Hi Fine,
If you have one of the original Obsidian models, it seems you can answer this for yourself. Find the effective length for any arm you are considering (the EPA-100 is 250mm), then measure that radius from the center of the spindle to see where an arc of that length will cross your armboard, allowing enough distance from the edge for mounting. If it fits within your armboard and there is sufficient clearance underneath for mounting hardware, it should work. As Jonathan said, most any 9" or 10" arm should fit.
Now, not to begin a debate with Stanwal but not everyone believes the EPA-500 arms were a sonic improvement over the EPA-100. I have not heard a comparison so can't offer a personal opinion but will only say that some prefer the 100.
The only reason for a different armboard would be a different size or shape for the mounting hole as the 100 and 500 have the same effective length. See this -
http://www.vintagetechnics.info/tonearms.htm
The match of a MC cartridge with any arm begins with the compliance/mass match. The dynamic damping system on the EPA-100 makes it more flexible than most other arms in this match, meaning it can be adjusted for a wider range of cartridges than many other arms.
If you have one of the original Obsidian models, it seems you can answer this for yourself. Find the effective length for any arm you are considering (the EPA-100 is 250mm), then measure that radius from the center of the spindle to see where an arc of that length will cross your armboard, allowing enough distance from the edge for mounting. If it fits within your armboard and there is sufficient clearance underneath for mounting hardware, it should work. As Jonathan said, most any 9" or 10" arm should fit.
Now, not to begin a debate with Stanwal but not everyone believes the EPA-500 arms were a sonic improvement over the EPA-100. I have not heard a comparison so can't offer a personal opinion but will only say that some prefer the 100.
The only reason for a different armboard would be a different size or shape for the mounting hole as the 100 and 500 have the same effective length. See this -
http://www.vintagetechnics.info/tonearms.htm
The match of a MC cartridge with any arm begins with the compliance/mass match. The dynamic damping system on the EPA-100 makes it more flexible than most other arms in this match, meaning it can be adjusted for a wider range of cartridges than many other arms.