Class-A operation uses one device
to amplify the full waveform.
Class-AB operation uses two devices,
each amplifing half of the waveform.

The inside of the Firefly

Connected with the JR2

Rackmount one or two Firefly DI's

Firefly™ Development

For years, folks have been asking Radial to build a tube direct box. What we did not want to do was simply put something in a box. Instead, we wanted to make a statement. Radial users have very high expectations, so our version of a tube direct box not only had to look and sound great, it had to be equipped with a solid feature set so that it could be used in the studio or taken out on the road. And it had to perform like a world class racing car.

Delivering absolute purity

Ask any audiophile and they will tell you that discrete class-A circuits are the most natural sounding. This is because class-A circuits employ a single full wave amplifier to do the work while class AB amplifiers split the waveform and amplify each section separately. Although more efficient, class-AB circuits necessitate combining the two waves together after they are amplified which invariably introduces zero-cross distortion.

As for efficiency, since we are not building a high powered amplifier, the Firefly is nowhere near as demanding. Thus using more energy to deliver the very best audio quality is a trade-off we feel is duly justified.

One of the most exciting attributes that sets the Radial JDV Super-DI direct box apart is the zero negative feedback (ZNF) circuit design. For years, design engineers have been trying to eliminate negative feedback as a means to 'free' the audio circuit from any form of phase cancellation. Negative feedback is used to stabilize the audio circuit to avoid run-away. The JDV employs some innovative thinking to get around the problem and this same front end circuit design has been implemented into the Firefly.

Adapting to all types of pickups

Next on the list was making sure that the Firefly could be used with any instrument in any situation. The most common pickups used today are magnetic such as those found on a traditional Fender bass, Gibson electric guitar or the type that fit into the sound hole of an acoustic guitar. Next in line are the active systems that are found in many acoustics or in today's active basses. And last but not least is the piezo transducer. This is often coupled with an active preamp such as on an upright bass or simply wired to the output jack as a means to save the instrument's natural acoustic properties or vintage value.

Magnetic pickups are basically electro-magnetic transducers that work very much like an audio transformer. When placed inside an electrical circuit they react differently depending on what they are connected to. This is why most musicians find that when they put a preamp (buffer) in between their magnetic pickup and the amplifier, it changes the tone. To solve the problem, Radial invented Drag Control load correction. This allows the user to adjust the load on the pickup so that the guitar or bass sounds as if connected to a tube amplifier. Active instruments combine a pickup with some sort of built-in preamp. Since the pickup is buffered inside the instrument, the Drag Control has no effect and it will simply be ignored when used with an active bass, acoustic guitar or synthesizer.

Although hard to believe, piezo transducers can actually sound amazing. Most musicians complain that they squawk or sound peaky when played hard. This is not so much the fault of the pickup, but the input impedance of the device they are connected to. When the input impedance is too low, they get all whacked out. Piezo transducers work best when presented with a very high input impedance. We designed the Firefly so that you when you bypass the Drag Control, the impedance jumps to 4 meg ohms, smoothing out the transients and peaks. The reason manufacturers avoid a high input impedance is that high impedance circuits tend to be very noisy... unless very carefully designed.

The Firefly Tube Drive

Before building the final prototype, we tested a variety of tubes. We looked at sourcing old discontinued tubes from the 1950s and looked at some of the ultra expensive esoteric tubes that are used in hi-fi systems. In both cases, we found ourselves asking how the musician on stage would deal with replacing tubes on the road or simply finding one 20 years from now. We decided instead to go with a standard 12AX7.

Although not perfect, the 12AX7 is by far the most popular tube made today. It is used in 99% of all guitar amplifier designs making it the one tube you can be sure to find. The only real challenge with the 12AX7 is keeping it quiet. To do this, we sourced a custom power supply that would provide sufficient voltage to the heaters and then made sure that the electronics around the tube were as quiet as could be.

Transformer coupled output

Radial singlehandedly brought passive DI boxes back in vogue. 15 years ago, the passive DI box had lost favour to the active counterparts due to the low quality transformers that were typically used. These cheap transformers gave passive DI boxes a bad name. By combining a laminated nickel core with ultra thin scatter winding techniques and a MuMETAL® shell, the quality of the signal improved dramatically. Today, Radial transformer based DI boxes are used by many of the world's top artists on stages and in studios around the globe.

Transformers are amazing devices. They do not distort like an active circuit... they saturate. This means that that act like a very subtle limiter producing a smoothing or rounding effect to the transient peaks. And because transformers block DC current, they inhibit stray DC currents from flowing around which can cause various pieces of equipment to interact. This often produces the hum or buzz commonly known as a ground loop. By incorporating a Radial transformer on the output of the Firefly, you benefit from a smoother tone with less noise.

Maximizing the functionality on stage and in the studio

The first Firefly prototype had two inputs. Radial staff member, Phil Coelho, suggested that the idea of having two inputs is great, but in order to effectively switch instruments on stage, separate volume controls for each instrument would be ideal. This way, the signal levels going to his amp could be balanced.

We then thought it would be cool to add a remote control. The JR2 was developed for this purpose 'borrowing' some switching technology that was used on some of the early Forest guitar amps. The cool thing about the JR2 is that it does not require any power. The power is remotely fed to the JR2 via a balanced audio cable which then sends a pulse back to the Firefly. Back then, the Forest amp used two switches: one to control channel changes, the other to turn on the reverb. We reallocated the switching so that one selected the inputs and chose to use the other to mute the Firefly outputs for silent tuning on stage. This of course necessitated adding a separate tuner out.

We then thought about effects. With two inputs, we had to come up with a way to enable the musician to insert effects onto both instruments. A standard TRS effects loop as found on mixing consoles proved to be the best solution. The TRS insert loop employs standard send & receive effects loop cables which are readily available at any music store.

For concert touring, being able to rack mount a DI is really important. Rolling racks in and out of arenas makes setups quicker and more efficient. And with today's high fuel costs, saving space is critical. We chose to make the Firefly as small as possible so that it could be rack mounted into a single space. For the studio, we decided to add a cool looking handle. This would enable the engineer to move the Firefly around the studio and bring it as close as possible to the source to maximise signal to noise.

Optimizing the tone for any instrument

A really cool trick that engineers use in Nashville when recording acoustic instruments is to introduce a high-pass (low cut) filter on the instrument. During the development stage we spoke to a number of big time Nashville engineers to get the inside track. What we discovered was that when you vary the low cut-off point to match the instrument's size, you get less interaction or modulation between the instruments. This results in a more balanced sounding recording.

Different Instruments used at certain levels

We opted to add a fully variable high-pass filter on the Firefly so that it could quickly be adapted to fit. Fully counter-clockwise delivers deep bass and rotating the filter clockwise makes it easy to size the Firefly for acoustic guitar, dobro, banjo, viola, violin or mandolin.

Routing the outputs

In the studio an engineer may use the PRE setting to record a second "clean" track in addition to the Firefly's XLR output by connecting the THRU output to a DI box.

When you look at the Firefly's back panel, you will see a ton of ¼" female connectors and a couple of XLR jacks. We chose to put all of the connectors on the rear panel to eliminate clutter. This is particularly important on hectic stages to help sort things out should something go wrong.

Next to the inputs is a high impedance ¼" instrument output labled THRU. This buffered output can be configured as either PRE or POST. When the THRU output is set to PRE the output captures the signal from inputs A or B after the Drag control but before the tube drive, insert loop and master level control.


Set this way the THRU jack is a super-clean unity gain output primarily used to feed the artist's stage amplifier. In the studio an engineer may use the PRE setting to record a second "clean" track in addition to the Firefly's XLR output by connecting the THRU output to a DI box.

When the THRU output is set to POST the output is identicle to the XLR output ensuring the same tone is sent to the players amp and the PA.

When the THRU output is set to POST the output is identicle to the XLR output ensuring the same tone is sent to the players amp and the PA. In most cases we anticipate that the artist will want to add the Firefly's warmth and harmonics to the signal path.

Finally, there are two XLR connectors. The first is a standard 3-pin XLR output while the other is a 4-pin XLR that is used to connect the power supply. By changing the pin configuration, it makes it impossible to connect the audio into the power slot or vice versa. XLRs are a great choice for power as they lock into place, are easily replaced and benefit by being rugged.

The balanced XLR output was designed to be both powerful and flexible. You will find that the Firefly's output is much hotter than a regular DI. We chose to 'rev up the engine' so that the Firefly could be used 'live' with an active PA cabinet or even drive a power amplifier. We also added a 180° polarity reverse switch on the output that would enable the engineer to better phase-match the amp on stage with the PA system or when used with acoustic guitars, could help eliminate feedback by 'electronically moving' the acoustical peaks out of the way. This, coupled with the high-pass filter, makes the Firefly particularly effective when it comes to solving resonant feedback problems on stage.

The Firefly... just about as good as it gets!

After several years of development, testing various prototypes, and changing up the feature set until we got it right, we are pleased to say that the Firefly is finally here. And the results are nothing short of spectacular. The Firefly produces a warm rich tone with plenty of even order harmonic overtones that is sure to please the most demanding artist.