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In Reply to: RE: the benefits of a transmission posted by user510 on August 14, 2007 at 21:36:29
but it would seem that the "idler" provides no "mechanical advantage" by itself. If it adds anything at all it would be as a coupling device. As the outer rim of the "Idler" engages both the motor capstan and the platter rim, the "idler" itself has a 1:1 "gear ratio", so to say.
Any "mechanical advantage" in an idler drive TT is a function of the ratio of the motor capstan diameter vs. the diameter of the driven rim.
Nobody here but us chickens.
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Follow Ups:
The mechanical advantage is the simple ratio between the driving pinion (pulley) and the driven platter. This we also see in a belt drive system. The "benefit" of using a simple transmission, such as in an idler driven turntable, is in the resistance to the minute slowing action caused by modulating stylus drag. This is why idlers tend to offer a more dynamic reproduction. Just like the transmission in your car when in first gear, it only wants to go forward.
-Steve
user510's system
as its ratio is 1:1.
Nobody here but us chickens.
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The final drive ratio is the circumference of the platter surface interface divided by the motor rpm and the drive pulley circumference. So even though the tire itself is driving 33.33 rpm a higher rpm motor should exhibit better resistance to stylus drag than a lower rpm motor of the same torque rating.
The devil is in the details....
the "idler" is just what it calls itself, an idler. Typically it is used in a gear train scenario to change the direction of rotation of the "driven" wheel (platter).
Repeating myself here, (sorry), the benefit from having a few wheels connected up within a driving system is the resistance toward any minute slowing action caused by modulating stylus drag. Hence the more dynamic musical reproduction of an idler turntable compared to any other type.
-Steve
user510's system
An idler provides a rubber gripping surface so that the motor can grip and drive the platter. If the rubber surface was part of the motor drive pulley or the platter, then speed would change as the rubber wears or compresses, but by making the idler out of rubber, and the drive pulley and platter out of metal, the drive ratio and hence speed, remains constant, despite the vagaries of the rubber idler size over time
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You are correct as the o-ring on the Verus motor wears the speed will indeed change. However, the o-ring is fairly hard and even with years of use the wear will be just a few thousandths. That means a net speed change of 0.0017%. Not a big issue.
I understand that the diameter of the idler wheel does not affect the speed of the turntable. I also understand that as the O-ring on the pulley of the Verus motor wears its outer diameter will decrease thus changing the speed of the platter. The Verus controller box allows a person to change the frequency of the signal going to the motor. Thus, platter speed can be set using a strobe disk. (Since platters have different outer diameters some method had to be built-in to allow most users to use the Verus motor on their particular brand of turntable.) Once the speed is set via strobe disk you can change to 45rpm or 78 rpm with the flick of a switch. Granted once or twice a year you may want to get the strobe disk out and check to see if the speed is still accurate. Some people may find that a major inconvenience. Personally I would not. I have to use the speed control on my Garrard 401 to reset speed everytime I change from 33 1/3rd to 45 or from 45 back to 33 1/3rd. I find that more annoying, since I play 45 rpm LPs fairly frequently, than having to get out the strobe disk twice a year to check speed accuracy.
Am I missing something in your objection to having to compensate for the wear of the O-ring by having to get out a strobe disk twice a year? Heck, because of my audiophilia nervosa, I used to have to do that with my VPI TNT-5 with SDS.
driving the platter directly from the motor. What I said is indeed the reason it was done that way in old idler tables like the Garrard.
In summary
- it provides the metal/rubber interface needed for grip
- it allows the dimensions critical to accurate speed, i.e. pulley/platter diameters, to be machined in metal and hence much more accurately than in rubber (this last is more important on models without speed adjustment capabilities)
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