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Tone arm improvements - no counter-weight (pseudo) magnetic levitation
Tone arm improvements - no counter-weight (pseudo) magnetic levitation
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Old 23rd July 2021, 11:10 AM   #81
hend1n1 is offline hend1n1  Singapore
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Join Date: May 2014
Quote:
Originally Posted by warrjon View Post
The EPA100 test was with two cartridges a Stanton 881s compliance 30 x 10-6 dyne at 10Hz and Technics EPC205 12 x 10-6 @ 100Hz. The Stanton in theory should not be a good match for this arm.

Here is a thread I created to document the EPA, the first tests were done with the EPC205 all others were the 881s.

Technics EPA100 modifications
Thanks great work!

Quote:
Originally Posted by warrjon View Post
The aluminium arrived today for the bearing housing (we are locked down ATM) Hopefully I'll have a working prototype in a week. This is going to be a 4 point arm in a gimbal arrangement, NO bearings. I have to grind and polish the pins, so there is a lot of work.
Re: ball races agree they are non ideal for tone arms.

With opposite point contact bearings I worry about thermal expansion loosening, pin point wears out quickly and may be damage prone. What's the 'best' contact surface material / geometry? And acoustic impedance diode / mirror effects? After all that's why we put spikes under speaker cabinets etc.!

So for the original run of 'Delta Inertia' spring arms I used off the shelf steel bearing pins (ground and polished hardened forgotten exact spec but mirror finish and tarnish resistant) housed in 'Oilite' (sintered brass to give slightly porous structure to hold medium viscous oil) bushes that I precision reamed to fit the pins. Very rigid with surprisingly low friction both due to oil film despite several mm2 of surface area.
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Old 23rd July 2021, 11:34 AM   #82
warrjon is offline warrjon  Australia
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Join Date: Feb 2011
Location: Terang Vic
The point of building this arm is to build a no-compromise tonearm with materials I have available.

I have 4 x sapphire Vee jewels for the pivot cups. These are designed to accept carbide pins, but I'm using tool steel ground at 55deg and polished to 1micron.

I completed the horizontal bearing housing today, pictured next to the Technics arm. This will be a very large arm. Built for rigidity.
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Old Yesterday, 09:00 PM   #83
spladski is online now spladski  United Kingdom
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Since the arm integrates the energy of the stylus, dissipating this energy becomes an issue since it excites resonances in the arm. One forum member reported improvements using a sorbothane counter-weight. There are also sand filled arms.

There is always a lot of focus on precision engineering but little on dealing with the shock wave that transits solids. The stiffer and denser the solid, the more efficiently the shock wave travels. If there was more effort to deal with that energy, less would be available to provoke vibration in bearings and elsewhere.

Magnetic levitation does not really address this issue and may make it worse, confining more energy to the wand.
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Old Yesterday, 10:30 PM   #84
warrjon is offline warrjon  Australia
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Join Date: Feb 2011
Location: Terang Vic
You are make an assumption I have not addressed tonearm energy. The most I mportant thing is the amount of energy reflected back to the cartridge.

This arm wand has variable wall thickness and taper to reduce refected energy. The removable headshell is made from the same section of aluminium and grain orientation as the wand.

It's made from 6061 T6. The plan is to make another from AZ31B magnesium.
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Old Today, 11:38 AM   #85
hend1n1 is offline hend1n1  Singapore
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Quote:
Originally Posted by spladski View Post
Magnetic levitation does not really address this issue and may make it worse, confining more energy to the wand.
'Mag-lev' geometry has gimbal bearings for vibrational energy to pass, same as most standard counterweighted arms do, so energy is not confined to the wand.

Indeed, mechanical vibrational energy routinely crosses over gaps in magnetic fields, that's how electromechanical devices such as motors work.

This 2003 forum thread explains how. Its about the Schroeder tone arm, that suspends the arm wand & counterweight on a wire tensioned by two attracting permanent magnets (i.e. no gimbal bearings):

“The magnetic field creates forces that act to damp the components of any vibrations that are perpendicular to the field. It does this by inducing currents, in the magnetic material - not the air, which are then converted to heat in that material (unless they are drained in an electric circuit of some sort). So, there is an energy coupling, but it's independent of the air.”
DIY Schroeder Tonearm???

This means 'Mag-Lev' geometry is even better at dissipating energy out of the arm wand than relying only on gimbal bearings or magnet gaps alone.
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Old Today, 02:16 PM   #86
spladski is online now spladski  United Kingdom
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My point was about dealing with the arm energy before it reflects or transits. The sand filled arm does this, though that approach is not elegant.

You can tell if the arm is damped by holding one end and letting the free end drop on to a hard surface like dense wood. If it makes a 'dink' sound, it is not damped. The sound changes as damping is added.

Last edited by spladski; Today at 02:18 PM. Reason: Typo
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Old Today, 04:22 PM   #87
hend1n1 is offline hend1n1  Singapore
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Default Acoustic impedance paths

Quote:
Originally Posted by spladski View Post
My point was about dealing with the arm energy before it reflects or transits. The sand filled arm does this, though that approach is not elegant ... a 'dink' sound is not damped ...
My point was to sink arm energy i.e. "a 'dink' sound" into the turntable before it reflects or needs damping. Rigid large surface area 2 degrees of freedom gimbal bearings and Mag-Lev geometry provide two low acoustic impedance transit paths for arm energy into the turntable mass.

(Note: Schroeder arm does not directly achieve this as it forfeits the energy path offered by gimbal bearings and instead only uses a magnetically tensioned flexible wire to partially restrict some of the 6 possible degrees of freedom and act as magnetically coupled low acoustic impedance path for arm energy into the turntable mass).

There are two sides in an electro mechanical transducer. The moving side. And the stationary side.

If the mass of each side of such a transducer is the same, then both sides will move the same distance / acceleration / velocity in reaction to grove modulations, and no signal will be generated.

This implies one side should have much more mass than the other, so we can measure the differences in relative distance / acceleration / velocity that groove modulations cause between these two masses.

If instead of sinking arm energy into the turntable, we dump it locally in the much lower arm mass, some groove energy will be locally turned to heat in that mass, and due to conservation of energy this must by definition mean its acoustic impedance is higher (i.e. absorbing) than without heating.

This means damping causes the cartridge body & arm wand mass (the heavy side of our electro mechanical transducer) to move more (its softer that's how it heats up) than if the arm wand were instead stiff and transmitted its energy across the bearings into the orders of magnitude larger mass of the turntable, where the same quantity of energy would cause a lower temperature rise for the same energy inputs. In turn causing lower mechanical disturbance i.e. better signal dynamic range.
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Old Today, 06:59 PM   #88
spladski is online now spladski  United Kingdom
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The material of the arm is unchanged from what ever it happens to be. If the arm wand has a sound, then this represents spurious deflection. Adding damping increases the overall mass.

A tapered arm with no damping will not perform any better than a straight arm with damping in my view. SME 3009 uses balsa wood but the implementation is poor because the wood has a square cross section giving minimal contact with the round cross section. Sinking energy just leads to more mass further in the structure without reducing the magnitude of the initial spurious deflection.
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Old Today, 08:47 PM   #89
hend1n1 is offline hend1n1  Singapore
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Quote:
Originally Posted by spladski View Post
...without reducing the magnitude of the initial spurious deflection.
Newton's first law tells us the "initial spurious deflection" has equal energy but in the opposite direction to the initial wanted deflection of the stylus & cantilever.

As such "reducing the magnitude of the initial spurious deflection" i.e its energy is not possible. We either:

a) convert "spurious deflection" energy into heat locally in arm wand / counterweight mass (i.e damping) where deflection magnitude of the mass will be greater compared to;

b) trying to transmit the energy to the much larger mass of the turntable, where deflection magnitude will be proportionally less, i.e. the initial wanted deflection of the stylus & cantilever will have a reaction interface that moves less = greater signal to noise ratio, increased dynamic range and reduced distortion.
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