New Turntable Advice

@lewm 

back when I was choosing between Lenco and Garrard, there seemed to be a strong consensus among 301 owners that an external motor controller worked wonders

I suspect the external motor controller concept for the Garrard 301 has died a natural death - like so many audiophile tweaks!  At the very least, I have never seen one offered in the stuff I read these days.

What I have learnt in the last few days is that the eddy current drag from the manual speed control might explain the so-called 'slam' of the table.  Once set, it applies a constant drag, against which additional drag from highly modulated passages is pretty insignificant.  Hence the music does not seem to slow down!

If the background drag was just bearing friction plus stylus friction, highly modulated passages would have a much bigger effect in proportion to the overall drag.

The use of an external motor control seems to require the eddy current mechanism to be disabled, so swings and roundabouts ...

An analogy might be a big V8 idling against the drag of a slush-box automatic. Switching the air-con on or off does not change the idle speed much.  But slip the box into neutral and then toggle the air-con and you can watch the revs change.

(In the good old days, US auto-makers like GM and Ford had Aussie production lines that stuffed big V8s into European-inspired sedans - mine would get up to city speed limits at idle)

I am not promoting the Garrard 301, just trying to understand why it is so highly regarded in some circles.  Now I have a plausible explanation of 'slam'

@richardbrand.

"Once set, it applies a constant drag, against which additional drag from highly modulated passages is pretty insignificant.  Hence the music does not seem to slow down!"

Precisely!. It will be slowing down but much less that if there was no predrag .

You described, much better than I, the efficacy of predrag. Of course the motor must be of good quality such that it doesn't become noisy when loaded thus. This is where torque ripple can be a problem as it is a function of current draw , torque output. 

In the Garrard's case the mechanism which sets the correct speed has the lovely positive side benefit of making the drive stiffer. The result, more startling transients, greater apparent dynamics. some may even call this "slam" My guess is that the technicians designing it knew about this

@lewm

What can affect speed of idler TT is wear on the idler wheel and stuff like that

Uniform wear on the idler wheel should have no effect on the platter speed - the idler just transfers its rim speed from the motor pulley to the platter rim.  This assumes it does not slip, and has no wear flat spots.  In normal use, the idler wheel is retracted until the table is switched on, which also mechanically engages the idler wheel via springs.

Provided the platter is not forcibly restrained while running, the hard rubber tyres on idler wheels should last for decades.

Wear on the motor pulley is more likely, though I think very uncommon in normal use.  Changing motor pulleys and idler wheels is straightforward unless you also want to change the idler wheel bearings!  The lower bearing housing is fixed with rivets made of unobtainium

@richardkrebs 

My guess is that the technicians designing it knew about this

By the time they got around to the 301, Garrard had been building turntables for about 35 years, starting with clockwork mechanisms.

The more I poke about, the more I appreciate the hidden engineering details.  For example the control knob for fine speed adjustment is mounted on the chassis, but the magnet it moves is pivoted on a sprung assembly.  To minimise unwanted vibrations between the two, the linkage is split into two, with an outer ring connected to an inner hub by three radial springs.

Turntables were just a sideline for Garrard, who for centuries had been the UK's leading jewelers, by Appointment to successive Royal Majesties!  Precision manufacturing indeed!

@richardbrand 

Yes, and if you want to advance the state of the art, dissect the older designs and take their learnings. Then apply them to the materials, manufacturing and tech capabilities we have today.

One example. A plain bearing should not have a highly polished shaft. It needs a tiny amount of roughness to ensure an even oil film. This is common in my industry, hydraulics, but is rare in the audio TT world. EMT knew this and when you look at the shafts of their classic TTs they appear dull...by design. This is engineering done properly. 

cheers.