of a Lifetime
|TURNTABLE DRIVE METHODS
|Over the years there have been
many methods used to rotate the turntable platter. This is brief description of
each method along with it's merits and weaknesses.
Used almost excusively for automatic changers throughout the 40's - 60's. Today, it is used only in school style phonographs. The benefit of Rim Drive is torque. It can handle heavy tracking forces without slowing down and it can turn the most complicated automatic gear mechanism without stalling. The weakness of Rim Drive is transmitted motor vibration(rumble) directly to the platter. Rumble can be reduced by using massive platters, but this is costly, and mostly seen in hi end or broadcast style turntables.
Lighter tracking forces and an interest in quieter rumble performance produced the Belt Drive system. Early versions of this system were exclusively manual turntables since the belt drive system is inherently a lower torque delivery system. Belt drive has the potential to be quieter than rim drive, but that depends on the motor isolation, not the belt. Vibration can still get into the platter through the mounting board and motor mounts. Early designs avoided this by using bouncy spring suspensions to isolate the arm and platter from the lower deck where the motor was moored. Very succesful designs these were; very quiet. The weakness of belt drive is torque delivery. Many designs will exhibit speed variations as a result of static friction and dynamic friction caused by the needle in the groove. Some 'tables actually vary up to 2%, speeding up as the needle progress from outside to inside groove. Though it may be psychologically lifting to have each record end on an up tempo, it is far from accurate. Static needle/groove friction effects can be corrected with Servos. See Below. Maximum torque in a belt drive design is acheived by routing the belt around the full outside perimeter of the platter.
All early turntables and still a few today use the Hysteresis Synchronous drive system. This is a good choice for fixed speed designs for the speed is locked to the power line frequency. Thus, the drive system has a good source of regulation. Speed stability is good and drift is very low. The weakness here is the difficulty to produce variable speed.
DC Belt Drive
This is a belt drive system which uses a conventional DC motor as the drive source. The advantages here are cost, variable speed potential and reverse rotation. The weaknesses are great however. These motor systems suffer from temperature drift and load sensitivity. I.E. the platter will speed up over time and it will slow down under stylus load. This is not a serious drive system. It is usually found on budget DJ turntables. DJ's typically que between two turntables and rarely play anything over a 3 minute song. In that short time, the speed variations do not show substantially. But play a full album side and they will. After the DJ makes some money, these tables are always traded in for better direct drive turntables.
DC Servo Belt Drive
These systems use a sensitive regulator to monitor the voltage to the motor. There is marked improvement in long term speed drift and static stylus drag sensitivity. FG servo gives better absolute accuracy
FG Servo Belt Drive
Frequency Generator Servos or Tachometers, offer the most sensitive way to control a DC drive system. The Tachometer provides a direct readout of motor speed and thus is able to control the motor most effectively. This drive system offers the best control of both long term speed drift and static stylus drag friction. It cannot however respond fast enough to dynamic groove friction. But if the platter is heavy enough, the flywheel effect will deal effectively with most dynamic groove friction effects.
Platter Update Servos
Some turntable systems use an updating system which usually comprises two magnets mounted on the platter 180 degrees apart. As the platter rotates, the magnets trigger sensors. The information is compared against some internal calibrated reference and an adjustment is made to correct the speed. This system can work at slow speeds(33/45) and with heavy platters. But at fast speeds(78rpm) it can literally take minutes before the platter speed stabilizes. With a lightweight platter, the updates can cause platter jitter during correction. Many inexpensive direct drive turntables use this and some belt drives too. The Jittering can cause a smearing of hi frequency sounds softening treble and blurring detail.
Direct Drive General
Often confused with Rim Drive, Direct drive has the drive motor integral to platter. The motor therefore, rotates at the exact record speed. Since the drive power emanates from the platter's center of gravity, direct drive systems are very quiet. And since drive is direct, they offer excellent torque. It is easy to implement variable speed with this design. The key to the sonic quality of differing direct drive turntables is linked to the reference and control circuitry. See Platter Update Servo above.
Quartz Lock Direct Drive
The quartz crystal reference ensures essentially zero speed drift with very high accuracy. The control system will determine the sonic quality.
This is perhaps the best drive system available today. Not only is it dead accurate and stable. But the ability to correct for both static and dynamic load friction is uncanny. This, thanks to the frequency generator servo. Very few 'tables use this technology due in part to it's complexity and also patent infringement consideration. We are intimately familiar with one design the Technics SL1200 MKII. With this system, there are simply no speed variations. You can, for instance, rub your finger on the edge of the platter and the platter will hold perfect speed. We consider this to be a world class reference turntable. The economical price is misleading Since 1,000's are sold monthly worldwide.
Understanding drive methods should help you choose the right turntable for your needs. As always, weigh the needs you have today with the demands you may have in the future. This is probably the last turntable you will purchase. Weigh the features carefully and be sure to understand the retailer's refund policy in case you are unhappy with your purchase.
KAB Electro - Acoustics
P.O. Box 2922
Plainfield, NJ 07062