What's the deal with idler turntables and do they have a place in modern HiFi?

I believe it is safe to say that maintaining absolute platter speed in the presence of an ever changing load (friction from needle tip) is physically impossible! One can try to minimize speed fluctuations - but will never eliminate them completely! 
Regulating designs need to measure a speed change first in order to correct the speed. 
Unregulated TTs like the Garrard try to minimize speed changes by employing high torque motors. The speed-torque curve of the motor needs to be as steep as possible to minimize speed fluctuations from a changing load. 
The over all flexibility of the drive system is another important factor. That's where idler drives have their strong side! 
The flexibility of any drive system can be measured by applying a defined torque to the platter while locking in the shaft of the driving motor. The resulting angular movement of the platter allows to determine the flexibility of the drive system. Belt drives with very long and flexible belts allow more platter movement - idler drive systems with direct coupled idler connection and stiff motor support allow very little angular platter movement with same amount of torque applied to the platter. 
These thoughts lead to the conclusion, that a high torque motor with steep speed torque curve plus a drive system with little flexibility between driving motor and driven platter is a must - no matter if speed regulated or not. Not all modern TTs follow these principles - but some do. And I believe that some modern designs are able to achieve these goals even without employing an idler drive arrangement. A short and rigid belt driving a small sub platter follows the idea of little flexibility in the drive system. In combination with a high torque motor the result will minimize speed changes from the ever fluctuating load caused by the needle tip ... 

Hi Cleeds,

I’ve seen this claim before, but never seen any proof of it. I’ve measured the speed of my turntable with it 22-pound platter as it plays an LP and never detected this sort of speed deviation.

I stand (partially) corrected ... primarily because I didn’t want to blow my own horn. I’ve seen stylus tracing induce speed instability on some turntables but not on Galibiers. Mass absolutely helps.

Cheers,
Thom @ Galibier Design

decibell

I believe it is safe to say that maintaining absolute platter speed in the presence of an ever changing load (friction from needle tip) is physically impossible!

Please explain how the speed of a 22-pound spinning platter can be altered by a stylus whose force is measured in grams.

Please tell us about the measurements you've made that support your belief.

Belt drives with very long and flexible belts allow more platter movement - idler drive systems with direct coupled idler connection and stiff motor support allow very little angular platter movement with same amount of torque applied to the platter.

There's no "angular platter movement" on my belt drive table. It's a VPI.

thom_mackris

... I’ve seen stylus tracing induce speed instability on some turntables but not on Galibiers. Mass absolutely helps.

Agreed! If the movement of a phono cartridge stylus affects the speed of a turntable, there is something very wrong in the turntable's design.

Stylus drag is surprisingly strong, platter´s very high mass helps but does not eliminate it.