Jadis JPL line preamplifier
After a yearlong scrutiny of the state of the art of line-stage design, two underlying principles suggest themselves. First, the Gods, in having decreed that man shall labor long and hard in search of the perfect preamp, must surely be crazy. In the trek toward sonic perfection, mistakes are frequently made. The attempt to coax the signal from the program source and nurture it to its full musical potential is fraught with labor pains. Like the political process, the audio signal is subject to corruption. Small sins early in the chain may become capital offenses by the time they reach the loudspeakers.
That there is only a handful of great-sounding preamps out there (line-level or otherwise) is evidence prima facie of the difficulty in caring for the audio signal at its formative stage. Aided by these few centurions of sonic truth, the music can bloom, filling the soundstage with the fire, drama, and power that only live music can communicate. Too often, however, the preamp sinks the ship, and the "illusion of live" descends from the realm of the plausible into the realm of yellow cling peaches. Most preamps can hope to simulate the flavor of a fresh peach only to the extent afforded by the canned variety.
Second, I feel it essential for a preamp to incorporate the magic of the vacuum tubeespecially where digital source material is concerned. In my experience, the ultimate sound of any CD player or digital processor is dependent upon the type of associated line-level stage. If you doubt this for even a moment, I invite you to audition the Theta DS Pre Generation III processor/preamp in my listening room. Through the DS Pre's own solid-state line-level stage, the sound quality deteriorates to the point that the bloom and dynamic breadth of the music are largely squashed. The resultant harmonic textures are convincingly solid-statish, the overall effect being to subdue the vital link between perception and belief. Route the DS Pre's analog output from Tape Out to a good all-tube preamp or even a good hybrid design, and the sound quality changes dramatically for the better. The illusion of a real musical event ebbing and flowing before my ears becomes enormously heightened. For whatever reason, it's clear (to me at least) that digital sources require a tube buffer prior to the power amp. This is yet another manifestation of "Futterman's First Law of Audio": Thou shalt use a vacuum tube as early in the amplification chain as possible.
"Wait a minute," I hear some of you complaining. "If you're so hot about tubed preamps, why has the Threshold FET-10/e line stage lasted so long in your reference system?" Good question. Let me remind you that nothing is sonically right if it's harmonically wrong. And my master tapes have long told me that the FET-10/e line-level preamp really got the upper-midrange/lower-treble tonality right. This is the frequency range that makes or breaks soprano voice, and the Threshold didn't let me down. But deep in my heart I knew that its mastery over soundstaging and dynamics was less than perfect. Memories of the Conrad-Johnson Premier Three periodically flooded my consciousness. The way the C-J sculpted image outlines was a sound to behear. Certainly, no solid-state preampincluding the Thresholdcame close in this respect.
The final straw was my exposure to the Convergent Audio Technology SL-1 preamp (reviewed in December 1992 by Jack English). During the several weeks the CAT resided in the system, I connected with the music like never before. My level of sonic expectation would never again be the same. The CAT showed me that harmonic integrity, palpable imaging, and dynamic range can be bundled together in one package.
It was into this environment that the Jadis JPL made its grand entrance.
The Jadis JPL is a thing of beauty, a beguiling French damsel. The gold inner fascia set off against the chromed chassis finish looks positively luscious. Sitting as it did amid some pretty drab-looking neighboring gear in Bright Star Audio's "Rack of Gibraltar," Lesley had no trouble at all picking it out.
"Oh, what's that?" she asked.
"That there, my love, is the El Dorado of preamps."
Having looked over the JPL's schematic, it's difficult for me to objectively identify the source of its sonic magic. The design, by Jadis's Andr;ae Calmettes, is pretty conventional. The four line-level inputs and one tape loop are routed through three 12AX7 dual triodes (footnote 1). Voltage gain on the order of 35dB is provided by the first two 12AX7s, which are cascaded together. The final tube in the chain, used as a cathode-follower buffer stage, allows the use of long cable runs to the power amp without the danger of treble rolloff. There's also a dedicated CD input that uses a single 12AU7 as a buffer stage (unity gain). This input is DC-coupled to the 12AU7's grid, while the regular line inputs are AC-coupled via a 1;uF capacitor. The quite beefy power supply deploys solid-state bridge rectifiers followed by a capacitive filter network. As a final touch, active regulation is provided for the tube plate voltages.
A large circuit board accommodates the entire active signal path, and construction quality and part selection appeared to be nothing short of excellent. Stereo volume and balance pots adorn the front face. The mute switchan especially useful feature for someone like meallows record and interconnect cable changes without adjustment of volume. The unit mutes automatically for a couple of minutes when powered up. For best sonic results, Jadis recommends that the unit be left on continuously.
So where does the JPL's magic live? In my opinion, it's in the details: the power supply, the selection of passive parts, and the execution of the circuit. You meter-readers out thereyou know who you are, you whose modus operandi can be summed up in the motto "parts is parts"please take note: As H. A. Hartley put it many years ago, the sonic difference between a Stradivarius or an Amati and a mass-produced fiddle is literally in the stuff of which the instruments are made. Ditto for the difference between a Steinway and a Yamaha. It's not easy to measure sonic differences between violins or pianos, yet the musical ear has no problem at all in instantly resolving such differences. Build the same circuit with Radio Shack parts and with premium parts selected on the basis of active listening tests. Those who feel that the Radio Shack version would sound as good as or even better than the audiophile alternative are directed to read Ben Duncan's "Harmonic Convergence" article in the October 1992 Stereophile (p.78), where he discusses the measured results of just such an experiment.
The JPL spent its time exclusively in my reference room, where it was complemented by the Sound-Lab A-1 ESLs and a variety of power amplifiers, most notably the Air Tight ATM-3 (review forthcoming) and the Fourier Components Sans Pareil OTL monoblocks (reviewed in June '92). I tried CD program material with both the CD and line inputs. At least with the Theta DS Pre Generation III, I found the line input to give me a fuller palette of dynamic shadings, so I stuck with the line input for the duration of the evaluation. I also used the JPL in conjunction with the Threshold FET-10/e phono preamp for all of the analog listening sessions. The JPL did benefit from being left on continuously, particularly in terms of detailing and textural purity. Still, I'm a bit nervous about leaving tubes to cook indefinitely. Tubes are thermionic devices, depending for their operation on electron emission from a very hot cathode surface. The mere act of electron emission means slow but sure disintegration of the emissive surface. It pains me to think of all those premium tubes suffering so.
Memorable first impressions happen occasionally in this business, but nothing like this. It would not be an exaggeration to say that I had to pick my jaw up off the floor. I was immediately and overwhelmingly won over by the Jadis.