JDV-Pre™ Development

"Eliminate all types of distortion and a funny thing happens… things start to sound better."

The perfect instrument preamp would be one that amplifies the signal without introducing artefact, distortion or hype. It would simply capture the instrument and send it along without adding anything or taking anything away. And although this may sound easy, it is in fact the single biggest challenge in audio circuit design. The JDV-Pre may in fact be the first instrument preamp to approach this ideal.

Distortion… the good, bad and ugly

Now you may ask: What's the big deal? Distortion can be cool. It can be, but only if you want it. If the instrument is distorted before you have a chance to manipulate it, there is no going back. There is no way to fix it in the mix or EQ it out. Your clean white canvas is now painted some unknown color and from this point forward, you are living with compromise. The JDV-Pre is all about preserving the original sound of the instrument so that the colors you add will be true.

To do this, one must carefully navigate a rocky road to avoid the pot holes. In audio, the pot holes are the various types of distortion that abound. Few realize that there are in fact many types of distortion. Harmonic distortion is the one we hear when things get fuzzy when we overload the audio circuit. The linear performance of an amplifier is its ability to retain the relationship between all frequencies from the input to the output. Phase distortion is the amplifier's ability to keep all frequencies in relative time with each other and is most audible in the low end. Inter-modulation distortion is the electronic equivalent of dissonance – a good piano is able to reproduce dissonance so that it becomes pleasing to the ear. Some call it jazz. Get all of this right and the result is a warm sounding amplifier with beautifully cascading even harmonics that work together in true harmony.

From the very beginning audiophiles have always preferred the sound of a class-A amplifier. The full wave amplification stage design delivers unmatched depth and clarity. This is largely attributed to the fact that Class-A amplifiers do not suffer from zero-cross distortion that is invariably introduced with AB designs when the positive and negative half-waves must be rectified and brought back together. The JDV-Pre front end is pure 'class-A' and assembled using full size discrete parts. By eliminating zero-cross distortion, problems such as phase distortion and inter-modulation distortion are minimized.

For years, audio engineers have been trying to find a way to increase gain without using negative feedback loops. It does not take a rocket scientist to figure out that when you combine negative feedback in an amplifier (as shown here with the signal going through R2) with the positive signal, something has to give.

Herein lays the very essence of the JDV… audio pioneer John Vrtacic discovered a way to amplify the signal without introducing negative feedback and phase cancellation; Radial purchased the design in 1998 and we have been using it ever since. This is the foundation that makes the JDV-Pre unique.

The next concern is headroom. Headroom is basically the amplifier's ability to handle an instrument signal without distortion. Loud instruments such as active basses and digital pianos that can easily generate upwards to 7 volts are particularly troublesome as they can overload just about anything. The key to an amplifier's ability to handle hot signals is limited by the internal rail voltage. To give you a sense of scope, a typical active direct box has about 3 volt rails. A professional DI box like the Radial J48 has 9 volt rails. A small Mackie mixer will have between 12 to 15 volt rails, while a large format recording console like an SSL or a Neve will typically have 24 volt rails. The JDV-Pre employs the +16/-16 (32 volts) in the Workhorse to deliver 30 volts… plenty of signal handling capacity! With so much headroom, dynamics are not compromised and harmonic distortion is significantly reduced at all frequencies. This includes deep bass around the 20Hz region where power is greatest and controlling distortion is most difficult.

The Low-Down on Loads
Another challenge in designing the JDV-Pre was that it had to work with all types of instruments. In other words, it had to be flexible enough to handle the sensitivity of a piezo equipped violin while being robust enough to handle any type of bass. Piezo instruments sound best with a very high input impedance. But this is a double edged sword: the higher the impedance, the more prone to noise it becomes. Most manufacturers compromise with a 1 meg-ohm input. And although 1 million ohms may appear to be sufficient, players and engineers often complain that their instrument sounds peaky, glassy or uneven. We quadrupled the input impedance to 4 meg-ohms to solve the problem and carefully designed the circuit to eliminate noise. With a JDV-Pre, you can connect a violin, contrabass or acoustic guitar without some form of booster preamp in between.

Magnetic pickups are different. Once connected, they become part of the electrical circuit. To address their special needs, the JDV-Pre is equipped with a function called Drag Control™ that enables the engineer to adjust the load so that it replicates the sound of connecting a guitar directly to a tube amplifier. The problem here is simple: most electronic engineers use scopes and meters to determine how things will sound. Musicians and audio engineers tend to listen using a very sophisticated interface called the ears and process this information using the brain. We found that solid state circuits sound different from tube circuits (go figure) and a lot of this had to do with how the pickup was loaded by the circuit. Drag Control emerged as a means to adjust the resistance and capacitance that is being applied to the pickup so that it is 'fooled' into thinking it is connected to a tube amp. It works and sounds amazing.

Adding a Microphone to the Mix
After we finished designing the JDV-Pre we realized we had omitted a major function: There was no way to connect a microphone to it. With so many companies such as DPA and Shure now making miniature instrument mics, we figured that it would be smart to add this feature.

To make this happen, we converted the XLR input on the 500 series rack or Workhorse into a microphone input. We then added phantom power capabilities via a small 'set and forget' switch above the card edge connector. To bring up the mic level so that it would match louder instrument levels, we then added a high turn ratio input transformer into the signal path. This passively boosts the signal without introducing noise while adding the warmth that is often associated with vintage style audio circuits. During listening tests we were astounded… the JDV-Pre sounds wonderfully warm with all types of microphones. This may in fact become a favourite mic pre for recording vocals!

Controlling Resonance
Finally, we discussed the problems that Nashville recording engineers have to contend with when layering acoustic instruments. It has to do with resonance and how each instrument can interact with the other. For instance, layering a banjo on top of an acoustic guitar on top of an upright bass can sound amazing… but each has a natural harmonic structure that makes each instrument unique. Layering the low frequencies together can often result in dissonance as the low frequencies resonate. The JDV-Pre is outfitted with a variable high pass (low cut) filter that begins at 5Hz (essencially off) and rises to 500Hz. This enables the engineer to carefully sculpt the bottom end in such a way that each instrument can retain its uniqueness while eliminating the problem resonance. Simple yet effective.

Tremendous care and attention went into the development of the JDV-Pre so that it would be equally at home in the studio or when used on the most demanding live stage. By reducing phase distortion, eliminating zero-cross distortion and increasing the headroom, a funny thing happens… things start to sound a whole lot better.

The JDV… the most natural sounding instrument preamplifier ever made.