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Understanding

Dual-motor turntables: why synchronisation matters

Some high-end turntables drive their platter with two motors, sometimes more. On paper, two synchronous motors fed by the same signal turn at the same speed, and the matter seems settled. In practice, turning at the same speed is not enough: the two motors also have to pull together.

Why two motors?

• For torque. A very heavy platter needs a lot of effort, especially at start-up. Two motors share the load, and each works further from its limits.
• To balance the forces. Depending on how they are positioned around the platter, two motors can balance the sideways pull the belts exert on the spindle, and therefore on the bearing.
• For smoother rotation. Each motor's small irregularities can cancel out rather than add up, provided the motors are properly adjusted relative to each other.

That last point is what makes synchronisation so important.

Same speed does not mean same effort

A synchronous motor turns exactly in step with the frequency that drives it. But its rotor does not follow the magnetic field to the exact angle: under load, it lags slightly behind, and the more torque it has to deliver, the larger the lag. Two motors driving the same platter, each through its own belt, therefore turn at the same average speed, with nothing guaranteeing that they deliver the same effort.

A small difference is enough (pulley diameter, belt tension, motor position, manufacturing tolerance) for one to pull a little harder than the other. The platter they share then becomes the link between them: in the worst case, the motors fight each other through it, one braking while the other accelerates. The result is vibration, uneven heating and premature belt wear.

A motor's torque is never perfectly constant

Over each revolution, the torque of a small synchronous motor ripples slightly, in step with its poles. Each motor therefore produces a small periodic vibration. With two motors, these two ripples combine: depending on the offset between them, they can add up and reinforce the vibration passed on to the platter, or partly cancel each other out.

That offset depends on the mechanical position of each rotor, but also on the electrical phase of the signal sent to each motor. And that phase is something a dedicated power supply can adjust.

Synchronising: three adjustments

• Frequency. Both motors receive exactly the same frequency, derived from a single reference. This is the foundation.
• Phase between the motors. An adjustable electrical offset between the first motor's signal and the second's, placing their torque ripples where they interfere least.
• Load sharing. An adjustment of the amplitude sent to each motor, so they share the load instead of one carrying most of it.

An adjustment you measure rather than calculate

The ideal phase depends on the turntable itself: how the motors are mounted, the pulleys, the belts, the platter. Two turntables of the same model will not necessarily share the same optimum, and it can change over time, for example after a belt change.

There is also a very practical point: on many high-end turntables, the motors sit in separate housings that are placed by hand around the platter, with no jig to position them. Their distance from the platter and their orientation are therefore never exactly the same, neither from one installation to the next nor even after simply moving or cleaning them. The mechanical alignment of the two motors is never perfect, and that is precisely why they need to be phase-shifted electrically relative to each other, to compensate for what hand placement cannot guarantee.

The only reliable method is therefore to measure: try different settings, observe the resulting rotational stability, and keep the best one. A validation measurement avoids keeping a setting that brings no real improvement.