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points where I could see a squarewave anomaly, and I corrected for it by adding appropriate networks and matching impedances—the usual kinds of things that electrical engineers do to optimize circuit performance. I tried to identify and eliminate as much of the squarewave aberration as possible.” Steve was warming up, just like his amp. “I have a marvelous computer program that is produced by a California company called Electronics Workbench. The program takes circuit elements and allows you to put all the components and all their dynamic characteristics into a computer. It’s as if you are working with test equipment, but you are doing it inside a computer. The software has allowed me to identify all points of squarewave aberration and to correct for them without doing anything in real space.” According to Steve, squarewave performance matters at every stage. “Twenty years ago, Julian Hirsch said something to the effect that every parameter of audio performance should be capable of being objectively quantified. I wasn’t so sure at the time, but now I think Julian was right. There is a great advantage to being able to quantify things instead of shooting in the dark. You don’t have to spend a lot of time experimenting. You can reduce development time and cost.” So where do some other designers go astray? “I think they approach things from a black-box perspective. They look at the input signal and they look at the output signal.” But it matters what happens along the way. “This was during my B&K years: I was sitting around one day. I had my oscilloscope on and my signal generator. It was generating a 10kHz squarewave. I just randomly took the oscilloscope probe and touched it to a place inside the circuit. I noticed that the squarewave response was really odd. Then I added a resistor in parallel with the compensation capacitor, and I noticed that the squarewave was dramatically improved. I heard the difference.” I asked about the KST-150’s lowimpedance limitation. “The KST-150 has four MOSFET output devices per channel. MOSFETs are self-protecting. That is, they are selflimiting. As they become very warm, the gain goes down and their electrical characteristics become unfavorable.” SAM’S SPACE I CONFIRMED THE KST-150’S CHARACTER: OPEN, AIRY, TRANSPARENT, HARMONICALLY RIGHT…AND FAST. What might make the MOSFETs become warm? Trying to drive verylow-impedance speaker loads, especially at low frequencies. The B&K ST-140 used just a single pair of MOSFET devices per channel. Had Steve given any thought to doing that with the KST-150 and reducing the amplifier power? “I feel that two pair per channel is optimum, but no more. As you add to the number of output transistors, you slow down the output stage, because the driver stage has to contend with delivering more current. The output devices also have an associated capacitance, which has to be charged and discharged by the driver stage. “I think it was you, Sam, who coined the term MOSFET mist. I found that this mist, or fog, is, in fact, their capacitive load. By minimizing the number of output devices, it’s possible to avoid the encouragement of MOSFET mist—transient distortion, if you will—due to the loading of the driver stage. “But, yes, to answer your question, I did try a single pair of MOSFETs per channel. I found that when I added an extra pair, took the power up, and doubled the current delivery, I didn’t lose any sonic performance. “When I went to six MOSFET devices per channel, that’s when the capacitive load and the current requirements of the output devices started to overtax the driver stage. If I try beefing up the driver stage, the additional components compromise the transparency of the circuit.” So, yes, less is more. And some amplifiers are dragged down by their own weight, as it were—made slower, less transparent, less agile. As for the KST-150’s ability to drive low-impedance loads, Steve had this to say: “It’s related to the self-limiting nature of MOSFETs and the fact that there are two pair of MOSFET output devices per channel.” Does the amp crap out below 3 ohms? “It doesn’t fail, if that’s what you mean. But performance does degrade.” It’s a tradeoff: speed, transparency, and economy vs the ability to deal with loudspeaker loads that probably shouldn’t be foisted on the public in the first place. I also asked Steve about balanced designs. According to him, a balanced design is the cat’s meow only if it truly is a fully symmetrical, mirrorimaged design. Unfortunately, such a design just about doubles the parts count and build cost. Many designs purported to be balanced ain’t, Steve pointed out. In any event, if you’re looking for a quiet amplifier, the KST- 150 fills the bill. Which would you rather pay, $5000 or $3000? How universal is the KST-150? It’s not confined to Quads, that’s for sure. I whisked the KST-150 down to our living room, where the Triangle Comete Anniversaire and the new Verity Audio Rienze loudspeakers are set up. It was an easy schlep—the Luminance weighs only 33 lbs. It’s not some audiophile monstrosity that’s so heavy you can’t lift it. I confirmed the KST-150’s character: open, airy, transparent, harmonically right…and fast. As for bone-crushing bass, not quite (hardly an issue with the minimonitor Triangles). One thing for sure: with the Rienzi speakers, the bass was tight, taut, fast. The Luminance just didn’t hammer out current from hell. If it had, it wouldn’t have sounded half as good. A monoblock version of the KST- 150, rated to deliver 400W into 8 ohms, should be ready by the time you read this. It’s a bridged version of the stereo amp: you could buy one KST- 150 and add a second one later. The modification has to be done at the factory. A preamp is set to follow; but, as Steve Keiser well knows, a preamp is even more difficult to design than a power amp because you’re working with signals of much lower level. Luminance dealers are few. Seek one out. Try to hear the KST-150. It pumps out more than enough power for most speakers, there are no expensive tubes to replace, and it costs $3000. It deserves to be listed in Class A of “Recommended Components.” If the KST-150 sold for $18k instead of $3k, hosannas of praise would be raining down from all sides. ■■ www.Stereophile.com, May <strong>2007</strong> 23
MMDesign marlen@spotnik.com YG uses Cimatron The best loudspeaker on Earth. Period. Prepare to be shocked! During the process of designing a loudspeaker, one often finds that component A measures better than B, yet B sounds superior. Most designers resort to the known excuse that “measurements do not tell the whole story”. They simply choose B. At YG Acoustics, we research more thoroughly - first we find new measurements that explain the conflict. Then, we develop a solution which sounds and measures better than both A and B. For instance - our proprietary tweeter alignment combines the frequency-realm behavior of a flat baffle with the time-domain response of a sculpted one; its dispersion uniformity supercedes both. The endresult - with a correct system and setup, the YG Acoustics Anat Reference Professional is indistinguishable from live sound. Period. KIPOD ANAT I I ANAT REFERENCE Handmade in U.S.A. Designed by Yoav Geva (Gonczarowski), winner of the Japanese audio Grand Prix Contact factory/showroom: Computerized Loudspeaker Labs LLC The exclusive manufacturer of YG Acoustics loudspeakers 4941 Allison St. #10, Arvada, CO 80002, U.S.A. Tel. 720-840-6441 E-mail: usa@yg-acoustics.com Web: www.yg-acoustics.com Dealers / Distributors welcome
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John Atkinson DESCRIPTION Two-way,
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