Turntable Motor Noise and Vibration

Thorens TD160S 110V AC Synchronous Motor

In my earlier post on turntable designs we took a look at the different drive techniques used to spin the platter. Lets take a slightly more detailed look at the effects of the motor design that is used to drive the more popular belt drive turntables.

First lets see what challenges actually face the design engineer in getting a platter to spin.  The job of the stylus and arm is to pick up the modulation of the vinyl groove. These changes in groove amplitude can be smaller than the wavelength of light.

Electron Microscope View of Vinyl Grooves

It stands to reason therefore that your spinning vinyl must not be vibrated in anyway or that movement will be picked up by the stylus. This means:

1. The platter must only smoothly rotate at a constant speed in a perfectly horizontal plane and not vibrate or move randomly in any plane.
2. The arm relative to the platter must be totally rigid but at the same time:
   • Be able to freely move in both the horizontal and vertical directions with zero “play”
   • Absorb any energy that is put into it from the stylus/cartridge assembly
   • Have a dynamic rigidity that allows the stylus to trace the groove without the arm moving
3. The cartridge must be rigidly attached to the arm.
4. Energy that enters the vinyl from the stylus tracing must be absorbed. (Primarily the job of the platter matt. This matt also helps isolate the vinyl from picking up vibrations from the platter)

For this post we shall only be considering the first point. Tonearm design was briefly reviewed in this post.

The first point above raises several issues in the design of the platter and its bearing and how the motor spins the platter. Modern material science together with current CNC machining technologies can create platters, spindles and bearings that are not only perfectly square to each other, but generate very low levels of vibration that result from the spindles contact with both the sides of the central main bearing housing and the bearings bottom thrust support. Many techniques including magnetic and air levitation are used to improve this critical thrust support area. In the ideal world all bearings would be designed to create a ‘frictionless’, ‘play’ free connection from the platters spindle to the turntables main (sub) chassis on which the tonearm and eventually the stylus are rigidly attached.

The above is the cross section of my own bearing design that I have pictured in several of my earlier postings.

This brings us to getting the platter to spin. I discussed the various approaches in an earlier post. Here we shall briefly examine some of the issues that surround the most popular drive technique of all; belt drive.

There are two types of motor used to drive a belt drive turntable:

1. AC synchronous motor – permanent magnet
2. DC motor – brushed or brushless

The most popular and least expensive approach is to use the single phase permanent magnet AC synchronous motor. These very popular, small, inexpensive motors rely on the frequency of the AC signal for their rotational stability together with the number of magnetic poles for their RPM. They contain a permanent magnet rotor and two sets of copper wire stator coils that surround the rotor, the start and run windings.

Exploded View Of An AC Synchronous Motor

The rotational force is created by generating a second electrical current signal that is phase shifted by 90 degrees when compared to the original. This phase shift generates the required rotating magnetic field.

Start Winding Capacitor

Thorens TD160S Motor Connectivity

The required phase shift is created by placing a capacitor in series with the start winding. The value of the capacitor is very important as the 90 degree phase shift needs to be accurate in order to minimize motor vibrations due to uneven magnetic fields.

Typical Motor Winding Current Waveforms

Unwanted vibrational energy from the motor may be transmitted to the platter via the belt or motor mount from any or all of the following motor issues:

1. The quality of the motors bearings
2. The mechanical uniformity of the motors stator windings and capacitor value
3. How the motor mounts to the turntables chassis
4. The concentricity and balance of the magnetic rotor and the motors pulley and surface finish.
5. The number of motor poles – the more poles the lower the motors RPM.

The motors rotational speed is easily calculated using the formula:

RPM=(120*AC frequency)/Pole count

Thorens TD160S 16 Pole Stator

Linn LP12 24 Pole Stator

In the case of the Thorens TD160S using 60Hz AC power the FIXED rotational speed is (120*60)/16 = 450RPM, for the Linn LP12 it is (120*60)/24 = 300. In Europe the speeds will be different as they use 50Hz power so the Thorens motor would rotate at 375RPM and the Linn motor would rotate at 250RPM. This leads to the need for different sized motor pulleys for platter speed changes between 33 1/3RPM and 45RPM and for different countries. This issue can be overcome by using solid state AC frequency control with or without servo feedback. See here for my post on the Phoenix AC speed control system. These systems provide a very stable single phase AC signal whose frequency can be adjusted electronically and is totally independent of the AC powers frequency.

If we examine my Thorens TD160S motor, at an RPM of 450 it rotates 7.5 times in one second (450/60) so its fundamental frequency of vibration is 7.5Hz. Harmonics of this frequency will also be created, all be they hopefully at a much lower level than the fundamental. So we get 15Hz, 22.5Hz, 30Hz etc. NOTE; that while the fundamental frequency is well below that found in vinyl recordings, the harmonics rapidly start to appear well within the audible range. Also any non-linearity or non-uniformity in the motors windings or mechanical assembly will create vibrational fundamental and harmonic frequencies according to the number of poles and RPM. These frequencies are all well within the lower end of the audible range. Excellent mechanical quality control and low rotational motor speed are therefore essential for the motors used in audiophile belt driven platters.

NOTE: For a given turntable model, those audiophiles who live in 50Hz countries should, on paper, have an improved noise performance as the motors rotational speed is lower than in 60Hz countries.

It is therefore clearly important to rigidly isolate the motor from the platter, while at the same time providing a compliant drive connection to it. This isolation is the job of the drive belt and the mounting technique used to isolate the motor from the platter assembly. The belts compliance, being made of some type of elastic material, typically ground rubber, will absorb some of the motors vibrations. It will also help even out the motors rotational force that is not perfectly constant. It is then the  job of the platters mass to assist, like a flywheel, and help smooth out any tiny variations in motor speed. Unfortunately all this is still not perfect and some motor vibration and rotational speed changes will still reach the platter via the belt.

Mechanical isolation of the motor can be achieved in a number of ways:

1. Mount it in a heavy, totally separate assembly, not attached to the main turntable – Project, EAT, VPI and others
2. Use a suspended sub-chassis for the platter that is isolated from the turntable motor mounting plate – Thorens, Linn and others
3. Resiliently mount the motor into the turntables main body – many lower cost turntables

In my case the Thorens 160S uses method 2 (like Linn) and I modified my motor mount (see below) to becoming resilient yet maintain its rigidity.

Modified Thorens TD160S – Rubber Resilient Motor Mounts

This brings us to DC motors with or without servo control.

These motors are not some magic panacea to replacing the AC synchronous motor. They still have all the mechanical vibration and isolation issues that AC motors have but can be controlled far easier, have far higher torque and generally lower vibration.

First of all DC motors provide much more torque than comparable AC motors and therefore their speed regulation is far less susceptible to changes in stylus movement and drag. They tend to exhibit less noise and vibration and their speed is far easier to control. They are subject to the same wear and tear and cost more due to their required power supplies and control. DC motors also exhibit an effect called cogging. DC motors basically use a three phase rotating field that is created electronically. This field can cause the rotor poles to step between stator poles rather than rotate smoothly from pole to pole due to an inconsistent level of magnetic flux. This stepping action or cogging tends to show up as a mudding of the lower registers. With modern motors, sophisticated control, very powerful magnets and skewed rotor structures this effect has been minimized to a level where it should not be audible.

There is no free meal, but generally speaking high quality, well controlled DC motors, will provide a superior lower noise drive when compared to equivalent AC synchronous motors.

A Brief note on motor speed and servo control

Electronic speed control can be applied in one of two ways:

1. To set absolute speed only
2. To set absolute speed and then monitor it in order to maintain it – servo control

In the first case electronics are only used to initially set the speed of the platters motor by using some form of electronic speed measurement such as a strobe. After which no electronic feedback is applied to the control system in order to adjust it for any further speed changes.

Servo control allows the rotational speed of a platter to be compared against some established known reference and adjusted if it speeds up or slows down. Both AC and DC motors can use this technique. Some servo systems are sensitive enough to detect the changing drag of the stylus as it encounters high and low levels of groove modulation. As to the benefits of this control, some audiophiles do not favor it. Such a system has to be able to react very rapidly to drag changes while at the same time get up to speed quickly with no overshoot of the platters speed. This is all but impossible unless the speed control is very slow. Critically damping such an electromagnetic/mechanical system is extremely difficult so speed overshoot is a forgone conclusion as the servo systems try to rapidly re-establish the correct speed. This overshoot can result in continual hunting as the servo system tries to keep the speed absolutely constant. It can be minimized or stopped depending upon how large the speed window is before the servo system begins to correct the speed inaccuracy and the rate at which the speed error is removed.

In the case of my own Phoenix AC control system it only makes small corrections each revolution, in typical increments of <0.0005 RPM. The system controls the speed within +/- 0.005 RPM. This represents an error of only 0.015% at 33 1/3 RPM (0.011% at 45RPM) and I am unable to hear it correcting even using a 1KHz tone. It is also important to note that if I disable the servo control the speed of the platter remains slightly more constant with no record, typically +/- 0.002RPM (less hunting). However, the speed is constantly either slightly too slow or fast (+/- 0.005RPM) with it increasing as the stylus moves towards the vinyls center. Manual control to a finer level is not achievable with the changing stylus drag and available motor torque. Either way the speed variations on my servo controlled TD160S are very slow and not audible (to my ears) on any instrumental, vocal, tail-out or fixed tone.

See my original post on servo control and methods here.

Summary

Putting all this into perspective is important. Motor and bearing noise, and speed variations are facts of life, and modern audiophile turntables have generally mitigated them to such low levels that only the most discerning ears with some of the higher end systems will appreciate the differences between turntable motor drive designs. With these differences being detected in several areas to include: the lower bass through lower mid-range impact and clarity, stereo imaging, depth of image and ‘openness’ of the music. Remember that there are many other pieces of equipment in your listening chain that will have a far greater impact on what you hear such as the arm, cartridge, amplifier, speakers and of course your rooms acoustics.

Happy listening.

My original post on Turntable Drives Techniques.

Take a look at the Thorens TD1600 belt drive series.

See my turntable upgrades here.