Comparing an active direct box to a passive DI is akin to comparing a condenser microphone to a dynamic. Both have benefits and advantages. But what few realize is that an active direct box is in fact a pre-amplifier. And as anyone in audio knows, you can find mic-preamps for $99 or spend as much as $5000, depending on the brand, hype, features and quality expectations you may have.

When the first direct boxes appeared, they were simple transformer based devices. The passive DI boxes took the signal from the instrument and split it between the musician's stage amplifier and the PA system, both changing the impedance and balancing the signal at the same time. A problem with passive DI boxes became apparent when musicians using low output instruments realized that the sound from their bass would be thinner than when connected directly. This effect became known as 'loading'.

This created a need for the direct box to buffer the signal or amplify it so that the draw going to the PA would not affect the signal going to the amplifier. This worked well except that in order to power the active direct boxes circuit, one had to either provide local AC or use a battery. Finding AC may sound simple but more often than not. AC is rarely where you really want it on stage and this means bringing out extension cords. Batteries do not fair much better. When fully charged, batteries work fine, but as the charge diminishes, distortion increases and invariably, batteries fail in the most inopportune times. (Like in the middle of a song!). This same problem was occurring with microphones. Folks wanted condenser mics on stage, but due to powering restrictions, their use was limited.

Enter the Phantom

In the 1960s, condenser microphone pioneer Georg Neumann invented phantom power. His concept was simple: if he could find a way to remotely power his microphones, they could be used along side dynamic mics on stages around the world. Phantom power presents a relatively high voltage to the microphone's internal amplifier that polarizes the capacitive plates (diaphragms). Following the same idea as high-tension power lines, a high voltage was employed with a very low current. This concept worked extremely well allowing the power totraveldistances - over a thousand feet (300 meters) and suppl the microphones internal electronics.

Some 20 years later, as low cost, high efficiency integrated circuits became available, the first phantom powered direct boxes appeared. These worked extremely well! Back then, virtually all bass guitars were passive instruments made by Fender such as the Precision or the Jazz bass, the common stage piano was the Fender Rhodes and acoustic guitars were outfitted with low output Barcus Berry pickup systems or magnetic offerings from Bill Lawrence. The reason these old DI boxes worked well was that these instruments had one thing in common: They all had a low output.

The world changed

Today, acoustic guitars are equipped with built-in active preamps with tons of output level. The majority of all bass guitars have one and sometimes two batteries in them and tons of on-board active electronics. Pianos and synthesizers have gone digital with more dynamic range than ever before and electronic drums and digital sampling is now standard on most stages.

The problem is that these high output instruments have so much level; they overload the direct boxes of yesteryears. Now you may ask: why not simply increase the level handling of the direct box? And rightfully so: we asked ourselves the same question. The answer is that when Mr. Neumann invented phantom power, he never once considered that his powering scheme would be used to supply an instrument preamplifier (ie: a direct box) and therefore, never provided sufficient current to do the job. Without current, there is no power without power, you have no headroom. Without headroom, the moment you drive a loud signal into the circuit, it overloads. It chokes. When you exceed the operating voltage, the signal gets clipped. In essence, it immediately transitions from say 0.05% distortion to 100% distortion. And worst of all, these chips and circuit designs which were optimized to work under limited current, produce odd-order harmonic distortion, which causes a harsh, undesirable tone.

A digital revolution

This of course led us to look for a solution. We needed more power to drive the circuit in effort to increase the rail voltage. Batteries were out of the question. Local powering was also out. We noticed a new technology being used in power amplifiers known as class-D or digital switching. These new high efficiency amplifiers are able to produce thousands of watts of power while drawing very little current. This pointed us to creating a similar solution inside the Radial J48.

What we are in fact doing is taking 48 volt DC phantom power, converting it to AC, then running it through a proprietary power transformer using a high-speed digital switching circuit. This provides us with almost three times more voltage found in other direct boxes and enables us to create a circuit that is not based on compromising tone for efficiency; it is based on reducing distortion and delivering pristine signal transfer.

This innovative circuit also uncovered another valuable advantage quite by accident: the transformer served to isolate the power which in turn, reduced hum & buzz caused by ground loops. Keep in mind that if a transformer is used on the XLR at the output, 48 volt phantom power cannot be used as the DC current will be blocked by the transformer. If transformer isolation is used on the input, the same loading effect as described earlier reappears and we may as well use a passive DI box as we have not solved the primary goal. By transformer isolating the signal 'inside the circuit', we achieve a wonderful balance of both performance and reduced noise.

We then took the Radial J48 a step further by adding a number of useful features: To begin, the input impedance at 220kohms is high, but not so high as to cause the instrument to sound glassy. Through experimentation and lots of listening, higher 1 MegOhm inputs offered by others work reasonably well with piezo pickups, but they tend to sound edgy with most basses and guitars.

We then added a smooth 80 Hz low-cut (high-pass filter) that would both increase headroom by reducing power-hungry bass while eliminating low-frequency resonance, the prime cause of acoustic feedback on stage. We incorporated features such as a ground lift (not at the XLR out but inside the circuit), 180º polarity reverse to help align the PA and reduce run-away resonance when stage monitors conflict with onstage amplification.

Finally, we packaged the J48 in a protective book-end design to provide the switches and jacks with a protective zone. Internally, a unique I-beam construction provides a rigid monocoque platform that eliminates any opportunity for the PC board to torque, further reducing field service problems. A full bottom isolation pad provides both mechanical and electrical isolation against the constant abuse on stage while reducing the opportunity for steel-on-steel contact with electronic devices such as bass ans guitar ampswhich of course could introduce system noise or electrical shock.

Great features, low distortion and natural sound combine to make the Radial J48 one of the most popular professional direct boxes used today.