Eminent Technology ET-2 Tonearm Owners

Richardkrebs,
From your explanation I conclude that you are putting an argument forward that the more components that are in the process that are inherently unstable, then based on mathematical probability you are suggesting that this may increase the efficacy of the air flow. You might like to research chaos theory. Another solution may be to use a rigid tube, but put lots of pins through the tube - if you do the calculations and modelling, it would be possible to have a totally chaotic air flow rather than patterns of eddies, which may be preferable.

Last year you said that the ET2 air bearing is rigid. I assume since you now claim that you can hear a difference with different pumps and air supply configurations, that you would now concede that the air bearing is not rigid.

I have studied engineering including fluid dynamics and the way I view the ET2 is that the air bearing is not rigid, and what you are doing with all this experimenting on air pumps and tubing is in fact minimising instability within the bearing. That is why you hear a difference.

I note that you are now running 17psi whereas previously you said that 12psi was optimum ( and that your arm was perfectly rigid ). Can you explain why you have changed your view. Have you removed the lead and put your arm back to standard with the decoupled counterweight put back in now ?

Dover
Below is a copy of a mail I received from Bruce T many months ago. You might find it enlightening reading.

"Richard,

The resonance of the air cavity is over 500Khz and does not manifest itself on the surface of the bearing, it is a well damped liquid bearing.
A statement - " the air gap allows movement at audio frequencies" - shows a lack of understanding regarding how a tonearm works.
In two out of three degrees of freedom (x,y, & z axis) a cartridge is completely free to move in any tonearm. How can it not move in the X & Y axis but magically pull and push in the constrained Z axis? The record is encoded in and the forcing functions greatest in the X (vertical) and Y (horizontal) axis.
A tonearm works because of mass present in all three axis with the forcing functions above the systems natural frequency. The tonearm components headshell, arm wand, are thousands of times heavier than the cantilever and stylus, so by a ratio of masses, they sit still.
To put this in perspective go run back and forth and jump up and down on a several hundred thousand pound untethered barge and watch the displacement of the barge, while it will move, its motion will be extremely small relative to yours and proportional to the ratio of masses.
I used three measurement methods when developing the tonearm, accelerometers, strain gages, and the simplest and most effective was the use of a second tonearm to play parts of the tonearm under test while playing a record. Measurements at the air bearing are more than 60dB (1 million times) below signal levels, lower than the pivoted tonearms I used for comparison. In reality the ET-2 has its highest inertia in the Z axis and appears rigid to the cartridge.
If a tonearm moved at audio frequencies it would reveal itself as dips in frequency response.
brucet"

The arm IS rigid at audio frequencies, however it is sensitive to supply pressure irregularities. These are two completely different phenomena.

re pressures. In my rig, I found 12 PSI to be optimum without an oil trough, 17 psi with one. There is no inconsistency.

06-04-14: Richardkrebs
The arm IS rigid at audio frequencies, however it is sensitive to supply pressure irregularities. These are two completely different phenomena.

Richardkrebs, your interpretation of Bruces email is interesting.
Could you explain what you think happens when there are supply pressure irregularities.
Conventional air bearing theory is that the dynamic stiffness of the bearing will be affected by the air pressure and the surface area of the bearing and other factors.
Are you suggesting that the large diameter bearing tube upgrade and high pressure manifolds have no benefit and that users would be better off attending to supply pressure irregularities ? This would be a game changer for most participants on the thread if it were true.
Are you using the 2.5 manifold and bearing tube or are you using the high pressure manifold on an ET2 ?? or are you using the original manifold on an ET2 ??
By the way, I agree with Bruce's observations - traditional tonearms with gimbal bearings are prone to chatter and noise, and the offset tracking angle introduces a raft of conflicting forces on both the cantilever and bearings. Notwithstanding that a unipivot is another story..

My thinking on why pressure irregularities have such a large effect is this. It is to do with the way the air enters the bearing manifold. This thru a small single hole. It then circulates around the sleeve and enters the 14, from memory, threaded and loosely plugged holes exiting around the spindle. Each bleed hole will see a slightly different inlet pressure because their is a dynamic flow around the manifold with associated pressure drops. If there are pulsations in the air stream this will be manifest as slightly different flow rates into the manifold and thus pressures seen by each of these holes and the flow rate thru them. This would cause the spindle to chatter. When I built the replacement manifold, I planned to interpose a finely sintered metal tube between the air inlet and the sleeve. The idea being to force the air to more approximately enter the sleeve area at all points simultaneously. I didn't do this because it made the diameter of the manifold too large and it would have fouled with the platter and I didn't think that it was a big deal. I do now.
The walker arm manifold has 4 air inlet points. Maybe they have looked at this issue and used multiple inlets to help reduce its effect.

Richardkrebs,
On the one hand you say that the air bearing is rigid at audio frequencies, but now you speculate that the arm is prone to chatter due to "pressure irregularities". You also claim that the pressure irregularities are audible.
The two theories are mutually exclusive - they cannot both be true.

06-06-14: Richardkrebs
If there are pulsations in the air stream this will be manifest as slightly different flow rates into the manifold and thus pressures seen by each of these holes and the flow rate thru them. This would cause the spindle to chatter.

You do Bruce Thigpen a disservice.

06-04-14: Richardkrebs
Below is a copy of a mail I received from Bruce T many months ago.

The resonance of the air cavity is over 500Khz and does not manifest itself on the surface of the bearing, it is a well damped liquid bearing.

I used three measurement methods when developing the tonearm, accelerometers, strain gages, and the simplest and most effective was the use of a second tonearm to play parts of the tonearm under test while playing a record.

Please advise what testing you have done that would confirm your theory. The ET2 manual clearly states that Bruce goes through a design process, supported by principles of physics and sound engineering, includes mathematical modeling, the production of prototypes and thorough documented testing before his products go to market. There is no speculation or guesswork involved.

The ET2 has evolved from the 1st incarnation that utilized low pressure low flow bearing, then with the advent of customers using higher pressure pumps ( the WISA300 for example) Bruce redesigned the bearing for high pressure low flow air supplies. Bruce will also custom build for specific air pumps and provide advice on cartridge compatibility with each of the options.

It is abundantly clear that each bearing has an intended set of parameters with regard to pressure and flow. You have discarded the original ET manifold housing, discarded the decoupled counterweight, discarded the horizontal moving mass targets that the ET design is predicated upon, and may well be running a pump that is not optimised with your particular bearing. Unless you can provide some documented testing as Bruce does on his website, your comments can only be described as speculative at best.