Prototype number 2 has a 10 mm diameter aluminium armtube with 1mm wall thickness. No surface treatment of any sort was applied.
The headshell looks a bit different because it was treated with hot alum solution for about 10 hours. I managed to break a tap in it and had to dissolve the stuck piece. Alum bath doesn't form a hard surface, so the only difference is cosmetic.
The much stiffer aluminium armtube mates better with a headshell, routing vibration away more effectively. Thus the headshell resonances are now better controlled.
The frequency of all the key resonances except the bearing ones have shifted about 60-120 Hz up. I would need to look closer into this, and I think trying different materials will shine some light on why it is happening.
Interestingly, the extent of bearing resonance stayed the same despite more vibration reaching this part of the arm. This proves the quality of original Ortofon's horizontal bearing design.
The counterweight resonances are high Q, and they are all over the chart. Now I understand why the Ortofon designers decoupled the counterweight and filled the stub with damping plastic too. To try and see how the counterweight configuration changes the picture, I have removed the conical ring with the scale from the counterweight and repeated the measurements. Here's the result, with the new configuration in blue:
The second biggest counterweight resonance is gone. But it looks like the "old" counterweight was quite effectively damping the even harmonics of the main armtube resonance.
Sonically, the "clean" counterweight configuration as shown on the right (without the conical scale ring) proved to be much better. Looks like a high Q resonance @1300 Hz is a lot more audible than a series of main bending mode harmonics.
Compared to Prototype 1, there is a lot less sibilance and high frequency glare despite virutally the same resonance at the critical 1000 Hz mark.
Prototype 2 sounds a lot like the original Ortofon AS-212 — but a bit brighter. There's a very pleasant and captivating "slam", a sense of propulsion and rhythm to the music. But still, here we are in "acceptable," rather than "great," territory. I think that better performance can be extracted from the same basic configuration.