Reviewers' ChoiceWhen I was a kid growing up in the pre-statehood Hawaiʻi, three generations of my family would spend each Sunday night after dinner watching The Ed Sullivan Show from New York, a magical city an ocean and an entire continent away from our islands. On the stage of CBS Studio 51, in the Maxine Elliott Theatre, at Broadway and 39th Street, all manner of clowns, comedians, magicians, and musicians would cavort and play, inspiring us provincials into laughter, tears, or quiet admiration. Besides the big stars -- the singers and actors of the day -- there were novelty acts like the Spanish ventriloquist Señor Wences with his talking, lidded box; the Little Italian Mouse, Topo Gigio, a hand puppet that bantered with the host; and Borscht Belt comedians galore who kept us in stitches.

One act in particular never failed to send me into fits of laughter, tumbling backward on our rattan carpeting, rolling in glee at the comedic spectacle. This was the spinning plates routine, derived from the Chinese circus and executed with hilarity and feigned frustration by a variety of slapstick geniuses. A middle-aged man in a droopy suit would set up long, willowy poles on stands or a table. Then he’d place a china plate atop one pole, which he’d wiggle so that the plate spun and was held aloft. He then repeated this until he had four or more plates spinning simultaneously on poles, some starting to wobble precariously as they lost speed and momentum. He’d have to run back to the first plate and wiggle the pole under it again, renewing its vigor and stabilizing its rotation. Then another plate would start to slow and wobble, so he’d run like hell back, wiggling the pole that was under it. But before it could stabilize, yet a third plate would wobble and then crash to the floor, making a loud, shattering noise, much to the performer’s exaggerated consternation. He’d run back and forth this way, catching a wobbling plate, wiggling a willowy pole, or taking hold of two poles simultaneously and wiggling them, while two other plates wobbled and crashed with a clatter to the floor. He’d grow more and more desperate, flailing and sliding around, stumbling into a pole and sending its plate crashing onto his head. In the end, there would be shards of china scattered across the stage, the poles fallen in disarray across the man’s shoulders and shoes, his hair a tousled mess, his face drawn down into a pitiful, Stan Laurel-like moue as he realized the disaster he’d wrought.

You could say that this is exactly what goes on inside a manually biased, push-pull tube amplifier. You get two pairs of tubes going that you’ve biased at idle points you think are stable. A few months down the road, one tube starts feeling its age, and its ideal bias-point drifts. So you rebias it to a slightly higher setting, increasing the current flow a touch until it stabilizes -- at which point a tube from the second pair starts to drift from its setting and you have to re-adjust it. Then another tube and another, until you can’t quite catch the right bias-point of any of them and one tube “runs away” -- that is, its plate voltage is suddenly too high, the plate reddens, and the tube blows. You’ve got a blown tube, a shorted amp, a fried bias resistor, a blown fuse, and a little puff of nacreous smoke wafting into your listening room. I hate it when that happens. Meanwhile, all your solid-state friends in the Saturday afternoon audio club are laughing at you.

VAC Signature 200iQ

You don’t have to worry about that any more. The Valve Amplification Company has come to the rescue. For their new Signature 200iQ tube amplifier ($14,000 USD), VAC has designed an auto-biasing system that not only auto-corrects bias drift, it senses and corrects the bias before it starts to drift. Moreover, the Signature 200iQ, a successor to VAC’s wonderful Phi 200 stereo amp, maintains each tube at its ideal idle current even while music is playing. Thus, each tube performs at its perfect current level at all times, ensuring a continuously optimal relation to the impedance of a loudspeaker as the dynamic levels of the music being played change from soft to very loud. (Dynamic peaks are notorious for creating distortion and bad sound.) This lengthens the life of the output tubes and prevents tube runaway from ever happening. No blown amp, no blown tubes, no damn comedy at all.

Background and development

Around midsummer, I enjoyed a long telephone conversation with Kevin Hayes, president of VAC, who explained that his company had been quietly at work on the problem of biasing output tubes for over 20 years. “Ever since early 1992 or ’93, we’ve been looking for a way to let an amp take care of its own housekeeping,” he said. “We wanted a means to set the bias point of each tube to its optimum performance level at all times -- a tall order.”

Hayes explained that this bias point, based on a tube’s “lq” (engineering shorthand for its quiescent current, or idle current), varies with warm-up, power-line voltage, the temperature of the output transformer, the tube’s age, the volume of music being played through it, and randomly as well. Variations can be minor, with some happening over long periods of time, but they all affect the performance of the tube, and create irregularities and distortions in the sound you hear. In the past, owners of VAC amps would set the bias point of each tube (indicated by an LED pinlight flashing green) just after start-up, recheck it after warm-up, and periodically check it again at intervals over the life of a tube. But as the tube aged, its bias point would drift, requiring it to be reset again and again to optimize its performance, protect the tube from getting too hot, and running away with current flow too high for safety.

During operation, however, each tube’s lq or optimal bias point would also drift, the pinlights flashing to orange or red as the tube responded to both the shifting lq and the volume of the music running through it. The lq, then, is a moving target, responding to numerous elements regarding electrical current flowing through the amp. Moreover, each tube is susceptible to what’s called rectification effect, which causes part of the audio signal to appear as a DC component whenever you try to measure a tube’s lq while it’s busy turning that signal into sound. Quite extreme during cutoff (or clipping), when the amp reaches its maximum power output, this means it’s practically impossible to determine the lq of a given tube while it’s in operation.

VAC Signature 200iQ

Earlier biasing techniques -- fixed bias, cathode bias, set and hold, etc. -- could only approximate an output tube’s optimal bias point. They couldn’t accurately rebias the tube, and failed to account for the rectification effect, an indeterminable thing that was a kind of electrical gremlin in the machine, frustrating all efforts to measure the lq point.

VAC’s patented technology (#8,749,310), the VAC iQ Continuous Automatic Bias System, does account for this elusive gremlin, steering around the rectification effect, preventing the moving target of a tube’s underlying and shifting lq from shifting at all, actually stopping it before it de-stabilizes. Just as the lq might start to drift, VAC’s iQ technology, ever alert and at the ready, gently shifts it back. In other words, were an output tube a spinning plate on a willowy stick, VAC keeps stationed a sober attendant with a high IQ (and no hilarity or monkey shines) right next to it so that, even before the plate begins to wobble or distort the wave of its spin, it gets a wiggle and a push of sorts, keeping its beautiful and exact spin rate and never toppling over, never crashing and getting smashed to pieces.

Hayes told me that, while VAC’s original design goal was simply to automate bias so as not to interfere with the sound, the result was an improvement in sound -- the tubes in the Signature 200iQ are maintained at their optimal lq at all times, eliminating distortions and noise due to dynamic swings in the music, powerline shifts, changing transformer temperatures, other random causes, and that pesky rectification effect. The iQ technology also keeps the DC of the push-pull output stage precisely balanced. What’s more, tube failures due to tube runaway are completely eliminated, making the new 200iQ a more reliable tube amp than any others -- save VAC’s own flagship, the Statement 450iQ, from which the Signature 200iQ was derived.

Description and setup

The VAC Signature 200iQ arrived in an oversize double box of heavy cardboard, suspended in sturdy foam cutouts within the inner box, and wrapped tightly in transparent plastic. Another nice touch was a rectangular piece of muslin-like cloth covering its very large faceplate, held there by a cat’s cradle of rubber bands. Also inside the box were its stock 6SN7 input and KT88 output tubes (four of each in their factory boxes), power cord, owner’s manual, and tube cage of perforated metal. The 200iQ is finished in a high-gloss, metallic wet-coat paint (black or silver are available) that quite handsomely sets off the two control knobs and LED bar, all of chrome silver.

Like the Phi 200, its direct predecessor, the Signature 200iQ is a 100Wpc power amplifier with speaker taps for various impedances. Though it can be easily converted for use as a 200W monoblock by flipping the Stereo/Mono toggle on the rear panel, I never used it that way. Fully balanced in its circuitry when the BAL/SE switch is set to BAL, the 200iQ has both balanced (XLR) and unbalanced (RCA) inputs, and its gains are a claimed 33dB single-ended or 30dB balanced. The input and driver stages are class-A, direct-coupled, low-mu triode circuits. The bandwidth specs are impressive: a frequency response of 4Hz-85kHz, +0/-3dB, and a full-power bandwidth of 13Hz-70kHz, +0/-3dB.

The amp’s shape is a new look for VAC, and stunning for its reversal of the company tradition of having the transformers in back and the tubes in front, in two rows. I asked Hayes about this change: “It’s not as radical as it might seem. The transformers went to the front in order to move the input jacks as close to the input tubes as physically possible, shortening the signal path length where it’s most delicate.”

The faceplate is strikingly reminiscent of the old Marantz Model 9, with a circular, Cyclopean meter (to indicate the current level) just above centerline; at the bottom, a silver bar that is inset with a warning LED for each output tube; and, to left and right, knobs for On/Off and for metering each tube.

At the far left of the rear panel is an IEC power inlet. Moving rightward from there is lots of silk-screened lettering for voltages, a warning, and the serial number. At center are Cardas rhodium (RCA) and Neutrik (XLR) input jacks, and the Stereo/Mono toggle. Next comes a row of rhodium ground and speaker posts by Cardas, with right- and left-channel taps for impedances of 1-2, 2-4, and 4-8 ohms. Above these, on the rear deck, are two rows of sockets for four tubes each; next to the input sockets are two switches that toggle each channel between balanced and single-ended operation (when set to SE, an internal phase-inverter circuit provides the opposing phase that normally appears on pin 3 of an XLR connector). It was a snap to install the KT88 output tubes, which are labeled in matched pairs. The input tubes are matched in staggered pairs -- I took care to match each tube’s label to the corresponding number on its socket.

VAC Signature 200iQ

Though it’s a struggle, I can lift the rear-weighted, 85-pound Phi 200 by myself by first positioning its rear end in the rack, then sliding the rest of it in fairly easily. The Signature 200iQ was another matter. It measures 18”W x 8.75”H x 17.25”D and weighs an awkwardly distributed 95 pounds, front-loaded with its transformers. This and its large faceplate made it impossible to carry and maneuver by myself. A friend helped me place the 200iQ on its stock feet on the bottom shelf of my five-shelf Box Furniture rack.

In my setup, while I took advantage of VAC’s stock Gold Lion KT88 output tubes (KT120s are also compatible), I substituted NOS 6SN7 signal tubes from my stash rather than using the new-issue, Russian-made Tung-Sol ones provided. For the inputs, I used a pair of 1940s-vintage vintage black glass Tung Sol roundplate 6SN7s; and for drivers, a pair of Sylvania 6SN7GT tall-bottle “Badboys” from 1952 -- both tube sets much coveted by cognoscenti. I connected the amp’s XLR inputs to my VAC Signature SE preamp with a pair of Audience Au24 SX balanced interconnects. Finally, I substituted an Audience Au24 SE powerChord for the stock one provided.


I let the Signature 200iQ run in a good bit before taking any critical notes. Fresh out of the box, its sound was somewhat harsh: midrange a touch opaque, highs occasionally brittle, with light bass. No surprise -- it had been run in for only about 20 hours at the factory, and the output tubes hadn’t settled yet. But only a few days later its sound took a great leap forward, and within a week I’d really started enjoying it. The impressions that follow are all based on notes I took after a full month of burn-in.

I tried difficult and detailed music. Listening to Mozart, a collection of his works performed by Concerto Köln under the direction of Anton Steck (CD, Archiv Produktion 00289 477 5800), I heard great frequency balance, strong orchestral tutti, and timbral contrasts full of inner detail in the textures of each instrumental section. Violins and violas were extraordinarily nimble, with impressive force and expressiveness in fortissimo passages and no loss of tonal integrity in string and brass sections. Overall, there was a sense of fullness and a populous soundstage, with fine layering and proper lateral positioning of instruments. Speed and the precision of timing were impressive, with no distortion or unwanted shadows of notes lingering beyond their natural time. In the Adagio of the Serenade for Winds No.10 in B-flat Major, K.361, “Gran Partita,” the burble of bassoons, the oboe’s arc of melody, and the woody resonance of the clarinet created a richness of timbral palette, combined with fine textural distinctions, that was exquisite to hear. In the Overture to Der Schauspieldirektor, the plethora of dynamic contrasts built up to an expressiveness so finely nuanced that I might have missed it as a predominant feature, had it not been for Concerto Köln’s precise and subtle talents and the Signature 200iQ’s technical prowess in conveying them.

Chinese pianist Lang Lang has taken the classical music world by Sturm und Drang this past decade, and his recording of Beethoven’s Piano Concertos Nos. 1 and 4, with Christoph Eschenbach conducting the Orchestre de Paris (CD, Deutsche Grammophon B0008725-02), presents an audio system with a noble challenge, combining as it does the difficulties of reproducing the tone and dynamics of a concert grand with those of an orchestra. The 200iQ didn’t break a sweat. The various instrumental timbres were retained in orchestral tutti -- the oboe clear against the violins, the double basses bowed gravely, with vigorous drumstrokes from the timpani. Eschenbach has the musicians play mightily, with fine sweep, in one passage, then as delicately as smoke in still air in another. Lang Lang’s piano sounded superb, from liquid arpeggios to delicate trills, with a tactile presence that had notes floating in air above and between the speakers. There was an organic sound across the piano’s range, weight and power in the left-hand notes, a sensuous bloom in the midrange, and treble sparkle in the air of my listening room. What emerged was the full drama of Beethoven’s score and Lang Lang’s magisterial interpretation of it. The launch of each note was a thrill -- clear, stirring, emotionally affecting -- and proved the 200iQ very capable of re-creating a fully expressive timbral palette at the most delicate and most powerful extremes of dynamics.

Luciano Pavarotti is no longer with us, but his recordings are -- treasuries of his emotional genius and technical prowess. His “Nessun dorma,” from Puccini’s Turandot, is still my favorite. I played the aria from his The World’s Favorite Tenor (LP, London, OS 26384), and heard that beautiful, silvery voice, powerful enough to fill a stadium, sailing out open and expressive throughout his wide ranges of pitch and dynamics. Pavarotti’s vocal power -- a noted system crusher that can as often produce ragged distortions as sublimity -- didn’t faze the 200iQ one bit. The amp remained unaffected as it delivered the requisite steadiness of sustained current to my Von Schweikert Audio VR-44 Aktive speakers. Years ago, when I began my audio journey, Pavarotti’s recordings were the most difficult for my rudimentary and mismatched systems to reproduce -- so many of my amps, tubed or solid-state, clipped and distorted, and speakers spat and broke up under his immense power. Not so with the combo of VAC amp and VSA speakers -- all the heroism and bravura of his astonishing voice came through with a fine, dramatic purity.

VAC Signature 200iQ

“Milkcow’s Calf Blues,” from Eric Clapton’s Sessions for Robert J (CD, Reprise 48926-2), fared equally well. Its sound was big, thumping, and room-filling, with fabulous amplified instrumental textures coming together on time without chaos or gross jumble, but creating a swelling tapestry of roiling, electronic sound. I heard Nathan East’s bass and Steve Gold’s kickdrum locking together à la John McVie and Mick Fleetwood in John Mayall’s Blues Breakers, and Clapton’s slide guitar screaming over them, recognizably virtuosic and tasteful. There was a precise, controlled crunch to Doyle Bramhall’s rhythm guitar, and Chris Stainton’s deft accompaniment on piano contributed a dash of honky-tonk, while Billy Preston’s Hammond B3 filled the air and sweetened the bluesy edges. Center stage was Clapton’s voice, punchy and properly distinct, with a solid image.

I was equally impressed with Eva Cassidy’s voice in “People Get Ready” and “Take Me to the River,” from her Live at Blues Alley (CD, Blix Street G2-10046). The former track, a bluesy interpretation of Curtis Mayfield’s gospel-inspired song, emphasizes thrilling octave leaps in her hollers, contrasting with whispery near-recitative reminiscent of Cleo Laine. I could hear Cassidy’s swift shifts of timbre -- from rich, relaxed alto to sweet contralto cooing that segued alternately into whispers and tight, affecting wails. In the Al Green song, Cassidy got all its rhythm and bluesiness right, her shouts and wails working with the recitative of the chorus, which swung with tasty emphases and the odd, off-beat stresses that art-house rocker David Byrne used to toss into the lyric in the Talking Heads’ cover. It was real head-bobbing music, with terrific bass punch and drumming weight, and proved that the 200iQ could really rock.

Perhaps the most startling sound I heard was that of the immortal Miles Davis’s Harmon-muted trumpet in “Basin Street Blues” from his Seven Steps to Heaven (CD, Columbia/Legacy CK 48827). It was explosive and piercing in a pleasant way, the notes lingering a bit in the air, a ghostly presence in my listening room. Every note was clear, tactile, evolving in time, plaintive and dynamic. The detail was such that notes bloomed as Davis played them, from attack impact to a subtle gathering in volume or a fading in aching diminuendo in long, sustained legatos and portamentos. There was a fabulous sense of touch and liveliness, the trumpet’s aural image solid and center-forward. It was as if Davis were there in front of me. The other players of his quartet were spread across the soundstage: Frank Butler’s drums deep at left, Ron Carter’s double bass near the center and noticeably behind Davis, and Victor Feldman’s piano at right. In my notes, I wrote, “I can hear Feldman’s foot let loose the pedal at tune’s end.” How’s that for “Is it real, or is it Memorex?”


Released in 2008, VAC’s Phi 200 stereo amplifier ($9990) has been one of my references ever since I reviewed it for SoundStage! Ultra in 2011. Like the Signature 200iQ, the Phi 200 outputs 100Wpc into 8 ohms, and though its dimensions and versatility (it’s bridgeable into mono and has both XLR and RCA outputs) are roughly comparable, it weighs ten pounds less and, in terms of technology, is a generation older: it lacks VAC’s iQ auto-biasing. Nonetheless, I was skeptical that the new 200iQ could be “better with the internal textures of notes, particularly in dynamic peaks with classical music,” as Kevin Hayes assured me it was. After all, I’d been happy with the Phi 200’s performance for over six years and couldn’t imagine how or where the 200iQ might exceed it. As far as I was concerned, the older amp was completely competent in terms of extension, resolution, speed, and reliability. It had also never blown an output tube, and I’d rarely had to rebias its tubes; in more than six years, only one tube had even begun to weaken.

To create the fairest comparison, I used the same cables -- an Audience Au24 SE powerChord and a pair of SX XLR interconnects -- and even the same tubes, swapping in the new Gold Lion KT88s for VAC’s Shuguang KT88-SCs in my Phi 200, and transferring the pairs of NOS Tung Sol and Sylvania signal tubes from the 200iQ to the Phi 200.

It was obvious from the beginning that there was a big difference between these two VAC amps. Through the Phi 200, Miles Davis’s muted trumpet in “Basin Street Blues” didn’t sound nearly as explosive, though it was just as clean and clear as with the 200iQ. The thrilling impacts of attacks were missing, and though Davis’s legatos were comparable, Butler’s brushwork on snare was softer, farther back on the stage, and his snare and hi-hat weren’t quite as brisk or snappy. Feldman’s comping was farther in the background, and though Carter’s bass seemed just as full, it was much less punchy and quick. Through the 200iQ, this was an exciting track full of live presence; through the Phi 200 it was just a fine recording, no more remarkable than many others. The Phi’s overall sound was noticeably rounder than the 200iQ’s.

VAC Signature 200iQ

With rock and blues, however, the Phi 200 held its own against the Signature 200iQ. “Milkcow’s Calf Blues” featured a similarly locked rhythm section with great weight and pulse, Clapton’s voice rising above the moil of two amplified guitars. His slide lead was alternately clear and snarling, and Bramhall’s rhythmic strumming sounded just as satisfyingly crunchy as with the 200iQ. Cassidy’s cover of “People Get Ready” had the same punch of electronic impacts from her band, and her voice was just as soulful and rangy: explosive in wails and hollers, poignant and exquisite in whispered passages.

However, there were telling differences with opera and orchestral music. In the Overture to Mozart’s Der Schauspieldirektor, there was a slight chalkiness to the tops of the Concerto Köln’s violins with the Phi 200, and tutti were a little washy in comparison to the 200iQ -- a touch more edgy, with a disappointing tinge of sourness. “How could I not have heard this before?” I wrote in my notes. Dynamic shadings and contrasts were fine, but the 200iQ presented a much more pleasant sound that I found more accurate to life. In Beethoven’s Piano Concerto No.4, Lang Lang’s piano sounded rich and clear, with bountiful harmonics, generous bass legato, and swift, precise, dynamic nuances in his arpeggios. But I also heard an unmistakable stridency and forwardness in the violins of the Orchestre de Paris that were not present with the 200iQ. The first, cresting tutti of the Allegro moderato sounded slightly opaque compared to the more open sound of the 200iQ, presenting a harshness that was fairly moderate but discernible, and that compromised the beautiful sound of all the strings playing together fortissimo. The textures of the Phi 200 were murkier and less clear and suffered from a slight smear of electronic distortion I’d evidently become inured to -- until I heard the Signature 200iQ. Finally, Pavarotti’s top notes in “Nessun dorma” had a bit of edge and after-ringing at high volumes with the Phi 200. It took away from the beauty of his magnificent performance, and, sadly, I had to turn the volume down. Kevin Hayes was exactly right. His new iQ technology had taken his newest stereo amplifier to another, much more glorious level of playback performance.


If you’re in the market for a powerful tubed stereo amplifier, and if you appreciate precision in sound, great looks, and the last word in tube reliability and performance, you must audition the Valve Amplification Company’s Signature 200iQ. I guarantee that there will be no hilarity, no disastrous crescendos of smashed plates or capricious inconstancies of bias settings, no anxieties or mournful, sad-sack grimaces of defeat regarding the stability of this very serious piece of electronics. Rather than your having to be an acrobat of constant readjustments and maintenance to preserve your precious output tubes, the 200iQ comes with the acrobat preinstalled -- VAC’s iQ system automatically takes care of all that for you, taming the electronic gremlins of bias drift and signal wobble, to present you with only pure sound, a rock-steady current to your speakers, and a thoroughly enjoyable listening experience.

Once heard, purity can never be unheard. VAC’s new Signature 200iQ presents a clarified purity with a sensuous ease I have decided I cannot do without. Despite all my loyalties to the Phi 200 for its gifts of great sound over the last six years, and despite its charming retro look and the wondrous, glowing allure of its front-mounted tubes, I have to make a change. The Signature 200iQ is my new reference stereo power amplifier.

. . . Garrett Hongo

Associated Equipment

  • Analog source -- TW Acustic Raven Two turntable and 10.5 tonearm, Zyx 4D MC cartridge (0.24mV); Ortofon RS-309D tonearm with Miyajima Zero MC cartridge (0.4mV)
  • Digital source -- Esoteric K-05X SACD/CD player
  • Preamplifiers -- VAC Signature Mk.2 SE, Zanden 1200 phono stage
  • Power amplifier -- VAC Phi 200
  • Speakers -- Von Schweikert Audio VR-44 Aktive with RST-5 ribbon supertweeters and Masterbuilt jumpers
  • Power cords -- Audience: Au24 SE powerChord, Au24 SE powerChord MP, Au24 SE powerChord LP, Au24 SX powerChord
  • Interconnects -- Audience Au24 SX (unbalanced and balanced)
  • Speaker cables -- Zanden Audio Systems
  • Power conditioner -- Audience aR6-TSSOX
  • Record cleaner -- Loricraft PRC4
  • Accessories -- Box Furniture S5S five-shelf rack and amp stand, Pottery Barn four-shelf hardwood console, edenSound FatBoy dampers, HRS damping plates, fo.Q Modrate HEM-25B and HEM-25S Pure Note Insulators, Acoustic Science Corporation SoundPanels, Zanden Audio Systems AT-1 Acoustic Tubes and AP-1 Acoustic Panels

VAC Signature 200iQ Stereo Amplifier
Price: $14,000 USD.
Warranty: Two years parts and labor; tubes, 30 days.

Valve Amplification Company
1911 N. East Avenue
Sarasota, FL 34234
Phone: (941) 952-9695
Fax: (941) 952-9691