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Audible Illusions L3A Line Preamplifier

Audible Illusions L3A Line Preamplifier

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.

 

I was pleased to see that the L3A features a remote volume control, a motorized Alps pot. My enthusiasm was a bit dampened when I realized that this pot is a master control that feeds the traditional, Modulus-style, left- and right-channel stepped attenuators. In effect, the signal goes through two volume controls before getting to the grid of the triode gain stage. Since the stepped attenuators are used to control both channel balance and volume, the user manual suggests setting the stepped attenuators at the 12 o’clock position as a starting point, but don’t be afraid to experiment in this regard. Input from a low-gain phonostage may require nudging the attenuator settings to 2 or 3 o’clock. In general, I found it sonically best to fine-tune the sound by setting the stepped attenuators higher and cutting back on the master control level.

Serious auditions commenced after a proper break-in period. To say that I was surprised by the musicality of the L3A would be an understatement. I don’t think that the engineer in me expected what is after all a “plain vanilla” audio circuit to sing so beautifully. Of course, it is probably a common misconception to prime oneself to anticipate great sound only from complex new circuits. Art related the following true story, which describes my reaction as well. “Back in the mid-1980s my friend Joe Grado traded me one of his tonearms and cartridges for a Modulus. He fell in love with it. Trying to understand how it worked, he takes it out to Sea Cliff and puts it on Sid Smith’s bench. He then calls me saying, ‘Art do you know what you’ve done? This linestage is wonderful! You’re going to sell thousands! But a word of caution; somehow you managed to capture the little Music Man in the design. Don’t ever lose him.’ That was great advice and I’ve tried very hard not to lose the little Music Man.”

 What captured my attention first was the precision with which the L3A enunciated transients. Both the leading and trailing edges were superbly delineated. As you can imagine, the payoff was exemplary clarity. A spectacularly lucid midrange aided in resolution of low-level detail. It felt as though a high-power microscope was focused on the inner layers of the soundstage. Complex passages were resolved without any veiling. Solid-state preamps can do this as well, but none that I’ve auditioned can fully sculpt image outlines the way the L3A does, or erect a spacious soundstage with the same conviction. In these respects, the L3A clearly and proudly displayed its tube heritage. The sonic character was slightly warm, but it never strayed far from tonal neutrality. As a result, timbral fidelity was superb. Harmonic textures were free from the gratuitous brightness or treble brashness that wreaks havoc with the reproduction of violin overtones or the upper registers of soprano voice. Much of the credit for this level of purity must go to the Russian 6H23n-EB.

The L3A never sounded inherently romantic or as colorful as a vintage tube preamp. Neither did it project that big tone sound that some 6SN7-based designs are capable of. It therefore begs the following question: Is it possible for a tube preamp to sound musically gorgeous without being overtly euphonic? The L3A made it perfectly clear that this is definitely possible. Any preamp that can caress musical lines with velvet gloves and convey the natural ebb and flow of harmonic textures is a winner in my book. Let’s talk about the frequency extremes. There was nothing to complain about in the bass range. Bass reach and pitch definition were terrific, limited only by the performance of the associated power amplifier and loudspeaker coupling. The treble range initially sounded a bit polite, lacking in air if you will. That was before I switched off the 100pF polystyrene cathode bypass caps (the Black Gate bypass caps are fixed and cannot be removed from the circuit). Art explained that these caps provide a means of voicing the preamp’s treble range to suit one’s listening tastes or system requirements. After the initial break-in period, AI advises the owner to remove the top cover and access the four red switches located at the rear of the PCB and switch all of them to the Off position. Doing so helped noticeably in opening up the treble range. However, apparently many owners prefer the sound with the switches in the On position, which would certainly make sense in the context of taming either a bright front end or hot-sounding speakers.

One of the most difficult attributes to gauge is macrodynamics, the ability of a particular component to convincingly scale the range from loud to very loud. Loudness after all is a perceptual quantity, the relationship between sound pressure level (i.e., intensity) and loudness being nonlinear. Except at very low intensities, the auditory system responds in a logarithmic fashion; a factor of two increase in loudness requires roughly a factor of 10 increase in intensity. Of course, spectral content is also a major perceptual factor as can be seen from a study of the Fletcher-Munson equal loudness curves. With these caveats in mind, it seems obvious that judgment calls in this regard carry a large margin of error. Still, I’m confident in declaring that, as far as macrodynamics are concerned, the L3A fell short of a perfect 10. It didn’t equal the dynamics of the much more expensive Pass Labs XP-30 line preamp or match the explosive dynamics of the inexpensive Berning microZOTL2.0 personal amplifier used as a line preamp. The situation was much different when it came to microdynamics. Nuances essential for fully fleshing out the music’s passion were reproduced in compelling fashion.

The L3A line preamp distills the best attributes of modern tube sound: swift transients, a detailed presentation, and natural yet non-euphonic textures. It is its insistence on the sonic truth that defines it as a true reference line preamp. The L3A’s ability to retrieve the music’s rhythmic drive and drama make it one of the most sonically persuasive line preamps on the market and one of the best I’ve auditioned over the years at any price. If your goal is to conjure up audible illusions on a grand scale, this is your preamp!

SPECS & PRICING

Gain: 30dB line out; 0dB tape & surround-sound processor outputs
Distortion: 0.10% at 1.5V, 20Hz–20kHz into 50k ohms
Frequency response: 2Hz–100kHz, +/-1dB, 
Signal-to-noise ratio: 90dB below 1.5V
Max input: 9V at 1% THD.
Signal phase: Main out, inverting; tape and s-s processor, non-inverting
Input impedance: 50k ohms, all inputs
Output impedance: Main, 1.2k ohms; tape, s-s processor, 150 ohms
Dimensions: Main chassis, 19″ x 3.5″ x 17″; power supply, 6″ x 5″ x 10″
Weight: 19.5 lbs.
Price: $3995


AUDIBLE ILLUSIONS
P.O. Box 2537
Daytona Beach, FL 32115
audibleillusions.com

Associated Equipment
Analysis Audio Omega and Basszilla Platinum Edition DIY loudspeakers; VTL Manley reference series 100/200 and First Watt SIT-1 monoblock amplifiers, First Watt F7, Futterman H3, and Atma-Sphere S-30 stereo amplifiers; Kuzma Stabi Reference, Technics SL-10, Revox B795, and Sony PS-X600 turntables; MacBook Pro laptop running Amarra V3.03 software, April Music Eximus DP1 DAC; ModWright modified Sony XA-5400ES SACD player: FMS Nexus-2, Wire World, and Kimber KCAG interconnects; Kimber KCAG speaker cable; Monarchy Audio AC-Regenerator; Sound Application power line conditioners 

By Dick Olsher

Although educated as a nuclear engineer at the University of Florida, I spent most of my career, 30 years to be exact, employed as a radiation physicist at the Los Alamos National Laboratory, from which I retired in 2008.

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