Solid State Power Amp Reviews

Fast semiconductor switches chop up that high voltage and apply it to a small transformer that both transforms the voltage to a usable level for the circuitry to sip from, and provides an isolation barrier from the mains supply. The transformer can be small because its size decreases as operating frequency increases. A little guy running at 60kHz is far smaller than one operating at a mains frequency of 60Hz. The transformer in the Klimax is smaller than a 1½" cube. At 50 or 60Hz it might deliver something around 5W. As implemented in the Klimax, it delivers (according to the specs) on the order of 1000W!

Fast rectifiers and a small coil and capacitor sit at the transformer's output to filter and convert the high-frequency waveform back to DC for the circuitry. "The reservoir capacitors are about the size of thimbles, but they give better filtering than those the size of beer cans as found in other very powerful amplifiers."

A neat trick: By controlling the timing of the switches, the output voltage can be held constant or varied as needed without resorting to voltage regulators. This all takes place well above the audible range—a good thing, as Mothra, er, Martha would say.

How fast are the semiconductor switches? They turn on and off in about 20 nanoseconds—20 billionths of a second! The overload protection for the switches can detect a potentially destructive condition and protect the switches in less than half a microsecond (half a millionth of a second)...but there are problems. The first is simply the inherent complication of such a device. After years of effort, Linn claims they now can build a more reliable component than the conventional power supply it replaces.

Another serious problem is keeping the high-frequency signals from morphing into electrical noise. "The scale of the problem is quite daunting," quoth engineer Miller. "If the high-frequency signal applied to the transformer was connected to an external antenna, it would obliterate radio reception for miles around, so please don't try this at home!"

Obviously, such behavior must be tamed. Ivor speaks with earnest zeal: "This doesna' happen by accident, but takes experience, precision engineering, and diligent quality control. Only then can the potential audio benefits of switch mode begin to be revealed and exploited."

Circuitry
The Klimax uses all-surface-mount technology and eight bipolar output power transistors. "Essential circuitry takes maximum advantage of the inherent linearity of bipolars (unlike MOSFETs), and manages their potentially slower switch-off characteristic." Designer Bill Miller chose class-AB for the Klimax, with circuitry "to control precisely the handover from one power device to the other." The Klimax is direct-coupled with a DC servo to minimize output offset voltage; the technique avoids capacitors in the signal path.

The amplifier has a simple "decision-making" protection circuit. Either the output current is safe or it's not, in which case the amplifier shuts down for several seconds. An unsafe output current is defined as a particular level above a certain threshold for a certain length of time, or an instantaneous current above an absolute threshold. If neither of these conditions are violated, the amplifier is delivering all the current requested by the load. "It's very easy to tell when our protection circuitry triggers; the amplifier stops."

The Klimax may be small, but it's quite powerful. It's rated at about 290W into 8 ohms, and a hefty 500Wpc into Linn's standard load of 4 ohms. "We don't do this to claim a bigger output power, but to show our amplifiers are intended and fully specified for operation into 4 ohm loudspeakers. Most Linn loudspeakers, of course, have nominally 4 ohms input impedance."

Despite the efficiency of the design, it still must dissipate considerable heat, and does so with natural and forced-air convection. You'll want to leave a little breathing room around the amps. Natural air convection works under most "normal" conditions, heated air rising up and out via the central heat exchanger. The internal fan trips when the amplifier is driven "enthusiastically" or if ventilation is poor. If the Klimax gets too hot, it'll shut down until its surface temperature drops to a safe level. Kathleen managed to pound one into thermal overload at SPLs that nearly popped the windows out of their frames. "This is to meet international safety regulations, not to protect the circuitry, which can operate reliably with the case very uncomfortably hot to the touch." Ivor just wants you (and K-10) to know that.

The fan draws cool air in through the back of the amplifier and blows it along the heat exchanger, venting through the front, top, and bottom of the case. The fan is very quiet in operation, and, when activated, can be heard only between music selections. The high-thermal-mass casework usually absorbs short bursts of high power input with a slow rise in temperature and the natural convection of the heat exchanger.

The Klimax's automatic internal AC mains voltage switch allows it to operate from 90 to 260VAC. When the mains supply is below 140VAC, two seconds after turn-on the power supply switches to low range with a just-audible click, and remains that way as long as there's power.