR input of a device capable to send the 48V of phantom power that it needs to properly work, as if the J48 was some kind of condenser mic.

Connections are really easy. Your guitar or bass guitar are directly connected the input jack, the XLR output should be connected to a mic level input, in a mixing board or pre-amp. Just be sure it's an input for mic level of signal.
If the resulting sound has lot of floor noise, the "ground" button helps to isolate the ground in this unit, removing ground loops.

Theoretically, the button "15 dB PAD" should be pressed down if we find an instrument generating a very high output signal (like an electronic keyboard, high output pickups).

The jack "thru" should allow you to route the (unprocessed) input signal to any other device. Typically, the pedalboard or the amp but, we can also route it to the input of some other pre-amp or Studio outboard gear.

The button "Rev 180º" reverses the phase of the output signal. If we have phase issues, that are usually heard as a comb effect while hearing the mix in mono mode, this button can fix that issue.

The button "Low Cut" is a filter that drops the level of frequencies below 80 Hz in around -6dB, what can be of help to dismish the hum of passive single pickups.

Sound Tests

The three following MP3 files have recording takes of an electric guitar and electric bass guitar, that were recorded in four different ways.
  1. Instrument connected to J48, with -15 dB PAD active. J48 output connected to a DI input with pre-amp in the Digidesign Rack 003 sound card (inputs 1 to 4).
  2. Instrument connected to J48, without the PAD active. J48 output connected to a DI input with  pre-amp in the Digidesign Rack 003 sound card (inputs 1 to 4).
  3. Instrument connected to pre-amp SPL Track One and, its output to a DI input without pre-amp of the Rack 003 (inputs 5 to 8).
  4. Instrument directly connected to a DI input with pre-amp of the Rack 003 (inputs 1 to 4).
Note: the original files were bounced in WAV format but, since they are 50 MB size, I've created an MP3 version to upload those in Soundclick. Since Soundclick allows a (free) maximum resolution of 128 Kbps, the resulting sound will be clearly inferior to the original WAV file but, I hope you can appreciate the differences between the 4 takes for each instrument.


Used guitar was a Fender American Deluxe HSS Stratocaster, moded to SSS and, loaded with a set of Bareknuckle's Mother's Milk pickups. Just the Neck pickups was recorded.

Since differences are more notable when the sound is being amplified, I've sent the clean track to Amplitube 3 plugin, using the amp model named '64 Vibroverb Custom, from the Amplitube Fender package. I leaved the default settings for this test.

The amp's track fader has been lowered in -3dB, to avoid clippings (and, even this, some is there).

Takes are being recorded one after the other, just changing the card input for each track, between take and take. All them go thru the same plugin, with exactly same parameters.

The input level has been set according to the output level of the J48 unit, without the PAD active. Rest of input faders had been adjusted to same level (approx). Click on following link:

In the first take (J48 with active PAD), the signal lacks strength and, I can hear the string too much for my taste. Sound has some kind of "metallic fuzz". Maybe, it could work for acoustic guitars.

In the second take (without the PAD active), the signal is really strong. Here, the sound is a bit better with, it has that "metallic fuzz", also. The output level makes to saturate the input very easily.

The third take corresponds to the SPL Track One input and, the sound is way silkier. That "metallic fuzz" goes out and, the guitar sounds with a slight roll off in high frequencies, taming the brightness but, making the sound more musical (as if it was routed to a tube amp).

The forth take corresponds to a direct input in Rack 003. Is the one that removes brightness the most, rolling off high frequencies but, it sounds very musical to me, also.

My impression is that to plug the guitar in the SPL Track One pre-amp delivers the best sound of the 4 takes but, sure, it's a matter of personal taste.

Bass Guitar

I've used a G&L Legacy JB-2, with everything stock.
The clean track was routed to an Amplitube 3 plugin, selecting the amp model SVT-CL, of the Ampeg SVT package.

I've followed same order for these takes as in the case of the electric guitar.
Please, click on the following link to hear the track:

The first take corresponds to the J48 with the PAD active. As in the case of the guitar, this take lets you hear the strings but, the effect isn't so dramatic as in the case of the guitar.

The second take corresponds to the J48 without the PAD. Also, the "metallic fuzz" is being tamed a bit and, the sound is a tad more round.

The third take corresponds to the SPL Track One. Once more, the "metallic fuzz" goes out and, the sound is more round and musical.

The forth take corresponds to the direct input of Rack 003. Once again, is the input with less brightness but, still very musical.

The take to choose will depend on the effect we are looking for. If we wanted a very acoustic bass sound, J48 seems the best option but, if we are worried about that "metallic fuzz" sound, the SPL Track One seems to me the best option.


  • The J48 enhances the acoustic aspects of the instruments. It's catching the metallic noise of frets and strings. Possibly, it has a better application for acoustic guitars or when you want a very "unplugged" sound. With single pickups it can sound very thin and excessively acoustic.
  • The pre-amps of the Rack 003 are good enough to preserve the dynamic and natural sound of the instrument. But, they tend to roll off the content in high frequencies, removing some brightness.
  • The SPL Track One pre-amp is, to my understanding, the best option for guitars, amps and vocals. Certainly, it rolls off some high frequencies but, the result is very musical.

Why these differences?

In the sound chain of an electric instrument, as the bass or guitar, one of things that weight the most is the quality of the inputs and outputs of the gears that are in the signal path.

Electric guitars need of a high impedance input and, a very low impedance output. The more the difference of levels between both, the best you get the natural sound of the guitar.

Effect pedals have variate impedances in input and output. Boutique pedals, with some excellent buffers can have inputs above 1 MOhm, with very low output levels.

The input in an amp has a high impedance also (around 1 MOhm)  and, for sure, the good instrument inputs of the Studio gear have impedances even higher.

If we take a look to the specifications of the used equipment, we can see that the device with lower impedance is just the J48 (with 220 KOhm), followed by the Rack (which DI inputs have an impedance above 300 KOhm) and, finally, by the Track One (having an impedance of 1 MOhm.
Respect of the J48 that means 4,5 times higher impedance level and, respect of the Rack it means 3,3 time higher.

The Track One output has a low impedance of 50 Ohms (really impressive). If we compare this output with the output impedance level typical in effect pedals, this means a relation of 20,000 to 1.

The output of the J48 is of 600 Ohm (more or less the typical level of a pedal effect). What means that the relationship between input and output impedances is 366.67 to 1.

So, clearly, respect of impedance levels, the Track One is clearly superior, while the J48 has the worst relationship of the three devices.

Now, respect of the response frequency, the J48 covers a range from 10 Hz to 80 KHz. Since the Human Ear hears a range between 20 Hz and 22 KHz, the J48 covers the whole range with ease.
The SPL goes a bit beyond it, covering from 10 Hz to 180 KHz.

The Rack 003 is just correct, covering the audible range of 20 Hz to 22 KHz.
The three are able to "hear" all those frequencies that the human ear can hear.

Respect of the dynamic response, I don't know the range of J48 but, I bet it's inferior to the other two devices, as it's happening with all the "important" values we are reviewing.

The Rack 003 has a dynamic response of 104 to 106 dB, while the Track One has a dynamic response of 115 dB. And, here is were you hear the nuances. The higher the dynamic response, more nuances in the sound, since you have a higher range of values to represent the sound.

About the Total Harmonic Distortion and the relationship sound / noise, the J48 has a value of 0.005% in the best of the cases, while the Rack 003 has a value of 0.0008%. Specifications for SPL are expressed in other way so, I cannot make a direct comparison in this subject.

Clearly, the J48 introduces more floor noise than the DI input of the Rack 003. As per my takes, the SPL should introduce same or lower level of noise than the Rack 003.

So, in my understanding, the J48 is a product that doesn't represents an improvement respect of the DI inputs of the Rack 003, while the Track Pro is clear enhancement.

It seems that what I can hear while reproducing the tracks has a clear technical explanation.

Do I need a DI?

To me, the answer is very clear: if you have a sound card with pre-amps of quality or, if you have a quality pre-amp unit, forget that DI.

If you have a Digidesign Rack 003 sound card, you can always use any of the different instrument entries (labeled as DI). The ones with number 1 to 4 are the ones that have an integrated pre-amp.

If you have a pre-amp like the SPL Track Pro or even better, you will earn in quality if you route your instruments thru the instrument input of the pre-amp and then, routing the output to one of the inputs of the Rack 003 without pre-amp (inputs 5 to 8).

The best you can do before buying some DI is to check the technical specifications of the instrument inputs of your gear. If they are lower that the ones of the J48 then, yes, you should be interested in that J48 DI.

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