I've been having a thunk about this. (cross posted on several fora)
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An incomplete list of things that affect the sound of a motor unit (some due to stray vibrations, some due to noise and some due to speed stability, and some due to... ?):
-Main Bearing (quietness, lack of play, amount of friction)
-Platter Mass (speed stability, storage of stray stylus vibrations, isolation of record from bearing)
-Motor (inherent vibrations, torque, power, mechanical isolation, damping, speed stability, inertia)
-Drive Type - Direct, Belt, Idler, 'Indirect' [see Teres rim drive] (Coupling of torque between platter and motor, lack of slippage, elasticity, inertia, isolation of motor vibrations)
-Motor Control (Tacho/ constant voltage/constant frequency/magnetic pickup from platter [see Sony Biotracer] etc can cause subtle shifts in speed stability- the 'hunting' of servo motors for example)
-Plinth (sinking vibrations from the motor/bearing)
This is not to mention the rest of the things affecting the TT's sound:
-plinth (flexing between bearing and tonearm, isolation from external vibrations)
-tonearm (damping, resonance, alignment, quietness of bearings, lack of play in bearings, wiring [believers only], effective mass, length, type [unipivot, gymbal, servo linear or passive linear])
-cartidge (stylus shape, cantilever length, material, suspension type, type [MC, MM, MI, strain Guage], Inductance, resistance, wire material, no of turns)
-phono stage (matching to cart [resonant tank circuit here], transformers, RIAA match)
-platter mat (coupling/isolation of vinyl+platter, damping)
And the rest of the system:
-vinyl (quality, cleanliness, damage)
-amplification (including phono stage amplification)
The main problem with vinyl is the tiny wavelength of the sound recorded thereon- in the region of the wavelength of light (look sideways-ish at the grooves while in sunlight- you can see the spectrum, due to the variations in the wall acting as a diffraction grating). This, IMHO makes the main problem play in the bearings affecting the relative position of the cart relative to the vinyl and also makes very, very small variations of speed much more noticeable- not necessarily ans wow or flutter, but in terms of immediacy as the stylus slows the platter down while traversing a particularly dynamic part of the disc.
All this is IMHO, with what I think affects vinyl reproduction the most.
That it works at all is a constant source of wonder to me. That it sounds good completely befuddles me- it's a lump of rock being dragged down a plastic gutter, at a non-constant speed, with forces and vibrations all over the place.
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Personally, I lean towards the rim drive (indirect drive) system, with a damped, medium-mass platter (around 5-10kG), a heavy, fast spinning flywheel coupled to the platter using an idler-type thing, with the motor controlling the speed through a high-speed servo (IMHO, the servos on DD TTs run at too low a frequency) or fixed frequency controller, or possibly a low-speed servo that normally acts as a fixed-freq controller. A linear air-bearing tonearm and a rigid, damped plinth complete the system.
I just find it fascinating- trying to get a good balance of compromises in an imperfect world. And, of course, everyone's preference is different.
James
Don't just copy it though...
