Eminent Technology ET-2 Tonearm Owners

Hi Ct0517,

Yes, that Rockport Capella II is still my main turntable. When ready, the ET2.5 will be set on the brass armboard opposite to the Rockport 6000. Actually, you can see a bit of the ET at the left edge of that photo already, just that the whole counterweight assembly was not installed yet!

BTW, the Rockport 6000 can also be installed on nearly all turntables!

The Kuzma looks like a nice arm, but one thing that always made me scratch my head is the air supply tube. We ET-2 owners pay so much attention to tonearm wire routing. 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. Does anyone know any details about the tube's fittings or if it hinders the arm's movement?

Comparison of lateral forces on Kuzma/Terminator/ET2/ET2krebs

For those who are interested in understanding the side forces on the cantilever :

Force = mass x Acceleration, where acceleration = mass/(velocity squared)

If the record is 12mm out of true, the arm travels 24mm in and out with each revolution, which takes 1.8 seconds.

On an eccentric record the acceleration will be the same for each arm –
0.0024metres / (1.8 sec x 1.8sec) = 0.00074 metres per second squared

The horizontal effective masses of the 3 arms mentioned in this thread are:

Kuzma has been quoted as 100g
Terminator 80g
ET2 25g

The force on the cantilever is as follows:

Kuzma = 0.1kg x 0.00074m/s2 = 0.000074 Newtons
Terminator = 0.08kg x 0.00074 m/s2 = 0.000059 Newtons
ET2 = 0.025kg x 0.00074 m/s2 = 0.000018 Newtons

Summarising then you can see that the increased mass of the Terminator and Kuzma arms increase the lateral forces on the cantilever by 300-400% over the ET2.

Now Krebs has modified his ET2 by adding 30gm of lead to the spindle. This adds 30g to the effective mass of the ET2. Krebs also couples the counterweight ( no spring ) which adds another 30g to the horizontal effective mass.

So Krebs has increased the horizontal mass of the original ET2 from 0.025kg to 0.085kg.
The Krebs modifications have increased the lateral forces on the cantilever by over 300%.
Furthermore he employs no dampening to control this mass and runs his ET2 at a lower pressure of only 12psi.

The arm moves in and out every 1.8 seconds.
This equates to a frequency of 100/1.8 = 55hz
The resonant frequency of the unmodified ET2 is roughly 3.5 – 5hz.

Krebs argument is and I quote:

Below this resonant frequency the cartridge is able to move the arms weight, start it and stop it, without cantilever deflection.

He is wrong. This statement defies basic physics.
Below 0.55hz the cantilever WILL deflect on an eccentric record.
I assume he has misunderstood the explanation given to him by Frank Kuzma, and seems unable to comprehend this.

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. Does anyone know any details about the tube's fittings or if it hinders the arm's movement?

Hi Ketchup,
My Rockport and my friend’s Airline shares similar arrangement in the airhost. Actually, they are not that stiff, and they can be detached at the point where they enter the bearing. So, you can twist them a bit to attain a hanging n shape for minimizing the effect on movement.

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.

If the record is 12mm out of true, the arm travels 24mm in and out with each revolution, which takes 1.8 seconds.

Hi Dover,

While it may not affect you conclusion on the lateral forces of the 3 arms, a record that is 12mm out of true (24mm in and out, that is nearly 1 inch!) is probably unplayable even with a pivotal arm. I think, on average, 1mm (2mm in and out) is more like it!

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.