Eminent Technology ET-2 Tonearm Owners

thoughts on two or three open questions & i-beam
@richardkrebs:
- I'm looking forward seeing pictures of your bearing tower!
- I did not yet try the elevated bearing position with the original ET bearing tower, as there will be drawbacks stiffness-wise because of the (practical but) rel. narrow tripod feet of the design.
- I think a 20 degree angled piece of good wood (with a slit / hole for the cartridge nuts) would do the major part of adaptation.
- arm lift adaptation could be tricky.

- magnetic damping: "My" magnets are kitchen magnets. They are round and have a simple collar-shaped pole piece which focusses and intensifies the flux around it. This will make a considerable difference in efficiency regarding damping compared to a normal "open" magnetic structure.
The magnets need to be placed really close, not more than 1mm on the closest place. The flat magnet shape vs round bearing tube  is obviously not ideal. My feeling is that it kind of cuts the peak of the resonance. It sounds and feels a step more stable - i don't like the sound of too much damping anyway.
It's difficult to extrapolate Bruces experiences & comment to different implementations without exact descriptions.

- i-beam: I checked the rotation pendulum formulas to be correct about this: If one doubles the length of the i-beam one halfs the needed weight, because of double the leverage. But the inertia grows with a square factor: 0.5 of the mass x 2^2.This results in double the inertia and 0.7 of the resonance frequency. 
The whole double mass double spring system of the ET is very ingenious but also quite complex. It is a 4th order resonant system instead of the usual 2nd order one. With one short attempt :-) i did not yet successfully find the correct CLCL model for simulation.In the most simple view there is an i-beam resonance and a bearing tube/cartridge resonance, with a zone where the i-beams spring and the cartridges compliance work in series (the i-beam springs damping can control the cartridge/bearing resonance, if the i-beam  resonance is well chosen... ;-).Problem is seeing / knowing what each one does. Simulation? Empirism?... :-)
My techno-intuitive thought on this is:
- The lower the resonance of the i-beam, the wider the frequency range over which the bearing/cartridge resonance can be controlled by it.
- the higher the resonance, the closer together both resonances and the more resonant interaction instead of control.

Does Bruce have the model? Or should I ask my more MATLAB-experienced son?

Chris,
When you get a chance, I sent an off topic question to you by pm a few days ago.
Harry

@pegasus. 
The bearing tower/manifold is in the concept phase at present.
Some features are almost locked in.....
> The air feed into the manifold will be like a flattened funnel to slow the airs velocity before it gets to the bearing sleeve. The original jet of air has to cause turbulence where it first contacts the sleeve. This funnel will probably be lightly packed with wool. The manifold will be undercut to allow for a multi wrap of filter paper around the sleeve to further smooth/ equalise air delivery to the capillaries. The manifold will also have, as per my current version, the cap screw and shim metal locking arrangement between the manifold and sleeve to lock the two o'rings.
> The manifold and tower will be a male/female VEE slider arrangement with a single capscrew locking them in place. This will ensure that the manifold stays exactly horizontal at all VTA positions. It also gives a very solid join between the two parts. A vertical capscrew between the two providing for VTA adjustment. The slider will be straight. 
> The tower will have one fixed, parent material, foot and two PVC adjustable feet. The PVC feet provide for alignment of the tower to the platter surface. The single, parent material, foot provides a "virtual" single point of contact with the arm board.
> Parent material will be either zinc or aluminium and will have machined galleries containing damping material.

This is a long term project. I will keep you posted.

Re radically raising the arm to 20 degrees. I get the stiffness issue. Would the wedge idea work as desired? Do we not want the vertical pivot point to be in line with the cantilever. Maybe I missed something in your initial or your latest discussion  on the topic.  

cheers.       

Thanks for the captivating infos!
- Although I agree on the idea of eliminating turbulence - isn't damping it not also introducing an additional flow resistance which will upset the  "balance" of implicite air flow resistances from air feed via calibrated air beaing openings to bearings air film? Personally I'd be very (self-) critical when changing this, I like to be double on the safe side about changing such crucial areas (at least with my limited understanding of air bearings).
In fact I'd contact Bruce about his thoughts regarding keeping the correct pressure on the distributed points/air openings when introducing additional flow resistance.

- raising the arm bearing: I meant lifting the back of the arm, and correcting with a 20 degree wood wedge between arm and cartridge (the wedge pointing to the front of the arm),

Pegasus

My techno-intuitive thought on this is:
- The lower the resonance of the i-beam, the wider the frequency range over which the bearing/cartridge resonance can be controlled by it.
- the higher the resonance, the closer together both resonances and the more resonant interaction instead of control.

Does Bruce have the model? Or should I ask my more MATLAB-experienced son?

Pegasus
The model for the original I Beam was in part influenced by business priorities. We know this. This limitation is no longer there.  

Why don't you send your theory and questions to Bruce direct and let us know what he says ?

brucet (at) eminent-tech (dot) com

I am interested in your, John and others impressions of Bruce' Long I Beam.

Harry and I seem to be in sync.
Also, still am interested in seeing pictures of your dual path wiring setup.

Cheers Chris