Eminent Technology ET-2 Tonearm Owners

Bearing Stiffness – Naim Aro vs ET2

04-21-13: Richardkrebs
c)....the air bearing employed on these arms is effectively rigid at audio frequencies. So they should look elsewhere when looking for the cause of compromised note leading edge performance.

04-16-13: Richardkrebs
I started thinking about this when Dover commented on the superior transient performance of his unipivot. The idea further coalessed when the tests were done with loosening the CW arm bolts. This would change the Q and possibly the res frequency of the CW assembly.

My comments on the Naim Aro unipivot were pertaining to the superior preservation of the leading edge of notes - this is quite different from “transient performance”. Unipivots are mechanically coupled, whereas an air bearing is not rigid and loses some of the leading edge. It has nothing to do with Q. The addition of lead mass will alter the dynamic stiffness and compromise the performance of the air bearing. Capturing the leading edge requires secure tracking and speed, both of which are compromised by the addition of lead mass or removal of the decoupling.

04-23-13: Richardkrebs
Stiffness
Many years ago I remember reading an audio magazine which tested the rigidity of the ET2 bearing. It may have been Martin Colloms, but I can't be sure. This was done, again from memory, where accelerometrs were used and a sweep frequency was applied to the spindle. The result showed a bearing that was stiff at audio frequencies.
This is explained by the design of the bearing (it's self centering characteristics) and its extremely high resonant frequency. Many times higher than the audio spectrum. Although the bearing uses air which we know to be compliant, at the frequencies of interest, the bearing medium is stiff.
I also show here a quote from an industrial air bearing manufacturer. While these a big load bearing devices, their design is virtually identical to the ET2

"Outstanding stiffness for small deflections Most engineers visualize an air bearing as being like a hovercraft, and they erroneously conclude that a bearing which floats on air cannot be very stiff. Actually these gas bearings are many times stiffer than a ball or roller bearing. Sapphire orifices within the bearing gap control the pressure in a film of air which is only 0.0003 inches thick. As a load is applied to displace the bearing rotor or slider, the gap decreases very slightly on one side, reducing the flow of air through the adjacent sapphire orifice. This results in a pressure increase in the gap on this side which pushes the rotor back to its original position. In essence, the air bearing is a servomechanism with closed loop control, and maintains a uniform gap in spite of external forces that may be applied. This results in bearing stiffness of millions of pounds per inch for small deflections. Stiffness is linear and does not change with temperature. In contrast, ball or roller bearings have almost no stiffness unless heavily preloaded. The stiffness of a ball bearing is not linear, and varies considerably with temperature."

The response above to my original post of 04-17-13 contains misinformation. The comments plucked from the internet are irrelevant as they pertain to ball bearings and air bearings. They were copied from the following website
http://www.space-electronics.com/Products/air_bearings.php
The Naim unipivot does not use ball bearings.

The Naim Aro is mechanically grounded whereas the air bearing is not. Unipivots are the most rigid coupling you can get in a tonearm. Air bearings have compliance and gimbal bearings can only be too tight (loaded) or too loose and can chatter.

In the Hifi News Review of the ET2 Martin Colloms concluded that the shape of the resonance passing through the air bearing remained intact. This is not per se empirical proof that air bearings are rigid.

I note that most users of the ET2 have increased the air pressure up to around 19psi and have reported improvements to the sound as the pressure is increased.

When the operating air pressure is increased, the following operating parameters are altered - the Q of the system, the dynamic stiffness of the bearing, the resonance frequency of the air bearing itself, the shearing forces are changed.
All of these changes will of course be in themselves be difficult to calculate as the results will vary depending on the resonances in the I beam and cartridge and masses involved.

This is precisely why Bruce Thigpen backs his physics and maths up with extensive testing.

Issues Created When Adding Lead Mass and Removing the Decoupling of the I Beam on the ET2

04-21-13: Richardkrebs
b)....a heavy arm, when and only when, connected to a low compliance cartridge is a high performance, viable alternative

This statement is not correct within the context of the ET2.
Adding Lead to the arm increases the horizontal mass.
Removing the decoupling on the I Beam increases the horizontal mass.

The ET2 is designed with a target horizontal mass to be used in conjunction with a decoupled I Beam & Counterweight.

Increasing the horizontal mass increases distortion due to the additional side loads on the cantilever & tracking is compromised.
Increasing the horizontal mass creates a large peak resonance in the bass that also affects tracking and increases distortion.

Bruce Thigpen
If the weight is coupled the system resonant frequency would be extremely low, a resonant frequency at 3Hz with a significant rise in response (6-12dB) results, which would affect tracking slightly because of the asymmetric position of the cantilever, we opt for splitting the horizontal resonance frequency into two points and lowering the "Q" which improves tracking.
More important than tracking, the intent was to reduce the modulation effects of low frequency energy (FM and AM) that increase distortion in the cartridge

When you add mass and remove the decoupling how big is the resonance in the bass?

Bruce Thigpen has measured up to a 6-12db lift in the bass when testing the removal of the decoupling from the I beam

04-23-13: Richardkrebs
Maths and Physics.

Amplitude
A few weeks back I posted a transmissibility graph showing the effect of excitation frequencies at various multiples of the resonant frequency. This graph can be used to show relative resultant amplitudes for known resonant and excitation frequencies.
For a standard ET2 using in my case a Shelter Harmony, we get a resonant frequency of 8.4 hz. On my heavy arm, this frequency drops to 5.3 hz. If we take the lowest frequency of interest to be 20hz we get multipliers of res freq of 2.4 and 3.8 respectively.
By applying these multipliers to the graph we can see that the system which resonates at 8.4 hz shows a small rise in amplitude about 15%. If we now compare this with the 5.3 hz example we see a much smaller rise around 5%. We have to extrapolate this answer, since it is off the scale of the graph. In other words at audio frequencies the heavy arm produces less bass boost.
You can also see that the damping applied has very little effect on the resultant gain as the lines are trending together. This means that even if we factor in a higher resonant amplitude for the heavy arm, we can see that while it alters things slightly, it has minimal effect.

There is some merit in a discussion of what happens at sub sonic frequencies but the arm with the lower multiplier (lighter arm) will face problems sooner as we decend below audible frequencies.

As explained earlier in this thread the maths quoted above is for a single pendulum. The calculations above are based on a singular pendulum. The calculations above do not take into account the fact that the ET2 arm & cartridge have multiple pendulum effects in the horizontal mode –
- the cartridge cantilever swings around the record pivoting at the stylus tip
- the cartridge cantilever swings around the cartridge at the suspension end
- the I beam

This is why when Bruce Thigpen measures the impact of coupling the I beam he can measure up to a 6-12db lift in the bass. The calculated numbers in the above post are are theoretical calculations for a single pendulum, which does not apply. No actual test results have been provided that support these numbers and conclusions.

Does this resonant peak really matter if it is below the audio spectrum ?

04-27-13: Ct0517
I usually hear only about audio designers trying to come below the audio spectrum – especially with a TT setup ?
That is what the conversations have been based on here as well ? 2hz – 6 or 7 hz. .

The fundamental resonance is created by the combination of the compliance/mass of the cartridge vs the effective mass of the arm.
Tonearm designers try to keep this as low as possible and minimize its amplitude.

The peak rise in bass response generated by the arm/cartridge does not rise and fall at one frequency. The peak resonance has a spread either side of that calculated peak resonant point.

Again I need to reiterate that Bruce has actually measured bass lifts of 6-12db when removing the decoupling.

Even if one ( wrongly ) assumed this resonance has no effect because it is out of the audio band, one would be wrong because the bass lift ( nasty peak resonance ) can impact tracking adversely.

This is why adding lead mass and removing the decoupling as advocated is wrong. Not only is it increasing inertia and side loads on the cartridge, it is also putting a lift in the bass frequency by removing the split resonance functionality that this arm uses to give a flat response. Adding lead mass and removing the decoupling will increase cantilever flex and tracking distortion.

does anyone know if there is a downloadable mounting template for the ET@. I have two of them, an I want to mount it on my custom Lenco, but I'd prefer not to start dirlling unless I know for sure what I am doing. I have dowloaded the manual, but it really doesnt give enough accurate info on mounting, mostly because you can buy a metal template.
anyone have any info?

Chris.

Good to read your findings on the gooseneck. They parallel what I heard when making the change here. You can expect more of the same when we find a way to bypass the o'rings in the manifold, along with a satisfying jump in dynamic contrast and low level detail.

Dover wrote:

"This is nonsense Richard. Your comments that the air bearing is rigid defies physics. Have you heard of compressed air. You can get 2300l of air into a dive bottle with an internal volume of 11 litres. Try that with metal.

Why not try to compress 1 litre of metal into a 50ml can at audio frequencies. According to your thinking this is possible.

Do you get any of this?"

The writer should get this, from Franc Kuzma:

'At hi-fi shows, we routinely ask people to pull or twist the Air Line tonearm on a Stabi Reference turntable. The whole suspended mass of 24kg (52.8 lbs) moves back and forth for 1/4!9 while the air bearing maintains zero friction! Most people are shocked.'