|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
74.130.29.193
In Reply to: RE: Thanks a lot, but still one question posted by Da He Hua on February 02, 2017 at 19:03:49
If you followed the alignment procedure so that the stylus follows the arc exactly and the cartridge aligns to the grid lines at each null-point when the stylus is on the null-point mark, then you have achieved perfect alignment.
If you need to know all the definitions of tonearm parameters, you should read Brian Kearns' Treatise on Cartridge Alignment - Part 1.
There are other interesting articles in the Setup Guideline and Vinyl FAQ.
Follow Ups:
I have kind of have decided to make a new armboard. I know some people online have suggested that the manufacturer suggested 214 mm mounting distance makes it very hard to align Jelco 750; they suggest to mount at 216 mm. So, should it make the new board 214 mm mounting distance, or 216 mm?
Any materials to recommend for the armboard? Wood? MDF?
You really need to know the effective length of your tonearm to make a valid decision on mounting distance. If you believe its effective length is different than 229-mm, you should measure it. One way to do this is to measure from the tonearm pivot to the center of the headshell between the two cartridge mounting slots and add 9.5-mm. If you get 229-mm, then 211-mm is the correct mounting distance. If you get a different effective length, you can calculate the correct mounting distance from the following equation.
You need to select two alignment null-point radii and I recommend 66-mm and 121-mm . These alignment null-points will give you the best performance for modern LPs with a modulated groove envelope in-between groove radii of 60.325-mm and 146.05-mm. For example, if your tonearm's effective length is 229-mm, the optimum mounting distance would be the square root of (229 2 - 66 x 121) = 210.84-mm. This is approximately 211-mm.
On the other hand, if you believe the mounting distance should be 214-mm, then your tonearm's effective length would need to the square root of (214 2 + 66 x 121) = 231.91-mm or approximately 232-mm. Consequently, you need to measure your tonearm's effective length before you can determine the optimum mounting distance.
Good luck,
John Elison
Using Conrad Hoffman's application, when the mounting distance is selected to be 214 mm (as recommended by Jelco), what inputs should I choose to get an overhang of 15mm and offset angle of 22" (as suggested by Jelco)?
You would need the following inputs:
Spindle-to-Pivot = 214-mm
Inner Groove Radius = 53.773
Outer Groove Radius = 138.453
Select: Lofgren A
Best regards,
John Elison
The other question is let's say we have determined the right mounting distance, how do I tell what angel to use? In other words, I assume it is not that any place on the armboard that is away from the spindle by the mounting distance is the right place. How do I know where to drill the tonearm drop in hole?
Yes, anywhere on the armboard is fine as long as the distance from the platter spindle is correct. You can take a string and tie it around the spindle and put a pencil on the other end so you can draw an arc on the armboard representing the correct mounting distance. Then, select a place on the arc to mount the tonearm so that it fits nicely on the armboard when in its armrest. You don't want it sticking out from the plinth because you might want to be able to close the turntable dust cover.
Good luck,
John Elison
http://www.jelco-ichikawa.co.jp/products/tonearm.html
Here is a graph of Jelco's tonearm alignment compared to a graph of the optimal alignment for playing modern 12" LPs. Knowledgeable tonearm manufacturers like SME and Graham use the Lofgren "A" alignment. Check the distortion numbers in the turquoise cells.
.
.
I just realized something that might be important. When I measured the P2S distance, I took the Linn Basik arm out and measured the center of the spindle to the center of the drop in hole of the armboard. My turntable CJ Walker 55 is a four-point suspension table. I just noticed that when the arm sits on the board, the board will be pressed down a little, which seems to also have the effect of pulling the center of the drop in hole away a little from the spindle. So now I believe the mounting distance of this armboard is indeed 211 mm as it should be, instead of 208 mm. I think the Jelco 750 arm is heavier than Linn Basik, so when Jelco sits on the board, it would press the board down a bit further. Maybe will pull the pivot away from the spindle a tiny bit more?
That's exactly why I suggested you use a Linear Offset jig which is entirely arm spec and mounting position agnostic and will avoid the issues of determining distances precisely in order to use an arc protractor.
It will be highly unlikely you could get your measurements much finer than ±0.5mm unless you have access to precision instruments.
Regards Anthony
"Beauty is Truth, Truth Beauty.." Keats
Well, that's why I suggested trying the arm and see if it works before going to the trouble of drilling a new armboard. Measure P2S once the Jelco arm is mounted, if that's possible. You'll need an accurate P2S to generate an accurate arc protractor.
Good luck,
John Elison
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: