Eminent Technology ET-2 Tonearm Owners

Richardkrebs

Thou doth protest too much, methinks

About what exactly.
You raised the issue of bearing stability and the placement of magnets. The mass you have added to the bearing tube destabilizes the air bearing. My question to you as to whether you have measured the reduction in dynamic stiffness in the air bearing as a consequence of your modification and how you intend to remedy this remains unanswered. You encourage people to try your modifications. These modifications could damage expensive cartridges and records. Are you inferring that these concerns about your modifications should not be raised.

Dover
Adding mass to the spindle helps to stabilize it.
To repeat, for a given resonant system, extra mass lowers the resonant frequency and reduces the amplitude of this resonance. This is F=ma again. Force the same, Mass up, means acceleration must go down.

I agree that adding too much mass is potentially a problem. Where we disagree is at what point this extra mass becomes a problem. Pivoted arms, as seen by the cantilever, have high effective mass due to the head shell offset. Yet nobody seems to worry about this. I would also suggest that anti skate is significantly more dileterious to the cartridges health.

Placement of magnets..... Below the bearings natural resonant frequency, it is effectively loose, above it is nearly rigid.
So the thought experiment is where is it best to place a restriction to movement which creates a pivot point in order to minimize unwanted movement of the wand. If the mag is placed wand end, the bearing clearance allows wand movement at frequencies below natural resonance but not as much as when the mag is placed at the counterweight end. This is not a bearing stability issue, it is just optimizing the bearing geometery. Like I said this the same principle employed with inverted bearings in TTs.

Yeah I got all that the first time. We'll never agree on the added mass approach. But anyone trying it should check their cantilever deflection on eccentric records to ensure the cartridge can cope.
I'm sceptical on the degree of movement in the bearing, but if there is movement then stylus drag will pull the bearing tube forward on that side of the air manifold.

IMPORTANT NOTE for Frogman/Chris
What folk need to know is that if you use 2 magnets either side of the tube ( forming an eddy current clamp ), the dampening will increase disproportionately. I have seen controlled studies where dampening of unwanted energy has reduced by 80+% with 2 magnets either side of a beam instead of 1. You place the magnets S-N Beam N-S

Your ultimate solution if you want maximum dampening would be 4 magnets, 2 pairs either side of the manifold in the "S-N Beam N-S configuration".

I could argue that if you use 2 magnets in a clamp arrangement, you might be better off clamping the counterweight end and letting the stylus drag of the wand pull that end of the bearing tube against the bearing wall naturally. That way you dont have conflicting forces created by the eddy currents and stylus drag fighting on the wand side.

When positioning magnets for eddy current dampening, closer is not always better, the magnetism has nodes that vary up and down with distance.

Chris - since I am not running the ET at present, would be good if you could test the eddy current clamp I describe in the above post, using 2 magnets clamping the tube at one point in "S-N Beam N-S" configuration.

Playing with magnets.
From yesterday the pic shows two
magnet setups

The top setup is superior. Three on top of one another not alongside each other closer to the spindle.

Dover will try the Eddy magnetic clamp later today. It is just 9am here Sunday right now.

If you look at the bottom pic you can see I do not have alot of space to work with for two magnets either side of spindle.

I will clamp a small platform there with some blue tac today to allow me to put two magnets up closer to the spindle and on either side of it. Will pick up some square magnets today to give the
S-N Beam N-S

------O---------- Beam/Spindle
S || N--N || S---- Magnets

Being an IT guy I hope you are impressed with my illustration.

I took a cue from you.

Cheers