Overhang, I already did, though I'm not surprised you missed it amidst all this other chatter! Check out this post: http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1137938333&openfrom&18&4#18 |
Mario_b asked: Why cant one set the cartridge overhang using linear measurements as per the arms manufacturer for the designed tracking arc: Then use compassed circles for Baerwald, Stevenson or whomever one divines to have the null points ones looking for, and use these to set just the stylus tangent? What am I missing?Here's what you're missing: this method assumes one can attain stylus-groove tangency at DIFFERENT pairs of null points with the stylus travelling on the SAME arc. That is not possible. On any given tracing arc, assume the cantilever is tangent at two points A and B. This is only possible because the cantilever is travelling through a given moment of arc, from which it receives a given angular shift. Now choose two new points, C and D, located on the same tracing arc but separated by a different moment of arc than points A and B. Make the cantilever tangent at point C and check its orientation at point D. It will not be tangent. Why? Because it experienced a rate of angular shift that was suitable for the (different) angular seperation of points A and B. Changing alignment schemes will always require changing the tracing arc. Tracing arc protractors (like the Wally or your proposed DIY) are wonderful and easy to use, but like any protractor they are specific to one particular alignment scheme. |
Doug, Thanks for taking the time to explain this. I'm digesting - my mind coming to grips with what you're saying like a cold engine. Instead of thinking and postulating, I'll apply these test that you have been good enough to provide in actual hands on experiments - and hopefuly break through to that Eureka moment that has eluded me up to this point. On a side note, I have used the TTB mirrored protractor as well as oak tag (the latter atop a thin dynaflex type record). I certainly do like the mirrored gander that the TTB device affords. One note of caution with it to anyone with less than nimble hands; the alignment channel grooves are deep and rough and can catch and wrench a stylus/cantilever in a heartbeat. Thanks again for your patience, Doug. Mario |
Dougdeacon, my rega300 installed on an AlexMkIII has the following geometry. Pivot to spindle at 222mm and stylus nulls are at 64mm and 120mm, which seems to be close to the figures that you have mentioned. Cartridge is a BPS. I thought that those figures might be of assistance here. I used a protractor that was published in HiFi World. Salut, Bob P. |
Mario, That's the spirit! I first did this just like you're planning to do, by trial and error. I tried to set up my OL Silver/Shelter 901 at the OL-specified 223mm using a TTB protractor. No dice. The stylus couldn't be aligned at both points. I sat down and thought through the geometry, slid my armboard in a couple of mm and, bingo! Problem solved. It was only later that I got my head around the theory (a little) by talking with real mathematicians on VA and reading some of the papers there. If you want real math, go there and search for posts by John Elison. Again, anyone can choose Rega's Stephenson geometry and have no problems at 222-223mm. I just like Baerwald's theory (lowest total distortion) better and appreciate the greater accuracy the TTB's mirror makes possible. Frank Schroeder uses a DIY protractor he printed on a piece of card, so if you prefer that you're in the best possible company. I just like the mirror. P.S. Whatever protractor you use, don't forget to disengage antiskating! Bob, Thanks for sharing that info. Null points at 64/120 are pretty similar to 66/120.9. I think the key is that you're at 222mm whereas my Rega style arm specifed 223mm, which definitely didn't work. Another variable is the stylus-mounting screws dimension. The farther the stylus is ahead of the plane of the mounting screws, the farther the arm can be from the spindle. I don't know your BPS but ZYX's and Shelters are both about 7.5mm, which is on the short end of typical. Some cartridges run as long as 9mm, which would probably make adjusting the arm unnecessary. |
