2. analog
  3. 1275859693

Ortofon RS-309D geometry

I'm curious about the geometry used by Ortofon for this tonearm. The published specs are the following:

Mounting distance = 311 mm
Effective length = 326 mm
Overhang = 15 mm
Offset angle = 19 degrees

According to the calculator at Vinyl Engine, the resulting null points are 64.787 mm and 147.484 mm, the latter being outside the playable area on the LP surface. Maximum distortion is 0.831% and average rms distortion is 0.564, both of which are quite a bit higher than Stevenson, Baerwald, or Lofgren B geometries with an effective length of 326 mm.

I'm think of using this arm as the second on a 2-arm table. Current users seem to really like it, I really like the idea of being able to swap between SPUs and regular MM/MC carts on a 12-incher with easy VTA adjustment. But the geometry and distortion numbers are giving me pause. Should I even worry about them? If I change the mounting distance to get closer to Baerwald null points and distortion when using regular carts, will I lose the ability to use SPUs because of their fixed position?

I'm a bit perplexed as to why Ortofon would choose what seems like such an odd geometry, especially one that seems to increase distortion by quite a bit. Perhaps I'm misreading something. Any comments are welcome, but especially from current RS-309D users.

Thanks,
Bill
wrm57

Showing 1 response by john_gordon

john_gordon
116 posts
 
Hello Lbelchev,
You asked

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?

To reduce distortion, the cartridge must be twisted. It can be left straight, but then the distortion is higher.

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?

"Best results" here just means "least bad" because the SPU can't be twisted. Ideally the arm should have a different geometry for LPs.

Why it is designed this way, I don't know. The only reason to make an arm like this is that, historically, long arms were used by radio stations to play transcription discs, which could be 16" in diameter,and ideally would use different offset and overhang from 12" LPs

One other note: if an old short SPU-A is used with this arm, and the arm mounted at around 293mm, it would be acceptable.
John

http://en.wikipedia.org/wiki/Transcription_disc
.
More to discover