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Understanding

Turntable speed stability: why the motor power supply matters

Mechanically, a turntable has a single job: spin the record at exactly the right speed, without the slightest variation. It sounds simple. In reality, it is one of the most demanding aspects of analogue playback, and the motor power supply plays a role that is often underestimated.

What the ear hears

A speed problem can take two very different forms.

• A constant error: the turntable runs slightly fast or slow, and the whole track plays slightly sharp or flat. A 1% deviation equals about 17 cents, a sixth of a semitone: audible to a musician's ear, especially when playing an instrument along with the record or comparing it to another source.
• Speed variations: far more annoying, because the ear is very sensitive to pitch that moves. There is wow, slow variations often tied to the rotation of the platter or pulley, heard as a wavering pitch on sustained notes (piano, voice, strings); and flutter, faster variations rarely heard as a change in pitch but rather as a less clean, less settled sound and a less stable stereo image.

How they are measured

The classic measurement uses a test record cut with a fixed frequency, usually 3150 Hz according to the DIN and IEC standards. An analyser tracks the variations of that frequency during playback and derives a wow and flutter figure, expressed as a percentage and weighted to reflect the ear's sensitivity. Very good turntables achieve figures in the range of a few hundredths of a percent.

Average speed, on the other hand, is easily checked with a stroboscope disc or a dedicated app. Beware: a record whose centre hole is not perfectly centred adds its own wow, once per revolution, regardless of how good the turntable is.

The drive chain of a belt-drive turntable

On a belt-drive turntable, the motor turns a pulley, the pulley drives a belt, and the belt drives the platter. This decoupling is a strength: the belt filters part of the motor's vibration, and the inertia of a heavy platter smooths out small irregularities from one revolution to the next.

But that filtering has its limits. Motor vibration that gets through the belt ends up in the platter, and then under the stylus. And a slow or constant speed error is never "filtered": the platter always ends up turning at the speed the motor imposes.

The role of the motor power supply

Most belt-drive turntables use an AC synchronous motor. Its defining trait: its speed is set by the frequency of the current feeding it, not by its voltage. Anything that affects that signal therefore directly affects rotation.

• Mains frequency is not perfect. It constantly varies slightly around 50 Hz (or 60 Hz depending on the country). A motor connected straight to the mains follows those variations.
• The mains waveform is polluted. Household appliances distort the grid's sine wave. Those harmonics turn into vibration in the motor, which can reach the platter through the belt.
• The phase shift is approximate. A two-phase motor needs two signals offset by a quarter of a cycle. When a simple capacitor creates that offset, it is only approximate and varies with component tolerance and temperature: the motor's torque becomes uneven over each revolution.

A dedicated power supply solves these problems by generating its own signal instead of reusing the mains: a clean sine wave, at a frequency derived from a quartz crystal, with an exact phase shift between phases. It also allows the voltage to be adjusted: a motor needs torque to start, but far less once the platter is up to speed, and running it with just what it needs reduces its vibration. Finally, it allows the speed to be changed electronically, without moving the belt on the pulley.

Open loop or closed loop

A quartz power supply delivers a perfect frequency to the motor. Yet the platter can still drift from the correct speed: the belt slips slightly and ages, the stylus drags on the record more or less depending on groove modulation, bearing friction changes with temperature. The power supply knows nothing of this: it works in open loop.

Closed-loop regulation adds a sensor that measures the platter's actual speed and continuously corrects the signal. The challenge is then to correct gently: overly aggressive regulation would itself add speed variations, which is exactly what we are trying to eliminate.

Turntables with several motors

Some high-end turntables use two motors, or even more, to share the effort of driving a very heavy platter. The motors must then pull perfectly together: if they are out of phase, they fight each other through the belt, which creates vibration and wears the belt prematurely. Electrically aligning the motors with each other becomes an adjustment in its own right.