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Luminance Audio KST-150 Amplifier Review

The $3,000 Luminance Audio KST-150 as per the eyes of Constantine Soo: a tradition of fulfilling expectations of generations

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Luminance Audio KST 150

Prologue

The 1980’s was a period of certain enlightenment for the audio industry. After a decade’s worth of joint engineering efforts between Sony and Philips, the industry finally introduced the compact disc technology to the market, injecting unprecedented excitement and energy into both the music and the hardware manufacturing business.

Just as the compact disc had made music available to an exponentially larger number of people from all walks of life, one miracle that happened elsewhere in the audio business also contributed to the happiness of a distinct group of people, although this group was infinitely smaller compared to the general public. The miracle was the B& K Components ST-140 amplifier, and the beneficiary was the audiophile community.

I was but a junior member among my audiophile friends at that time, who were all men in their 40s to 60s. Teac was debuting their CD separates, and Krell was still in its infant years. Speakers of lower efficiency were on the rise, and continuing dismay from a considerably wide spectrum of audiophiles at the solid-state sound of the time helped drive many back to the world of SETs and horn speakers.

B&K ST-140

The B&K ST-140 was the stellar exception to the solid-state sound of the time. “Musical” was the word most widely used by audiophiles from all budgets in describing their feeling after listening to it, and, unlike today, there weren’t that many affordable audiophile solid-state amplifiers out there. The fact that it retailed for only $499 made it simply irresistible, and propelled it into an iconic audio essential that an audiophile “worthy of his salt” must surely acquire, even if it would just be sitting next to the system idling a good part of its life away. To many, the ST-140 was the only answer if he couldn’t stand the horn alternative and was adamant in using his less-efficient speakers.

24 years after the release of the popular B&K ST-140 stereo power amplifier, Steve Keiser, its designer, is joined by Rick Schultz of Virtual Dynamics and Mike Tseng to launch a new company, Luminance Audio. The first brainchild product to be released by this new company is the Luminance Audio KST-150. See Dagogo’s “From B&K ST-140 to Luminance Audio KST-150” commentary for an indispensable glimpse into the mind of Steve Keiser.

A stereo power amplifier that weighs 33lb and is rated to put out 250Wpc into a 4Ω load, the Luminance Audio KST-150 is a balanced, cascade-based, 3-stage design with a common mode rejection ratio of 90db+, and a declared objective of being among the top three fastest amplifiers with wide bandwidth and no distortion.

Steve incorporated an ultra high-current-gain complimentary push-pull cascaded amplification stage to drive a quadruplet of high-gain power MOSFETS per channel to attain a substantial damping factor of 100. As this damping factor is sustained fully by the driver and output stages from 20Hz to 20kHz, the KST-150 is said to be able to control a loudspeaker’s woofer movements and tweeter movements with equal vigor.

Being one of the fastest amplifiers, the KST-150 is rated for a slew rate of 250V/ms, an approximate figure also incidentally claimed by the $11k Audia Flight 100 power amplifier that I reviewed in October 2005, plus a substantial advancement over the industry norm of around 40V/ms, thus supposedly able to represent complex passages with higher accuracy. While Luminance Audio concedes to the comparable slew ratio as also offered by digital, switching amplifiers, the company is of the opinion that the switching process in digital amplifiers disrupts the continuity of the musical signal.

On the design parameters of the KST-150, Steve Keiser has the following to offer:

“The basic design premise behind the KST-150 is to insure an equal excellence in both time and frequency domain and output square wave response, which maintains a high degree of adherence to input waveform fidelity.

The primary test I used to evaluate the time domain response accuracy of the KST-150 was to inject a 10kHz square wave input into the active circuit, and to perform a spectral analysis which displays the relative amplitudes of each harmonic component of the square wave input at the output. Any observed deviation that the output exhibited relative to the input was determined to be distortion, and I adjusted the internal circuit parameters of the amplifier until the lowest distortion reading was obtained. This design process took about six months of intensive work and was interspersed with extensive listening tests, which correlated the connection between the measured improvements with perceived sonic benefits.

Specifically, to obtain the wave response fidelity measurements, the KST-150’s slew rate was maximized at 250 volts per micro second, its frequency response bandwidth was extended from DC to 630kHz. Overall negative feedback loop bandwidth is constant from DC to 63kHz and open loop bandwidth and closed loop bandwidth frequency domain time constants are matched using 1% tolerance polystyrene capacitors. All resistors on the amplifier circuit board are of 1% tolerance. The input stage uses a differential amplifier which inputs the signal from the source as well as from the output and performs the necessary voltage gain, which is 26dB. The driver stage is a fully complimentary push pull cascaded common emitter amplifier, which in turn drives a complimentary, two parallel push-pull source follower Hitachi MOSFET output stage. The power supply utilizes a total of 56000 mfd of storage capacity and is parallel distributed, meaning that total of six 8200 mfd capacitors are used connected in parallel, instead of the usual two. The input AC is terminated with 3-pole AC line filter to remove power supply line noise.

The important, overall design philosophy approach is to correlate with as much listening improvements as possible from years of identifying which technical parameters were actually relevant and which were not. By selectively deciphering the correlation of component quality and circuit specification to the resultant sound, we then summarized these results into what is currently the KST-150 amplifier.

Of course, the success or failure of the theoretical assumptions made on the KST-150 will be determined by the auditioning listeners’ perceived sound quality of the amplifier, in their evaluation of the sonic merits of my overall design approach.”

The KST-150 employs 28dB of local feedback for DC offset stability, which is gradually rolled off above DC at 12dB per octave to the point whereas only 0.3dB’s worth of feedback remains at 1kHz.

The chassis design of the KST-150 is representative of a minimalist’s conception, and the heatsinks are thankfully fitted at the back of the amplifier. Although the faceplate is adorned minimally with the Luminance Audio logo, being a Steve Keiser design, it carries the promise of the ordinary-looking, disproportionately overperforming B&K ST-140. When Mike Tseng finally queried me one day, on the necessity of upgrading the KST-150’s look, clearly concerned that Steve Keiser’s design would not be attractive enough in today’s market, I asked him to simply hold the price for as low and as long as possible, instead.

The heatsinks at the rear of the KST-150 resembles what Quad once came up with, each situated at the far left and right of the rear panel. Two RCA inputs are positioned at the center between the heatsinks, along with the power inlet and fuse holder. The 5-way speaker binding posts located on the outer sides of the amplifier were a little problematic, in that the space provisioned for twisting the posts would be too cramped for larger hands. While you’re at it trying to keep the cost low, we need a little redesign to yield more space around the speaker posts, Mike.

My recent encounter of the Furutech XLR Audio Reference has been rewarded with consistent assurance of performance whenever it was used, regardless of MSRP’s of associated equipment. Hence my additional, innocent wish for an extra set of XLR inputs on the KST-150. Again, you know the drill, Mike.

Audition

The first preamplifier assigned to the KST-150 was the $1,490 Monarchy Audio M24 tube DAC/preamp. The M24 is definitely the best value for its price point for the level of performance, and its interaction with the $3k Luminance Audio culminated in the price-tag shock of the year at the very least. Just as Sandy Greene surmised in his review of the M24 this March, I found this little tube DAC/preamp possessing stellar sonic quality despite its budget pricing, so much so that there are few contenders in the same price range, if any.

The Monarchy Audio provided an essential suite of properties for the Luminance Audio to work with, namely a lively midrange, expeditious transients and a very well-behaved spectral delineation. For what it induced the $3k Luminance Audio to do, I wholeheartedly endorsed this little DAC/preamp.

Accustic Arts’ $7,700 CD Player I Mk2 and Audio Note’s $10k, M5 preamplifier propagated the KST-150 to an even higher level of refinement. With the Accustic Arts and Audio Note, the KST-150 sounded considerably more dynamic, with livelier tonalities and more expansive sound-scapes.

For instance, having the M5-coupled KST-150 drive the 86dB/4Ω Apogee Duetta Signature divulged a refined spectral delicacy, albeit a less flamboyant tonal persona and a comparatively restrained bottom-end dynamics as compared to that from the Linn Klimax Chakra 500 Twin. The KST-150 solicited impressive bass extension from the panels, at the same time exhibiting remarkably expeditious transients for its price, drawing colossal dynamics from the Apogees at the up-turning of the volume knob toward the reproduction of climactic passages.

This level of dynamics and tonal density is sonic properties highly reminiscent of the sound from more capable and refined power amplifiers, such as the $11,000 Audia Flight 100, and the Audio Note M5 preamp’s regulation of the KST-150 provided definitive clue as to the $3k amplifier’s potentials.

The KST-150’s definitive moment arrived when it drove GamuT’s $12,000, 89dB/4Ω L5 loudspeaker, with the Accustic Arts CD player and Audio Note preamplifier at the helm. Despite its budget pricing, the KST-150 was not prone to chaotic displays of sonic palettes in the most ardent of power demands. Its 4Ω output of 250Wpc proved most indispensable for the GamuT, and was the budget amplifier in a long time not only able to propel the 89dB/4Ω GamuT L5 to producing colossal bottom-end scaling and transients, its interaction with the L5’s customized ScanSpeak tweeter went so far in its depiction of violinist Gil Shaham’s lamentation in Paganini’s Violin Concerto No. 1 (Deutsche Grammophon), as embossing the listener in an oil painting-like complexion with full and arresting disclosures in the fabric agitation of the bow and strings.

The KST-150’s top-end was utterly free of the minute electronic artifacts common among lesser amplifiers, albeit not at a level as resolving as that of the $9.5k Linn Klimax Chakra 500 Twin. When driving the ribbon Apogee, the KST-150’s portrayal of instruments rich in high-pitch contents, such as triangles and strings, possessed very nicely balanced spreads of energy that gave convincing substance and spatiality to the performance. More importantly, each sonic palette rendered was free of the momentarily stimulating sheen rampant in many budget amplifications that would quickly become fatiguing.

It was perhaps the proclaimed transient capability of the KST-100 that was its most predominant virtue. For even when driving the GamuT in playing heavy-weight, orchestral pieces like Philip Glass’ composition of the opening title for the movie Mishima, there was unmistakable underlying current supported by impressive scaling diligence, rendering a backdrop of the furious bass drum credibly sized.

Since the KST-150 costs only $3,000, I experimented with various accessories on the amplifier, and met with profound success with a quartet of the Harmonix Reimyo BeauTone TU-66ZX Tuning Feet ($865), and the Isoclean Focus ($1,000) power cord. The KST-150’s dimensionality and microdynamics were exceptional for its MSRP, but the Harmonix Reimyo BeauTone further extended the amplifier’s ability to produce an upper midrange of such clarity as to effectuate a bona fide jump in performance envelope.

Isoclean’s Focus accorded extra dynamic transients and low-level detail resolution to the music via the Luminance Audio, so much so that paces in rock music became livelier, and the more intricate, intrinsic subtleties of background instruments crystallized as never before.

Conclusion

Since the day of the B&K ST-140 from the 80’s, the economy has seen its price indexes skyrocketed, and $499 in today’s economy can no longer buy you an amplifier with enough output devices and all the necessary components to drive anything less than 100dB efficient, despite the technological advancement of today’s industries. Although loudspeakers of higher efficiencies are becoming more widely available, efficiency in loudspeaker continue to be regarded as secondary by most manufacturers.

From the performance standpoint, it is a necessity to utilize as lavishly designed an amplifier as possible to coerce the maximum potentials from a pair of high-performance loudspeaker. To many readers, an amplifier’s role in bridging the source and the transducer is so critical that it needs to be the highest caliber component in a system.

For $3,000, the Luminance Audio KST-150 kept up with higher-caliber electronics, such as the $7,700 Accustic Arts CD Player I Mk2, and the $10,000 Audio Note M5 preamplifier, with which the Luminance Audio KST-150 churned out dynamics and resolution that qualified it as an extraordinary design, and it was at its best when driving demanding speakers, like the 4Ω GamuT L5. Steve Keiser’s latest design is an amplifier of mind-easing price tag that provides mind-easing music-listening sessions.

To companies on the opposite ends of the operating economy and scale, their respective successes can be mutually inclusive in any market, and in the case where a company’s creation of a $30,000, cost-no-object design may usher in an astounding level of market penetration envied by all, the rationale of another in creating a sub-$3,000 amplifier can be of equally sound market strategy, when it contemplates the vast consumer base for a $3,000 amplifier versus the $30,000 one.

Rick Schultz and Mike Tseng believe in Steve Keiser’s design ingenuity, and their vision is proven in my system, as well as that of the Ascendo E loudspeaker driven by the KST-150 during the 2006 Los Angeles Home Entertainment Show.

Even the most financially affluent among us will succumb to the cryptic temptation of making some of the most brilliant minds in this hobby come up with an unprecedented accomplishment for the cheapest of change. In this regard, Steve Keiser’s latest concoction as embodied in the Luminance Audio KST-150 is destined to grant him another quarter century of fame. But the story goes on from here.

Rick and Mike sent a new prototype amplifier to me not too long ago, one equipped with a miniaturized version of a new circuitry that will be four-times as massive as that of the KST-150 in its full implementation, will require the breadth and width of an entire KST-150 chassis to house just one channel, and will sell for twice of the KST-150.

I have been listening to it and found it to be absolutely superb. According to Rick, it is possible that this exciting, new machine will carry the designation of SOL somewhere to reflect a vital portion of the technologies put into it …

Manufacturer’s Comment:

On behalf of my partners Rick Schultz and Mike Tseng, I would like to extend the deepest appreciation to editor Constantine Soo for his comprehensive, thoughtful and articulate viewpoint regarding his experience listening and evaluating the performance of the KST-150 amplifier.

Indeed we are overwhelmed by his most lavish and supportive commentary. We believe the editor has accomplished an outstanding level of excellence in maintaining objectivity as well as fully elucidating all relevant points of reference when describing the individual characteristics of the KST-150’s sonic performance. Constantine proportions the relative value of the KST-150 when he correctly alludes to superior articulation and inner detail of the Linn amplifier. This specific comparison and contrast makes it possible for the reader to form a relevant understanding and framing of expectations when the opportunity to audition the KST-150 is made available.

I agree with two critical observations the editor has made: the speaker binding posts are too close to the heatsink and a lack of balanced input. Both of these will be addressed by the partners in an upcoming meeting. The primary objective is to hold the line on the price and if modifications can be implemented without incurring excessive cost, they will be done. A single-ended only input approach was chosen because when the circuit was evaluated incorporating both single-ended and balanced modes, a compromise in sonic performance was noted. To implement a circuit that would accept both balanced and single ended inputs without compromise would have increased the retail price by approximately a thousand dollars. I firmly agree with the editor regarding the inherent superiority of balanced circuit operation. I will continue to endeavor to find a cost effective implementation for both.

I am resolutely gratified that editor Constantine Soo can maintain effectual and friendly interactions with the manufacturer, and yet relate the highest standard of integrity with regards to impartiality with respect to objectivity when reporting his observations. This blend of characteristics make DAGOGO the most unique of audio publications I have ever encountered.

Respectfully submitted,

Steve Keiser

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