In late 1972, Audio Research Corporation released what would become one of ARC’s bestselling preamplifiers of all time: the SP-3. While available, the SP-3 earned praise for its low noise levels, wide soundstages, and awe-inspiring transparency. More than one audio publication described its overall sound character as “a straight wire with gain.” ARC sold thousands of SP-3s at $595 USD before discontinuing the model in 1976, but what I find interesting is how much of the SP-3 remains evident in more recent ARC models. By today’s standards, the SP-3’s faceplate of brushed bronze over satin black, rotary analog controls, and big pushbuttons look dated, but the three-box segregation of the faceplate and orientation of controls aren’t all that different from those of ARC’s SP-20 preamplifier ($9000, recently discontinued). Then there was the SP-3’s tube complement: six 12AX7s in the analog stage with two 12AX7s in the tone control circuit. This arrangement in the analog stage, albeit with different tubes, can be found under the hood of the subject of this review: ARC’s Reference 6 preamplifier.
The Reference 6 ($15,000) replaces Audio Research’s Reference 5SE ($12,995 when available). In discussions with ARC’s Brand Ambassador, Dave Gordon, I learned that the Ref 6 shares very little with the Ref 5SE, and is actually based on their current flagship preamp, the Reference 10 ($33,000). At first glance, the most obvious upgrade is the appearance. As in ARC’s G and Foundation series, the exterior of the Ref 6 was designed by the visionary Livio Cucuzza. The Ref 6 is structurally more advanced than the Ref 5SE: the front and side panels are all machined from thicker slabs of aluminum, and the chassis they’re bolted to has been stiffened. As in the Ref 10, the Ref 6’s top and bottom plates are made of transparent acrylic instead of aluminum; I asked Gordon if this is done purely for aesthetic reasons:
While an aluminum top cover is offered as a no-cost option, we have found that many products, including our Reference preamps, sound better when the metal cover is removed. The acrylic covers sound like no cover is installed. These acrylic panels, in combination with our heavier chassis, provide more structural rigidity. And this, in tandem with our isolation feet, helps to dissipate electrical and mechanical interferences.
Measuring 19”W x 7.8”H x 16.5”D and weighing 37.5 pounds, the Ref 6 not only looks and feels more substantial than the Ref 5SE, its ergonomics are better. Two large knobs bear a striking resemblance to those on the SP-3 and have been put back where they were on that model, where they belong: Input at left, Volume at right, flanking a large, dimmable, vacuum-fluorescent screen that displays the volume level, signal source, and mute status, all easily readable from across the room. Attractive, gloss-black trim surrounds the knobs and display, and highlights ARC’s logo and the model name. At far left and right are ARC’s trademark handles, now redesigned to be more comfortable to grasp and to match the texture and direction of the brushing on the faceplate. Beneath the model name but still within the black surround are six round pushbuttons with brushed-aluminum tops: Power, Menu, Enter, Mono, Invert, Mute. All but the Menu and Enter buttons are toggles and, to my surprise, felt a bit loose and flimsy to the touch. The Ref 6 can be had in aluminum finishes of Natural or anodized Black.
Most of the Reference 6’s rear panel is equally divided into upper (Right channel) and lower (Left channel) sections. At left are matching arrays of four balanced (XLR) and four single-ended (RCA) inputs, and to their right are balanced and single-ended sets of one Record Out (XLR, RCA) and two Main Outputs (XLR, RCA). At far right are an RS-232 control port, an IR input, and a 12V Trigger Out, and below these a fuse bay and a three-bladed, 20A IEC power inlet. As on the Ref 160M, a 20A connector has been used instead of the standard 15A because ARC believes it provides a better connection and sounds better.
Peer through the Reference 6’s acrylic top panel and your eye is immediately drawn to the six-pack of 6H30P dual-triode tubes that drive the analog section. These high-transconductance tubes are wired in parallel to reduce noise and improve linearity; as Dave Gordon walked me through the signal path, he told me that the tubes’ clever implementation is but one of several advances over the Ref 5SE. For starters, the Ref 6’s gain stage is DC-coupled to a buffer, and includes a tube regulator designed to help reduce high-order distortion artifacts to provide a more open sound. Driving the heavily revised high-voltage power supply are the two remaining tubes, a 6550WE and 6H30P. The new power supply includes a larger, quieter power transformer, and each of the nine low-voltage rails is separately regulated. When I asked Gordon if any of the Ref 6’s tubes can be replaced with other types, he had this to say: “The 6H30 tubes are unique and cannot be swapped-out with any other tube. The 6550 in the power supply could technically be swapped-out with a KT88, but it will degrade sound quality. We use very specific graded 6550 tubes in the power supply.”
The Ref 6’s new circuitry is fully balanced from input to output, pure class-A, uses zero feedback, and is printed on newly designed circuit boards. When I asked Gordon about the new boards, he paused, then apologized -- the boards are proprietary -- before quickly changing the subject to the Ref 6’s other advances over the Ref 5SE; e.g., the custom-designed capacitors in the audio circuit, which throughout is tidily threaded with pure copper Ohno Continuous Cast (PCOCC) wire. The Ref 6’s volume control is based on a resistor ladder network, has 103 steps of attenuation, and is controlled by a microprocessor for utmost precision. The volume, source selection, display brightness, muting, and polarity can also be controlled using the small but weighty aluminum remote control. ARC specifies that the Reference 6 provides 12dB of gain via its balanced outputs and 6dB via its single-ended outputs; 1.7uV RMS of residual noise, IHF weighted, noise with volume at “1” (109dB below 2V RMS output); and less than 0.01% distortion at 2V RMS, balanced output.
Controlling the Reference 6 and navigating its menus using the front panel or the remote was intuitive. Features I liked were the ability to use any input as a home-theater bypass; when this option is selected, the Ref 6’s volume control is switched out of circuit and signals are passed through at unity gain. I also appreciated the Ref 6’s tube monitor, which lets you track tube life (the 6550WE is specified to last 2000 hours, the 6H30P 4000 hours), and the defeatable auto shutdown, both aimed squarely at lengthening tube life. What I didn’t like so much was that inputs can be named only with preprogrammed identifiers -- there’s no way of inputting my own custom names.
As with all ARC products, the Ref 6’s case, knobs, handles, circuit boards, key capacitors, transformers, and wiring are all designed in-house, then outsourced for manufacture in North America. Each Ref 6 is hand-assembled in ARC’s 20,000-square-foot plant in Maple Grove, Minnesota, then tested, power-cycled, burned in for 24 hours, and tested again, all before undergoing a critical listening session. If a unit is rejected during listening, it’s returned to the bench tech to find out why it failed, where it’s corrected or repaired. Then it’s listened to again. Assuming it passes that second audition, its panels, knobs, handles, and top and bottom plates are attached. After a final visual check, each unit is photographed before being packed for shipping.
The Reference 6 arrived on a shipping pallet in a large white cardboard box, alongside a pair of similarly packaged Reference 160M amplifiers, which I reviewed in February. Like the Ref 160Ms, the Ref 6 was securely held in place in its box with inserts of molded foam. Above these inserts, were a shelf of cardboard sandwiching a small box containing all of the tubes and a screwdriver for removing the top panel.
With the top panel removed, installing the tubes was a snap: Each tube is labeled with its corresponding socket on the circuit board. Tubes inserted and top panel replaced, I set the Ref 6 atop one of the two isolation shelves previously occupied by my reference Simaudio Moon Evolution P-8 preamplifier, and connected it to the rest of my reference system: PS Audio DirectStream DAC, Simaudio Moon Evolution W-7M monoblocks, and Simaudio Moon Evolution 780D DAC, using Kimber Kable Select KS-1116 interconnects. Each component was fed power via a Torus AVR 20 power conditioner with Clarus Crimson power cords. In comparing the stock ARC cable to my Clarus cords, I found the ARC cord performed comparably.
Before settling in to some serious listening, I played music through the Reference 6 and Reference 160Ms for a little over 200 hours, to ensure that all three components were equally broken in. Swaying a bit from the norm, I decided to evaluate both products simultaneously. I used the same recordings, and swapped interconnects and speaker cables among the various components in the signal path. This approach not only made possible direct A/B comparisons with my reference gear, it highlighted whatever differences I heard that were made by the Ref 6 and Ref 160Ms, separately and together, as well as if there was any ARC “house sound.”
The Reference 6 sounded great straight out of the box, with only marginal improvements in soundstage size, transparency, and refinement after those 200 hours of break-in. By the end of that time I was hearing a preamplifier that added very little to the music sent through it other than a welcome dose of sophistication. I expected to hear some more of the warmth and softening in the bass that the Ref 160Ms clearly added to the sound, but the Ref 6 contributed very little of either, sounding smooth, open, and tonally quite similar to my reference Simaudio P-8 preamp ($11,000, discontinued). In fact, in Supertramp’s Crime of the Century (16-bit/44.1kHz FLAC, A&M) I was surprised to hear levels of resolution, depth, air, focus, and slam that were comparable to the P-8’s. When I focused on Rick Davis’s harmonica in the opening seconds of “School,” notes dissipated into space a little more effortlessly through the Ref 6. The harmonica’s image was slightly larger and faintly more focused through the P-8, but the Ref 6’s depiction of it was ever so slightly more finessed and delicate, almost as if I were listening to a harmonica made of brass instead of stainless steel.
Davis’s voice was locked at center stage, as expected, with ample space around him, but I couldn’t hear the porosity in his voice as I do through the P-8. I enjoyed this. The Ref 6 seemed to have an uncanny ability to add just the right amount of fluidity to voices -- they sounded more organic, less analytically derived. The sound of Dougie Thompson’s bass guitar was another ear-opener: tight, punchy, utterly controlled. I heard no loss of depth or sense of flab, and the control of transients was downright visceral. Naturally, this prompted me to poke the bear a bit, but no matter how hard I pushed the Ref 6, it maintained its refined, nuanced composure -- something that can pose a challenge to preamplifiers, regardless of cost.
I moved on to “Gold Dust Woman,” from Fleetwood Mac’s Rumours (24/96 FLAC, Warner Bros.). Before my very ears, the Reference 6 drew a highly resolved image of Stevie Nicks’s voice etched at center stage and sounding velvet-smooth and full of dimension in front of Mick Fleetwood’s drums. Swapping between the Reference 160Ms and my Simaudio Moon Evolution W-7Ms, I could clearly hear differences in sound between the two pairs of monoblocks, which illustrated just how transparent the Ref 6 was. I also realized that, between the Ref 6 and Ref 160Ms, the Ref 6 was more precise. In my review of the Ref 160Ms, I wrote that “I rarely experience an amplifier that reminds me what classic rock sounded like when I was a kid.” With the Ref 6 connected to my Simaudio W-7Ms I lost some of that quality, but that wasn’t necessarily a bad thing -- what was lost in terms of nostalgia was replaced by a sense of truthfulness. Simaudio’s P-8 is all about accuracy and realism -- it’s a precision tool that treads the fine line between just enough and too much of a good thing. For the most part, the P-8 gets it right, but at times I find myself wanting to pull back on the reins a bit -- the P-8 can sway to the analytical side, depending on recording quality and volume level. This never happened with the Ref 6, whose consistent yet subtle warmth enabled the sound to remain faithful and alluring without ever sounding artificial or stressed. Lindsey Buckingham’s acoustic guitar, for example, was projected into my room just as precisely as it was through the P-8, but with a slightly diminished sense of steeliness. I also got the impression that Fleetwood’s increasingly intensive thwacks of the drums toward the end of the track were more fleshed out through the Ref 6 -- I appreciated how the nuances of each thwack were slightly more present, and took longer to dissipate.
Equally as engaging was the resonance of Kacey Musgraves’s voice in “Space Cowboy,” from her Golden Hour (16/44.1 FLAC, MCA Nashville). After swapping cables to compare various teams of components, I realized that this nuance was most evident when listening to the Reference 6 and 160Ms together, and was almost nonexistent through the Simaudio P-8 and W-7Ms. Even more curious was that these resonances were more focused, and therefore best appreciated, with the Ref 6 tethered to the W-7Ms. My best guess is that the solid-state P-8 fastidiously preserves whatever signal it’s fed, while the Ref 6, perhaps due to its tubed gain stage, sustains transients just a hint longer, thereby producing a greater sense of nuance. These faintly prolonged transients were then precisely amplified as they made their way through the Simaudio amps, but through the Ref 160Ms became a wisp blurred as they were compounded. But it’s important to note that I heard this only in the process of performing back-to-back comparisons of multiple combinations of components, and even so, it was only just audible.
Returning to the W-7Ms and focusing on other aspects of various tracks on Golden Hour, I found that the Reference 6 consistently excelled at communicating the scale, texture, and dimensions of Musgraves’s voice and her band’s instruments. Todd Lombardo’s banjo throughout “Slow Burn” was expertly articulated and replete with delicate pick slides, and I loved how robustly yet concisely Daniel Tashian’s bass guitar was enunciated in this track.
Because bass reproduction is supposedly the Achilles’ heel of most tubed gear, I went for broke and cued up “Unforgivable,” from Armin Van Buuren’s A State of Trance 2008 (16/44.1 FLAC, Armada Music). The Ref 6 maintained the track’s pace, rhythm, and timing without missing a beat. The bass line was commanding yet controlled, and the various synthesized effects -- cymbals, ticks, pops -- remained fast and well defined without ever approaching the splashy. There was no loss of resolution, no slowing or dimming of the fast, dynamic tempo, and no loss of depth or resolution in the bass. In short, there was no audible hint that I was listening to a tubed preamplifier.
In my time with it, I quickly learned that Audio Research’s Reference 6 preamplifier is one of those rare products capable of producing a quality of sound greater than might be suggested by the sum of its parts. The Ref 6 also offers outstanding build quality, instinctual ergonomics, and sexy appearance, all for only $2005 more than the cost of its predecessor -- a 15% premium. Moreover, it’s the first tubed preamplifier I’ve heard that can match and in some ways exceed the performance of my reference, the solid-state Simaudio Moon Evolution P-8. This tells me three things: 1) some seriously astute engineering is going on under the Ref 6’s hood; 2) even at $15,000, the Ref 6 is a heck of a value; and 3) I’d be an idiot to return the review sample. Audio Research’s Reference 6 has just become my new reference preamplifier. Wholeheartedly recommended.
. . . Aron Garrecht
- Speakers -- Paradigm Persona 7F
- Subwoofers -- JL Audio Fathom f112 (2)
- Amplifiers -- Parasound Halo A 51 (five-channel), Simaudio Moon Evolution W-7M (monoblocks)
- Preamplifiers -- Anthem AVM 60, Simaudio Moon Evolution P-8
- Digital-to-analog converters -- Cocktail Audio X45Pro, PS Audio DirectStream, Simaudio Moon Evolution 780D
- Sources -- Oppo Digital BDP-103 universal BD player; Intel NUC computer running Windows 10, Roon
- Interconnects -- Analysis Plus (USB), Clarus Crimson (S/PDIF) and power cords, Kimber Kable Select (balanced)
- Speaker cables -- Kimber Kable KS-6063
- Power conditioner -- Torus AVR 20
Audio Research Reference 6 Preamplifier
Price: $15,000 USD.
Warranty: Three years parts and labor; 90 days, tubes.
Audio Research Corporation
6655 Wedgwood Lane N., Suite 115
Maple Grove, MN 55311
Phone: (763) 577-9700