Lets look at things again.
I am officially retracting my retraction to Thekong regarding the ET2.5. Here is why.
When I realised my obvious mistake in thinking he was going to place a rigid CW arm on an ET2, when in fact he was proposing to use one on an ET2.5, I looked at the figures for this arm.
On 03/14/13 Chris posted the Horizontal resonant frequencies for the two arms. ET2 5-6hz ET2.5 2-3hz.
I already knew the horiz mass of the ET2, so calculated the mass of the ET2.5 assuming the same cartridge mass and compliance was used for both arms( a reasonable asumption)
Res freq is proportional to the square root of the inverse of the mass. In other words 4x mass = 1/2 res freq. Since the ET2.5 freq range was approx 1/2 the ET2, it implied that the ET2.5 was 4 x heavier, making it over 100 gm. This surprised me but in the absence of any other information, I took it on face value.
From experience it would be problematic if Thekong were to add a rigid weight to this apparently already heavy arm. Later on in the thread, 03/14/13, Chris published the actual weight of the ET2.5 being only 8gms heavier than the ET2. This means that ET did not use the same cart parameters when giving Chris the original info. Thekong, your call obviously but if I were you I would try a fixed CW arm provided it is really rigid.
Re my question re interaction between the two spring rates, cart and CW. I raised this question, not because I had suddenly "understood" how the ET2 worked but because the idea that having two springs driving the same structure could actually have a down side. 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. Potentially reducing any interaction between the two springs, but not eliminating it.
When I fixed the CW and added further mass and used a low compliance cart, there was an unexplained positive side effect. Focus and sharpness improved. Transients were better. It was some years ago when I made this change and while liking the improvement, I didn't put too much thought into 'why'. While I did not post these positive results. My question to the fellow posters was " could the interaction between these two springs be a problem." I don't know, but it is a valid question and the graph I posted suggests that they do "talk" to eachother. My thinking was that there may be a link between the transient improvements I acheived and the use of a swinging CW.
When I first purchased the ET2, I set it up as per the manual, to the point of obsession. As with all of my gear, I wanted to extract every tiny bit of performance. In standard from, it was best as per manual, with the slight adjustment of swinging the Ibeam down as Chris has posted.
Years later I started to experiment. The result is what I have now. It is just my opinion, no more or less valid than anyone else, but I believe that it is significantly better than a standard ET2, when using low compliance carts.
We can debate this from first principles all we like, but the proof of the pudding is in the hearing. What does it sound like, how does it perform? This is evaluated with our ears.
I will not be degrading my arm by converting it back to standard form.
