Kinergetics KBA-280 power amplifier
Before that, in 1990, Dick Olsher reviewed one of Kinergetics' early power amplifiers, the KBA-75. The KBA-280 is a modern update of that design, still specified for class-A output-stage operation but now sporting considerably more power. It is also a significant cosmetic upgrade on the earlier amp. With its four solid, sculpted corner posts (each with a Sorbothane foot for vibration isolation) and lack of visible heatsinks, it's one of the handsomest amplifiers I know of—and compact (though hardly a lightweight) for a powerful class-A design.
The front panel has only a single switch—Standby/On (if plugged in, the amplifier is always in Standby, at a minimum)—and standby and power LEDs. Around back, the real estate is a bit more crowded. There are balanced and unbalanced input connections, with switches to select between them for each channel. (My bench tests indicated that these switches do not completely short out the input not selected.) The five-way output terminals are directly below the input jacks—a little crowded for the long-suffering reviewer or dealer, but unlikely to bother anyone else. The output jacks showed early signs, during the review period, of a tendency to rotate when tightened; use with care.
The three-prong power connector, a remote On/Off jack (triggered by 12V), and five fuse holders occupy the opposite side of the rear panel. Four of these externally accessible fuses are for the power-supply rails. (Why do amplifiers with easily removable covers—like the four-screw Kinergetics—have external fuse holders, while amplifiers whose fuses are mounted internally have covers secured by 16,000 fasteners? Along with the meaning of life, this question is pondered by those who spend parts of their days manacled to test benches.)
Below the input/output terminals is an exhaust vent, with heatsinks incorporated internally. But the KBA-280 lacks the heatsink area one might expect in a class-A design. Instead, an internal fan vents out this rear-panel exhaust opening. The fan is relatively quiet, but can just be heard from the listening chair in a quiet room with no music playing. While a lot of hot air does come out of that vent—no reviewer jokes, please!—the rest of the amplifier's case stays remarkably cool.
While many high-end electronics manufacturers use high-quality parts in their designs, Kinergetics is the first one I know of to actually list their suppliers in the owner's manual. The input connectors and internal wiring are by Cardas, XLO, and Kimber, the toroidal transformer by Plitron of Canada, the power capacitors by Sangamo and Philips, bypass capacitors by Wima and Roederstein, power-supply bypass capacitors by RelCap, solder by Wonder. The circuit boards are glass epoxy.
The power supply consists of separate low- and high-level sections, the former for control, the latter for output current. Separate regulated, low-impedance supplies with separate transformers are used for the low-level section. The high-level section is anchored by a large, 1kVA-rated, toroidal power transformer. Separate rectifiers and filter capacitors (a total of 40,000µF) supply each channel.
The output stage is designed around a 200W-rated NPN transistor rather than the usual NPN-PNP complement. There are 13 of these NPN devices in each channel: one for current control, two for current drive, and 10 for power output. Coupling is direct, with no capacitors in the signal path; the DC is kept near zero by DC servo circuits. The KBA-280 uses minimal loop negative feedback.
Kinergetics' signature design feature has long been their patented, so-called "KDP" circuit, designed to reduce what the company calls "hysteresis distortion." Kinergetics claims that all parts, including passive ones, exhibit nonlinearities that can compromise performance. Actually, all distortion is caused by nonlinearity of some sort or another, and though most engineers would put the blame on the active parts of a circuit (which are always nonlinear in some part of their operating range—the trick is to stay on the linear portion), passive devices are not entirely blameless, either. In typical amplifier designs, negative feedback is used to minimize the distortions that cannot be eliminated from the basic design (and no active parts, in particular, are completely free of nonlinear distortions).
Kinergetics' explanation of how they solve this problem sounds suitably high-tech, but to me it sounds as if it operates in much the same way as local feedback. From the owner's manual: "We then create an electronic model of a capacitor's nonlinear distortion and put it into the negative input of the differential amp. The positive (real) and the negative (created) distortion now cancel each other—distortion removed!" (The term "hysteresis distortion" is unique to Kinergetics in this context. It is usually used to describe nonlinearities in ferrous materials such as transformer cores.)