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From the B&K ST-140 to the Luminance Audio KST-150

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Being a digiphile, I was on an extended “tube trip” in the 80s and 90s, an euphemism coined by Arnold Marr, then-audiosales guru of Music Lovers of Berkeley, California. Even though several audio friends of mine became owners of B&K Component’s ST-140, not to mention the fact that their units sounded musically involving just as a tube amplifier would, I just had to experience the modern tube sound, so I bought the Music Reference RM9 II instead.

Still, I reckon that I could just as easily have gotten the reliable and musical ST-140 for myself, and saved. The onslaught of positive verbatim from those friends of mine that ensued persistently in the following decade didn’t help a single bit.

Now, the person who designed the B&K ST-140 is out and about again, and this time, he is introducing new circuits of his own design. The “K” of B&K, Steve Keiser is now the chief engineer of Luminance Audio, a partnership that includes Rick Schultz of Virtual Dynamics fame, and Mike Tseng, savvy businessman and proud father of a concert-pianist daughter.

For Dagogo’s readers, Steve has graciously taken significant time out of his hectic schedule to share his adventures in all things audio exclusively with Dagogo’s readers in the following paragraphs. We at Dagogo are privileged for this opportunity. -Constantine Soo

The Beginning

When I was 8 years old, I began to listen to music on my mother’s radio. On my ninth birthday, I received an AM-FM portable radio of my own for Christmas and proceeded to indulge in listening primarily to “top 40” music at least two hours a day.

My music listening from there expanded to saving up to buy a 45 rpm single every month to be listened to on my parents’ Magnavox console. I managed to assemble a meager first system at age 14 consisting of a Dynaco PAS3x pre-amp, along with a Stereo70 power amp, Garrard turntable and a pair of Dynaco A25 speakers. While in college, I attended electrical engineering school which formalized my approach to circuit analysis and design. During senior year, I did an independent study with a sponsoring professor where I undertook the task of designing and building an audio power amplifier, which turned out to be the B&K ST-140.

Graduated in June 1981, and in August 1981, I met John Beyer, the B of B&K. He was looking for a stereo at the time and wanted my advice on which components to purchase. I suggested the idea that I was interested in starting my own company and producing an amplifier.

Beyer heard the design and really liked it, and we formed a business partnership that was known as B&K Components in November, 1981. In May 1981, John and I personally assembled 25 amplifiers and went to the Chicago CES show for our debut.

During my tenure at B&K, John Beyer acted as sales and business director, and I performed all technical functions, which, at that time included mostly testing and inspecting all production units of the ST-140. After two years, B&K was able to hire a technician to perform final checkout and occasionally service a broken unit. The failure rate on the ST-140 ran about 5%, so frequency of repair was relatively low and incidents were rare. Since B&K, specifically, John, had no interest in a pre-amplifier, I left and began a career in computer engineering. Computers were up and coming in a big way at the time and I joined a local company doing digital design and software engineering. I am still with this company to this day.

The Luminance

The ST-140 remained in production from 1982 to 1992, after which it was replaced by Ed Mudka’s design, the ST-1400. In 1994, I had the opportunity to meet Brent Riehl of Audio Machine and Design in Lincoln, Nebraska. Brent was looking for a power amp design to build and manufacture, so I submitted my latest design which he made 50 of, but due to financial shortfalls, he was unable to commit to producing more. In 2002, I spoke to Rick Schultz of Virtual Dynamics, a common friend of Brent and myself. Rick was acquainted with Mike Tseng and we decided to forge a partnership together to manufacture, market and distribute what is now called the KST-150.

The initials “KST” is representative of the first letter in the last names of the three principle people involved in Luminance, Keiser, Schultz, Tseng.

Ever since I could distinguish differences between audio equipment which measured “indistinguishably identical”, I wondered what objective or measurable criteria would most significantly contribute to what we hear. Actually, this has been the “burning question” many audio engineers continue to investigate, with the idea that if electrical engineering parameters could be successfully correlated with what we hear, innovative advances in acoustic sound reproductive science could be implemented more efficiently, thus a quicker step towards recreating an indistinguishable difference between live music and that which is reproduced via audio component technology.

In 1985 I stumbled across something I feel is significant that I would like to share. I was “doodling” or just casually measuring various internal points within an amplifier circuit that I was working on at the time. There was no particular rationale for the measurements that I made but it just so happened that I was feeding a 10kHz square wave into the input and measuring the square-wave response using an oscilloscope.

What I found was when I measured the square wave response at the feedback terminal, it looked nothing like a square wave at all. More like a mountain range. I wondered to myself of what the significance was of this and how many other designers had stumbled on this same finding. I also wondered if I were to make alterations in the circuit, how that might change the sound.

So I contacted a few notable designers and they were not familiar with the phenomenon. I then proceeded to make an objective design goal to eradicate the circuit of the main feedback loop by matching the time constant between the feedback network and the internal compensation network of the main gain stage. This resulted in a genuine square wave being seen at the main signal points throughout the amplifier circuit. I eagerly connected the amplifier up to system and listened. Low and behold, what I heard was a significant improvement in all areas of audio performance. What I noted in particular was an absence of “transistor sound” characteristics. Most notably, I noticed a more open and vaster landscaped soundstage, greater detail and information content, and a lack of hardness and grain structure.

This comparison was done between the modified circuit and the original. Curious. I then set out to have other listeners take an audition and see what their experience was. Their response was universally positive, and I then crafted the final version of the B&K ST-140 upon this discovery and its practical implementation. Since that time until the present, I have utilized the design premise of measuring square wave response “inside” amplifier circuits and making sure by adjusting component values that square-wave response is seen everywhere uniformly throughout the entire circuit. Actually, some of the design parameters employed to attain this result can be quite challenging, such as the circuit needing to be very “fast” open loop, thus requiring very high slew rates, which in turn, means optimizing circuits for very wide bandwidth with stability.

One thing I have noted is that I have been able to attain cleaner square-wave response using the described technique on tubes rather than solid-state. Concerning the KST-150, I have tried to design the circuit using these techniques in hopes of achieving an amplifier more closely resembling a straight wire with gain in that there is an absence of sonic coloration and audible characteristics by allowing the input signal to be modified without change.

In Reality…

The industry has changed since the B&K days. The addition of home theatre has had an enormous impact in that now a video component has been added to the total home entertainment experience as well as the addition of surround sound.

Actually, this means that dedicated music listening audio systems are loosing favor to home theatre, since the various surveys that I have seen have revealed a steady decline in two channel audio for the last 20 years. The number of dealers retailing audio-only two channel equipment continues to dwindle, and the perspective buyers and market for two channel equipment is also depleting. Still, there remains an opportunity for manufacturers who are manifesting the highest caliber of performance they can possibly muster from their designs, and these companies can have a shot at being successful. It is exceedingly less likely, though, that an up and coming manufacturer (like Luminance) can be financially successful in the long run, because the available market is about 75% smaller in the U.S. than when B&K was launched.

On the other hand, there exists today an international market which has largely supplanted the U.S. market for most companies. So, factoring in the availability of the entire world at manufacturers’ disposal, the market share to date is probably about 75% that of when B&K was launched, so the probability of success for new companies is only slightly diminished as compared to 20 years ago.

Product quality and advances in component performance continue to improve. I am amazed to pull out an old B&K ST-140 and give it a listen and feeling disappointed. Sound reproduction has come a long ways in 20 years. Take, for example, diamond tweeters and other exotic new cone material that continue to push the “performance envelope” further and further.

In this sense, I believe the prospects for the industry have greatly improved because of thoughtful publications, such as Dagogo, who emphasize and focus on perceived subjective sound analysis, believing that the human ear is far better calibrated than any test instrument when the gestalt of the listening experience is considered. Remember the days when editors Julian Hirsch and Leonard Feldman pre-dominated the publishing market share with their “everything sounds the same or sounds good when it measured well”? These two editors heavily defended their position that amplifiers were not a source of coloration given certain measurement thresholds were attained – no need for tubes any more.

The recent resurgence in tube equipment I feel is the result in editorial emphasis on listening, a position I agree with wholeheartedly. There is a place for “tech stuff”; but I think its importance must be placed in context of the big picture, which is “does the technology serve the intention of the music?”

The final arbitrator in the end is the listening experience, and it is here where I am most interested in your listening impressions of the amplifier relative to other designs with which you are familiar, to see how well the aforementioned design criteria hold up in the real world. I hope I haven’t been too technical, but I wanted to convey in much detail as possible the design details involved.

Watch for a Review on the Luminance Audio KST-150. -Ed.

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One Response to From the B&K ST-140 to the Luminance Audio KST-150

  1. Mark Bajkowski says:

    Your square signal distortion remedy sounds incredibly useful. Since there is a significant number of ST-140s still in operation (myself included), could you elaborate on “I then crafted the final version of the B&K ST-140 upon this discovery and its practical implementation” and what was the resulting circuit or component modification?

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