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From the B&K ST-140
to the Luminance Audio KST-150
Steve Keiser
May, 2006
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FOREWORD
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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
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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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DAGOGO©
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