Audio Research Reference 6SE Preamplifier Review
Craig Joyce luxuriates in the silky sound of this high-end tube preamplifier…
Reference 6SE Preamplifier
Audio Research has long been making superbly finished high-end amplifiers with an industrial bent, and its new Reference 6SE two-channel preamp is no exception. It sits in the middle of the company's line-up – above the LS28SE and below the multi-chassis Reference 10 line stage.
The new look of this Audio Research preamp line builds on the design cues of the Reference 5, with a subtle new fascia design that's both stylish and minimalist. The unit has an aluminium case and weighs 17kg, with dimensions of 480x198x419mm (HxWxD). Two large grab handles are provided for moving it carefully into place. A pair of microprocessor-driven rotary encoders adorn the front of the unit, with a handful of tactile buttons to switch between menus and input modes. The striking vacuum fluorescent display is clear and readable across the room but may be dimmed via the front panel configuration at the owner's discretion.
The business end of the device has a single differential gain stage with an output stage buffer with zero feedback. Four sets each of balanced (XLR) and single-ended (RCA) inputs are fitted, two pairs of balanced and single-ended outputs, a separate record output and an RS232 interface for integration with a home automation system. Lastly, an IEC19 plug provides input power. Being a line stage, there is no phono input or DAC included. Audio Research says most owners don't run turntables, or if they do, they use separate phono stages.
It works in Class A mode, and employs a vacuum tube regulated power supply. Fully balanced from front to rear, it's a quintessential high-end design that uses three 6H30 dual-triode tubes per channel, which drive the analogue audio circuits, and a single 6H30 and 6550WE for the power supply. Significant modifications have been made from the previous generation Reference 6 to the new 6SE – no less than eighteen capacitor and fourteen wire changes, we're told. Audio Research works closely with two US capacitor companies and co-operatively designs them until they have together achieved something they are happy with, the company says. This attention to detail is leveraged across the other products in the stable.
What of the commonly held audiophile belief that point-to-point cabling in valve preamps is superior to circuit board manufacture? In the Reference 6SE, Audio Research hand-solders every signal component in every board because it claims it sounds superior. “Those who believe that point-to-point wiring sounds better have never heard a well designed circuit board. All boards sound different—you can't just specify what you want and expect it to sound better. It took us about eighteen months to evaluate and choose specific board materials, trace materials, thickness, etc. And, board layout is incredibly important.”
Interestingly, the company attempts to allow upgrades to keep previous generation products up to the latest specification. So it's possible to upgrade a Reference 6 to a Reference 6 SE specification, for example. To do so, units are sent back to the company and disassembled. Old parts are removed and replaced, before the unit is thoroughly bench-tested. Only then does it goes to warm-up and some critical listening in the reference system. If the updated unit passes listening tests, it is then packed and returned to the owner. The retail price for this update is $3,500 USD plus freight, and handily Audio Research then adds one year of additional warranty to the updated device. Pleasingly, the company continues to support and service most products that they have offered since 1970.
Tube devices are different to solid-state devices in that they run to a clearly defined operating lifespan. Tubes could be considered consumable components in a preamp of this nature and will require periodic replacement. The 6H30s should last at least 4,000 hours (their lifespan is specified as 10,000 hours, but Audio Research believes this is unrealistic for optimal audio performance), and the 6550 in the power supply should be replaced before 2,000 hours. In this time of increased global tube demand, customers should take solace in the fact that the company has been able to maintain a large supply of tubes for both production and spare parts.
To many, an appeal of a tube preamp comes in the ability to tailor the sound profile to their preference by 'rolling' other tubes. When asked about tube rolling, Audio Research's Dave Gordon told me: “We design with what we believe are the best tubes available in a given circuit – as long as it is in current production – to achieve the results we expect. There is no other substitute for the 6H30, so tube rolling is not possible. The KT88 is electrically interchangeable with the 6550 that we use for regulation, but installing a KT88 in the power supply will degrade the performance of the 6SE.”
The Reference 6SE supports a home theatre bypass capability (Processor Mode) assignable on any input to integrate the preamp into a dual-purpose home theatre/stereo listening room. The preamp allows 'hot swapping' of connections, meaning you do not need to turn the unit off when changing interconnect cables; if the device senses a low line voltage, the preamp displays 'Low Line' and automatically mutes to protect the system.
The Reference 6SE has an auto-shutdown mode to protect the life of the tubes, so it's possible to safely leave the device on all the time. This is the fastest way to break in a new unit, and was the path taken by myself to get the suggested 200 hours on the tubes before I attempted critical listening. During startup, the preamplifier begins a warm-up sequence that lasts approximately forty-five seconds, during which 'MUTE' flashes on the display.
While preparing to install the preamp in my system, I noted that the balanced output impedance is quoted as being from 669 ohms (at 1 kHz and 20 kHz) to 1,394 ohms (20 Hz). I also noted that the unit is recommended to use a power amplifier with at least 20k ohm input impedance and 2,000pF maximum capacitance. This gave me cause to pause, as my McIntosh MC1201 power amps are rated at 10k ohm input impedance. Raising this with Audio Research, their engineers stated that the recommended input impedance of a downstream amplifier should be 10k ohm, not 20k ohm - “We need to correct this in our published specifications. The real-life reality is that the 6SE will easily drive both of your amplifiers simultaneously (with a net effective impedance of 8.3k) without a problem.”
My reference system is based around streaming hi-res digital audio to an MSB Technology Premier DAC, with a pair of McIntosh MC-1201 monoblock amplifiers powering JBL K2 S9900 loudspeakers.
The Reference 6SE is a high-performance preamplifier that resolves the finest detail in any recording. It possesses a lovely, natural tonality and creates a real sense of space and life, something that's missing in all too many solid-state designs. As with the best tube preamps, it sounds stellar on vocals, with an excellent, natural tonality and general ease of presentation. Yet it doesn't commit the sins of some such designs, and retains the structure of the stereo soundstage in accurate proportions with excellent instrumental separation.
The standout attribute of this preamplifier is its midband purity. Lilac Wine by Jeff Buckley showed a palpable sense of emotion, the preamp providing vivid and accurately rendered vocals with beguiling reverb trails. Tonally everything was just right, and dynamic shifts were pleasingly brought to bear. I also heard significant low bass, a muted kick drum and brushed cymbals offering the track the buoyancy it needs to become transcendent. At serious listening levels, it was as if the band was in the room with me.
Although this preamp displays the characteristically warm sound signature of a tube design, it isn't overly rich and coloured nor appears fluffy and opaque. Instead, the Reference 6SE is open enough to deliver an uncanny realism to what it plays. The pretty, tremolo-loaded guitar work of Guest List by the Eels sounded like a real valve guitar head amp in the room. Vocals were emotive and raw with limited double-tracking; clear but absolutely organic. When the harmonica came in across the middle eight, it sat high in the sound stage, right where you would expect it to be, while guitars were realistically rendered on what would otherwise be a fairly straightforward song.
It may be sweet and smooth sounding but rhythmically slow; it is not. The classic indie rock of The The's Kingdom of Rain did not disappoint, with Johnny Marr's crisp, bright acoustic guitar handled deftly, with a liquid bassline burbling beneath. The delicate piano riffs were very pleasing to the ear, with each note demonstrating a fast attack which elegantly illustrated the careful and restrained use of compression during the production of this album. Drum hits were precise and imaged well through the recorded acoustic, and the busier sections of the music were presented without fuss or clutter.
The Reference 6SE is also able to convincingly recreate the spacious acoustics of fine recordings. For example, White Picket Fence from Welsh electronic producer Koreless sounded positively otherworldly. Through this preamplifier, piano and harpsichord samples interplayed with cut-up ethereal vocal samples to create an expansive and emotional landscape. The dense sampling was writ large in the stereo space, feeling like multiple tracks in parallel rushing simultaneously like a waterfall, each designed to rip your face off. Such electronic trickery really benefited from its lovely tube warmth.
You might not think that an obscure cover version of the Are You Being Served? theme would become a reference selection in a review, but the Coil produced version is a real test of stereo imaging and separation. One of the last songs that the band recorded with John Balance before his untimely death in 2004, I was thrilled to hear it so beautifully presented through this Audio Research preamp. Processed drum sounds panned and pounded across the stereo field with military precision, before the synthetic strings came in and offered a level of morose sophistication.
In many lesser preamps, this song would be an audible mess, yet the co-existence of heavily treated electronics, strings and marimba gave the Reference 6SE no challenges in painting a full stereo image that engaged multiple parts of your listening brain simultaneously. The angelic voice of François Testory was beautifully intertwined with John Balance's sampled voice, and ascended continually as the song reached its outcome.
Last but not least, the Reference 6SE has all the guts and solidity that you would expect from a preamplifier of this price. Its handling of low frequencies is quite superb, as Don't Know How by Andy Stott showed.Adub-infused electronic track with some proper bass-heavy material, it came over with power and poise – with no sense of this preamp struggling to drive my power amplifiers whatsoever. Even at room-shaking volumes, I got vast tracts of fast and articulate bass; the insistent kick drum and bass guitar work pressurised my listening room in a tight and well-defined manner. Alison Skidmore's heavily treated vocals provided a stark and bright counterpoint to this steamroller of a mix.
A very expensive preamplifier by any standards, I would say that Audio Research's new Reference 6SE still represents good value for money, as it's one of the very best such products that I have heard. Compared to any solid-state design, it delivers a subtle shift in the overall sonic palette, changing the presentation slightly to one that accentuated different instruments to my reference solid-state design. It loves instruments with fast transient attack, imbuing them with remarkably positive characteristics.
Indeed, there is something that tubes bring to the table with recorded music that is absolutely moreish. Instruments like pianos and acoustic guitars seem to sound more alive and real, and electric guitars shine and shimmer in a magical way. This preamp had me running the gamut of my music collection, looking for tracks it would really show off – and it drew me back to many a past favourite album along the way. I was especially enamoured of music heavy in piano, harpsichord and minimally processed guitar work as well as records with a great room sound. Try to pick any Steve Albini engineered record, and you won't be disappointed…
Overall then, if you are considering a tube preamp to evolve your playback chain or to introduce a slightly different type of zest to your system, it's unlikely you will be disappointed with this.
With an engineering degree in digital signal processing and a storied career in IT networking and cyber security, Craig loves to push the boundaries of audio technologies. An aficionado of live music with personal detours in music production and event promotion, Craig is a long time enthusiast of post punk, electronic and experimental music.
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