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Radial Engineering Firefly

Radial Engineering Firefly

Radial build some of the best-performing and most reliable DI boxes, switches and interfaces. Not even their staunchest supporters would accuse them of styling frivolity though, so the orange steel enclosure of the new Firefly Tube Direct Box, with its smoothly curved black steel handle and air of insouciant elegance was a pleasant surprise. Otherwise, the Firefly is typical of Radial: it boasts more controls, capabilities and connectors than I’ve seen before on a DI box.

There are two selectable inputs, A and B, each of which has a dedicated Trim control, allowing you to keep two instruments plugged in and balance their levels (a useful trick in a stage environment.) The latching Select control switches between inputs and this function is paralleled on the optional JR-2 footswitch and JR-2DT desktop remote, which also enable you to mute the active channel. Next to the Trim controls sit the switch and recessed Load Adjust potentiometer of Radial’s innovative Drag Control. This allows you to vary the Firefly’s input (load) impedance from 22-500 kΩ to suit your instrument’s pickups. Radial reckon that for single-coil pickups the 12 o’clock position is equivalent to plugging into a Fender valve amp. When Drag is off, the input impedance rises to 4MΩ, which will suit most non-preamped piezo pickups and any active inputs (guitar preamps, keyboards).

A Low Cut knob brings in a fully variable high-pass filter between 25 and 500 Hz. As well as cleaning up low-level subsonics and rumbles, this allows you to limit the Firefly’s low-end response when using it with acoustic instruments. There’s no point in having 50Hz available if you’re playing violin, for instance! A Level control suggests there’s some boost available inside the Firefly — something which is oh so true! One point to note, though, is that there’s no legending on the scales surrounding the rotary controls, so you’ve no idea where you are in terms of level or frequency. This makes repeatability a bit more hit and miss than it might be.

Radial Engineering FireflyIt’s useful to understand this box’s signal flow. From either input, the signal passes through the Drag control, is buffered and then passes through its Trim control. The signal is then split into three parallel feeds for the buffered, high-impedance, always-on Tuner output; the source selector switch for the transformer-isolated, high-impedance Thru output; and, last but not least, the Class-A signal chain that runs from the 12AX7 tube stage to the balanced XLR output. The 12AX7’s two halves are configured as a Class-A common cathode amplifier and a cathode follower that buffers the signal before it passes to a Class-A high-voltage FET buffer. The Insert send/return stage (accessed via a TRS jack and relay) comes next and is followed by the Level control. From there, the signal passes through the Class-A stages of the high-pass filter, a second high-voltage FET buffer and, finally, to the transformer-balanced XLR output.

In addition to the XLR, the Firefly’s rear panel carries the input, output, insert and JR-2 jacks, three recessed switches for phase inversion, ground lift and Thru (pre or post tube chain), source selection and the five-pin external power supply inlet.

Radial Engineering FireflyThe Firefly is a joy to use: it does exactly what you’d want and expect from a tube DI box and then some! At lower input and output levels the XLR output is clean, but with just a (pleasant) hint of valve character from the tube stage. Opening up the input Trims brings that valve character to the fore, and winding up the Level will push the transformer into saturation. With a maximum gain of +22dB available, and given a suitable source, the Firefly is capable of driving an active PA loudspeaker or even a power amp. Having an insert loop is really useful, as it means that you can play through your normal effects and feed an amplifier or monitor system from the Thru, whilst at the same time giving the engineer a clean feed from both the Tuner output and, if you’re not using it, the Thru. When deciding if this product is for you, the price will probably be the main consideration: like many of Radial’s products, the Firefly Tube Direct Box delivers uncompromisingly professional performance, but at a professional price. Radial do offer more affordable DI boxes but I’m very tempted to hang onto this one! 

Firefly £658.60, JR-2 Footswitch or JR-2DT Desktop Remote £119 each. Prices include VAT.

Firefly $559.99, JR-2 Footswitch or JR-2DT Desktop Remote $99.99 each.


Frequency response: 20Hz to 20kHz

Maximum gain (quarter-inch input to XLR out): +22dB

Noise: -98dBu

Dynamic range: 100dB

Equivalent input noise: -100dBu

Input impedance (Drag control off): 4MΩ

Input impedance (Drag control on): Variable from 22 to 500 kΩ

Output impedance: 200Ω (nominal)

Low-cut filter: Variable from 25 to 500 Hz (-3dB point)

Size: 5.75 x 8.25 x 1.75 inches; 2.75 inches with handle. (146 x 210 x 45 mm; 70mm with handle.)

Weight: 3.95lbs (1.8kg)

Power supply: ±16V, 1600mA