Eminent Technology ET-2 Tonearm Owners

This post includes feedback from Bruce Thigpen

Dear gentle reader,

Over the past month Richardkrebs has argued the case for adding substantial lead mass to the ET2 and replacing the decoupled counterweight with a fixed counterweight.

In Richardkrebs recommended setup he advocates increasing the horizontal mass by over 300% from 25g to 85g by adding lead to the bearing spindle and coupling the counterweight rigidly to the arm. This is well outside the design parameters carefully formulated by the designer, Bruce Thigpen.

The laws of physics are very simple:

Higher mass = higher inertia ( resistance of the arm to movement )
Higher mass, means that when the arm moves back and forth on eccentric records it places higher lateral forces on the cantilever.

The added lead mass will cause the cantilever to flex more on eccentric records as the arm oscillates in and out.
Furthermore as the arm oscillates back and forth the side loads on the cantilever will increase by over 300%.

I have raised the issue of the additional loads on the cantilever from the added lead mass, and resultant increase in cantilever deflection.

Richardkrebs has consistently denied the laws of physics by claiming there is no deflection because the resonant frequency of the eccentric movement ( 0.55hz ) is below the arms resonant frequency; 3.5-5hz for the unmodified ET2..

To quote Richardkrebs

02-15-13: Richardkrebs
I have a view on linear arms in that the rules for pivoted arms and effective horizontal mass do not apply. In fact I have added a lead slug inside the bearing spindle 25 mm long with its OD equalling the ID of the tube.

03-11-13: Richardkrebs
the resonant frequency due to the combination of a typical low compliance cartridge and horizontal effective mass was in the region of 2.5 -3.5 hz.(this has been published by them elsewhere), this is well above the 0.55 or 0.75 hz for 33 or 45 rpm eccentric records. Therefore the cartridge does not "see" this movement.

03-12-13: Richardkrebs
Below this resonant frequency the cartridge is able to move the arms weight, start it and stop it, without cantilever deflection. I do not need to talk to cartridge manufacturers to confirm this. Do the math.

03-13-13: Richardkrebs
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.

A fellow Audiogon member has contacted Bruce Thigpen, the designer of the ET2.

Bruce Thigpen has confirmed that Richardkrebs assertion that ‘the cartridge is able to move the arms weight, start it and stop it, without cantilever deflection’ is wrong.

The following are quoted from the correspondence with Bruce Thigpen:

the cartridge will "see" .55Hz mounted in any tonearm, more so in one with higher horizontal inertia

I don't think Kuzma means the stylus does not deflect at all at .55Hz, that would defy physics

I hope this puts an end to this matter as it is becoming boring having to sift through gobbledygook, pseudo science and rubber band conflations.

On the other hand some of the more entertaining highlights posted have been:

Claims that rotational forces of a pivoted arm are the same as the linear forces of a tangential arm

The Morch adds what appears to be considerable mass at a radius out from the pivot point. In so doing they have made a flywheel.
As viewed by the cantilever this is no different to me adding mass in the linear plane to the ET.

Comparing tone arms and cartridges to a rubber band:

All you need is a rubber band representing the cartridge suspension …
The groove modulation is simulated by rapidly moving the rubber band up and down.
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.

Being called a scaremonger

Your scaremongering may have dissuaded people from trying a simple reversible mod

And of course, meeting a legend

I am the only person here who can speak with any authority on the subject.

Bruce Thigpen has clearly put a lot of thought and experience into designing a low mass air bearing arm that includes a decoupled counterweight to optimize the arm and cartridge.
If you read his manual and patents he starts with a low mass arm, and then brings the effective horizontal mass up very gently by providing variable spring rates on the decoupled counterweight. This is formulated to keep the differential resonances between horizontal and vertical in sync with the compliance of the cartridge 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, transparency and articulation.

I would not recommend adding lead. Adding mass creates a risk of damage to my expensive and irreplaceable cartridges - Ikeda Kiwame and Dynavector Nova 13D.

For the Richardkrebs of this world – here’s a simple test. It will only take a few minutes. Put your gumboots on and fill them with lead shot. Now try and move your feet sideways, out and in in 1.8 seconds. That’s what your cartridge sees.
Lead filled boots are not required to get the best sound from this outstanding tonearm.

I do not doubt that Richardkrebs beliefs are sincerely held. However they defy physics and are clearly wrong.

Hi Thekong

Thanks for posting that pic and allowing us to be part of this !

What a cool setup with those armboards and counterweight. It appears that the rod is “welded” to the square which attaches to the spindle end ? These are grounds I have not stepped on. Richards comments about flexing are very interesting.

Can you tell us what cartridge you are using and the psi you are running ?

We have been discussing the advantages of the single, double, triple leaf spring counterweights here a lot. I am assuming you have a stock single leaf spring that came with the ET 2.5? Most of us I would think use the leaf spring counterweight. It would be valuable for me and others, if later on at some point; after tweaking and listening with your custom fixed counterweight; you could put on the leaf spring counterweight and tell us the differences you hear.

Anticipation builds. :^) Thanks again for including us in this.

Cheers Chris

Richard/Dover

If both of you guys ever decide to join forces and start up a cover band, please let me know. I will personally make the trip to NZ to hear you. I would not miss it. As long as you close the set with a little Jethro Tull.

Richard/Dover - Do either of you play
flute?

Dover.

I don't think that personal attacks advance this thread, so lets both agree that we stick to our opinions on the subject or post relevant information.

Shown here is a link to the Math on driven harmonic oscillators, a mathematical representation of an arm/ cartridge assembly. It shows in both formula and graphical terms what I have been trying to say. The Math is a bit of a struggle but fortunately the graphs show the results.

http://en.wikipedia.org/wiki/Harmonic_oscillator#Driven_harmonic_oscillators

We can see from the sinusoidal graph that the Transmissibility, for input frequencies that are say 25% or less of the resonant frequency, is 1. This means that there is total transmission of the input frequency into the structure. It moves as one. In other words the whole arm moves. At input frequencies above 25% up to resonance we get increasing gain and this area should be avoided.

For input frequencies that are 300% of the resonant frequency we get transmissibility of around 15%, unless the structure is highly damped and we all agree, I think, that lots of damping doesn't sound good.

So at 3x the resonant frequency we are loosing around 15% of the groove modulation, as the arm is still at this point moving back and forth sideways slightly.

This is not a problem provided this 3 x resonant frequency is not a valid audio signal. Actually you would need to extend the graph out to around 6x resonant frequency before the transmissibility was approaching 0. Until we reach that point, part of the low frequency goove modulation goes into moving the cartridge and arm sideways and not into generating an output voltage.

It was the discovery of this characteristic that led me to look into possible performance improvements in the LF area of the ET2. Since if the resonant frequency was say 6 hZ we would not have total transmission of LF modulation until we reached say 36 hz.


Kuzma state that the horizontal resonant frequency for low to med compliance carts is in the range of 2.5 to 3.5 HZ, with an effective mass of 100gm.
Resonant frequency is inversly proportional to the square root of the mass. So my arm at around 85gm would fall into the range of 2.7 to 3.8 HZ

If we take the mid point for these resonant ranges 3 hz and 3.2 hz respectively, we are, in my opinion, in the Goldilocks range for the Kuzma and my arm. In that it is sufficiently high to avoid gain caused by eccentricity, since 0.75 Hz for a 45rpm record is less than 25% of the resonant frequency. But low enough to give virtually complete conversion of desirable groove modulation into output voltage. 19.2 hz (3.2 x 6) being at the lower end of what most systems can produce.

If we look at the same numbers for a standard the ET2 we get a resonant frequency range of 5 to 7 hz for a 25 gm effective mass. This is comfortably above the 0.75 hz eccentricity problem, but if we take say 5 hz as the resonant frequency, we see that it will not be until we reach say 30hz before we have complete conversion into an output voltage. I dont think that this is desirable.

Making the arm lighter still will extend this frequency upwards.




http://en.wikipedia.org/wiki/Harmonic_oscillator#Driven_harmonic_oscillators