This project began quite innocently as Chris Brady and Thom Mackris were mulling over the idea of building Thomas Scheu's turntable kit . This turntable has been compared favorably to the Platine Verdier in both the Sound Practices Magazine mailing list, a.k.a. The "Joe" List as well as the Analog Addicts mailing list. Sheu's prices for bearing, platter, sub-chassis, and base are very reasonable for world class components (approximately $650), but the fact that Chris's brother, Bryce is a CNC machinist with extensive experience machining high quality bearings got the two of them thinking.
Later that weekend, another local Colorado DIY fanatic, Ken Schei joined the fray. Shortly after words, Igor Kuznetsoff and Steve Zettel joined up, and the race was on. A passing reference on the Joe List to this "Scheu" project resulted in Alex Mitaru coming on board. Alex had ordered Scheu's (German) plans and had begun the process of building his 'table from these plans. Not reading technical German, his project stalled, and he was eager to join forces with this growing band of Analog devotees.
After looking at Scheu's plans, the team quickly decided that we would either roll our own table from scratch, or buy Scheu's kit. Machining Scheu's table ourselves from his plans made no sense, considering Scheu's reasonable pricing structure as well as our inability to locate someone literate in technical German. Besides, we all had an itch to design our "own" turntables and felt that we could equal or surpass Scheu's design. The combined insights and synergism of this team have resulted in a very promising design. The name Teres is Latin meaning polished, smooth, fine and elegant.
As we studied Scheu's inverted bearing design, it became readily apparent that the reason for this architecture was to reduce lateral thrust on the bearing surfaces. Conventional (upward opening) turntable bearings are less dynamically stable (they tend to teeter-totter like a top on its point) and create larger side loads on the bearing sleeves and so are more prone to a "wow" type of distortion by allowing (a microscopic) teetering and wobble of the platter. In addition the pull of the drive belt is well above the bearing causing additional side loading of the bearing.
Thomas Sheu's approach was to place the inverted bearing in the platter thereby placing the platter center of gravity at or below the bearing midpoint. This also made it possible to align the pull of the belt with the center of the bearing. This is clearly a superior approach that would result in better dynamic stability. However, the inverted bearing does not allow the use of an oil sump which provides consistent lubrication and a high degree of viscous damping.
The Teres bearing design is a traditional upward opening bearing. However, as in the Sheu design the majority of the bearing is placed inside of the platter. This provides the same dynamic stability as the Sheu bearing and also allows for the use of an oil sump along with it's inherent benefits. The bearing spindle is machined from a single piece of stainless steel. A hardened ball bearing inserted into the bottom of the spindle shaft rides against a delrin thrust plate. Several materials were considered for the thrust plate. Teflon would certainly provide the lowest friction, but it is not rugged enough to handle the weight of a high mass platter. A brass thrust plate would be the most rugged but probably not as quiet as the delrin. Delrin seems to be the best compromise for this application. Since the thrust plate is removable it is a simple matter to experiment with different materials. The bearing body and the spindle shaft are precision machined to very tight tolerances. The large 3/4 inch spindle shaft has been polished to a a superbly smooth (4 - 8 micron) finish. The bearing body is machined from solid brass and since an oil sump is used there is no need for using oil impregnated bronze bearing material for the bearing.
The bearing spindle has a large surface area to maximize viscous damping. Surface area for the Teres bearing is 75cm^2 compared to the Platine Verdier at 60cm^2. The bearing body is machined from brass.
This bearing design is exceptionally quiet with plenty of viscous
damping. This is combined with the superb dynamic stability shared
only by inverted bearing designs like the Sheu.
Platter
The Teres platter is a massive 3 inch thick block of acrylic. Acrylic is well damped and very stable making it an ideal material for a platter. Machined to close tolerances the platter has a friction fit onto the bearing spindle. This provides excellent stability but also allow for a small degree of acoustic decoupling of the bearing and platter. If desired this decoupling can be increased by adding a thin layer of a lossy material between the shoulder of the spindle and the platter. The platters high mass provides enough inertia for good rotational stability. Pictured are the as machined platter with a frosty appearance and the optional polished surface.
To drive the platter a precision Maxon model 110189 DC motor was selected. The Maxon 110189 is an exceptionally smooth running motor with good torque, low rotational inertia, precious metal brushes and sleeve bearings.
It seems that a properly implemented DC motor drive has some advantages over an AC synchronous motor. The most obvious advantage is smoothness of operation. AC motors create much more vibration requiring very good isolation of the motor from the turntable and also the use a of compliant belt to avoid coupling vibration to the platter, while the vibration can be dealt with why not just start with a quieter solution. DC motors also have far less rotational inertia reducing the possibility of interaction between the motor and the platter causing oscillation.
The difficulty with DC motors is maintaining correct speed. Using a strobe disk the DC voltage can be adjusted for correct speed but over time the speed will drift so frequent re-adjustment is required. A servo loop or PLL may be employed but then there is the problem of the circuit making constant small adjustments resulting less than precise speed control. However, all of these problems are resolved with Manfreds remarkable controller described below.
For the motor controller Manfred Huber has kindly made his superb design available to the group. Manfreds design is an 'intelligent' regulated power supply. The output voltage is controlled by a micro-controller (Atmel 89C2051) and a serial 12Bit DAC. The controller measures the rotational speed via a stroboscopic disc which is attached to platter. Two control algorithms have been implemented. The first is a relatively fast PI controller which brings the platter quickly up to speed. After the speed has stabilized the controller automatically switches to a *very* slow integrative algorithm that holds the desired speed. This second algorithm can only change the output voltage at a rate of some tens of millivolts per minute. The supply sounds exactly the same as a good voltage source but with the added benefit of automatically maintaining the correct speed.
The regulator circuit consists of a current source feeding into a shunt regulator that is controlled by the micro-controller. This sophisticated design provides superb performance.
Some nice features of the controller are:
Manfred's controller design offers outstanding sonic performance combined with great ergonomics and ease of use.
A precision aluminum pulley was custom machined for the Teres as off the shelf pulleys are generally not of sufficient quality for this application. The pulley is superbly concentric with a run-out spec of less than two ten-thousandths. To retain the near perfect concentricity the pulley has a friction fit on the shaft and no set-screw is used.
To mount the motor a custom machined 3/8" thick, 4.5" round motor mounting plate is used. The motor controller PCB is conveniently mounted on the mounting plate and uses the mounting plate as a heat sink. This results in a complete, compact assembly that only needs an AC power source (14vac @ 650ma) and a connection to the strobe disk sensor. A matching motor pod has also been produced to complete the motor assembly.
The majority of the Teres design was hammered out via e-mail between the early participants. A compilation of these e-mails is provided here for those interested in more details about the design and more particularly the rational behind the design decisions. The Teres list has also become a forum where a number of analog related topics have been discussed beyond turntable design.
To make the data more manageable the digests have been broken out by topic. Within each topic area the digests are further divided by date into chunks of less than 200k bytes. However, the volume of data is still enormous. Proceed only "if ye be men of valor".
The design of the bearing and platter was done primarily by the following list of participants. However, as the project progressed many others, too many to list, contributed valuable information about all aspects of turntable design and construction.