High quality AC turntable motor

By lowering the voltage on an AC synchronous motor, the "pulsing" is not as strong allowing the inertia of the platter to better smooth the jitter. That's why the VPI SDS is so effective. It uses full voltage to help get the platter moving, then drops the voltage significantly after a few seconds.

Interesting to refer to motor mechanics in terms of digital distortion. The "pulsing" you refer to is known as cogging, which refers to the rotation of the rotor through the fixed poles (stator) on a synchronous motor. Depending on the rotational speed of the motor, the number of poles varies; a 300 RPM motor will generally have 24 poles, while at 600 RPM, there will be twelve poles. The overall torque of the motor is determined by the field strength of the magnets; stronger magnets have more cogging and hence more torque.

I find that with most 115VAC synchronous motors, run at their approximately nominal frequency (50/60Hz), it's usually better to smooth out the cogging by running the motor at approximately 65-75% of its rated voltage. For example, I run my 600 RPM/115VAC motor at 55Hz for 33RPM with a voltage around 75VAC on one phase, and around 69VAC on the other. This optimizes the torque to cogging ratio such that the motor doesn't introduce vibration through the drive chain, yet maintains enough torque to overcome all stylus drag during heavily modulated passages.

The SDS is a decent controller, but its lack of parametric adjustment (phase angle, harmonic distortion, and individual phase amplitude) makes it nothing more than a glorified speed controller (though nice for a VPI owner to not have to swap pulleys). It does little, however, to address the differences found even in sample to sample runs of the same exact motor. Without being able to optimize every parameter of the motor, you can never really null out cogging, nor get the best performance out of any given motor.

Palasr,

Can you point us to any turntable controllers that have all of the features you mention above?

I use Mark Kelly's Synchrotron AC-1 controller, which contains all of the aforementioned features (and then some) but has long since been unobtainable; this was a DIY Audio kit project from several years back. You bought the raw boards, bought the parts, understood the design's theory and you were basically on your own; you built the unit from the chassis on up - not a build for the novice. I admit, I've been spoiled by the controller's abilities.

Another possible candidate in the making is this gentleman's controller, based around a PIC/Atmal chip:

http://www.soundbound24.blogspot.ca/ (scroll down to the Bi-Onda controller). He seems to be on a hiatus, but I'm hoping he continues on the fascinating path he's on; his is the most promising and versatile design I've seen in a while.

I would gather that without upping the power handling of the output amplifiers, both this controller and my Kelly can't drive anything larger than say, 7 watts (the average synchronous motor in a TT is usually 3-5 watts).

There is also the Sander's speed controller over on Lenco Heaven: http://www.lencoheaven.net/forum/index.php?topic=2042.0 This appears to have applicability toward higher wattage motors (15-20 watts), and shaded-pole induction types as used in the Lenco, Garrard and others. I haven't gone through the entire post to tease out the details of the circuit, but it appears to cover the essential parameters. I'm still exploring the salient characteristics in this lengthy thread.

I've never had a Walker controller open to see what makes it tick, but I'm guessing it's about the same as the SDS. While a Linn Lingo has a nice sine wave synthesizer, it also doesn't allow for any parametric adjustment - despite the fact they're loaded with (unlabeled) trim pots.

While I advocate complete parametric motor adjustment, it's also a catch-22. You need some test gear like a multimeter and an oscilloscope to really maximize performance; a little too far this way or that with some settings and you can really mess things up (as in your motor stalls, vibrates and you become sad because your table no longer spins). This, I believe, is the reason nobody has a desire to market a motor controller which contains all the necessary parametric adjustments: the support issues would instantly turn into a complete nightmare since most folks simply aren't technically savvy enough reap the benefits or dig themselves out of the inevitable holes that would result.

Thanks Palasr. Much appreciated.