New Turntable Advice

@lewm 

And as you adjust V to regulate speed, torque goes down, until at some point torque disappears and the platter coasts to a stop unless braked

The Garrard 301 does not adjust speed by changing V (Voltage?).  Instead, drag from eddy currents induced in an alumium disk are adjusted by moving a permanent magnet.

A separate mechanical brake which pushes a felt pad into the rim can stop the platter about as quicky as the motor can start it - in under a second.

While playing, the motor keeps the platter spinning by supplying enough torque to overcome main bearing friction, idler wheel friction and that eddy current drag operating on a disk spinning at 1,000-rpm.  Oh, there is the friction of the stylus which might be around a gram or less?  Vinyl is pretty slippery.

You can adjust the drag and immediately see the effect on the strobe markings on the rim of the platter (extra cost when new) but I for one can see no slowdown effect when the stylus is lowered, nor speedup as it traverses to the record centre.

Similarly, I can’t see extra stylus drag when playing highly modulated music even doubling the normal friction drag without ejecting the stylus from the groove!

For @badbruno the as-new specifications are

• Wow: less than 0.2%
• Flutter: less than 0.05%
• Rumble: almost non existent

These are far worse than for example the direct drive $300,000 Wilson Benesch GMT One table where they are unmeasurable!

I am just curious as to why the Garrard 301 seems to be so highly rated!  The fast start and stop times may be one reason, and the lack of any (need for) feedback for speed control may be another

@richardkrebs 

I have never understood the naysayers criticizing DD TTs and their use of feedback yet they happily listen to amps with local and global feed back...its the same stuff..almost

Is it?

An ideal turntable should rotate at constant speed.  An amplifier that produces a constant output would be boring in the extreme - you would hear nothing!

Amplifiers produce a highly variable output voltage which ideally is a multiple of the input voltage - the straight wire with gain ideal.  It is logical to compare the input and output voltage and apply corrections, whether described as feedback or feedforward.

For feedback or feedforward to work, there first has to be a measured discrepancy.

You argue the case very well for direct-drive versus belt drive, but I don't think you covered idler drives, in particular those with no feedback for speed control?

RB, Look up “effect of AC voltage on torque of an induction motor”.

@richardbrand

Re feedback, IMO it is fundamentally the same. We are comparing an output to an input and correcting if needed. Sure in a TT we are looking for a constant output, while as, you say, in an amp we are looking for a constant multiple of the varying input. But in both cases we are correcting, if necessary, a difference in output compared to input. I think that effectively we are saying the same thing.

I have very limited experience with idler drives. so temper what follows with that in mind.  

This is how I see it. If the idler is using a synchronous motor, there is what could loosely be called feedback in that the rotor is following the stator's rotating field synchronously. The rotor lags behind this field in phase but travels at the same average speed. If a higher torque demand event occurs, this phase angle increases a little in response but on average the rotor still rotates at the desired speed. Reduce torque and the angle decreases but, on average it still rotates at the desired speed. The rotating field is thus compliant. So microscopically this angle is modulating about the mean torque demand which also modulates the current demand. The result is tiny momentary speed changes. The platters inertia helps smooth this but does not totally eliminate it. 

The same principle applies  with a BD or DD. The drive is preconditioned to the present torque demand and the energy stored in the platter is fixed, at that moment in time. Take energy out and the platter must slow down because all drives are in some way compliant, feedback, if used, is not instant and the platter's moment of inertia is not infinite. The core difference being a BD has an extra compliant link in the drive chain. You could argue either way whether an idler wheel is compliant or not.

Years ago I observed this vicariously by scoping the motors current demand in a Goldmund Studio. It was not steady when simply spinning a platter and was less steady when playing a record.

You will not see this speed change by looking at a strobe. And the GMTs speed errors are measurable. 

Cheers.

@badbruno I am one of the demographic you have discouraged from posting, as Subjective Analysis is my thing, as for some reason I am recognised having a discerning ear and a system that pretty much reveals the nuances as each layer of the Onion is revealed.

I know lots of Techie' Info, but am not at liberty to discuss it publicly it is not my IP.

Other who you encourage to partake, are in the larger volume with a business interest and can only be limited with their IP, how far that extends beyond Manufacturers Published Data and investigation of those performance measurements I can't forecast.

I don't see much more evolving as there are lookers in, that like to steal ideas for their own remuneration purposes.

I can be easily wrong though, back to my Earl Grey and Cucumber Sandwich.