Audible Illusions (AI) belongs to an elite group of manufacturers who have been at it for over 30 years. That sort of longevity reflects an uncommon degree of commercial success and consumer acceptance. Any audiophile who was around in the 1980s and 90s most likely has had a close encounter with an AI Modulus preamp. From the start, company head Art Ferris’ guiding principle has been that “simpler is better” and he is still convinced that simple, well-designed products have a place in high-end audio. Over the years, AI had charted an evolutionary path forward slowly upgrading and refining its product line the old-fashioned way—via better passive parts and power-supply enhancements. About ten years ago Art closed AI’s California factory and relocated to his childhood home in Daytona Beach, Florida. His plan was to service AI’s existing customer base by providing repairs and updates, and building a few new units each month. That plan didn’t work out so well. In his own words: “Unfortunately, trying to run a business this size out of my house became impossible. Since my wife would’ve left me if she kept seeing a constant stream of boxes coming and going, I had no choice but to build a new factory to produce our new designs and restore domestic tranquility.” In addition to re-designing the AI product line, Art has been building a new production facility in Ormond Beach, Florida. He says that while the new factory is smaller than the California operation, he feels it is more efficient with better production continuity.
The L3A is the first preamplifier AI has submitted for review in over 15 years and represents a perfect example of Art’s design approach. An external power supply is used to isolate and shield the audio circuitry from the power transformer’s stray EMI. It connects to the main chassis via a thick umbilical cord. There’s only a single voltage gain stage, using a twin triode configured as a common cathode amplifier without either global or local negative feedback. That’s as simple as it gets in tube electronics. In place of a cathode follower, the two triode sections are wired in parallel with the concomitant benefit of reducing output impedance by a factor of two to a reasonable 1200 ohms—not quite cathode follower territory but sufficient to accommodate moderately long cable runs and facilitate compatibility with low-input-impedance power amplifiers. The use of a single gain stage means that the L3A main outputs (but not the buffered tape and surround-sound outputs) invert signal polarity. Since no phase-inversion circuitry is used, be sure to reverse speaker cable polarity at either the power amp or speaker terminals in order to maintain correct absolute polarity.
Each cathode resistor is bypassed by a 470µF “Black Gate” cap to maintain maximum gain and obtain the lowest output impedance possible. It turns out that AI is sitting on a large stockpile of these caps left over from the good old days when they were used exclusively in the M2 preamplifier series. The Rubycon Black Gate, now out of production, was considered by many, yours truly included, to be the best-sounding electrolytic available. This cap is itself bypassed by a 100pF polystyrene cap that can be switched in and out of the circuit (more about that later). Black Gates are notorious for demanding a long break-in period (50 hours according to Ferris), but since the L3A also uses proprietary film-foil coupling caps that require a minimum break-in period of 100 hours, plan on at least that many hours before the L3A fully settles in.
By the early 1980s several preamp manufacturers followed Audio Research’s lead and shifted from the ubiquitous 12AX7 to the 6DJ8/6922 frame-grid tube; although it should be pointed out that the Marantz 9 amplifier was the first high-end product to use it, as early as 1960. Winding the grid tightly onto a rigid frame allowed closer cathode-grid spacing without the risk of shorts, the payoff being high transconductance. In the beginning, AI embraced the 6922, the industrial version of the 6DJ8. Then, in the mid-1980s, Art discovered the Russian 6H23n-EB, really a premium version of the 6922. Not only is it a good-sounding triode, but it also happens to be rugged, exceptionally linear, low in microphonics, and, according to Art, very stable in his zero-feedback tube designs. Subsequently, all of AI’s Series 3 preamps since 1989 have this Russian tube installed in the linestage. Note that AI no longer recommends Russian-branded (e.g., Sovtek) 6922s due to poor quality control. AI will continue to use NOS Russian-military 6H23n-EB tubes until AI’s Russian distributor can no longer supply them, at which time Art says he will probably switch to the Russian 6H30 tube. While the two triode sections are connected in parallel, the cathodes are split and biased using individual cathode resistors. This enables the preamp to have more drive.
The L3A evolved from the L3 linestage preamp, which was introduced about three years ago. Modifications were primarily to the external power supply. It now benefits from a 30% larger toroidal power transformer. An unusual but really nice touch is the large common-mode filter choke located at the input to the power transformer, which greatly minimizes low-frequency noise on the incoming AC line. All power supplies, including the high-voltage B+ and filament supplies, are now regulated, ensuring stable voltages regardless of AC-line variation. AI has been a proponent of keeping its preamps continuously in standby mode; when the front-panel power switch is off, filament supplies are kept on, while plate voltage is reduced to 10% of full value to allow just a trickle of current through each tube. Art credits the late Bill Johnson at Audio Research for this suggestion since without some current flow cathode poisoning is a serious possibility. The most severe type involves the growth of an interface resistance layer between the cathode and its oxide coating.
Early Modulus One preamps using 6DJ8s and 6922s were switched on/off in the conventional way, and Art was recording premature tube failure in these units when compared with preamps that were left on continuously. His tests established that the thermal shock involved in cycling a tube on and off was a definite cause of premature tube failure. All Modulus preamps produced since 1989 incorporate this standby circuit. Art tells me that during the past 30-plus years of servicing preamps, some of the units received were as old as 20 years and yet still had their original tube complements intact—compelling evidence that the standby circuit works. However, while many of these old tubes still measured low in noise and microphony, listening tests revealed that there was a definite softening of high-frequency response, though this is likely a common phenomenon in all audio tubes used for extended periods. AI recommends that, for optimum performance, customers should change tubes every two years. There is now a rocker switch on the front of the power-supply chassis. This switch is designed to completely shut off the preamplifier instead of leaving it in continuous standby operation. If you’re planning on an extended vacation or if a major thunderstorm is brewing, fully turning off the preamp makes perfect sense.