Thoughts on Turntable Design by George Merrill
The Energy Management design principles are incorporated in all the Merrill product designes. In the design of the AR subchassis and Heirloom the arm mounting platform is a nonremovable part of the sub-chassis allowing coupling the released energy to a large homogeneous dissipating area. This area was many times larger than that of any turntable produced at that point in time and probably most now. This is one of the main reasons for the sonic integrity and cult status of these products. The PolyTable utlizes the same principal.
The Spindle Bearing
The Drive System
A synchronous motor (synchronous motors are frequency locked, not feedback locked) of just enough torque (small motors equal low noise- large motors add more noise) is my choice. Some say much torque is necessary to overcome groove loading. Groove loading will, to a very minuscule degree, reduce the speed. Most any quality turntable with a heavy platter will have enough inertia to keep the micro speed changed invoked by signal modulation groove loading to a level completely undetectable.
The direct-drive motor: The name indicates the absence of isolation from the motor to the platter. Direct drive turntable motors can exhibit cogging the amount of this cogging is determined by the pole number and the inertia (weight) of the platter. Hunting can be a problem if the feedback servo is lacking in definition.Direct drive motors do not use an oil well bearing.
The rim drive (idler) is plagued with problems. The reason it came into existence is; in the past, motors with torque enough to rotate the platter were of the induction type and operated at high speed (1800 rpm). The Idler drive allowed enough speed reduction (16-33-45-78 rpm platter speed) and made for a compact design. More bearings and a hard drive wheel that generates and transmits noise are a real detriment to sound quality. Some early turntable manufacturers added a belt to the equation ( Thorens TD124) to tame the motor noise presented to the platter. But this did not eliminate the rolling wheel and second shaft bearing noise.
The test to verify if motor-induced noise plays a role is simple. Useing a synchronous motor belt-drive turntable. The motor is first powered directly by the AC supply line. After listening, the direct AC supply line is subistuted with a motor drive supply (DMD) that generates an internal sign wave, thus isolating the motor from the AC line noise. The difference is stunningly apparent because eliminating any type of noise transmitted from the motor through the belt then into the record support platter always improves the sound.
My Secrets to Turntable design
By George Merrill
In order to design the AR modifications and the Merrill Turntables I designed and constructed a device I called the Vibration Intrusion Table along with a vibration pickup transducer. The operating frequency is from .1 HZ to 100 K HZ. The VIT allowed me to analyze important design points. Testing the ability of a turntable to effectively decrease the mechanical input from the platform supporting the turntable. The intrusion of airborne acoustic energy. Testing the different parts within the turntable for resonance frequency and top.
As a side note, the results from tests of mechanical intrusion lead to the design of the Stable Table support stands and the Energy Absorption Plate (EAP). These products were manufactured over 35 years ago.
This brings me to the first point of the paper. Which type of turntable, suspended or non-suspended, is better at isolating energy from both airborne and mechanical intrusion from the support platform. The tests prove a conventional suspended turntable is generally better. The (springs Heirloom) (Rubber elastomer Merrill-Willians) (Sorbothane Isolation Pucks PolyTable Signature) allow good isolation from mechanical input vibrations. If properly designed, the sub-chassis will act as a tuned low pass filter. In order for the filter to perform effectively, the total mechanism resting on the suspention must have a balance between weight and self-resonance. For instance, most effective suspended designs have a less bulky platter. Thus allowing for a more supple suspension, lighter springs allow less energy intrusion into the sub-chassis. The suspended turntable can be designed to control energy to very good degree. This is the reason the old AR was such a good design and to this day still a highly regarded product.
There are three major drawbacks to the spring-suspended turntable.