Integrated Amp Reviews

All my earliest hi-fi memories involve tube amplification: as a young girl, staring at the tubes' glowing filaments and listening to music with my audiophile father. I was mesmerized by the glow of those tubes, too hot to touch, but even more so by the music, which often was classical or opera. How those tubes worked was a mystery to me, but I knew they played a big part in the magic coming from the speakers.

Those early tube amps, however, were unrefined and often a maintenance challenge. Visually, they were more functional than luxurious, and even if you managed to avoid shocking yourself while setting the bias—or perhaps building a kit—they required more dedication than many modern audio consumers might wish to provide.

Sarasota, Florida–based Valve Amplification Company (VAC) aims to produce amplifiers that address both those issues. Visually and in terms of craftsmanship, VAC amps aim for luxury, and they're easy to own and use—almost as easy as solid state.

VAC is also known for amplifiers that push the boundaries of technology—and scale and price; all of the above are on display in the immense, pricey Statement 452i iQ Music-bloc mono/stereo amplifier, reviewed by Michael Fremer in Stereophile's May 2020 issue (footnote 1). There's a similar (and similarly pricey) integrated amplifier, the 450i iQ.

Still luxurious but much more modest in scale and price are VAC's Sigma series, which currently comprises just one product, the Sigma 170i iQ.

Technology and design
The Sigma 170i iQ integrated could be considered VAC's entry point, but that doesn't mean it's basic or skimps on details. Nor, at $10,000 in its base configuration, is it inexpensive. Then again, VAC products are hand-built and intended to last a long time. "My goal for the customer is to be able to leave your VAC component to your grandkids in your will," Kevin Hayes, VAC's president, cofounder, and lead engineer, told me in a telephone interview. "In 30 or 40 years, replace the power supply capacitors." The tubes, obviously, will need to be switched out before then.

The VAC Sigma 170i iQ integrated is a push-pull design with an 85Wpc output rating into 4, 8, or 16 ohms. It uses KT88s—two per channel—for its output stage and 12AX7s for the preamp and optional phono sections. The output stage is Ultralinear. "I think with the KT88 tube, the Ultralinear mode, properly done, sounds far and away better than the triode mode," Hayes told me during our call. "That comment applies only to this specific tube and to our output transformer, not to some other tube type or somebody else's output transformer." Hayes uses current-production tubes, including the Russian-made Gold Lion KT88s, for longevity and sonic consistency. Tube rolling is not discouraged, but the amplifier was voiced with the stock tubes. Aesthetically, the Sigma 170i iQ makes a statement, but it isn't bling. It feels luxurious, with its small, sturdy throw switches and heavy chrome knobs for source switching and volume control. The chassis is machined from aluminum and finished in black powdercoat. The faceplate is covered in multicoat black gloss lacquer with either gold or silver metal flake; the review unit had the silver.

The three throw switches are for power on/off, mute, and "Cinema"—and for selecting cartridge loading for the optional phono stage. Fuses, located in a small drawer adjacent to the IEC connector on the rear panel, are user-replaceable. Four front-panel LEDs indicate tube issues. Otherwise, front-panel lights indicate Power, Mute, and Cinema.

Hayes prefers old-fashioned switches to relays. "I can't find a relay as good as a good, high-force mechanical switch," he said. The volume control is old-school as well: an Alps motorized potentiometer, made in Japan. Transformers are custom-made for VAC in the US. The power transformer "floats"—mechanically, not electrically: It's decoupled from the chassis in a gelatinous material for isolation and mechanical-noise reduction.

The Sigma 170i iQ's optional, built-in phono stage ($1500) works with moving magnet and moving coil cartridges. Gain is fixed, specified as 37dB for MM and 57dB for MC. There are three user-selectable settings for MC loading: 100, 200, and 450 ohms. Each channel incorporates two twin triodes (12AX7A), and the circuit uses active RIAA equalization. My review unit came with the phono section, and I spent much of my time with the 170i iQ listening to vinyl.

The four inputs—all single-ended RCA unless you choose the $800 XLR option—are all line-level, unless the phono option is specified, in which case Input 1 (RCA) is given over to phono. There's a volume-controlled RCA Preamp Out and another RCA pair at the opposite end of the back panel, marked "CINE"; it's an input that bypasses the volume control. As its name suggests, it is intended for use with a home theater system or sources that have their own volume controls, as many DACs do. Otherwise, the back panel has several taps, shown in ranges, to connect speakers of various impedances: 2–4, 4–8, and 8–16 ohms.

The manual says that when hooking up your speakers, you should connect the red speaker wire to "Common," because the Ultralinear output signal is inverted. "The phase of that signal is automatically inverted," Hayes said. Rather than add complexity to the circuit, VAC decided the user could simply invert the loudspeaker leads.

There's a substantial remote control with rubber buttons for volume and mute. The user manual is user-friendly but not dumbed down. It covers the 170i iQ's essentials, plus some information on tubes, clearly and intelligently.

Hayes told me that although the "iQ" autobias system is intended to make VAC amps easy to use—iQ is a pun on intelligence and quiescent current, aka iQ—he has found that this method of autobiasing actually improves the sound. "Even as an experienced engineer, I can take my best meter and my best adjusting screwdriver, and I will never get the same result as the iQ system can," he told me. According to a white paper from VAC, some other manufacturers use a "set and hold" logic controller to set tube bias soon after the amp is turned on. A concern about this approach, Hayes said, is that a tube's idle current changes significantly after the first half-hour or so of operation.

It also fluctuates during dynamic swings in music, with sustained loud volumes, and with power line voltage fluctuations. Tube current can also simply drift. VAC's iQ autobiasing system, Hayes said, is unique in its ability to hold the true idle point steady in all playing conditions: "That's important, because if you allow the actual idle point of the output tubes to vary, you move away from the optimal match between the tubes and the load (loudspeaker), which can alter sound character, power delivery, and distortion." A VAC white paper claims that the iQ Intelligent Continuous Automatic Bias System maintains each tube's underlying current within 1% of the set target under all conditions, in real time. "And so being able to hold that point steady is like putting a really firm 20-foot concrete support under your turntable. It just matters," Hayes said.

The iQ Bias Control System performs two other functions. If there's ever a "runaway" tube drawing excessive current, the circuit shuts it down to prevent amplifier damage; one of those red LEDs on the front panel illuminates when this happens. If a tube is getting weak, a green LED lights up, alerting the user to replace it when convenient. Meanwhile, the user manual says, the iQ circuit will continue to make the most of the tube.

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Footnote 1: $150,000/pair configured as monoblocks, as they were reviewed.