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Old 23rd July 2006, 12:48 AM   #1
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Default More unipivot questions...

Hey all,
I'm still planning on building a unipivot arm, and after some reading I had some ideas. The audiomeca and graham arms claim to be "perfect mass" or "neutral balance" -- is this, like it sounds, too good to be true? It seems relatively simple; set the pivot point at the arm's center of gravity and VTF is constant no matter the angle.

I was thinking that by having the pivot descend from the arm, one could use a low-slung counterweight for lateral stability while retaining constant VTF, if the CG could be made exact. This way, the CG could also be at record height for all the benefits there.

Also, for a while I've been curious about making a "balancing bird" arm, a two-pronged arm based on this:
Click the image to open in full size.

It seems that the unused "arm" could be used for both lateral stability and anti-skating, with an adjustable "cursor" a la the Mayware, for example.

What do folks think? I'm a total n00b and not versed in physics, but these both seem like workable ideas...
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Old 23rd July 2006, 08:29 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
for unipivot arms, if the CoG coincides with the pivot, then there will be no stability in your arm.
Theoretically it will rest at any angle.

To be stable the CoG must be below the pivot and then gravity provides a restoring force as soon as the arm is forced away from it's "at rest" position.

Further, the CoG must be moved forward towards the record contact point to give some measure of stylus force. To avoid this a spring /magnetic system can be arranged but the forces must be balanced to ensure a couple does not force the arm out of alignment.

A high level of rotational inertia will help prevent the cartridge twisting when record playing. This dumbell weight is better at the cartridge end to reduce resonances.

Finally, for a low level of effective mass the rear weight should be compact and VERY close to the pivot. A very dense material here will help considerably. Lead is cheap to obtain and shape. Tungsten is denser but other disadvantages may conspire against you. Osmium or depleted uranium might be considered the ultimate.
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Old 23rd July 2006, 10:30 AM   #3
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Quote:
Originally posted by AndrewT
Hi,
for unipivot arms, if the CoG coincides with the pivot, then there will be no stability in your arm.
Theoretically it will rest at any angle.

Right, but that seemed to be beneficial according to Graham? That way there wouldn't be extra force put on the stylus when warp riding...

"The third, and most desirable system for tonearms, is Neutral Balance. With this system, the pivot point and the CG of the moving system are in the same plane. When the arm is raised or lowered, there is no opposing force trying to return the arm to a rest position; the pivoting system doesn't really know or care if the stylus is at the record surface level or a half-inch above or below it; as a result, there is no opposing force to the arm as it is traversing record deflection during play. The only downward tracking force is that of the adjustable counterweight, which remains a constant. --"

http://graham-engineering.com/_wsn/page2.html

They use magnets to stabilize it laterally, although from their pic it doesn't seem like it would help when the arm was in any position except rest.

I also don't really get how there can be downward tracking force from the counterweight in a neutral balance system. Wouldn't that make it a stable balance?

Thanks for the help and again, please excuse my n00bishness...
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Old 23rd July 2006, 11:15 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
just read about the Phantom. Thanks for the link.
I called it unstable, Graham calls it neutral, he's right.
But my observation still stands. It will come to rest at any angle.

Phantom uses magnets to hold a horizontal pivot axis.
A contact system (bearings/knife edges) will have some compliance and most arm strive for a low compliance (high stiffness).
The magnet system MUST have a high compliance and would lead to very inaccurate tracking of the recorded information.
Phantom use heavy damping to decrease the dynamic compliance and try to keep the arm close to the magnetically defined rotational axis. This may sound quite good. Lots of arms use damping at various levels to improve performance.

Now to your counterweight query.
Starting from neutral balance with the COG of the counterweight at the same height as the record contact point and also at the same height as the unipivot contact point. That's three theoretical points in exactly a straight horizontal line. Screw/adjust the counterweight towards the pivot along that horizontal straight line and you have applied the tracking force.

However you no longer have a neutral system of balance.

If the stylus is lifted to a new height the literature claims that neutral balance keeps the force at the stylus the same.
But in the phantom with the three points mentioned earlier all exactly in a straight line as the arm is tilted, the horizontal distance of the arm COG moves closer to the pivot. The complementary horizontal distance from the stylus to pivot also reduces exactly in proportion. You may think that neutral balance has been maintained, but no. The critical distance for the stylus is the inclined length from pivot to record contact point and the critical angle for the stylus is referenced to the now inclined line from unipivot to cartridge. The stylus to unipivot distance remains as was before tilting. The ratio of stylus to unipivot distance referred to arm COG to unipivot distance does NOT remain constant for various angles of tilt of the arm.
For movement of the stylus while tracking information from the record the phantom arm does not achieve neutral balance.

I have not thought through the complete mechanism, but I suspect that a spring or magnetic system for stylus force may be able to use neutral balance and achieve constant stylus force as erroneously claimed for the phantom, but I further suspect that the mechanism will be complicated to achieve constant force at varying arm tilt and that the system may introduce added friction that is unavoidable.

Will these errors in Graham's claims make any significant difference to the sound coming off the record? I don't know, but maybe a Mk2 is coming along to address the factual errors in their claims.
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Old 23rd July 2006, 12:18 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

When building an arm yourself it is best to take the path of least resistance.

KISS, keep it simple stupid. pivot point = CofG is not simple, it is stupid.

Grahams claims are essentially waffle, they are not as important as they imply.

All AT's points are very pertinent, what you want is stability.

I'd copy the Mayware formula 4 arrangements but add a lateral
outrigger to the base of the bearing hub. Note the MF4s clever
use of the rear counterweight to also set lateral balance.

/sreten.
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Old 23rd July 2006, 12:41 PM   #6
EC8010 is offline EC8010  United Kingdom
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C of G must be below pivot point to provide a restoring force to keep the arm level. But not much below. Have a look at how I approached the various problems Unipivot pick-up arm
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Old 23rd July 2006, 05:33 PM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by EC8010

C of G must be below pivot point to provide a restoring force to keep the arm level. But not much below.

The centre of gravity is fractionally below bearing height to maximise the frequency of rocking (which makes it easier to damp).

Hi,

It is one approach, but that doesn't mean others are not valid.

Anyone remember the old Mitchell Transcriptors unipivot ?

My opinion is a unipivot should have high torsional inertia, effective
mass in the three modes,vertical horizontal and twisting should be
diffrent and the twisting effective mass should be highest.

Not that it has to be like that - I just think it should be.

/sreten.Click the image to open in full size.
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Old 23rd July 2006, 06:15 PM   #8
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But we're saying the "neutral balance" approach isn't valid? Or that Audiomeca and Graham aren't true NB systems. Is there any benefit to what they do vs. Mayware, Hadcock etc.? I was hoping to do something a little bit outside the norm...
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Old 26th July 2006, 04:00 PM   #9
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Hi AndrewT,

Could you elaborate what are the of disadvantages Tungsten ?

THanks
IK
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Old 26th July 2006, 05:35 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
Tungsten is expensive, and may be difficult to machine.
Finding a supplier may also be difficult particularly in the size and shape for a counterweight or two.
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