The science of turntable design by jrlaudio - diyAudio
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Old 17th June 2013, 08:35 PM   #1
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Default The science of turntable design by jrlaudio

On vinylengine jrlaudio has posted a thread Some Thoughts on Turntable Design.
This is a must read. I never read a better discussion on the subject. To jrlaudio, please bring this discourse to diyaudio forum.
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Old 17th June 2013, 09:48 PM   #2
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Old 18th June 2013, 08:53 AM   #3
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Interesting that he gives the reduction in tracing error as the absolute % reduction in error, not the relative reduction. If tracing error was 10% and you reduce it by 0.1% who cares. But if it was only 0.3% to start with then its a worthwhile reduction.

That's tricksy presentation.
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Old 18th June 2013, 10:30 AM   #4
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Looking through it further there's some good stuff in there, but some contradictions based on assumptions. Like Gimbal vs unipivot. At one point he's saying speaker spikes couple best then later he's saying unipivots don't couple well to the armboard. Which is it, are spikes good or bad, where's the data to back up that assumption.

He goes on to rail on a certain arm manufacturer that uses damping claimed to reduce the resonant frequency of the arm, obviously headshell damping doesn't do this. But he misses the opportunity to state why if unipivots are less well coupled to the armboard as energy sinks, that a unipivot with a fluid well at the pivot end would be a great way of sinking energy from the arm.

He's certainly cherry picking.
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Old 18th June 2013, 01:19 PM   #5
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damping, unless it also adds mass doesn't change the resonant frequency, it reduces the Q.

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Old 18th June 2013, 02:04 PM   #6
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I meant reduce the 'level' of the resonant frequency, not the actual frequency.
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Old 18th June 2013, 02:04 PM   #7
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Quote:
Originally Posted by sq225917 View Post
Looking through it further there's some good stuff in there, but some contradictions based on assumptions. Like Gimbal vs unipivot. At one point he's saying speaker spikes couple best then later he's saying unipivots don't couple well to the armboard. Which is it, are spikes good or bad, where's the data to back up that assumption.

He goes on to rail on a certain arm manufacturer that uses damping claimed to reduce the level of resonant frequency of the arm, obviously headshell damping doesn't do this. But he misses the opportunity to state why if unipivots are less well coupled to the armboard as energy sinks, that a unipivot with a fluid well at the pivot end would be a great way of sinking energy from the arm.

He's certainly cherry picking.
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Old 18th June 2013, 02:53 PM   #8
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Originally Posted by bear View Post
damping, unless it also adds mass doesn't change the resonant frequency, it reduces the Q.

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That's not true.

Damping does reduce the resonant frequency, because it influences the rate at which energy can be transferred between the energy sinks on the system. Any resonant system consists of two or more energy sinks and a time dependent mode of energy exchange between them. The system will resonate at the frequency at which the energy exchange is most efficient. A tonearm / cartridge combination is a torsion spring / rotating mass resonator and the two sinks are the strain energy of the spring and the kinetic energy of the mass: when one is maximised (eg the spring is maximally stretched) the other is zero (the mass is stationary) and vice versa.

Mathematically, viscous damping can be represented as the presence of a damping force which is proportional to velocity so Fdamp = c.v .
The effect of the damping depends on the mass; there is a damping factor γ = c/2M where c is the constant of proportionality in the first equation and M is the system mass.

I will assume you don't want the partial differential equations that get from here to the result, which is that the damped resonant frequency is given by ωd = SQRT(ωo^2-γ^2)

Net result is that ωd < ωo, so in the presence of damping the frequency is reduced. It's not a huge effect, a damping factor of 0.1 will slow the resonance by about 5%.

A good reference is Damped Harmonic Oscillator at Hyperphysics.

Last edited by Mark Kelly; 18th June 2013 at 03:22 PM.
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Old 18th June 2013, 03:25 PM   #9
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γ in the above is html code for the Greek letter gamma, it's not displaying properly on my browser, hope it's OK on yours.

ω is OK, go figure.
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Old 18th June 2013, 03:51 PM   #10
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Quote:
Originally Posted by Mark Kelly View Post
Damping does reduce the resonant frequency, ...

Net result is that ωd < ωo, so in the presence of damping the frequency is reduced. It's not a huge effect, a damping factor of 0.1 will slow the resonance by about 5%.
Yep, wikipedia's article on Damping agrees, ωd < ωo for the underdamped case (0 < ζ < 1). Image attached.
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File Type: png wiki_damping.png (36.4 KB, 138 views)
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