The Turntable
a turntable drive has the task of turning the record with utmost constancy of speed. Add to this immunity against internal and external disturbances.
Rumble is a kind of inner disturbance, that may couple Bearing noise into the music signal due to direct mechanical connection.
External disturbances are for example airborne- and impact-sound, or electromagnetic coupling.
The constancy of the speed is a result of the mechanical quality of the platter and bearing and the drive/motor system.
On the drive side two concepts prevailed.
- The Belt-Drive - where a AC or DC motor rotating with 200-500U/min drives the platter via a transmission belt, a thread or a foil strip (tape). It makes regular use of the range of industry standard motors. By slip and strain the precision of the drive is limited on one hand, on the other hand is hunting of the motor reduced by the transmission ratio to lower values. Typically one combines heavy platters with high rotational mass and rather low-torque motors to reduce fluctuations in motor speed by sheer rotary inertia. Electronic variable speed drives can be found -if at all- almost exclusively in simple forms of motor control. An exception was Pilips who even implemented Quartz PLL servo drives in their AF-8XX and AF9XX series, utilizing speed sensors attached to the main bearing.
- The Direct-Drive - where a AC or DC motor is directly connected to the turntable platter. The motors are low-rpm types, designed specifically for this application.
Here the motor quality determines the accuracy of platter speed 1:1. Therefore electronic controllers were introduced at an early stage, culminating with the introduction of the Quartz PLL servo
in the 1970s. The advancement of electronic control was accompanied by a steady reduction of platter mass. Increased use of lightweight plastics eventually costed on playback quality of the
devices due to higher sensitivity against airborne and impact sound. The quality rating regarding look and feel dropped. This together resulted in an Image-decay, so that the basically superior
direct drive almost disappeared from the high-end sector.
A small, now growing-again fan base still holds up the flag.
The Tonearm -
The task of the tonearm is to guide the pickup ideally at a fixed position over the record groove surface. This is opposed by the constantly changing horizontal position of the groove and radial run-out of the record disc. At the same the tonearm must therefor be as smoothly moving as possible.
Pivoted and tangential tonearms have prevaild.
- The pivoted Tonearm - leads the pickup at the tip of a mostly 9´ to 12' long beam in a circular section trail over the record surface, and that´s horizontal and vertical pivot
point is located outside of the turntable platter surface.
Because of the difference between tangentially cut record and the circular shaped replay path additional distortion components occur (tracking errors) that must be minimized by optimizing the
geometric dimensions of the tonearm and the precise mounting and adjustment of the pickup stylus.
There are arms with straight, J or S-shaped arm tube forms. There is also a wide range of bearing designs such as single points, gimbals, conical tips or blades, and combinations thereof.
- The tangential Tonearm - leads the pickup in a straight line across the Vynil disc, similar to the cutting head the record producers use. The beam is shorter than for the
pivoted tonearm, in some embodiments even so short, that a part of the tonearm´s mechanics need to be swiveled over the platter for operation.
The optimization of a parameter causes virtually always the deterioration of another parameter. Thus for e.g. the reduction of the inertial mass of a short arm tube is indeed positive, but a
radial run-out of the record would rather appear like speed variation/flutter. Here too different types of bearings occur, from trolleys, ball-, tip- or blade bearings, up to contact-free
friction-less air bearings.
The pivoted tonearm is the dominant market variant without this design offering a special acoustic benefit. Similar to the drives, there is a group of loyal followers granting tangential tonearms
fundamental sonic benefits.
The PickUp cartridge
The pickup is an electromechanical transducer which mechanically scans the tiny grooves utilizing a microscopic stylus gem. The vibrations of the stylus are transferred via the cantilever to an
electromechanical generator that converts them into an electrical signal.
There are a number of different transducer types of which the electromagnetic principle has prevailed. Piezoelectric transducer, capacitive or optical transducers have, despite partly distinct
advantages never gained much prominence.
The electromagnetic transducer can in turn be divided into three sub-groups, the Moving Magnet (MM), the Moving Iron (MI) and Moving Coil (MC)
- Moving Magnet, MM-pickups vibrate one or two magnets, that are attached to the cantilever, in the air gap of the pickup coils.
Shure MM:
1) Stylus 2) Magnet 3) Cantilever 4) Pole pieces 5) Coil winding 6) Connector pins
Due to the typically high number of turns of the coils and the soft-magnetic pole pieces these pickups produce relatively high output voltages of ca. 2 to 5 mV at a cutting speed of 5cm/sec at
1kHz. However, the impedance values are in the range of 400-700mH plus 400-1200Ohm. Together with the input impedance of the phono amplifier and the cable capacitance the electrical bandwidth
limits at about 12-25kHz.
A major advantage is the replaceability of the stylus insertions.
- Moving Iron, MI-pickups are built very similar to MMs. However, here is a magnet fixed to the pole pieces and soft iron rods, or platelets are attached to the cantilever. This allows to use more powerful magnets, so that the number of turns and thus the inductance and Impedance of the coils can be reduced, while keeping the moving mass sufficiently low.
The electrical parameters are similar to those of MMs, whereby for example Ortofon VMS are typically located towards the upper end of the impedance values range (2M Red 630mH-1,2kOhm, 5 mV), while Grado pickups present significantly lower inductance values (Prestige 45mH, 475Ohm, 5 mV). This allows for significantly increased electrical bandwidth limits beyond 20kHz.
- Moving Coil, MC-pickups utilize a inverted generator system. Here tiny coils are mounted on the cantilever and vibrate in the air gap of a powerful magnet. The number of turns of the coils must be small in the interest of low mass. It follows that the devices are not only low impedance, but also supply signal voltages lower by a factor of 10-20.
The electrical parameters fall in the range of several dozens µH and less than 100Ohm, at signal voltages in the range from 0.15 to 0.5mV. The low impedance allows electrical bandwidth limits up
to and beyond 50kHz. But the small signal voltage asks for higher amplification factors of the amplifiers, or the use of audio transformers.
A disadvantage is that the stylus assemblies typically can´t be exchanged, but are mounted fixed.
Phono-preamps
In a sense a Phono stage belongs into the category of preamplifiers.
But it is not common nowadays that a amplifier features an integral phono stage. Instead You rather find highlevel preamplifers and dedicated external highquality Phono stages.
Phono is also special in that it requires the highest gain-/amplification-factors within the signal chain and it requires equalizing filters for amplitude and phase linearization. The Vinyl uses preemphasis and is cut in a way to lower the noise floor and to allow for sufficient playback time.
At playback the preemhasis needs to be equalized mirror-like.
Best known is the RIAA-recommendation of 1928 that features a double staircase-shaped response at three distinct frequencies of 50Hz, 500Hz and 2120Hz. A fourth frequency was added to the RIAA-IEC in 1963, forming a 20Hz subsonic filter.
Neumann, a german cutterhead manufacturer introduced another frequency of 50kHz which limits the power fed into the cutterheads to safe levels.
The RIAA-curvature is just a recommendation, no norm. Hence exist a couple of differing recommendations and it is by no means sure that a LP is always and correctly cut after the RIAA-recommendation.
In the end, while the mastering the recording engineer works with numerous equalizers and effects anyway. If for example the groove distances needed to be reduced, as it is the sometimes the case with long movements of classical pieces, the recording engineer could lower the bass volume. Even with a perfect RIAA playback equalization such records would sound thin and shy of bass. Essentially every phono preamp or preamp required a compensation control knob for just this situation.
