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ohfourohnine
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Drive Belt Cantilever alignment.

Been scratching my head about this one since December. Art Dudley moves the drive motor on a Linn TT so that the path of the drive belt is aligned with the axis of the cartridge cantilever and gets improved musicality and higher output. Except for Roy Hall's TT's, very few turntables with good reputations subscribe to that design approach. Given that it worked at least once and is simple and cheap, why isn't this approach more popular? Obviously, I'm missing something.

Buddha
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Re: Drive Belt Cantilever alignment.

Hi, Cheapskate!

I'm still trying to wrap my head around the "why" of this, myself.

I seem to recall the the Empire turntable line used this configuration.

I have a Michell Gyrodec that I did this with back in the late 80's. I'll try and get a pic to put up.

I gotta go draw myself some picture of the different things that line up in different ways and get a better idea of what's what.

ohfourohnine
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Re: Drive Belt Cantilever alignment.

You're right about the Empire, Buddha. The cap which covered the drive motor was at the left front (7 o'clock). The Roy Hall MMF - 7 and 9 also follow that configuration. ProJect, VPI, and Clearaudio, to name a few avoid that 7 o'clock position and go to 9 or 11.

Scooter123
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Re: Drive Belt Cantilever alignment.

From a mechanical engineering standpoint, the only effect of doing this would be to place the load points for the platter spindle bearings in alignment with the toanarm pivot. Considering the grade of the bearings used in most quality turntables this really shouldn't have any effect on the sound quality. Frankly I think that someone may have bought into some marketing hype.

Personally, I think that a drive motor should be placed as far as possible from the path of the playback cartridge and the wiring for the tonearm. That would reduce the exposure of the cartridge and wires to any magnetic field produced by the motor. So I would put the motor somewhere in the left rear of the turntable.

Buddha
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Re: Drive Belt Cantilever alignment.


Quote:
From a mechanical engineering standpoint, the only effect of doing this would be to place the load points for the platter spindle bearings in alignment with the toanarm pivot. Considering the grade of the bearings used in most quality turntables this really shouldn't have any effect on the sound quality. Frankly I think that someone may have bought into some marketing hype.

Personally, I think that a drive motor should be placed as far as possible from the path of the playback cartridge and the wiring for the tonearm. That would reduce the exposure of the cartridge and wires to any magnetic field produced by the motor. So I would put the motor somewhere in the left rear of the turntable.

Maybe Furutech can come up with a motor demagnitizer.

Jan Vigne
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Re: Drive Belt Cantilever alignment.

VPI didn't always opt for the left hand pull of the driving belt. In the early versions of the TNT three pulleys were employed in order to reduce the wobble of the main spindle. Two were dummy pulleys with the right hand pulley in line with the direction of tavel of the groove in relation to the stylus. The second released the belt at a right angle to this plane of travel. And the third was attached to the motor shaft which was located to the left of the platter, at a point farthest from a pivoted tonearm's center and in the same plane of travel found in a linear tracking arm (think master cutting lathe here). This nine O'Clock motor location slightly reduces induced noise entering the tonearm but somewhat forgets the dynamics of the pivoted tonearm (think skating here). The three pulleys on the TNT were (partially) meant to reduce the wobble of the spindle as the motor met with the constantly changing demand for increased or decreased torque/pull due to warped or off center records or dynamic passages in the recording where stylus drag momentarily slowed the velocity of the platter. Damn that variable pitch to a record groove! You trade a few minutes of increased play time for additional distortion all along the stylus path.

When the platter is being driven by an elastic belt the benefit of this indirect drive mechanism becomes its own worst enemy as the spindle momentarily returns, or at least makes the attempt to return, to an "undriven" position when platter velocity changes only to be yanked back toward the motor position once the belt slack tightens and the motor/belt is once again driving the platter at a constant speed. The result of this never ending driven/undriven wobbling action in the main spindle is similar to a slightly off center disc and results in image instability, loss of fine detail and ambience, increased disc noise as random movement is generated in the stylus assembly as it follows this back and forth wobble of the groove walls along with other various problems associated with the stylus constantly "hunting" for the proper orientation in the record groove. (And you thought anti-skating was the worst thing you had to worry about!) The wobble of the platter also affects how the belt initally makes contact with and releases from the edge of the platter which will affect the sound quality of the table giving a poorly defined sense of speed consistency which again results in loss of image specificity; tansient attack and decay; bass response, extension and impact; decreased transparency, etc. In short, many of the same "hunting" problems associated with a poorly implemented servo driven motor become apparent, though to a lesser degree, when the motor pulley, belt and platter are not held firmly and consistently in a single consistent spatial relationship to each other.

The solutions to these problems are supposedly presented in increasingly sophisticated designs which try to resolve the many dynamic conflicts which occur in LP playback systems to a more finite level. Throw in a bit of micro-vibration set in motion by the mechanical and air borne feedback that inevitably results from playback and you can see how rapidly the difficulties mount for a turntable designer. Unfortunately, many of the individual solutions to one problem are resolved only by presenting two resulting drawbacks to the whole mechanism. In a LP based system isolation and coupling are antithetical comrades to each other as are motion and stability. Toss in the many variations possible in associated tonearms, cartridges, support systems and so forth and the tolerances a designer must allow just to build a marketable high end product make the task seem all but Herculean.

VPI seems to have abandoned the dummy pulley operation in favor of an umbilical connection to an outboard motor which drives a slave pulley which in turns drives the platter. This places one more buffer between the motor/belt/platter assembly which, as I'm sure you can guess, solves one problem while presenting other problems of its own. And, unfortunately, in high end audio, and particularly in high end LP playback based audio, the lesser of two (or even three) evils is typically resolved at expotentially increased monetary input.

The wobbly spindle/platter conundrum results from the basic problem of choosing whether to drive the platter with a high or low torque motor which has been a Catch-22 ever since Mr. Vilchur disconnected the platter's direct connection to the motor over fifty years ago. Ever since that time most belt drive table designers have chosen a fairly low torque (at speed) motor for its lack of affect on bearing wobble - among other things. A very low torque (at speed) motor won't affect the spindle wobble as drastically as will a high torque motor for obvious reasons. However, as you can estimate, a very low torque capacity will affect the ability of the motor to turn the platter at all, and most certainly from start up. (Anyone remember the Connosieur table from the 1970's with the kick start on/off switch? A simple solution to a complex problem.) The desire for a high torque motor at start up which then becomes a low torque motor at speed has resulted in some interesting placebo devices over the years, not the least of which is a massive platter which "acts" as a flywheel. The problem with such devices is how much start up torque is required to get them to speed and how elegant (read expensive) will the solution become. On a smaller "micro-wobble" level, AC line conditioners on turntable motors have much the same desired effect by minimizing the wobble of the motor pulley caused by incoming line problems.

To think of how the platter/stylus/cartridge motor assembly must properly fit together you have to imagine that at every nano-second in time the stylus is sitting in the groove as if all things were frozen in time and space, not moving at all. But, of course, the stylus assembly must be driven by the movement of the record groove in order to produce a subsequent output voltage. If that were not the case, LP playback would be quite simple and a twenty dollar table would sound as good as a $100k table. Unfortunately, at every nano-second in the existing dynamics of LP playback, the previous nano-second of stability is lost and the next nano-second awaits the dynamic potentials and failures of the system. Possibly in Bizzaro World these potentials and failures are not a problem.

In what amounts to a Bizzaro moment, the realization that microscopically tight tolerances are not always the desired goal of a shaft which must turn in a sleeve, the "sloppy" bearing is a stroke of genius! Consider for a moment that tighter and tighter tolerances do not allow for the realities of movement (read friction and its associated partners) and can actually work against the goal of low noise and smooth, consistent motion. If the shaft is to remain fixed in place along with the platter and stylus in our hypothetical ideal, tight tolerances work fine, once things are set in motion we once again have a counter intuitive problem and solution.

One of the few designers to really address the constantly changing dynamics of the wobbly platter/spindle/motor/belt/ stylus/cartridge assembly is Bill Firebaugh who designed his Well Tempered table to have a spindle that is loaded only when actually playing a disc. Check the Well Tempered web page for more information regarding the problems facing a designer when confronted with the micro-wobble of the platter. The Well Tempered Table was, to my knowledge, the first table to incorporate a weighted, detached motor assembly in an attempt to achieve one level higher isolation, and in turn coupling, from the dynamics of the motor's own inability to drive the platter smoothly without introducing additional noise. Yet, as always seems to be the case, Firebaugh's solution is merely one more solution which results in compromises in other areas of performance; a one for you, two for me affair. Still the WTT solution to spindle wobble is unique and ingenious.

Buddha
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Re: Drive Belt Cantilever alignment.

Hi, Jan, thanks for a great post.

I, too, keep thinking about the Well Tempered approach. It truly is/was genius.

I also thought about the VPI triple belt set-up, but I admit that it never caught my ear, and didn't make sense to me, given what we acknowledge as "spindle play."

With three belts, I get the image of the spindle trying to be held "suspended" within the tolerance spaces of the thing in which it sits. It seems much less secure or "dependable" in that configuration that it would with just one direction of belt pull.

Conversely, as you mention with the WTT, the spindle is allowed to "make up its mind" quite early where it will be, and ends up, ideally, mostly in one place.

With those additional flywheels on the VPI, my mental picture is that of the spindle being pulled in three directions by three elastic belts. I always thought that would make for more wobble rather than less. At any given moment, the physical forces at play could send the spindle in who knows how many directions following multiple vectors.

I'm not even so sure I dig the sound of the VPI with that "middleman" flywheel that drives the platter as the flywheel is being driven by the motor.

Drive belts are elastic, so what benefit would it be to have one belt from the flywheel see the platter and have some elasticisty in its reaction, and then also face another belt-interface going back to the motor. It seems you would lose more by interfacing all these compliant factors than you would gain in decreasing vibration.

Oh, well.

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Then, platter mass becomes something to talk about.

High mass = more inertia = harder to be altered by the stylus/groove interaction; but it also makes it harder for the motor to make up for any effect of the stylus/groove interaction.

Maybe more massive platters make for smaller variations with a longer duration of effect?

Lighter platter = less inertia = more easily influenced by stylus groove interplay; but more readily "corrected" by the motor.

Light platter = larger amplitude of change, but with shorter duration and a greater relationship between motor quality and speed stability.

A lighter platter seems like it would "couple" the motor to the platter more instantaneously, though.

How about a light platter with a massive motor?

I'm trying to think of an example.

Then, to keep going, just how compliant do we want our drive belts to be?

Oy, it goes on forever...which is great fun, eh?

Jan Vigne
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Re: Drive Belt Cantilever alignment.

You've managed just a few examples of the "if I give you this one benefit, I'm also going to take away these two desirable qualities" school of audio. It is all about tradeoffs. But the drive belt orientation is not snake oil.

ohfourohnine
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Re: Drive Belt Cantilever alignment.

Jan: Thanks for the great response. You've cured my headscratching and allowed me to go back to listening to the music. That's why I bring gear related questions here when I find them running around in my head.

Buddha: The lightest platter I ever encountered was on a B&O linear tracker they marketed with some success about 25 years ago. It was stamped aluminum about as thick as a credit card. Light weight overall appeared to be the design goal for that TT. Unfortunately, despite its bold engineering approach, that TT didn't beat even the modest little AR for overall performance and musicality.

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