Stereo Power Amplifier
Back to the Future
If you have been around high end audio for any length of time, you are familiar with the brand name B&K. If you’ve been around as long as I, you are even more familiar with the model number ST-140. Back in the early eighties, solid state in high-end amplifiers was in the early stages. Most audiophiles responded to solid state then much as they did to the CD when it first arrived – they hated it. Or, to be more accurate, the sound of it and for very similar reasons. Transistors sounded distorted, dry, etched and ultimately unmusical.
Two guys named John Byer and Steve Keiser got together to form a new company whose name was culled from their last names – B&K. The “B” did most of the marketing and the “K” handled the actual design of their first product; the B&K ST-140. It turned out to be the first solid state amp that audiophiles could get their ears around. While still not as rich as tubes (that battle still rages to this day), the ST-140 was said to at least be “musical” – high praise back then and a true breakthrough. Even today, a vintage 140 is a desirable find on the audio boards.
What Steve had discovered is that square sound waves looked a lot different as they passed through different stages of the amp. What was supposed to look perfectly always looked more like a 10 year chart of the S&P 500, particularly at the feedback junction. Steve attacked this condition and eventually was able to concoct a circuit whose square wave output looked much more correct throughout the whole circuit to the outputs.That turned out to be the big breakthrough that brought solid-state sound kicking and screaming into the rarified high-end realm.
Fast forward to 2002. “K” had left B&K and was introduced to Rick Schultz of Virtual Dynamics, the cable maker, and Mike Tseng. ”K” proposed that the three of them form an amplifier company based on a working prototype circuit that he felt would make a noteworthy contribution to the audio market place. Thus was born Luminance Audio. After more years of perfecting the new design and making it market ready, the KST-150 was born. Again, the KST moniker represents the first letter of the creator’s last names. This time Steve Keiser got top billing.
For those you of who believe in "Buy American", you will be glad to know that the KST-150 sports a "Made In USA" logo.
Lingering Luminance Looks
The KST-150 is a stereo power amplifier whose rated output is (surprise) 150W RMS continuous into 8 Ohms, both channels driven over a frequency range of 20 Hz to 20 kHz with less than 0.1% total harmonic distortion. Pretty specific specs. The rest of the specs are also impressive, but the one that stands out the most is one not usually touted or viewed as all that important; slew rate. Slew rate represents the maximum rate of change of signal at any point in a circuit. Limitations in slew rate capability can give rise to non-linear effects in amplifiers.
The red trace represents a true square wave, the green the effect of a slow slew rate on its reproduction by an amplifier
Very basically, slew rate is a measurement of speed. Slew rate for this amp is a claimed 250 volts per microsecond (vps). Think of that as "miles per hour" in your car. That amounts to near record setting speed in the audio world, and certainly the highest anywhere near this pricepoint. For example, Krell’s best power amp, the Evo 1 lists for $25,000 and has a published slew rate of 100 vps – less than half as fast. How does that 250 vps translate to sound? We’ll find out in a minute.
There is not much to describe about the looks of the Luminance. It’s your basic black box with the Luminance logo in dark gray, a rocker on/off switch and the now ubiquitous blue LED in the center that is way too bright – one of my few pet peeves. I like to listen in the dark and when an unestiquishable LED lights up the whole frickin' room, constantly announcing only that the unit is on, it causes me to run for my trusty roll of black electrician’s tape to cover it up - which I did post haste.
The plainness of the amp is not necessarily a bad thing. What is little known by audiophiles is that casework for an amplifier is very expensive and can amount to a large percentage – up to 50% and more – of the cost of the product. You pay for those jewel-like boxes for the most part. Putting the most bucks in things that make the amp sound (and not necessarily look) better was a conscious decision. Said Steve, “We wanted to keep the price as low as possible and the quality as high as possible”.
I don’t know how much extra this would cost, but perhaps Steve, if you just laser cut the cool Luminance logo in the front and put a soft dark blue light behind it, it was add quite a bit of eye appeal for not a lot of bucks – not retooling required. In other words, illuminate the Luminance!
Around back things are a bit tight, but workable. If your speaker cables are spades and you have fat fingers, because of the close proximity of the heat fins, hookup takes some deftness. But, if you are not a reviewer and don’t change cables frequently, that’s a one time issue. I use bananas, so it really wasn’t a big deal anyway. I hear a redesign is underway to fix that issue.
The 150 offers no balanced circuitry. Steve said the sonic benefits were small compared to the additional cost such circuitry (if done with sufficient quality) would have entailed.
A hallmark of a Stereomojo review is that we try to always ask the designer what his design goals were for the product. One reader questioned that policy and asked, “Aren’t all amps designed to do the same thing – sound good?”. Well, are all cars designed to do the same thing – take you from one place to the next? Yes and yes, but it is obviously not that simple. There is a rather large difference between a Yugo and a Lexus, a Corvette and a Chevy Suburban or a Ferrari and a Jeep 4x4. You wouldn’t want to race on the F1 circuit in the Jeep or take the Ferrari off-road. You wouldn’t want to run a NASCAR Ford in the Indy 500, either. There is also a difference in amps, such as a 2 wpc SET and a 1,000 wpc McIntosh.
I asked Mr. Keiser is his design of this new amp was based at all on his previous ST-140; “The KST-150 is not really based on the the St-140 circuit at all.The St-140 circuit was a single pole compensated circuit with 60Khz bandwidth and a slew rate of 14 volts per microsecond. My investigative research since my involvement at B&K has revealed the necessity in designing overall bandwidths that are very wide and circuit speeds that are very high”.
When I queried Mr. Keiser as to his criteria for this amp he said, "This set of design parameters became the primary design criteria for the KsT-150 in addition to the preferred requirement that the steady state distortion measurements constitute an essentially inaudible contribution and that overall circuit and product cost be affordable. This set of requirements necessitated the implementation of overall negative feedback as well as class AB output stage. The big question became was how articulate circuit characteristics in which a feedback loop system could effectively emulate an open loop system. In my investigation into circuit performance began I inquired into the prospect that a circuits subjective performance could largely objectively quantified if the appropriate set of circuit performance and characteristics could be correlated with what we heard. In a rather casual interlude in the daily routine in 1987,I connected an oscilloscope probe to the feedback terminal of the St-140 with a square wave input. The measured waveform at the feedback terminal didn’t resemble a square wave at all. Curious, I wondered if there would be any sonic effect if I corrected the circuit elements in such a way as to obtain a square wave performance at the feedback terminal of differential front end. After making the necessary circuit adjustments I listened and was amazed at the improvement. It became apparent from that moment on that internal square wave performance became the primary correlative objective performance specification with that of audible consequence that I had been looking for. Over a period of years I was able to distill all relevant circuit nodes that had significant effects on perceived listening impressions. When I had examined all relent and measurable circuit points for proper observable square wave performance, I decided it was time to implement the final design into a producible product. KST-150 was born.”
The Sound of the Luminance
The unit we received was well traveled and well burned in, so I just let it cook for about an hour (and until night fell) before strapping into my chair. I started out with the Halcro pre/dac in front. Then, after doing my electrician’s tape thing, I popped in the Official Stereomojo Reviewer’s CD.
I must digress for a moment. I am not a drummer, but I have played drums on occasion and I took a course required for my music degree called Class Percussion where one is required to learn to play every orchestra percussion instrument from snare drum to triangle to tympani. Flams and paradiddles. In bands in which I’ve played, I also on occasion sat way to close to the drummer where every chart sounded to me like a drum solo. I say that simply to state that I know first hand what real drums and cymbals sound like, feel like, and sometimes smell like – depending on the drinking/smoking/hygiene of the drummer. Cymbals, like every other instrument have a basic, fundamental pitch. Even on some very expensive amps and speakers, crash cymbals sound more like a short shot of white noise or the sound a wet finger makes when it touches a hot iron. So, it was with a great deal of surprise that the first thing I noticed when listening through the Luminance was that crash cymbals on the Tricycle cut were reproduced with their fundamental pitch intact. Hmmm. Even my Halcro Class D, 400 wpc MC20 didn’t do that as well, and one of the qualities of a Class D amp is supposed to be speed. Through the KST 150, cymbals did not sound like white noise and their long ring-outs were there, too. They had a real body and presence to them that was not an artificial boost in frequency. This seemed to warrant further exploration.
One of the most demanding recordings I know of for speed is “Friday Night Live in San Francisco” by Al DiMeola, John McLaughlin and Paco Deluca (SACD) – three of the best guitarists on the planet. Here they are live and executing some of the most incredibly jaw-droppingly fast acoustic guitar technique ever recorded. The 150 had no problem reproducing the lightening-like pick on strings sound – sometimes all 3 players simultaneously. “Lively” is an understatement. It was remarkable and left me almost breathless just listening to it. This quality was apparent in every recording and every genre I played; drums, plucked strings, anything struck and most thankfully, piano. That fast slew rate was slaying them all. You really need to hear this phenomenon if just once.
“Okay”, you say, “But what about the rest of the amp’s sound”? Glad you asked. In my “Publisher’s Biases” article , I say, “If something can get strings, solo piano and vocals right, chances are everything else is pretty good, too". The KST-150 can do strings and vocals, too. We already know it does piano unusually well. The overall presentation is one of the most un-solid state sounds produced by a solid state I have heard. No, it does not sound like tubes, but try to buy a good tube amp that makes 150 watts per side for $3k. It did remind of a good hybrid amp – the kind that has a couple of 6922’s in the pre section and solid state in the output, but I know of none at any price that have the 150’s speed. The amp is dead quiet at idle and throws a big, nicely layered soundstage which is also pretty quiet in terms of grain – very good for the price. That soundstage is also nicely balanced between a forward image and one that is recessed. It does not impose its will but reproduces the stage that is present on the disk. Excellent.
There is no offending glare and the response sounds very linear from top to bottom. Detail is fine and neither over cooked or etched. Vocals from Pavarotti (rest in peace) to Jennifer Warnes were neither warm or cool, perhaps leaning a little to the cool side. By the way, if you have not heard the new Jennifer Warnes 20th Anniversary reissue of “Famous Blue Raincoat” (review upcoming) you simply must. I had the pleasure of meeting Bernie Grundman at the Rocky Mountain Audio Fest who did the original mastering and the new, updated reissue. He generously gave me a copy and it is astounding.
Bass performance is very solid and powerful without calling attention to itself. It does not brag about its low-end prowess, rather it just replicates the bass present in the recording.
I also used the new TP 2.1 tube preamp from Horizon Audio in front of the Luminance. Much better. Now there was no coolness at all and virtually no solid state dryness or other such artifacts. Solo instruments and vocals had a bit more roundness and body. The TP 2.0 is still in the review process (the first ever), but I can say that it sounds very, very good so far and it is a wonderful mate for the Luminance.
During the review process of the Luminance, I was fortunate enough to acquire a hotly sought after world-class pre-amp by master builder Gary Dodd.
Hand made and powered by rechargeable batteries that fire up two 6DJ8 tubes, the Dodd is considered among audiophiles in the know to be one of the best you can buy - including our own Technical Editor Danny Richie, in whose ears I trust. Gary sells them direct for $3,300.
Gary also built these stunning 275 pound monoblocks for Danny. I was able to spend the better part of a Sunday putting these and the Dodd preamp through their paces through a pair of 8' tall pair of Danny's LS9 line array towers. It just doesn't get any better than this.
The Dodd allowed the Luminance to strut its stuff unencumbered by any noise or sonic limitations. While it was not a match for the $50,000 Dodd mono's, its similarities to it exceeded its differences - without all the heat the enormous Dodd's generate.
The first review of the Dodd pre is coming soon. Here,of course.
Whatever preamp I used, the KST-150 produced power that seemed more than the rated 150 per. It was coherent and cohesive at high levels and I never reached its limit with the 90db efficient Gemme’s in the big room.
While Mr. Keiser did much research and evaluation via computer, the sound was ultimately tailored by ear. Like the classic ST-140 before it, the KST-150 oozes musicality with an unusual rightness to leading edges which lends an extra of believability. It makes many other amplifiers sound as if they are rhythmically challenged. Sort of like me trying to dance Salsa at a wedding reception. Not pretty. Meanwhile, the rest of the presentation is as neutral as a Swiss diplomat. There are no obvious errors or omissions. It treads lightly between lushness and over articulation.
As one who has spent a great deal of time in recording studios, I think the KST-150 would be a very good choice for mixing and mastering. I think if Misters KS&T marketed this amp to the recording studio industry, they might sell a ton, though its lack of balanced capability might be a drawback. The amp I was sent had quite a few battle scars and had been in many melees with UPS and Fedex and come out victorious, proving that it is at least rugged and perhaps even bulletproof. It certainly never faltered in the hundreds of hours I
abused used it.
The Luminance KST-150, via its potent, overachieving power, neutrality and life-like speed, has proven itself to be an affordable reference quality stereo power amplifier. And that is exactly why it is staying right here at the house of Mojo. I can't think of a higher recommendation than that.
The Luminance KST-150 proves that speed does kill, because this is one killer power amp! It breaks new ground in bringing ultra-high performance, especially in terms of speed, to the affordable audio arena. Though it makes appearance/performance tradeoffs, we think the decisions were good ones in bringing overall excellent value to the fore. Because of the absence of typical solid state sound, this would be an excellent choice for those who want something closer to tubes without sacrificing output power or the added maintenance a tube amp can demand. A tube preamp from the likes of Triode or AudioSpace would provide the extra tube roundness if so desired without breaking the bank. The Luminance is a true reference quality power amp at a bargain price of $3,000.
For outstanding acheivement in a high quality stereo power amplifier at an excellent price, the Luminace KST-150 receives our
In addition, the Luminance KST-150 gets our
Congratulations to Steve Keiser, Rick Schultz, Mike Tseng
and all the people at Luminance Audio