Integrated Amp Reviews

Audiophiles are often misunderstood as mere hobbyists; in fact they hold a profound reverence for sound. Japanese jazz kissas, where vintage vinyl, turntables, tube amplifiers, and horn speakers have drawn audiences since the 1950s, exemplify this passion. Western audiophiles share a similar fervor, though our pursuit of sonic perfection often manifests in a more discreet, almost monastic, lifestyle (footnote 1).

Shinobu Karaki, a 65-year-old electronics designer, is the founder and chief designer of Japan's Aurorasound. A music lover first, an audiophile second, and an aspiring jazz and classical guitarist, Karaki crafts phono stages, step-up transformers, preamplifiers, power amplifiers, and integrated amplifiers for his Yokohama-based audio company.

Prior to founding Aurorasound in 2010, Karaki held the position of Digital Audio Group Business Director at Texas Instruments Japan, where he amassed nearly three decades of experience in research and development. Earlier, in his formative years, Karaki attended Osaka University and Yokohama Polytechnic, earning degrees in electronic engineering and business. His expertise enabled him to design and manufacture a range of high-performance amplifiers and DACs, resulting in the small bounty of products currently available from Aurorasound.

Despite its modest size—the company employs four people, including Karaki and his wife—Aurorasound has cultivated a global following, with products distributed in 12 countries across Asia, Europe, and the United States. The lineup, available in the US through a small dealer network serviced by California distributor HighEnd Electronics, includes three phono stages, a multicurve phono equalizer, two step-up transformers, two preamplifiers, and a $20,790 pair of tubed monoblocks. This review focuses on the latest addition to the company's offerings, the Aurorasound HFSA-01 Integrated Tube Amplifier ($3680).

Design
The HFSA-01—short for High Fidelity Stereo Amplifier One—reverses the usual hybrid approach, using 10 Texas Instruments OPA604 FET op-amps to drive four Sovtek EL84 pentode tubes running in Ultralinear, class-AB, push-pull mode and outputting a maximum of 14Wpc with an 8 ohm load. Why reverse the more typical hybrid approach? Karaki has his reasons.

"HFSA-01 uses tubes and semiconductors," Karaki explained over email. "The input stage, tone controller, and phono EQ stage are all semiconductor designs. Also, the phase inverter and driver for the EL84 tubes is a semiconductor circuit. Only the output stage [uses] the EL84 tube. A semiconductor has very low distortion, good S/N ratio, and longer life than tubes. The EL84 tube is easy to drive at low bias, so a semiconductor can drive the tubes easily, and it makes an ideal phase inverter for both push-pull tubes. Small size, lower cost, longer life, less failure, and low power consumption are advantages over an all-tube solution. The sound character mainly depends on the tubes, so HFSA-01 has real tube sound."

I asked about the sound he was seeking. "I wanted to develop a unique and original design for a tube power amplifier that has push-pull tubes and an output transformer. The Texas Instruments OPA604 semiconductor can make perfect positive and negative signal for a push-pull design and can also drive a 600 ohm load. The EL84 tube's bias level is 12–13V, so this construction makes an ideal push-pull amplifier for an EL84."

The HFSA-01 is a compact 14" wide × 5.7" high × 12.5" deep and weighs just 23lb. The top cover, shell, and bottom plate are made of 1.2mm-thick iron, suggesting it means business. Its fascia and rear plate are 3mm-thick aluminum, anodized with a hairline finish. Chunky mahogany trim adorns the amplifier's sides, projecting out on each side of the fascia. The HFSA-01 is sturdily built, and its retro appeal "looks the dickens" as my mom would say. It resembles a '50s integrated tube amp from H.H. Scott or Dynaco, I'd say.

Lifting the lid unveils Karaki's artistry, which is evident in the pristine epoxy and copper-print circuit boards; its precise wiring with premium Furukawa Electric and Beamex wire; and ceramic and gold-plate tube sockets to support the Sovtek EL84s.

A hulking power transformer, custom-created by Aurorasound and Japan's General Industry KK, dominates the insides, wrapped in a copper band to reduce EMI. Also taking up significant space (and increasing weight) are a pair of iron-core output transformers. Populating the circuit boards are premium components including Rohm silicon carbide diodes (used as rectifiers), an Alps volume pot, Dale resistors, and a selection of KOA and Nichicon (Muse, KX Series Radial, Fine Gold) capacitors.

A "CR" tone control is used for "easy adjustments to achieve more rich bass and comfortable treble to meet listeners' taste even with small-size speakers," the website states. A CR tone control is a basic, passive circuit that uses only capacitors (C) and resistors (R) to adjust the frequency response of an audio signal. The tone controls can be bypassed with a "Tone/Direct" switch for optimal signal integrity.

Those tone controls double as EQ curves for the MM phono input, enabling playback of pre-RIAA records. Rather than altering the RIAA stage, Karaki made it possible to use the tone controls to adjust tone consistent with various EQ curves: Decca, Columbia, AES, NEB. The included "Old Disk Easy Compensator," an etched plastic panel that fits over the tone control knobs, allows you to easily set the tone controls to the correct positions to imitate the sound of those EQ curves.

Which implies, of course, that the HFSA-01 has an onboard RIAA phono stage; the Aurorasound website, in Japanese, describes it as "an NF-type circuit"—presumably that's negative-feedback, as opposed to a passive CR or LCR circuit—"with excellent SN characteristics using Linear Technology semiconductors." The HFSA-01 also has a feature craved by many old-school audiophiles but missing on many modern preamps and integrated amps: a mono/stereo switch. All these features add up to a versatile integrated amplifier despite the HFSA-01's apparent simplicity.

Karaki made sure that the semiconductor and tubed sides are well isolated: "The power supply section is completely isolated between the vacuum tube and semiconductor to completely eliminate any interference, regardless of volume level and frequency range." While there's only one power transformer, Karaki explained, "the tube section power regulator and semiconductor section regulator are all divided and isolated. No influence together."

The front-mounted controls of the HFSA-01 could hardly be simpler: flanking an Aurora logo, four large dials labeled "Bass," "Treble," "Volume," and "Selector" (the latter with options "Phono," "CD," "Line 1," "Line 2"). Below, a circular power button trimmed in red light, those tone/direct and stereo/mono switches, and a ¼" headphone jack share space with the words "Vacuum Tube EL84 Push-Pull UL Operation Hybrid Direct Coupling."

The rear panel echoes the front panel's spartan design. A cautionary note warns against blocking the ventilation openings, in Japanese and English. On the back panel, two rows of four vent holes are situated above four pairs of RCA inputs (with many more on the top and sides). To the right are the L and R speaker terminals and separate 4–6 and 8 ohm taps. Four solid feet, machined from aluminum and cushioned by rubber rings, ensure a sturdy, stable platform. Which adds up to a solid, compact, handsome unit, like a small Abrams tank ready for duty.

Setup
Given the amplifier's relatively light weight and the absence of any app or network connection, setup was easier than sliding a pizza out of the oven. Speaking of pizza ovens, the HFSA-01 ran very hot, though I smelled no odors—or none beyond the usual ones rising up from my musty Greenwich Village street.

I experimented with a variety of cables. After extensive comparisons, I settled on AudioQuest Pegasus (2m) interconnects from phono stage to the HFSA-01, with, alternately, ArgentPur AgPur12 (3m) and Auditorium 23 (2m) speaker cables. The AudioQuest ICs provided exceptional force and resolution. The ArgentPur AgPur12 speaker cables performed similarly to my Triode Wire Labs cables, creating an uncolored tonal balance with a hint of warmth in an atmospheric, large-scale soundfield. The Auditorium 23 cables tilted the sound toward a richer tonal balance and deep, round bass with a soft, harmonic glow. AudioQuest William Tell speaker cables (3m) revealed the finest detail, drama, and superior overall speed but produced a slightly forward treble.

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Footnote 1: Which makes sense: The US equivalent of jazz kissas are mostly found only in larger cities, and often the patrons there can't be quiet. Apparently, if you want to truly listen to recorded jazz, you need to stay at home.