Heart of any table is the drive system and having settled on speed stability as the key performance parameter then it was a very short step to direct-drive. Executing that drive system was another matter. The Monaco’s motor is based on a 12-pole DC design with a ceramic rotor, originally developed for use in the microprecision field of IC and chip production. The underside of the platter carries an encoder disc with over 4700 individual lines that are optically read over 4000 times a second, the results being fed to sophisticated DSP control circuitry. Where this differs from most solutions is that rather than simply reading any error and correcting it, the circuit is able to predict the results of its action, thus applying just enough corrective “nudge” to avoid overshoot and re-correction. Thus it doesn’t just keep the speed (well) within the under/over limits, any changes are minimal and smooth, rather than sudden or jerky. Perhaps it’s better to describe it as maintaining rather than correcting the speed, a notion that gives a better sense of the progressive nature of its action. The result is an incredibly smooth and cog-less drive system. This is a world away from the on-off servos that marred early direct-drive performance. It also avoids or minimizes the kind of cyclic errors that can afflict belt drive systems, both through slippage but also concentricity issues with drive pulleys and platters. Any drive system will be prone to error, but the important thing to note here is the nature of those errors and that they vary with different systems, rather like the nature of distortion generated by a valve as opposed to a solid-state amplifier. So whilst belt drives tend to exhibit relatively high levels of cyclic error as well as individual random events, the Monaco’s direct drive system virtually eliminates cyclic errors, confining itself to specific random events. Causal linkage is always an attractive but dangerous path, but this lack of cyclic error and the smearing and rhythmic imprecision that goes with it is exactly what you’ll hear when you listen to the Grand Prix ‘table.
This complex and highly controllable motor system is in turn built around a sophisticated pressurised oil-bath bearing, in which the platter’s rotational action pumps lubricant up the shaft of the bearing, creating a system in which there is zero horizontal contact. The vertical axis is supported by a ceramic ball resting on a proprietary hybrid alloy thrust-plate, coupled to the bearing shaft by a damped support rod, the whole assembly submerged in the oil reservoir. Those surprised by the physical contact maintained in the vertical axis should reflect on designer Alvin Lloyd’s reasoning: any fluid, flexible, magnetic or air interface introduced at this point will inevitably introduce a vertical spring rate, which will in turn impact on the reading of vertically encoded information.
This brings us to the final element in the drive system, which is of course the platter, a precision machined magnesium alloy disc coupled to a phosphor-bronze mass ring that both damps the whole and optimally locates its centre of gravity. GPA are reluctant to discuss the surface finish of the platter save to say that it is extremely rigid, thus preventing any vertical deviation of the record and subsequent loss or distortion of vertical amplitude information. In this regard, the low mass, self-damping and extreme rigidity of the magnesium exceeds the performance of all but the most sophisticated and costly composite constructions. Its selection again underlines the emphasis on sound engineering and the appropriate use of materials rather than simple, cost no object extravagance, an approach that informs the entire design of the Monaco.
Taken as a whole, this system has consistently delivered speed accuracy better than 0.002% peak error, measured under the most exacting conditions and with no measurable rumble. This compares to the 0.001% claimed for the (also direct drive) Sirius III – although no parameters or measurement protocol are published to support that figure. The next best (and rather more relevant) published claim for speed consistency is in the region of 0.005% and that comes from the belt-driven Continuum. But GPA go a stage further, applying a 3-Sigma protocol to the platter itself, rather than the motor, thus eliminating downstream inaccuracy in the drive system and measuring actual speed consistency on a nanosecond by nanosecond basis. This means that no more than three individual instances of speed variation approaching the chosen parameters are acceptable within a thousand samples, a dramatically more accurate standard than the RMS method usually applied. Do the maths and you discover that in practice, it’s entirely possible for the Monaco to play an entire 20minute LP side without deviating from its speed by more than 0.002% on a single occasion, confining any speed deviation to far lower levels – an astonishing degree of speed consistency.