Tip 23: The Antique Sound Lab KI22-FOX Stereo Integrated Amplifier
Do you own any consumer goods made in China? Before you reflexively answer in the negative, let me point out that this is a trick question, as most people probably do. From fashion clothes to well known American brand names such as Procter Silex and Altec multimedia speakers, production has gradually been shifted to Mainland China. With the steady expansion of Chinese capitalism it was only a matter of time before the export stream grew to include high-end tube gear. Antique Sound Lab (ASL), now distributed and serviced in North America by Divergent Technologies (DT), offers a full line of both classic as well as innovative tube designs. Some of these designs are customized to DT's needs and specifications, as is the case with the KI22-FOX, which is an upgraded version of the basic ASL KI15-FOX: the output transformers are higher grade, the volume control is an Alps pot, and Infinicap coupling capacitors are now used in current production. My review sample incorporates all of these improvements except for the new coupling caps. In either case, what you have here is an inventive integrated amp built around the Russian 6C33C-B power triode.
When it comes to appearances, the KI22-FOX amplifier clearly belongs to the "plain Jane" school of industrial design. Its FOX designation must apparently refer to all of the technical sophistication under the hood. Make no mistake about it: this amplifier blazes a new trail in Single-Ended triode amplification practice. And it would be fair to say that I've fallen under its spell. At its asking price of US$1,549, many audiophiles are going to have a hard time accepting it as more than a budget offering without any pretensions of encroaching upon high-end audio's inner sanctum. The fact is that at the low-power end of the spectrum it destroys domestic competition under $3K. It's sad but true that consumers find it difficult to overcome the perception that an overpriced product from an audiophile approved "designer label" brand name automatically equates with sound quality. In fact, the reverse is often true. My policy is to keep an open mind and let the sound quality speak for itself.
It's possible to tell an awful lot about the sonic potential of a particular tube product by simply inspecting its tube complement. In this particular case, I was quite pleased with what I saw. It is really a breath of fresh air to find something else besides the ubiquitous trio of 9-pin miniature types – also known as 12AX7, 12AU7, and 12AT7 dual triodes - used for the voltage gain and driver stages. Instead, the FOX opts for Sovtek brand 6SL7 and 6SN7 octal type dual triodes. The 6SL7/6SN7 tube alignment follows along classic glass design lines. Whether it's the beefier construction, or the fact that they operate in a higher current regime, octals to my ears beat out miniature types in terms of dynamics and linearity. Here a high-gain 6SL7 is used for the first two stages: input voltage gain and cathode follower buffer. The driver stage is configured as a symmetrical totem pole and uses a pair 6SN7 dual triodes wired in parallel. For some years now, I've been enamored with the harmonic richness of a good 6SN7. Add a 6SL7 at the front end, and you get a lovely synergy in terms of harmonic expressiveness. According to Tash Goka at DT, various New – Old Stock (NOS) 6SN7 types have been evaluated, but that ultimately it is more logical to switch to the new production Valve Art metal-based 6SN7 in the near future, when it becomes readily available in quantity.
The output tube is the 6C33C-B, a triode originally designed in Russia for voltage regulator duty. There's no mistaking this tube on a crowded chassis! It's fat, intimidating, and comes decorated with three distinctive glass nipples on top of the glass envelope. This Russian tank of a tube has become quite popular for OTL applications because of its very low plate resistance. The output stage topology deployed by ASL is quite unusual in that the output transformer is inserted between cathode and ground instead of in the customary plate circuit. The cathode follower output circuit affords SE designs a much lower output impedance and a better damping factor than is feasible with the standard plate connection. See my review of the Cyrus Brenneman Cavalier amplifier for more details about this type of circuit. Global feedback is applied from a tap on the secondary winding of the output transformer. Speaker connections are provided for 4, 8, and 16-ohm load impedance.
The power supply uses AC for filament heating, and solid-state rectified DC for everything else. A Pi filter network is used in conjunction with the B+ supply for the 6C33C-B output tubes. In addition, the fixed grid bias voltage for the output tubes is regulated. Overall, this is a beefy power supply, and that is absolutely unheard of at this price point - even in more expensive amplifiers.
It quickly became apparent that this was no typical SE triode amplifier. Just about all of the SE amps I've auditioned in the past 10 years had struggled with bass definition. The bane of SE amplification is high internal impedance, typically on the order of 2 ohms, that results in poor control over bass transients. SE bass mush is what it really is. And I remember what the crafty Bob Carver once told me in defense of the high-Q bass quality of his Amazing loudspeaker: "I like rolling bass." Well, excuse me, but I prefer tight, defined bass lines - just like the real thing.
This same high source impedance also leads to frequency response deviations due interactions with the speaker's impedance magnitude. There is nothing new here that hasn't been known for over fifty years, and the common engineering solution is to apply local and global feedback in order to reduce the source impedance. Local feedback applies automatically in a cathode follower output stage. And in the case of the KI22-FOX both types of feedback are used to drive the source impedance down and improve bass control. I've used this amp with the Hammer Dynamics Super 12 as well as my own BassZilla loudspeaker, which are both bass-reflex designs. This type of bass alignment needs bass control, and the FOX provided bass discipline in spades! In both cases, bass lines were extremely well fleshed out with excellent pitch definition. A jazz bass has nowhere to hide. The likes of Ray Brown or Percy Heath stand out in a mix and sustain a well-recorded jazz combo. One example that comes quickly to mind is Cannonball Adderly's "Know What I mean?" with Bill Evans on piano and Percy Heath on bass (Riverside RLP-9433). Not only were timing and pitch inflections clearly resolved, but bass timbre was also captured with a delicious palette of harmonic color. There was plenty of current drive into an 8-ohm nominal load. With the BassZilla high-efficiency loudspeaker, bass impact was just south of stupendous. And this thing was pretty darn quiet. Hum and noise were sufficiently low in level as to be unobtrusive.
Treble extension and power bandwidth in a SE triode design are mainly limited by the quality of the output transformer. Many SE designs, including the Cary 805 with which I'm very familiar, sound rolled off in the treble with blunted transients that soften the impact of the music. Sonically, it's as if soundstage lighting were suddenly tinged yellow and warmer in character. The tonal effect of bandwidth limiting is a shift in tonal focus toward the midrange. In contrast, the ASL integrated amplifier sounded open and spacious in the treble registers. Transients were unfolded with speed and control. Tube glare and brightness, the artifacts of odd order harmonic distortion, were minimal. Even when pushed hard, its sonic character remained civilized.
The midrange is where SE triode magic is self evident – at least with a compatible loudspeaker. It's a question of being able to nurture the music's microdynamics and low-level volume and pitch modulations to full bloom. Ironically, the music's emotional power and expressiveness are hidden in the low-level details. For example, subtle modulations in a singer's voice can infuse a song with either hope or sadness. The waveform differences look pretty minimal on a scope, but their impact on the human auditory system can be dramatic. The key is to preserve these cues and their relationships so as to allow the auditory system a chance to fully experience the music's sensuality. Yes, it's still the first watt that sets the stage, and SE designs continue to show the way. Right out of the box, the FOX displayed decent midrange expressiveness and plenty of coherency. By midrange coherency I mean that the transition from the lower mids to the upper mids was balanced, and did not change in character when instruments spanned this range. Still, there was something missing. The soundstage could have been more expansive, front to back and side to side. The soundstage could have also used a cleaner window, as some veiling was evident. These slight losses in transparency and spatial resolution made it a bit difficult to step into the original acoustic. It took more effort to accept the illusion of live, and more effort equates with less pleasure. Finally, recall my recent review of the WAVAC MD-811, which consistently weaved an organic musical tapestry without any electronic artifacts. The FOX was not as relaxed nor as natural sounding as the WAVAC, sounding a bit more artificial in the midrange.
The Sound – Transformation
Well, that's where matters stood until I decided to experiment with tube substitutions. My primary motivation was an article published last year in Vacuum tube Valley Issue #11 ( www.vacuumtube.com) by Charlie Kittleson and Eric Barbour titled "Listening to 6SN7s." The sound of various 6SN7 types was evaluated, at least in the context of the VTV Octal Line Stage. The VTV listening panel goes on to describe the Sovtek 6SN7GT as "a bland and primitive sounding tube. Not very musically involving and nothing special. Used by tube amp OEMs due to its cheap price and ready availability." Ugh, after such an unappetizing verdict, which by the way I agree with, it was time to try something else. Out came the Sovteks and in went my vintage 60s, brown-based, Sylvania 6SN7GTs.
Power the unit on and get ready to orgasm!!! I've been tube rolling for many years, but never before have I experienced a more dramatic sonic transformation than this one. All of my previous reservations were obliterated. In fact, I'm still shaking my head in disbelief over the magnitude of the change. For starters, the soundstage expanded to fill the front of the listening room with a wonderfully cogent, transparent, and romantic midrange. Almost as if with the stroke of a magic wand, the music became infused with extraordinary verve. What an addictive experience. Album after album, the message was clear: this puppy can tango seductively with the best of them. It seemed to consistently unleash the music's dramatic underpinnings. I was surprised time and time again by its fresh rendition of music I was very familiar with. Instrumental outlines were sharply focused in space. Low-level detail floated to the surface with exceptional clarity. The decay of various reverb signatures was clearly resolved, down to the noise floor of the recording. It would, however, be a mistake to overly analyze the sound of the FOX, because its greatest strength when fortified by a quartet of good 6SN7s is an overall coherency of time and space rare in any amplifier at any price point. What that means in practical terms, is that you are easily transported into another dimension, which is what should happen much more often in this high-end audio business of ours. For the record, I did experiment with a couple of NOS substitutions for the Sovtek 6SL7, but without any meaningful sonic gains. My advice would be to leave the Sovtek 6SL7 alone, and focus instead on upgrading the 6SN7 tube complement. It may cost you upward to $200 to obtain a premium set of NOS tubes, but don't skimp here, since it makes all the difference in the world. I hope to report at a later date on the sonics of the new Valve Art 6SN7.
It gives me great pleasure to share this finding with a wide global audience. The Antique Sound Lab KI-22 (DT version) when fortified by a suitable complement of NOS 6SN7s is an intensely musical performer. Driving a suitable loudspeaker such as the BassZilla, the FOX facilitates a wonderfully emotional musical experience – a rare happening at any price point. However, considering its US retail price, the FOX qualifies as a major miracle. It is the prototypical giant killer - a clear winner at any price point. High-end audio has just become much more affordable! Kudos are due to the Antique Sound Lab team, and especially designer Joseph Lau for taking the road less traveled to the Promised Land.
Technical Specifications for the Ki22FOX DT
Output Power 22 wpc