I have the AS309-S and I had the same questions. Ortofon were happy to give an explanation - see below.
There are two schools of thought concerning this arm the figures that are suggested by Ortofon Japan s engineers, versus the suggestions by our chief of cartridge engineering (in Denmark ).
The recommendation from the Japanese engineers is:
Mounting distance = 311mm
Eff. Armlength = 326mm
which gives overhang 15mm)
Cartridge parallel to headset, i.e. offset angle like the arm itself (19 deg)
Denmarks recommendation is:
Mounting dist = 316,6mm
Eff. Armlength = 329mm
Off.set. angle = 16.5 deg
Denmarks rec. gives Baerwald alignment.
As you see the cartridge need a slight twist with respect to headshell.The stated eff. armlength is easily achieved with Ortofon headshells and most cartridges.
Using a SPU (in G-housing) gives an eff armlength of 329mm
Off.set angle is of course determined by the arm itself, i.e. 19 deg.
A mounting distance of 314mm will then give the best result.
Louis @ Ortofon
Just I see the new 12' Ortofon TA-210 technical data (geometry seems correct):
Effective armlength: 329.0 mm
Mounting distance: 316.6 mm
Offset angle: 16.5 degrees
Overhang: 12.4 mm
By the way Ortofon have changed the RS-309D technical data from 326 mm to 329 mm eff armlength (but in mounting template it is still 326 mm).
I have two questions:
1. Can we adjust the RS-309D without twisted the cartridge in the headshell to change the offset angle from 19 deg. to 16.5 deg?
2. Why the eff armlength of 329mm, off.set angle 19 deg. and mounting distance of 314mm will give the best result with SPU cartridges, but do not work with "normal" cartridges?
Thank you in advance.