Comparing Direct Boxes

There are three common questions folks ask us when considering a direct box:

The answer is ‘it depends’. A direct box is primarily used to allow you to run a long cable without adding noise or losing signal quality. High impedance signals tend to be much more prone to noise and keeping cable lengths under 8 meters (25 ft) is recommended. A DI box converts the high-impedance of an instrument to a balanced low impedance signal. This enables the instrument signal to travel distances of 100 meters (300 feet) without adding appreciable noise. The output of a DI box is mic level – thus the balanced signal is treated just like a microphone.When playing live, the mixing desk is usually positioned in the house (front of house or FOH position) which is often 50 to 100 meters away. Most instruments such as bass guitar, acoustic guitar and keyboards are connected to a DI box and mixed at FOH. For the sound engineer, capturing the sound before it is processed by the artist on stage usually makes it easier to amplify the signal as it can be optimized for the room.

In the studio, recording from a separate room (isolation booth) enables the engineer to capture a direct sound from the instrument using a direct box and also add a mic in front of the amplifier to capture the tone generated by the amp. When recording in the control room, you may or may not need a DI box, depending on the tone you are looking for. Most mixer inputs have a ‘one size fits all’ type of input that may not flatter your instrument. A good DI box is optimized to interface with your mixing desk or recording system by presenting the instrument with the right input impedance. This can take the edge off the instrument for a smoother, more natural sound.

This is like asking what kind of microphone should I buy. Just as there are all kinds of microphones for all types of applications, there are a number of DI box types for the same reason. Just as you could use a dynamic microphone like a Shure SM58 to capture just about anything, you could buy a Radial JDI and be pretty much set. But sometimes, using a condenser mic can give you the extra reach to capture more sparkle from an acoustic guitar or more detail from a voice when recording. An active DI box like the Radial J48 is almost the same.Like dynamic microphones, passive DI boxes tend to be able to handle more signal level without distortion while condenser mics – like active DIs – generally produce a wider frequency response while being more sensitive. And just as most studios and live stages are equipped with a selection of dynamic and condenser mics, both passive and active DI boxes are often used side by side. The choice usually comes down to personal preference.

A simple rule of thumb:

  1. If the source is active, use a passive DI box
  2. If the source is passive, use an active DI box

So if you have an old Fender Precision bass with magnetic pickups, an active DI box will probably work best as the buffer will generate plenty of signal for the PA system while reducing the load on the instrument. If on the other hand you have a new, high output active bass with a 9 volt battery inside, you can turn up your volume with confidence knowing that your passive Radial ProDI will easily handle the signal without distortion.

Over the years, industry standards become common place. For instance, most engineers prefer to run an acoustic guitar through an active DI like the Radial J48. On AC powered devices like keyboards, electronic drums, DJ mixers and other active sources, most engineers prefer to use a passive DI like the Radial ProD2. This is because passive DIs can handle more level without distortion and they are also really helpful when it comes to eliminating hum and buzz caused by ground loops.

We recommend that you follow the 5:1 rule: if you spend $1000 on an instrument, you should probably invest $200 in a direct box. So if your guitar is worth $500, sending $100 on a direct box is probably the right ratio. In our view, there is no point in spending $200 on a DI box if your guitar is only worth $200.Think of it this way: would you spend $2000 on a great guitar only to play through a $30 guitar amp? Probably not. A DI box is a unity gain preamp. Preamps can cost anywhere from $30 to $10,000. And just like guitars, basses, keyboards and studio preamps – direct boxes come in all shapes, sizes and quality levels based on your expectations. There is no shortcut to quality. Radial DIs are used on the world’s top stages and in the best studios because they are designed to deliver the utmost quality day in and day out. We are confident that once you listen to the difference, you will come to appreciate the benefits.

Radial DI boxes come with a 3-year transferable warranty which means that if you decide to sell your Radial DI within the warranty period, the warranty will still be good for the next owner. And because Radial DIs are in demand, they tend to have a much higher resale value.

As described above there are no hard fast rules. It comes down to taste and application.


Comparing passive DIs for instruments

When comparing passive DI boxes, begin by considering the instrument or audio source, the number of channels required for a particular setup and the feature set. You will find that most passive DIs share features such as an input pad and a ground lift. All Radial DIs have built-in filters to reduce parasitic noise caused by magnetic fields and radio frequency waves.

The primary difference between the various models is the type of transformer that is employed. We spend hours listening to various transformers to find the best one for a given application and price point. For instance, the industry standard Radial JDI employs a USA-made nickel core transformer with dual faraday shields and a MuMETAL® external can. MuMETAL® provides upwards to 10,000 times greater shielding than aluminum. This combined with the steel outer shell protects the sensitive high impedance signal from outside magnetic fields from power transformers that can pollute the signal. The JDI, JD6 and Duplex are exceptionally linear from 10Hz to 40kHz. The ProDI employs a similar custom-made transformer that is made off-shore that is also equipped with a MuMETAL® shield. The ProDI, ProD2, ProD8 share the same Eclipse transformer. It is linear from 20Hz to 18.5kHz and works extremely well for live touring. The Stage-Bug SB-2 employs a copper shielded transformer that delivers great performance from 40Hz to 18kHz making it ideal for weekend club dates and jamming with friends.

While the industry standard Radial JDI is primarily intended instruments such as bass, acoustic guitar and keyboards, the two-channel Duplex adds greater connectivity for AV applications where you may need to connect to CD players, video machines and so on. The same follows with respect when looking at the Radial Pro series. If you are using the DI in a live situation and the PA system is band-limited to 18kHz, you may find that the ProDI will work perfectly well. If on the other hand you are recording in the studio you may want to capture the very highest harmonics, the JDI may be a better choice. The Radial JDI is used by Tony Levin, Chick Corea, and The Who while the Pro series DIs like the ProD8 are used by U2, Rush and the Rolling Stones.


Comparing Single Channel Passive Direct Boxes
Features ProDI™
JDI featured image
Primary application Instrument Audio-visual Instrument
Transformer type Eclipse ET-DB2 Eclipse ET-DB2 Jensen JT-DBE
Frequency response 20Hz~20kHz 20Hz~20kHz 10Hz~40kHz
Input connectors ¼” ¼”, RCA, XLR, 3.5mm ¼”
Thru connectors ¼” ; ¼”, RCA, 3.5mm ¼”
Balanced output XLR-M XLR-M XLR-M
Stereo to mono merge No Yes Yes
Input pad Yes Yes Yes
Ground lift >Yes >Yes >Yes
Rack mount No Yes – J-Rak Yes – J-Rak
Desk mount Yes – ProClamp Yes – J-Clamp Yes – J-Clamp


Comparing Two Channel Passive Direct Boxes
Features ProD2™
Primary application Keyboard Audio Visual Multi-Purpose
Transformer type 2 x Eclipse ET-DB2 2 x Eclipse ET-DB2 2 x Jensen JT-DBE
Frequency response 20Hz~20kHz 20Hz~20kHz 10Hz~40kHz
Input connectors ¼” ¼”, RCA, 3.5mm ¼, RCA, XLR”
Thru connectors ¼” ; ¼”, RCA, 3.5mm ¼”
Balanced output XLR-M XLR-M XLR-M
Stereo to mono merge No No Yes
Input Pad Yes Yes Yes
Ground Lift Yes Yes Yes
Rack mount No Yes – J-Rak Yes – J-Rak
Desk mount Yes – J-Clamp Yes – J-Clamp Yes – J-clamp


Comparing active DIs for instruments

Radial produces a number of active DI boxes to address various requirements. Selecting some, such as the J33 for a turntable, is easy… while choosing between others can be confusing. This section aims to simplify the process by bringing various active DI boxes together for comparison.

For the most part, active DI boxes are employed on instruments. Like condenser microphones, active DIs tend to have more reach over passive counterparts due to the sensitivity of the input. But as with all amplifiers (an active DI box is in fact an amplifier of sorts) these can distort if the input exceeds the available headroom. More headroom can be achieved by increasing the size, or power generated, by the supply. For instance, a DI box like the JDV employs a 30 volt, 400 milliamp power supply that has incredible headroom, but it needs to be locally powered. This may not be convenient on a live stage. Thus the reason most live engineers prefer to use an active DI box that derives its power from the 48 volt phantom power generated by the mixing // console. Phantom power generates 48 volts but only 5 to 10 milliamps of current. Innovative powering schemes such as the one employed inside the J48 elevate the headroom to an acceptable level. This enables most instruments to be connected without fear of distortion and is likely the reason the J48 is used on the biggest stages by the biggest acts around the globe.


Comparing Active DI Boxes for Instruments
Features Pro48™
J48 feature image
Circuit type Active FET Active FET Class-A discrete Class-A discrete w/ tube drive
Max clip level +12dBu +14dBu +19dBu +24dBu (aux out)
Power supply 48V 5milliamps 48V 5mA +/-15V 400mA +/-16V 1600mA
Power source Phantom Phantom DC power brick DC power brick
Instrument inputs (1) ¼” (1) ¼” (2) ¼” (2) ¼”
Thru/Aux outputs (1) ¼” (1) ¼” (4) ¼” (1) ¼”
Insert TRS No No No Yes
XLR balanced out Yes Yes Yes Yes
Selectable input No No Yes – 2 chan. Yes – 2 chan.
Remote switch No No No Yes, option footswitch
Stereo to mono merge Yes Yes No No
High pass filter No Yes, 80Hz Yes, 80Hz Yes, 5Hz ~ 1kHz
Input impedance 220kΩ 220kΩ 100k~1megΩ 100k~1megΩ
Piezo input No No Yes, 4 meg Yes, 4 meg
Pad -15dB -15dB -20dB Variable
Ground lift Yes Yes Yes Yes
Rack mountable Yes, J-Rak Yes, J-Rak Yes, SA19-RA Yes, SA19-RA


The J48 and Pro48 are primarily designed for live stages. These employ similar audio signal paths and derive their power from 48V phantom. The Pro48 is basically a trimmed down version that eliminates the stereo to mono merge function and high pass filter while reducing the size. This enables us to reduce the price to make it more affordable.

The JDV is primarily designed for studio. The over-sized power supply and class-A circuit delivers tremendous dynamic range, and because of the unique zero-feedback design, it produces a very open sound. This has made it a favourite with artists like Marcus Miller and Victor Wooten who are known for their incredibly dynamic playing. Drag control load correction further advances the tone – allowing the artist to optimize the sound of the circuit by properly matching the instrument’s load. The JDV is all about purity.

The Firefly is a tube direct box designed to introduce character by driving the signal through the 12AX7 tube and Radial output transformer. This adds warmth and depth that folks often characterize as ‘vintage sounding’. The tube circuit necessitates a special high output power supply to provide sufficient voltage to the heater (tube filament). As the newest in the family, it enjoys extra features such as Drag control, a fully variable high pass filter to eliminate resonance, dual selectable inputs and remote controllable mute function.


Selecting the right DI box for a violin, viola or upright

Most string players tend to use a piezo pickup that mounts directly on the bridge of a violin, viola or upright bass. Although piezo manufacturers rarely publish the optimal impedance for their device, piezo transducers sound fuller, warmer and less peaky when they are connected to a very high input impedance. So if you are looking to produce the most natural sound of the instrument, a high input impedance in the range of 5 to 10 meg ohms is ideal. Radial offers several products that meet this criteria.


Comparing the right DI box using a piezo transducer on a Violin/Viola pickup
Features SB-4™
Input impedance 5 meg 10 meg, 1 meg, 220k 10 meg, 220k 10 meg, 220k
Channels 1 1 1 2
Equalizer No No Yes Yes
Low cut filter (HPF) Yes, switch Yes, variable Yes, switch Yes, switch
High cut filter (LPF) No Yes No No
Effects loop No No No Yes
Polarity reverse Yes Yes Yes Yes
Ground lift No Yes Yes Yes
Instrument amp out Yes, buffered yes, thru-put Yes, buffered Yes, buffered
Balanced DI out Yes Yes Yes Yes, pre & post


The difference between these four products comes down to controls and features. For example, the StageBug SB4 is very basic with an input and two buffered outputs to feed the stage amp and the PA system. The PZ-DI takes this up a step by providing a variable low cut filter for precise low frequency control. The PZ-Deluxe adds a 3-band EQ for tone shaping while the PZ-Pre gives you a second channel for another instrument plus an effects loop and power booster for soloing.


Using effects on a violin or viola

If you plan to use effects pedals – as commonly used with electric guitar, there are a couple of ways to approach your setup. The first thing to consider is your clean tone. If you connect the piezo to a guitar pedal, you will likely find that the buffer inside the pedal will range from 10k to 1 meg ohms. As described earlier, the higher the input impedance the better. For example, a Boss GE7 tuner has an input impedance of 1 meg-ohms which is a bit low, but will work.

From the input buffer (first device in line), if you can then drive your various effects pedals… and then send the signal to a direct box. The choice of direct box depends on a few factors, but as a rule of thumb, when the signal is already buffered, we generally suggest a passive direct box like the Radial JDI or ProDI. A passive DI will generally be a bit quieter than an active one due to the noise cancelling benefits of the transformer isolated signal path.

The PZ-Pre has the advance of a built-in effects loop. This gives you a 10 meg-ohm input impedance, full on board EQ, a low cut filter to eliminate resonance, and lets you add effects when you need them – or take the out o the signal chain when not in use.


Comparing DI boxes for laptop computers and other devices

Today, the use of computers is omnipresent in the audio world. Computers are used to create sound files to back up the band’s performance, to play back videos and for walk-in and walk-out music during a concert. The problem with computers is that they are often the cause of buzz and hum in audio systems.


DI boxes for laptop Good Better Best


Radial produces a variety of solutions for computers, sound cards and consumer music players. These are broken down into two groups, one being passive, the other active. Many stage techs prefer passive DIs like the ProAv2 as they do not require power and the transformer isolation benefits by eliminating stray DC currents which in turn can cause hum and buzz on the audio line. Active DIs benefit by enabling the electronic designer to add features such as buffers for additional gain and blocking capacitors which can filter out noise without affecting the audio signal path.


Passive DIs
Features SB-5™
Application Laptop AV / Laptop AV / Laptop Multi-Purpose
Signal path Stereo & mono Mono Stereo Stereo
Max signal handling -10dB +4dB +4dB +4dB
Max gain -20dB -10dB -10dB -10dB
Frequency response 40Hz ~ 15kHz 20Hz ~ 18.5kHz 20Hz ~ 18.5kHz 10Hz ~ 40kHz
Input connectors 3.5mm TRS cable 3.5 RCA, ¼” 3.5 RCA, ¼” 3.5, RCA, ¼”
Output connectors (2) ¼”TRS, XLR (1) XLR (2) XLR (2) XLR


Active DIs
Features JPC
Application AV / Laptop Consumer to Pro USB Digital Phono Turntable
Signal path Stereo Stereo Stereo, mono Stereo
Max signal handling -10dB -10dB Digital source RIAA specialty
Max gain -10dB +8dB -10dB -10dB
Frequency response 20Hz ~ 20kHz 20Hz ~ 20kHz 20Hz ~ 20kHz 20Hz ~ 20kHz
Input connectors 3.5 RCA, ¼” 3.5 RCA, ¼” USB Type 2 RCA
Output connectors (2) XLR (2) XLR (2) XLR (2) RCA, (2) XLR


The direct box is a device that has been around for nearly 50 years, but only recently have folks begun to really pay attention to how they work or which type will do the best job for a given task. At Radial we pride ourselves in making a wide variety of DI boxes to suit every budget and application.