VAC Sigma 170i IQ integrated amplifier

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.

Footnote 1: $150,000/pair configured as monoblocks, as they were reviewed.
Valve Amplification Company, Inc.
1911 North East Ave.
Sarasota, FL 34234
(941) 952-9695

tonykaz's picture

Not hidden by out of focus camera work or Photo manipulation like is common for Chinese Audiophile stuff.

A Quality product made by Loyal Employees at a Fair price.

Is $10,000 a fair price? , the reviewer seems to think so. ( seems pricy to me )

Tony in Venice Florida ( about 25 miles from this outfit )

Ortofan's picture

... by the time you add on the prices of the optional XLR upgrade, the optional phono stage and the optional tube cage.

Would you still choose this VAC amp when, for the same or even lower cost, you could instead have either the combination of a McIntosh C22 preamp and MC275 power amp or the combination of a Luxman CL-38UC preamp and MQ-88UC power amp?

tonykaz's picture

Hello Mr.Ortofan,

I would want or desire this Amp if it's Sound Quality is outright/downright outstanding and I could justify it's price.

I already own liquid gold amplification systems so I'd have to be dazzled enough to reorganise my outstanding to make room for this piece.

but before that:

I'd be taking a close look to see what's holding my gear back that I hadn't noticed and attempt to revise & improve.

I'm impulsive, I might just love these VAC folks and their gear!

Owning a system Made in Sarasota Florida would be spiritually wonderful for me!

Do they have a scatch & dent department?

Tony in Venice Florida

Anton's picture

We should be thinking about the price of that unit in comparative terms. Your examples are p100% apt.

georgehifi's picture

Ouch!!!!!!!! "Talk about going back in time of no damping factor"
6.5db frequency response variation in the audible range. And that's with a simple simulated Kantor speaker load. Hate to see what happens into something like some nasty Wilsons or something harder.

"Using our definition of clipping, which is when the output's THD+noise percentage reaches 1%, the amplifier with both channels driven with a 1kHz signal clipped at 11.8Wpc into 8 ohms"

And on the lower taps you'd have less wattage, do they just want to drive things like Klipsch-Horns etc

Cheers George

tonykaz's picture

Yes, lets do Horns!!

Tony in Venice Florida

ps. you make good points, I like reading your thoughts!!!

Jim Austin's picture


I thought I'd chime in here, first to say that I really like he collegiality demonstrated in this and quite a few other threads lately. Disagreement is fully compatible with courtesy (not that I'm seeing a ton of disagreement here.

But I'd also like to comment on the content of your post. This is obviously an old-school amplifier with high-quality construction and parts. Choices were made. Output impedance could have been reduced with the judicious use of negative feedback. The designer chose not to go that route, finding that the disadvantages outweighed the advantages--or, to put it more positively, that the SQ advantages of the sound even with the possibility of gross frequency-response variations was so good he left it alone. Those seeking lower output impedance can find it even within the VAC line.

I've heard this amplifier only at shows, in unfamiliar systems, so no opinion I express should be taken too seriously. With an amplifier like this, much care must be taken to matching it with the right loudspeaker; while it's possible that one might get synergy with less-demanding Wilsons, that's a crap-shoot--not an obvious match. If that's your cuppa, probably best look elsewhere. It should however make an excellent match with, eg, a pair of DeVore Orangutans, or Audio Notes, or anything that loves tubes and doesn't care much about high output impedance (or thrives on it). I heard this amp at a show (Tampa?) with the little Acoras, in a small hotel room, and thought the system sounded great.

One more point to make, which I'm sure is already familiar, but I'll put out here anyway: For an amp like this, "clipping" means something quite different than it does for a classic solid state amp. There's no sudden rise in harsh distortion. Here, "clipping" is just a word coupled with a more-or-less arbitrary number.

There's room for more than one model of excellence in the world, is my main point.

My Best to all,

Jim Austin, Editor

thatguy's picture

I think it is so great that companies will make unique products like this that have such a small niche. To me, some of these rise to the point of being functioning art.
It isn't something I'd ever own but, just like exotic super cars, it is fun reading about and fun to know someone had the passion to build it.

Now, back to listening to my home built 6L6GC single ended tube amp.

Ortofan's picture

... with a sufficiently high output impedance, such that it will result in audible variations in frequency response when connected to a typical speaker, would it not be beneficial to potential customers for them to publish a list of those speakers they have used during the development of that amplifier, and to identify which ones they have found to be relatively more, or less, compatible?

LTig's picture

.. because the user has no control over it, other than choosing a matching speaker.

dc_bruce's picture

I don't think it's praiseworthy for a manufacturer to describe this as an "85 watt" amplifier when it appears not to meet even 1/2 of its rated specification. Even if you subscribe to the "Julia Child" theory of assembling a stereo system ("a little bit of this, just a dash of that, and voila it all sounds wonderful"), I don't think its fair of the manufacturer not to advise the purchaser that this amp needs to be partnered with speakers that are 90 dB+ efficient and have benign impedance curves or he/she needs to be satisfied with modest loudness levels in a small room. And that goes for dealers, too.

I won't even comment on the price tag, except that it is not axiomatic that expensive parts + expensive labor costs = great sound quality. If it were, I -- an English major -- would be capable of building a great-sounding amplifier.

It would be interesting to compare this amplifier sonically to a properly assembled Dyna Stereo 70/PAS3x combination with fresh capacitors, something I did assemble more than 50 years ago. The Stereo 70 uses EL34 output tubes in ultralinear configuration, not KT 88s. The PAS3x uses a quad of 12ax7s.

Herb Reichert's picture

the definition of amplifier clipping is simply when the top of a waveform is "clipped" off by the limits of its power supply.


By this standard EE definition this amplifier probably delivers more watts than shown here. (Even thought 85 watts seems very optimistic for KT88s.)

According to Bob Cordell (Designing Audio Amplifiers pp 516-524)"... amplifier clipping is exacerbated by negative feedback."

Consequently, low-feedback tube amplifiers are said to clip more 'gracefully" than solid-state and 'appear' in practice to deliver more than their rated power.

just saying


Jim Austin's picture


It's simple enough to define "clipping" from an operational/theoretical point of view. The other perspective is, "what does it look like when you measure it?" That's what I was addressing. If you look at this distortion vs power for this recently published Counterpoint amp review (solid state):

and compare it to the VAC

You see that the first as a kink followed by a more rapid rise. In the VAC, the closest equivalent to that kink happens at, what, 2.8W from the 8-16 ohm tap? This is not a 2.8W amp!

But because of the kink and the fast rise, that same feature does define clipping on the solid state Counterpoint amp. Notice that it hits 1%--JA's usual criterion--just slightly above the power where the kink occurs. For the VAC, in contrast, there is no kink, but the amp hits 1% at roughly four times the power where it starts rising. What's more, there are no sudden changes in the distortion--just a steady rise--up to 50W, where the measurement stops. Gentle clipping!

Not shown here, but if you look at the 4-8 ohm tap measurement--fig.7 in the VAC's measurements section--you can see that it does have an eventual kink--at about 80W--well above 1% distortion. Looked at from this perspective, you could almost say that the clipping point--the kink--is never reached in those higher-impedance-tap measurements!

Best Wishes,

Jim Austin, Editor

Ortofan's picture

... increasing distortion from the VAC amp become audible?

John Atkinson's picture
Ortofan wrote:
At what power output level does the increasing distortion from the VAC amp become audible?

There's no straightforward answer to this question, as it depends on the nature of the amplifier's harmonic distortion, the kind of music being played, and the impedance and sensitivity of the loudspeakers.

I created tracks on our no-longer-available Test CD 2 - see, so listeners could test for themselves at what percentage of pure second, third, and seventh harmonic become audible with a 500Hz tone. I find that 0.1% or even 0.03% of seventh harmonic will be audible, due to the fact that the frequency of the distortion is very much higher than 500Hz. However, 1% of second harmonic is only just audible with the pure tone because the distortion is both musically consonant and has a frequency much closer to 500Hz.

All these harmonics will be less audible with music, due to masking, as long as the amplifier is not also creating intermodulation distortion products that will have no harmonic relationship with the music. In that respect, this VAC amplifier performs reasonably well, as its intermodulation distortion at moderate powers is relatively low (fig.13).

John Atkinson
Technical Editor, Stereophile

Jim Austin's picture
Kind of obvious, but often mentioned: Audibility depends on absolute level, not only relative level. It's harder to hear 1% in quiet music (or tones) than it is in louder music (or tones). Jim Austin, Editor Stereophile
LTig's picture

... is the definition of the old German high fidelity norm DIN 45500.

Glotz's picture

Cutting through right to the truth!

Gentle clipping IS different than hard clipping. Real world or on paper.

Both amazing posts! Thank you for level-setting and a bit of clarifying.

Fstein's picture

Lirpasound announces $79 amplifier, states previous price of $159,000 a joke no reasonable person would believe

a.wayne's picture

I missed the Party ....!