Integrated Amp Reviews

Celebrated for its contributions to cuisine, music, and art, Italy boasts a rich cultural heritage and a history of profound significance to the world.

From the 1088 establishment of the University of Bologna, the oldest university in the Western world, to the invention of the radio by Guglielmo Marconi in the mid-1890s, Italy has been at the forefront of innovation and artistic pursuit. Modern Italy stands as a testament to its vibrant past.

"The Italian Renaissance, which began in Florence in the 1380s, marks the first moment when Europeans stepped away from the traditional and medieval pious Christian view of man to embrace values and attitudes they had discovered through studying the culture of the ancient world," wrote William ("Bill") Adams, a former professor of Italian history, in an email. "Those values, belief in the dignity and worth of man, in human development through education which, having spread through Europe, led to the Scientific Revolution and the Enlightenment."

In 1996, Bill and I journeyed to Tuscany, where we stayed in a mountaintop castle nestled within the ancient walls of San Gimignano. Returning to Italy in the summer of 2024, I embarked on a tour with kindred spirits. Our travels took us from savoring perfectly al dente pasta in Rome to indulging in boot-sized portions of tiramisu in Tuscany. We also ventured into the Treviso countryside, where we toured the Unison Research (UR) factory. This is where I had my first exposure to Unison Research's S6 Black Edition Integrated Amplifier ($6999).

But before we get into the blood and guts of this review, some Unison history.

Founded in 1987 by Giovanni Maria Sacchetti, Unison Research shares a facility with Opera Loudspeakers, founded by Giovanni Nasta. The two companies have close ties: Riccardo Nasta, Unison Research's production manager and Giovanni's figlio (son—I looked it up), contributes to the fine-tuning of Opera's loudspeakers. Bartolomeo Nasta, managing director of both companies and Giovanni's altro figlio, patiently answered my questions about the S6.

In their Treviso shop, Unison Research crafts audio equipment with a dedicated team of 11 specialists supported by three external engineers. The company's staff is rounded out by a social media manager, a marketing and automotive manager, and an external digital team. Among UR's most sought-after offerings are its Simply Italy amplifiers (reviewed by Sam Tellig in Vol.35 No.8; Vol.37 No.1 but not online) and the complete Unico line, including the Unico Primo integrated amplifier, which I reviewed in 2016. John Marks reviewed the original S6 in Vol.36 No.8, which at the time sold for $5395.

Introduced at High End Munich 2024, the S6 Black Edition is a single-ended design, its three KT77 tubes per channel are connected in parallel, operating in a class-A, Ultralinear configuration. The amp's output transformer, comprised of seven winding sections, is designed for maximum power transfer into an impedance of 6 ohms.

The original S6 uses EL34 power tubes, each channel of the S6 Black Edition employs three Genelex Gold Lion KT77 tubes to generate 40Wpc (specified at 5% THD), driven by a Genelex Gold Lion 12AX7/ECC83 dual-triode tube. The S6 Black Edition stands 13.75" wide, 17" deep, and 8" high. It weighs 55lb. Upholding Italian tradtion, the S6 Black Edition is gracefully appointed, its curved transformer cover giving it a streamlined Moderne sheen. Its two large stainless steel control knobs are curvaceous and eye-catching.

The bias meters, RCA jacks, and speaker binding posts are manufactured in Asia, but the bulk of the S6 Black Edition's parts are made in Italy, including its case, bottom, and top plates—all powder-coated iron. Its MDF control panel is finished in piano-black lacquer, tube bases are ceramic with gold-plated contacts, and the aforementioned knobs are brushed stainless steel. The tube grilles and stainless steel plates that surround the tubes are plated with black nickel. The feet are black anodized aluminum with rubber bases. The remote control is wrapped in a milled cherry case with an aluminum control panel with laser-engraved text.

The S6 Black Edition is dual-mono, partly hard-wired, partly PCB, with separate left- and right-channel circuit boards. Electrolytic capacitors inside the S6's anode power supply carry recognizable brand names: Rubycon, Nichicon, Nippon Chemicon. In the Black Edition, Nasta wrote, "strategically placed polypropylene capacitors have been added to the power supply, improving transient response and signal dynamics." The power supply has also been enhanced by decoupling the anode voltage of each tube to ensure their independence.

Polypropylene caps in the signal path also have recognizable brand names: Mundorf, Wima, TDK.

All three transformers utilize ferrosilicon laminates, thin layers of iron-silicon alloy stacked and bonded together. "Power and output transformers are of our own design, manufactured under our strict specifications by an Italian manufacturer," Bartolomeo Nasta wrote over email. "Power transformers are resin-potted to prevent vibration and to better dissipate heat."

The first amplification stage is feedback-free, and there is no global feedback. Feedback is utilized on the second triode—the second amplification stage. This feedback "allows more control of power delivery to the speakers without altering the musicality of the amplifier."

The S6 Black Edition uses "ionic polarization" for the first signal triode, an approach Unison Research has been refining for decades. "This polarization, coupled with the absence of global feedback, results in improved micro-detail and a natural dynamic sound," the UR website says. Nasta added in an email that this ensures a "detailed, fast sound combined with very low distortion with very natural harmonic decay, similar to that of a musical instrument."

The original S6 used an autobias system for the power tubes; a user-friendly biasing system has been introduced in the Black Edition. The first model used ECC82s as signal tubes; these have been replaced with 12AX7/ECC83s. The circuit boards have been upgraded.

"The input selection circuit has been meticulously redesigned using microrelays with gold-plated silver contacts to ensure stable contact and to prevent crosstalk with other nonselected sources connected to the amplifier," he added. "The return grounds of the inputs have been implemented using Return GND technology. The signal not in use is enclosed in a small path and confined near the RCA connectors. Service power supplies in the Black Edition are individual for each electrical load so that there is no interference in sound between the various parts of the device."

"For the input selector, we used the same approach as in our Reference and Absolute amps," Nasta wrote. Armored relays with gold-plated silver contacts are used to switch inputs; these are manufactured by Fujitzu. "Signal selection by relay allows greater separation between channels. The signal to the unused inputs is closed on a load resistor near the input itself through a special return ground. As a result, the inputs not in use (signal and ground) don't interfere with the audio signal, and the signal doesn't propagate through the boards. The same principle is applied between the right and left channels so that they are as independent and isolated from each other as possible so as not to lose detail."

Sacchetti, Nasta, and their team meticulously refined the S6 Black Edition's design to minimize noise. Their efforts included optimizing (in addition to the input selector) signal paths, board placement of components, and cabling. On-board components were strategically shielded to mitigate the influence of magnetic fields emanating from the transformers and power supplies. These enhancements are said to result in a lower noisefloor, increased output power, and, consequently, greater dynamic range.

"The S6 Black Edition was created to exceed the musical performance of the standard S6 version," Nasta told me in an email. "Compared to the standard S6, we gained more power, better sonic detail, and the lowest possible noisefloor. "In general, when we design an amplifier, we look for instrumental and musical performance as sound scene reconstruction. We try to recreate a sound event as close to reality as possible, without artifice, that can be listened to for the long term. All with a touch of Italian design."

So far, this all sounds sophisticated but analog-purist: old school. But that's not the whole picture: The S6 Black has a built-in DAC. The "DAC3" module uses a Sabre ES9018K2M DAC chip. Inputs include USB (supporting PCM up to 384kHz and DSD to DSD256), TosLink, and S/PDIF.