Vinyl Turntable Drive Techniques
Over the years there have been several motor coupling techniques used to rotate the turntable platter. This is a brief overview of each method along with their advantages and disadvantages.
There are three basic types of motor drive, the second two of which have various methods of motor control:
- Idler wheel or rim drive
- Belt drive
- Direct drive
Idler Wheel/Rim Drive
This design was very popular throughout the 40’s, 50’s & 60’s, found in virtually all radio broadcast facilities and widely used for those popular automatic record changers. The benefit of Rim Drive is its torque, though modern direct drive turntables can now match this. It can handle the heavy tracking forces of the time without slowing down and control automatic gear mechanisms used for arm control etc. without stalling. Its major disadvantage is the level of transmitted motor vibration to the platter. This can and was reduced by using very heavy damped platters, but this is costly, and this technique is now all but defunct. Some audio enthusiasts still seem to prefer the particular sound of these antique turntables.
Belt Drive
Overview
The desire for a lot less rumble, improved speed stability and the reduced tracking weight for cartridges produced the Belt Drive system. The original turntables were manual as the belt drive system provides inherently low torque. The coupled motor noise is much lower based upon how the motor is mounted as the belt isolates the platter from the motor. So designs were developed where the platter chassis is separate to the motor chassis, being suspended on springs and dampers. This lower torque delivery results in speed variations from both static and dynamic friction from the stylus in the groove. This will change from the outer to the inner grooves and upon the groove amplitude. This design concept was and still is the most popular and cost effective design for audiophile turntables. The only real issue is that the belt needs to be very accurately manufactured and replaced every few years so that it maintains its performance.
There have been numerous improvements in obtaining both higher motor torque and improving rumble (bearing noise), wow (low frequency speed changes) and flutter (high frequency speed changes) from both redesigned central bearing systems, routing the belt round the outside of the rim to sophisticated motor control techniques.
NOTE: ALL motors produce an effect called ‘cogging’. It is a juddering effect as a result of the motor shafts rotation not being perfectly smooth and is a function of the number of motor poles and the way it is controlled. Modern control systems, particularly for direct drive turntables, have refined this issue to be almost non-existent. However some audiophiles claim that they can still hear the effects of ‘cogging’ even in modern well designed turntables.
There are several ways the belt drive motor may be controlled:
- Synchronous
- DC Motor
- DC Servo Motor
- Frequency Controlled
- Rotational update
NOTE: DC motors may be of a conventional design and use brushes or brushless design that requires a multi-phase (typically 3) winding, driven by a sophisticated electronics circuit that generates a rotating magnetic field; the basis of the Direct Drive turntable (see below).
Synchronous
All original belt drive turntables and several today use Synchronous drive motors. This is a good low cost choice for fixed speed designs as the motors rotational speed is locked to the power line frequency. So speed stability is only as good as your power lines frequency. Speed stability is generally good and drift is very low. This type of design can support fixed speeds of 33.3, 45 and 78 by moving the belt to different sized motor pulleys, but variable speed is problematic and requires a sophisticated motor control system.
Phoenix Engineering (now not in this buisness) produced an add-on microprocessor controlled digital AC high voltage power source to drive synchronous mains motors. The Falcon PSU uses Direct Digital Synthesis (DDS) to create an extremely accurate and clean high voltage AC sine wave that significantly improves motor speed stability, and allows the user to precisely adjust the turntable platter rotational speed. DDS technology allows the Falcon power supply to generate signals with very high resolution, less than 35µHz steps. Its performance is totally independent of any mains AC frequency or voltage level fluctuations.
This precise frequency regulated power supply can be coupled with the RoadRunner that provides platter rotational speed information (see below) that is used by the Falcon PSU to dynamically control the platters speed. My own Thorens TD160S turntable uses this EXCELLENT combination, see here.
DC Motor
This uses a conventional DC motor as the drive source. Its advantage is cost and simple variable speed control. Unfortunately this design has a number of weaknesses that make it unpopular. It suffers from temperature drift and load sensitivity and is not a popular drive technique.
DC Servo Motor
These systems use a speed encoding system to monitor the speed of the motor (or platter) and compare the voltage created against some known stable reference. Any errors are used to control the motors DC supply voltage and compensate for the speed variations. See voltage servo below. While this improves long term stability its response is generally too slow to compensate for speed changes due to dynamic stylus drag.
Frequency Controlled
Frequency controlled feedback offers a far more sensitive technique and a faster response time to control a DC motor. These servos or tachometers provide a direct indication of the rotational speed of the platter, compare it against an established frequency reference and create a voltage control signal that modifies the motors DC voltage. The feedback frequency can be used as a direct readout of the platters speed and the technique produces excellent long term speed stability and with good design can respond rapidly to small changes in rotational speed. See frequency servo below. These designs may be enhanced to support Phase Locked Loops (PLL), see PLL below. However, the platters rotational mass and ‘flywheel effect’ is still the major governing force keeping the wow and flutter to a minimum and overcoming stylus dynamic force changes.
Rotational Update
Some turntable designs use an updating system which usually comprises one or two magnets mounted on the platter 180 degrees apart. As the platter rotates, the magnets generally trigger Hall effect sensors. The time interval between triggers is used to generate a count from a very stable known frequency. The count is then compared against some internal calibrated reference and an adjustment is made to the motors control voltage or frequency to correct the speed. This system can work at all speeds and with heavy platters. Heavy platters still take time to either slow down or speed up so some inexpensive designs use much lighter platters. While this provides a faster response time it can cause platter jitter and smear high frequency sounds.
The RoadRunner by Phoenix Engineering is an add-on microprocessor controlled digital tachometer used for monitoring turntable platter speed to 3 decimal places of resolution. The internal time base is a temperature compensated crystal oscillator (TCXO) accurate to 2.5 PPM (±0.00025%) with less than 1.0 PPM aging per year.
The RoadRunner Tachometer provides a direct digital readout of the platter speed after only 2 platter revolutions and updates the display on each revolution afterwards. This provides a significant improvement over strobes and other devices that are only indicators of fast/slow, not a measurement device. Strobes also lack resolution; often you need to track the “drift” of the marks over several minutes and compute the speed using a calculator. At 33 RPM, a speed error of 0.01 RPM represents a drift of only 0.0113” per revolution.
This tachometer uses a single hall effect sensor and a single very small magnet attached to the underside of the platter to sense its rotation. The magnet & sensor assembly are installed under the platter or sub-platter edge, see here .
Direct Drive
Overview
This design is based upon the brushless DC motor. Direct drive has the motor integral to platter as shown above. The motor therefore has to rotate at the exact record speed. These designs can offer excellent torque, very fast well damped response to dynamic stylus drag and very low wow and flutter. Variable speed is also easy to provide. These turntables are more susceptible to ‘cogging’ but modern motor and control designs have virtually eliminated this issue.
Direct drive (and belt drive) motor control maybe achieved in a number of ways:
- Voltage Servo
- Frequency Servo
- Phase Locked Loop (PLL)
See here for an interesting white paper from VPI on the design of their direct drive HW40 turntable.
Voltage Servo
The original and simplest control loop of a brushless DC motor is the Voltage-servo. Drive current flows in only one of the commonly used three phases of the motor. The other two sets of windings generate voltages that are proportional to the turning speed due to the generator effect, hence Voltage Servo. Rectifier diodes and a filter sum and smooth these voltages. This speed dependent voltage is then compared to a very stable known voltage the error of which then controls the motor drive amplifiers. Changes to speed are made by adjusting the voltage reference level.
This design is the least expensive of the three, provides less control precision and tends to drift with time and temperature.
This design has been used in several Technics turntables and the infamous DUAL 701.
Frequency Servo
In this design the motor drive amplifiers drive all three motor windings with a rotating field. A sensor attached to the motor or platter spindle generates a speed-related pulse signal that is converted to a smoothed DC voltage that is proportional to the pulse frequency. This voltage is again compared to a known very stable voltage reference and the resulting error applied to all three motor drive coils via the motor drive amplifiers. Changes to speed are again made by adjusting the voltage reference level.
This technique provides a control signal that is more precise and less prone to drift.
Phase Locked Loop (PLL)
The phase locked loop control (PLL) system provides the tightest and most refined control available for a motor. The primary reference is a very stable high frequency quartz crystal oscillator whose frequency of comparison is created through a frequency divider that allow for motor speed control by adjusting the comparison frequency.
The oscillator provides the phase and voltage converter with a highly stable clock signal as reference. Its frequency is used to set the motors speed and is therefore adjusted to set the turntables fundamental speed: 331/3, 45 or 78 RPM. The frequency to voltage converter provides for a second speed-related signal. The phase & voltage converter compares the timing relationship of the signals and converts the difference into a DC-voltage which is then compared to a reference voltage that drives the motor drive amplifiers. The phase & voltage converter is comparing the timing of the edges or phase of the waveforms and the system ‘locks‘ them to each other to produce zero error. The frequency to voltage converter provides the preset speed loop while the phase & voltage loop provides the fine lock control.
Examples of Quartz PLL-servos are the Technics SP-10MK2, SL-1000, SL-1200, SL-1210 and SL-1500.
Other Important Turntable Parameters
Speed, Control, Drift, Wow & Flutter
- You need at least 331/3 and 45 RPM for LP’s and singles. Those enthusiasts of older recordings will also want 78RPM.
- Ideally you need some degree of speed adjustment as nothing is perfect and adjustment of pitch is often used by musicians.
- Long term variations in speed are not desirable but if very gradual cannot be heard and can be compensated for using the speed control.
- Wow is a slow change in speed, typically lower than 4Hz due to control hunting, mains AC frequency variations or even changes in the dynamic stylus force.
- Flutter is a rapid change in the speed, typically higher than 4Hz usually due to control instability (jitter) issues, or other motor (cogging) or mechanical issues.
Rumble and The Bearing
Most low frequency noise from turntables comes from its central main bearing. Although worn motor bearings, poorly machined pulleys, dirt or wear on the belt will certainly add to this issue. There are various designs of the main bearing all of which try to minimize noise and vibration transferred from its thrust bearing and spindle friction to the platter. They range from simple and very popular brass housings using oil baths to PTFE linings, to the more exotic magnetic levitation and air bearings. Ignoring the very uncommon exotic solutions, the performance of this bearing assembly is dependent upon the quality of the materials, machining and the method used for the thrust bearing that takes all the platters weight.
The above photograph is off my own designed turntables center main bearing. If you look carefully you can just see the top grey PTFE press fit bearing insert in the brass housing. The second insert sits 3/8″ up from the bottom of the shaft. The ground shaft and ball thrust bearing are nitride hardened steal. The bottom of the brass housing contains a ‘bath’ of Moly-Graph EP grease which the ball and bottom face of the shaft sit in.
Chassis Suspension & Turntable Isolation
The isolation of the turntable and arm from both its motor and surroundings is important to stop unwanted air born or structural vibrations from affecting the stylus tracing. Numerous techniques and designs are used to reduce these problems the most common of which for belt drive turntables is a separate chassis for the turntable and arm that is suspended on three damped spring assemblies. Thorens, Linn, Ariston and Acoustic Research (both out of business) all use(d) this technique. Other methods include a totally separate motor assembly, pressured air and plinth isolation feet.
In all cases the turntable plinth should be solidly mounted in such a way that floor and air born vibrations have minimal pickup by the turntable. In my case it sits on a 25lb slab of granite that is mounted in a rack that sits on an physically isolated section of my A/V rooms wooden floor. This is more of problem for those of us with wooden walls and floors, poured concrete floors and brick/block walls are much more forgiving.
The Platter and Matt
No matter how well the turntable is made and controlled, noise and vibration will still exist. This is further reduced by:
- High mass platters that may be made from layers of different materials that may also be damped with various compounds.
- The application of a matt between the platter and the vinyl record. This matt also impacts how the energy is absorbed from the record as the stylus traces the groove and generates noise within the vinyl. Its selection can have a significant effect on overall sound quality and many designs and materials are used to create a whole range of these interchangeable matts. At least one turntable uses a vacume seal between the platter/matt and vinyl in order to reduce vinyl warping and provide intimate contact to the matt.
Summary
Both Belt and Direct Drive designs can provide audiophile ‘state of the art’ performance. You pays your money and you takes your choice. However, belt designs are far more common than direct drive designs due to the complexities and hence costs associated in obtaining technical performances that match or exceed the simple design of a belt drive turntable. At $3,999.00 the re-release of the Technics SL-1200G is the ‘Daddy’ of direct drive PLL turntables dating back to its original release in 1972. However, this totally revamped design provides a significantly enhanced technical performance and of course an arm. That being said, $4,000.00 can buy you one hell of a good belt drive turntable from many companies to include Linn, VPI, Rega and Thorens. For those on lower budgets checkout Music Hall, Fluance and EAT turntables.
Read my post on Turntable Motor Noise and Vibration.
See here for the upgrade of my Belt Drive Thorens TD160 Super.
Interested in the basics of tonearm and phono cartridge design then checkout these posts:
