Rockport Technologies System III Sirius turntable and tonearm Page 2

So Much for So Much!
It takes sixty vendors and six months to create and assemble a Sirius III, which, after testing, must then be broken down and crated for shipping. The basic components consist of the active pneumatic isolation system and the stand on which it sits, the plinth and associated machined parts, the air-driven spindle/bearing/motor drive-unit, the electronic motor-controller unit, the air-bearing tonearm system, the air-supply regulator, and, finally, the compressor.

Starting with the foundation: The four active piston isolators are integrated into the tops of a massive, square-legged steel stand. While the isolators bear some resemblance to Vibraplane "tabletop"-type devices, they're far more sophisticated contraptions. The Vibraplane is "active" and thus self-leveling; the Rockport's system provides a progressive damping coefficient that increases as the 200-lb plinth assembly's motion increases. In other words, the more you push it, the "stiffer" it gets and the more it resists being displaced. Not that Payor expects it to be "bumped," nor does he encourage one to demonstrate this for friends (but he did show me how it works).

The system is designed to isolate 1G of acceleration by 120dB, according to Payor. With its approximately 1Hz resonant frequency in both the horizontal and vertical planes, it is said to virtually eliminate footfall- and structure-borne bass energy. Payor claims it to be "the most advanced isolation system employed in any audio product." Watching it in action, I believe him.

Also associated with the plinth is a meticulously machined, constrained-layer-damped aluminum/stainless-steel composite spindle-bearing/tonearm-mounting plate—a thing of precision beauty probably more costly to manufacture than the average turntable, and critical to the design's functionality. It secures the drive system and tonearm to the plinth and defines the spatial relationship between them.

Drive System
Once the stand has been placed in position, the drive system is carefully lowered into place. It's an engineering masterpiece designed and built specifically for this application by an outside vendor, with all air-bearing surfaces ground by a company called Professional Instruments. The air-bearing, eddy-current motor drive, and optical encoder (rigidly attached to the bottom of the drive shaft) are integrated into one compact 25-lb unit. Clearance in the air gap is measured in millionths of an inch, with machining tolerances 10 times smaller!

In Payor technotalk, the spindle bearing is a "hydrostatic air bearing with an integrally mounted, pure-induction motor and coaxially mounted optical encoder that generates velocity and position signals used by the drive system controller. The spindle is a fully pre-loaded Rayleigh (or step) compensated radial bearing with a scavenged thrust axial bearing. It operates at a high pressure and low flow (footnote 1) with a Reynold's Number far below the onset of turbulence." Total radial and axial error motions are claimed to be well below 0.000005". Other so-called "air-bearing" drives use air only in the axial direction, with the radial load supported by a standard sleeve-and-shaft assembly, and thus make mechanical contact unavoidable.

Once the spindle/motor assembly is in place, the platter is gently lowered onto the spindle bearing shaft and the finished turntable begins to take shape.

Next come the electronics. A machined aluminum front plate contains the pushbutton switches and indicator LEDs for On/Off, vacuum holddown, 331/3 and 45rpm, and speed sync. A shelf below the plinth holds the motor control unit, which Payor says costs him "around $15,000" alone. Why so? The temperature-controlled unit (held constant to within 0.5 degrees C), housed in a heavily walled, milled-aluminum chassis with massive integral heatsink, includes a six-layer circuit board crammed with parts. It is, essentially, a real-time analog computer.

The motor controller receives velocity data from the encoder, compares them to a precision analog clock uniform to approximately two parts per million, then "smoothly adjusts the direct and quadrature vector magnitudes from maximum-torque production during acceleration and deceleration to a highly viscously damped, low-torque production state when rotating at the preset commanded speed." In other words, it's able to start fast, stop fast, and stay at the correct speed by means of low torque and high damping. The motor simultaneously acts as a torque producer and a viscous brake—a neat trick made possible by the unique motor design and the sophisticated controller.

Some turntable designers believe a spinning air-bearing platter with ultra-low friction isn't really controlled, or even controllable. This design debunks that belief. When the platter is spinning at 331/3 or 45rpm, it's virtually impossible to slow it down or speed it up with your finger. The more you try to impede the spinning, the more torque the motor/servo system applies to correct the error. It's an amazing device to feel work.

Footnote 1: The significance of high pressure and low flow will be clear to those who followed the controversy swirling around my review of the Rockport Series 6000 tonearm in the May 1996 Stereophile (Vol.19 No.5). Some air-bearing skeptics and captured-bearing detractors claim that high-pressure devices that force air into confined spaces cause high-frequency resonances ("turbulence") as the pressurized air seeks an escape route. Payor and the designers at Precision Instruments vehemently refute this charge, pointing out that these bearings are used in precision machine tooling (eg, nuclear weapons), and any such resonances would be evident in the finished products.
Rockport Technologies
229 Mill Street
Rockport, ME 04856
(207) 596-7151
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