When the Radial JDI was introduced in 1996, it revolutionized many preconceived concepts of what a direct box is and how a passive DI can, in many instances, be a better choice than it’s active counterpart.

The central task of a direct box is to convert an instrument’s signal from high impedance to balanced low impedance. This enables the signal to travel long distances without adding noise and provides a proper load to the mixing console input for improved audio performance.

Active direct boxes employ electronic circuits to change the impedance from hi-Z to lo-Z and balance the signal. Passive DI’s employ a simple passive transformer to do the work. Active DI’s require batteries, 48V phantom-power or external power supplies to power their circuits. Passive transformer-equipped direct boxes do not require any power to perform their task. The advantage of a transformer over typical active circuits can be summarized as ‘a great device to reduce system noise’ in a sound system.

A transformer is an ingenious device that works by converting an electronic impulse, generally referred to as a current, into a magnetic field and then back again into a current. This ‘magnetic bridge’ has the distinct advantage of decoupling the source instrument from the receiving mixing console or mic preamp.

When you hear a hum or buzz in the sound system, this is usually caused by stray voltages traveling through the wire causing havoc at the receiving end. This effect is commonly referred to as a ground loop. The most effective solution to this common malady is to place a transformer between the two offending devices.

If the solution is so easy, why aren’t all direct boxes passive? The trick lies in the quality of the transformer. The Radial JDI, the Duplex, and the JD6 all employ the world’s finest audio transformer, Jensen, to manage the task. The problem lies in the fact that high quality transformers are expensive. These employ hand-stacked and laminated nickel core materials, precise winding schemes and expensive mu-metal Faraday shielding, all of which adds up to big bucks. Cheap transformers employ iron core materials and as a result, tend to saturate quickly, causing harmonic distortion and phase shift resulting in groups of frequencies exiting the transformer at different intervals or out of phase. Bottom line – cheap transformers sound bad.

The quick solution is to employ an active circuit to balance and convert the impedance. Although this works and active DI’s have their advantages, they do not provide any isolation and therefore are ill-equipped to eliminate ground loops.

ProDI uses a specially made custom audio transformer.

The Radial ProDI and ProD2 direct boxes employ a custom-made transformer that is fashioned around the Jensen transformer we use in the more expensive Radial JDI. No, the ProDI is not quite as good as our industry standard JDI (see comparison below), but considering it is half the price, the performance is nothing short of impressive!


Radial ProDi's custom designed audio transformer exhibits virtually flat response throughout the entire audio range.

Technical Comparison	Radial JDI	Radial ProDI
Frequency response	5Hz to 50kHz	10Hz to 30kHz
Phase distortion at 100Hz	0.1 degree	0.5 degree
Linear response -3dB down	15Hz to 30kHz	20Hz to 15kHz

Feature Comparison	Radial JDI	Radial ProDI
Inputs & outputs	¼" in/thu, XLR out	¼" in/thu, XLR out
Input pad	Yes, -10dB	Yes, -10dB
Merge function	Yes	No
Polarity reverse	Yes	No
Ground lift	Yes	Yes
Speaker signal pad	Yes, -30dB	No
Mil-spec PC board	Yes, 2-sided	Yes-1 sided
Internal I-beam construction	Yes	Yes
Book-end outer shell	Yes	Yes
Full-bottom rubberized pad	Yes	Yes
Multi-sided screening	Yes - 3 sides	No - top only
J-Rak mountable	Yes	ProD2 - Yes ProD1 - No

The best feature of the ProDI (and ProD2) is 'plug & play' ease of use. All you do is connect the instrument to the ¼” input(s), the ¼” through jack(s) to the stage amplifier(s) or mixer and the XLR output(s) to the mixing console and you are done. Any distortion? Hit the -10dB pad. Any noise? Just depress the ground lift switch.

You will find that with acoustic guitars and basses, the natural tone of the instrument will shine through with a smooth and musical rendering. For electrically powered devices such as keyboards, the ProDI will immediately eliminate troublesome ground loops while delivering solid dynamics with a full rich sound.