Vinyl Asylum

Posted by B.K. (A) on January 10, 2008 at 15:12:36

In Reply to: RE: Mark Kelly Synchrotron AC-1 Drive Controller posted by Dave Garretson on January 10, 2008 at 06:30:55

The Mark Kelly Synchrotron AC-1 is the ultimate upgrade for anyone using a turntable powered by an AC synchronous motor.

I have a beta version of the Synchrotron AC-1, and I have to say that without qualification this is the best PSU you will find for driving a synchronous motor powered turntable on the affordable side of $1000.

For those who are not familiar with the Mark Kelly turntable PSU, it is available in kit form directly from Mark himself. The PSU comprises three PCBs which together provide a split phase sine wave generator for driving a 110V AC synchronous motor.

Now, one could be forgiven for wondering why there is a need for another such power supply; there are, after all, several such products already on the market: Heed Orbit, Hercules, Linn Valhalla and Lingo, Project Speed Box etc; but the Mark Kelly Synchrotron AC-1 includes a number of vital additions over other commercially available turntable power supplies such as the Heed Orbit PSU etc.

First up, there is the fact that the power supply produces the two phases to drive the motor directly. Many of the currently available turntable power supplies, such as the Project Speed Box, provide a single phase output and leave the phase splitting to a passive network located at the motor. This arrangement doesn't work very well, because a typical passive phase splitting network branches the input into a pair of outputs: one which connects the input directly to the first motor terminal, and another which connects the input to the second motor terminal via a series capacitor. The series capacitor does provide a phase shift, but a series capacitor cannot provide the correct phase shift (90 degrees is the required value). Moreover, the series capacitor reduces the amplitude of the voltage applied to the second motor terminal. The result of the different voltage amplitudes on the motor terminals and the incorrect phase difference is to add a significant amount of 100Hz vibration to the motor.

The second benefit of the Mark Kelly Synchrotron AC-1 is that the frequency is adjustable; this may not seem like such an important adjustment, after all isn't the pulley on a synchronous motor machined to give the right rotational speed for a 50 Hz or 60 Hz input? But actually, speed adjustment is a vital feature of a turntable power supplies particularly if you are using a turntable which has seen changes to production over the years. My measurements carried out on several Pink Triangle PT TOO turntables and a number of Thorens TD 160 turntables let me to the conclusion that many synchronous motor driven turntables are running at the wrong speed. For example, the effect of belt tension on the platter speed should not be underestimated: it is easy to produce a deviation in the order of 1% by changing the tension and width of the drive belt. Even changing the oil in the bearing housing can have some effect on the speed of rotation of the platter, so the lack of frequency adjustment on most power supplies for driving synchronous motors is a major omission in my opinion.

To verify the above, I tried running one PT TOO below the correct speed and above the correct speed, and the effect on the music was very apparent. Incidentally, I was not born with so called ‘perfect pitch’, but I found it quite easy to observe that a speed deviation of 1% too slow imbued the music a ponderous and deliberate mood, whereas a deviation of 1% too fast made the music sound pacey yet artificial (did someone say PRaT?). With the Synchrotron AC-1, the speed is adjustable to a precision of less than 0.02 %, so concerns about the turntable running at the wrong speed can be banished forever.

Next there is the fact that the phase between the motor windings can be adjusted about the theoretically optimum 90 degrees. In principle, this is not needed either, except that for may synchronous motors the alignment of the metallic teeth which produce the rotating magnetic field in the stator is not perfect, and an adjustment of the relative phase of the voltages applied to the two terminals can correct this problem.

Finally, there is the method by which the sine wave is produced. In private correspondence Mark Kelly comment that producing a signal by analogue methods (such as using a Wien bridge oscillator) has the benefit that a pure sine wave without unwanted harmonics can be produced but that creating a second signal with a 90 degree relative phase is difficult to achieve, and that accurate frequency control is difficult to maintain; on the other hand, producing a signal by digital methods enables the precise control of frequency and phase, but requires very high order filters to produce a pure sine wave output. So the solution was a hybrid, the signal is produced digitally, but by sampling the sine wave at a higher frequency (400 Hz instead of 100Hz), the result is the best of both worlds, accurate control of frequency and phase, and a relatively benign residue of harmonics which are suppressed using a relatively simple low pass filter circuit.

I have been using the Synchrotron AC-1 PSU to drive a Pink Triangle PT TOO which is partnered with a Grado Signature tonearm and a Denon DL103R for the past 3 months. The speed is exactly right at both 33RPM and at 45RPM, and the motor is absolutely silent. It’s so quiet that even if I touch the motor with my hand, I can only tell if it is running or not by observing whether the platter is spinning. I have never had a synchronous motor run so silently in any turntable that I have owned over the years. I put in some time and effort to restoring this innovative turntable to its former glory throughout 2007 and can now say with confidence that the turntable sounds better then it did when it came off the production line about 20 years ago!

The picture below shows the PT TOO turntable; the Mark Kelly Synchrotron AC-1 is housed inside the original metal housing for the PT TOO PSU, there’s nothing much to see, it’s just a black metal box, so I have hidden it at the base my rack.

BK