Eminent Technology ET-2 Tonearm Owners

Dover
The fact that it takes considerably more force to accelerate a heavy arm sideways is self evident.

What I have constantly said is that this force will not be enough to deflect the cantilever while tracing an eccentric record, provided the resonant frequency of the arm / cartridge system is above 0.55 hz for a 33 rpm and 0.75 hz for a 45 rpm record.
The analogy I used earlier is a good practical test to show this phenomena. For those interested it would take 5 minutes of your time.
All you need is a rubber band representing the cartridge suspension ( spring) A weight, representing the arms effective mass, attached to the rubber band such that it bounces up and down at a few HZ, representing the resonant frequency of the combination. Remember the resonant frequency is a function of the arms effective mass and the cartridges suspension stiffness ( springiness)

The groove modulation is simulated by rapidly moving the rubber band up and down. ( The cantilever driving the suspension) Do this at frequencies higher than the bounce frequency and you will see that the weight stays still. In other words the cantilever is moving and the arm is not.
Now move the rubber band up and down at a frequency lower than the bounce frequency. This simulates an eccentric record or the lead in, lead out grooves. Now the weight moves up and down in total sync with the rubber band. The whole arm is moving and the rubber band is not stretching or retracting. The cantilever is not deflecting.

Thekong,

Yes I agree with you. I only used the 12mm as that was what was used on the Kuzma video Richardkrebs referred to.
The key point as you have indicated is that the Kuzma and Terminator arms will have 3-400% higher lateral force on the cantilever due to their horizontal effective mass being 3-400% higher.
In my view this isn't great and I would worry about using medium to high compliance cartridges with these arms. From your comments I can see you are very careful on setting up your arms.
Will be very interesting to see what you think of the ET2.5 when you have had a chance to set it up. I would encourage you to try my set up recommendations, using my decoupled counterweight suggestion - which means running the I beam very very loose, and tuning the bottom end response by slowly adding dampening to the movement.
Bruce Thigpen has clearly put a lot of thought and experimentation into the decoupling methodology and the low mass.
If you read his manual and patents he starts with a low mass arm, and then brings the effective horizontal mass up very very gently by providing variable spring rates. This is to keep the resonances between horizontal, vertical in sync with the compliance of the cartidge and the Q of the system. The Q is related to the dampening of the oscillation - the use of magnetic dampening will shift this slightly. Very small adjustments can give quite dramatic changes to the sound, especially in speed and articulation.

Hi Thekong

Yes please post a pic of your ET2.5 with the modified counterweight when you can; would love to see it and look forward to your ET 2.5 impressions as well. I have had my ET2 on the left side of a table. I found it very awkward both from a handling and viewing (the cantilever) perspective.

Then again I don’t get driving on the left side of the road either.....

I am curious about the Rockport and Kuzma based on what Ketchup and yourself said regarding the tubing.

Ketchup
What about that stiff plastic tubing on the Kuzma? It seems like it would have a huge effect on the arm's movement even if both of the fittings swiveled.

Thekong
I can’t say they have no effect on movement, but they certainly don’t hinder it, at least not to the point of causing mistracking.

Based on my ET2 experiences, the arm can be lined up properly and if the pump is decent it will track fine. But nowhere near its potential if the wiring is affecting its travel. So would not the best air bearing design/solution mean no wire(if it was possible) and tubing attached to the moving pieces, giving the cartridge/armtube the greatest freedom ? I'm just sayin' as a user.

The Rockport and Kuzma tubing makes me think of my past experiences with my VPI JMW 12 tonearm. That tonearm’s wires are used as its antiskating method. Their positioning pushes the tonearm back toward the outside.

Could the tubing being in a loop at the centre of the Rockport and Kuzma arm, be acting as a type of dampener for the arms motion in both directions ?

Cheers

Could the tubing being in a loop at the centre of the Rockport and Kuzma arm, be acting as a type of dampener for the arms motion in both directions ?

Hi Ct,

I think the tubing is just a necessary evil in the Rockport / Kuzma design. As for whether it affects the tracking, when I adjusted the arm to free floating (i.e. 0 VTF), I can put the arm in any position without it going in or out. So, I suppose the affect is minimal.

However, there is one interesting point that I still can’t understand. When moving the arm in and out by hand, the feeling of the ET is smoother (or I should say requires less force) than the Rockport! While I don't have the spec. of the 2 bearings, I have always assumed that the Rockport has a tighter tolerance / air gap. This can be shown when trying to move the arms without the air supply. While you can still move the ET quite easily, a lot more force is required for the Rockport in this condition.

But, when air pressure is supplied, shouldn't both of them be virtually frictionless? I think if the Rockport indeed has friction, the cartridge would probably mistrack. Or could it be the horizontal mass that we have been talking about? Based only on the look, if the horizontal mass of the Airline is 100g, I would estimate the Rockport is about half to 1/3 of that, so in the 35-50g range, not that far away from that of the ET!

Ct,

Here is a photo taken by my friend recently, not very clear, but you can have a general idea of my ET counterweight assembly, with a threaded rod for the weight.

http://i1280.photobucket.com/albums/a500/der_yeti/IMG_1567.jpg