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Tonearm Mounting/Geometry Question....

108.215.210.6

Posted on July 5, 2012 at 14:33:39
Mr Blue Sky
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The measured length from the center/pivot point of my tonearm to the stylus tip is 219mm. According to what I think are correct calculations, this would mean that the pivot-to-spindle/mounting distance would be roughly 200mm to achieve proper Baerwald alignment. Is that right?

 

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Yes, maybe..., posted on July 5, 2012 at 17:52:32
tketcham
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Hi, Mr. Blue Sky,
Your conclusion that the pivot-to-spindle distance is 200mm might be correct. That's if you measure an overhang of 19mm and the headshell will accomodate an offset angle of 25.25 degrees. (Refer to the Vinyl Engine alignment calculator link below.) You could also check Vinyl Engine's tonearm database to find your make and model of tonearm and see what the design parameters are for effective length, pivot-to-spindle distance, overhang, and offset angle (or better yet, linear offset) and see which alignment geometry the tonearm will best accomodate.

Regards,
Tom

 

Forgive me if this is a dumb question, but..., posted on July 5, 2012 at 18:15:09
Mr Blue Sky
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..what is "overhang" and how is it measured? I am only interested in Baerwald alignment. The headshell is fixed position mount...a vintage arm...so I can't adjust anything except the mounting position. All I know is that the stylus tip to the pivot point of the arm is 219mm. It's a Rek-O-Kut S120 arm. The factory specs are on the Engine, but they do not allow for proper Baerwald alignment. The figures are incorrect.

 

RE: Forgive me if this is a dumb question, but..., posted on July 6, 2012 at 10:02:01
dtut
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You could try this.
Doug

 

Not sure I understand...., posted on July 6, 2012 at 12:06:32
Mr Blue Sky
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What are you suggesting here? The unique mounting plate? Please explain. Thanks! Also, how is the arm? How does it sound/track? I've not heard mine yet.

 

RE: Not sure I understand...., posted on July 6, 2012 at 15:18:09
dtut
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Mr. Blue Sky,

I wanted to get my 120 working and mounted with my L34 Rondine Jr., but like you found, alignment is difficult so I built the sliding base to set overhang - the distance from the spindle to the stylus point. That worked, but I couldn't quite get the right angle for the Baerwald alignment so I made an adapter which allows the cartridge to be rotated in the head shell. The alignment is a bit fussy, but it works. The key is a machine screw which drops through a fitting in the hole at the top of the head shell to a flat aluminum bar which carries the cartridge.

I like the look and simplicity of the 120. It's built like other Rek-O-Kut products - no frills, no foolin' around - and I'm very pleased with the way the table and arm sound. The arm tracks very well.

Tom - Thanks. The solution was born of desperation.


Doug

 

Desperation is the mother of invention..., posted on July 7, 2012 at 05:59:53
tketcham
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Hi, Doug,
You came up with a great solution without having to modify the original tonearm and headshell. As you discovered, trying to force a square alignment into a round geometry can be frustrating. (Pardon the lousy pun.) The photos are especially helpful.

Mr. Blue Sky - The slotted mounting plate acts like a sliding base similar to what SME uses for their tonearms. Instead of moving the cartridge forward or backward to get the stylus at the proper position you slide the whole tonearm. But the similarities to the SME tonearms ends there...

Modern SME tonearms are designed for the Lofgren A (Baerwald) alignment and you slide the tonearm to put the stylus at the proper null point position(s). The headshell is fixed because you don't need to slide or rotate the cartridge. The stylus will be at the right angle when it's at the right position.

But the S-120 tonearm is designed for a different alignment than the Lofgren A alignment. The tonearm has a different geometry (tonearm position/length and headshell angle). Just sliding the tonearm to put the cartridge at the Lofgren A null points gets you only part way there. You still need to rotate the cartridge to have the right geometry to fit the Lofgren A alignment. That's where the headshell adapter plate comes in; it allows you to rotate the cartridge to get the right stylus angle. You'll then have the correct combination of tonearm position/length and cartridge angle. (The headshell angle is still wrong but you compensate by rotating the cartridge.)

As Doug mentions, it's not going to be as easy as just sliding the tonearm like an SME. Once you find the right tonearm position (by sliding it) to put the stylus at the proper null points you'll need to rotate the cartridge to find the right stylus angle. But it shouldn't be any more difficult than trying to set up a cartridge in a slotted headshell, where you have to find both the correct position and angle simultaneously.

Enjoy the project!

Regards,
Tom

 

That is a..., posted on July 6, 2012 at 10:23:22
tketcham
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Great idea!

Tom

 

No dumb questions, only dumb designs..., posted on July 6, 2012 at 04:32:18
tketcham
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Good morning,
John's reply gives you plenty of information to consider. If you could find one of those Rek-O-Kut mounting kits it would make things easier to accomodate the cartridge you're using. But it still might not make things work for a Lofgren A (Baerwald) alignment. I have read that many of the Japanese tonearm models produced in the past used a Stevenson (or modified Stevenson) alignment geometry. And your S-120 tonearm appears to have those design parameters. Based on the limited information provided, I ran a few scenarios using the Vinyl Engine calculator. John's estimate that the offset angle is probably close to 21 degrees is a very good estimate. While not a true Stevenson alignment, the S-120 appears to have an alignment that minimizes distortion at the end of a record using a DIN inner groove of 57.5mm and then oddly minimizes distortion in the middle portion of a record. Using an offset angle much different than 21 degrees ends up with even worse alignments. Refer to the Vinyl Engine alignment calculator...

Rek-O-Kut S-120 aligment

Unless you extend the cartridge out past the headshell to increase effective length and then rotate the cartridge, you're probably stuck with an odd alignment. But if you could find a way to extend the overhang by about 4.5mm and rotate the cartridge 3 degrees you'll get alignment null points closer to a Lofgren A setup...

Modified Rek-O-Kut S-120 aligment

That's a lot of modification. Good luck. I'd just use the mounting distance of 209.5mm and live with the alignment of the tonearm. Or get a different tonearm.

Regards,
Tom

 

RE: No dumb questions, only dumb designs..., posted on July 6, 2012 at 07:15:27
John Elison
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> I'd just use the mounting distance of 209.5mm and live with the alignment of the tonearm.

That's the worst idea of all.

Mr Blue Sky has the right idea to mount the arm at the correct distance for Lofgren's A alignment and simply adjust the offset angle appropriately. He can then download Conrad Hoffman's program for making a custom arc protractor and end up with a very good alignment with null-points at 66-mm and 120.9-mm.

 

Worst may be best..., posted on July 6, 2012 at 08:23:30
tketcham
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Hi, John,
I don't agree that using the tonearm "as is" is the worst idea. Trying to make that tonearm work for a Lofgren A alignment may turn out to be an exercise in frustration. But heh, it's worth a try.

Regards,
Tom

 

RE: Forgive me if this is a dumb question, but..., posted on July 5, 2012 at 20:13:49
John Elison
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Overhang is the shortest distance from the center of the platter to the stylus when the tonearm is moved inward across the platter spindle. It generally is not measured directly because most tonearms cannot be moved inward over the spindle. However, it is also the difference between the effective length of the tonearm and its mounting distance.

Effective length is the distance from the stylus to the pivot point of the tonearm. According to Rek-O-Kut, that distance should be 223-mm for your S-120 tonearm. Of course, it can vary slightly because different cartridges have different distances from stylus to cartridge mounting holes. There is no standardization among cartridges. Therefore, you say you measured your effective length to be 219-mm, which is quite possible if you are using a cartridge with a very short stylus-to-mounting-hole length. Most tonearms have mounting slots in their headshells to allow the cartridge to be moved forward or rearward in order to compensate for different stylus-to-mounting-hole lengths.

Rek-O-Kut states the following parameters for your S-120 tonearm:

Effective Length: 223-mm
Overhang: 13.5-mm
Mounting Distance: 209.5-mm

Rek-O-Kut does not state an offset angle, but my guess is that it cannot be more than about 21-degrees. These parameters do not provide a very good alignment. Many tonearm manufacturers do not understand alignment geometry and Rek-O-Kut seems to be one of them. Your choice for Baerwald's alignment is therefore an excellent choice. I believe it provides the lowest overall tracking error distortion between an inner groove radius of 60.325-mm and an outer groove radius of 146.05-mm. The alignment null-points will be 66.0-mm and 120.9-mm. Use these in the equations below to calculate mounting distance.

Baerwald's alignment, which we now refer to as Löfgren A, requires an offset angle of almost 25-degrees, so you will need to angle the cartridge inward in the headshell. There is usually enough slop in the mounting holes to allow for this. If not, you have to determine how to accomplish this or you cannot get a true Löfgren A alignment.

You also need to be very certain in your measurement of effective length. Check the distance from the stylus to the back surface of the headshell. According to Rek-O-Kut, this distance should be 50-mm for an effective length of 223-mm. Since you measured an effective length of 219-mm, this distance should be 46-mm on your headshell. If this is not the case, remeasure your effective length until you get a very accurate measurement. Only then can you calculate the correct mounting distance for Löfgren A.

Good luck,
John Elison



 

RE: Tonearm Mounting, posted on July 5, 2012 at 16:21:45
M3 lover
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Mr Blue,

The measurement for pivot point (tonearm) to stylus tip includes the sum of overhang (typically 15 mm) and pivot to spindle distances. So given your measurements the pivot to spindle distance would be 204 mm. Consider that with stylus alignment 1 mm will be a huge distance.

Now for the Baerwald, or any other popular alignment, it will be a combination of setting both overhang and offset angle (see John's equations). The alternative alignments are based on different assumptions for minimizing distortion. Since a pivoted arm scribes an arc and the mastering cutter head traveled on a straight line tangent, compromise is necessary. Choosing an alignment formula means lowest average distortion across the whole record side or lowering distortion toward the end of the record where it becomes more critical.

I'd suggest a search here on the various alignments to help decide if you want the Baerwald or some other option if you are unclear about that.

"The piano ain't got no wrong notes." Thelonious Monk

 

Sounds right to me..., posted on July 5, 2012 at 15:56:06
John Elison
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Here are the tonearm alignment equations as a function of null-points. Simply select the two alignment null-points you like and crunch the numbers.



 

If I had known math ..., posted on July 5, 2012 at 17:27:56
reelsmith.
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...was going to be involved I'd have bought a CD player.

;-)

Dean.


reelsmith's axiom: Its going to be used equipment when I sell it, so it may as well be used equipment when I buy it.


 

CD players rule! ;-) /nt\, posted on July 5, 2012 at 20:32:56
John Elison
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Oh, John...., posted on July 6, 2012 at 08:07:22



 

RE: Oh, John...., posted on July 6, 2012 at 09:50:29
John Elison
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I think vinyl must be an acquired taste. I just can't seem to make it work for me anymore.

Anyway, I just ordered a Hagerman Cornet2 to see if I can put the magic back into vinyl. I'm thinking tubes might be the answer. What do you think?

Best regards,
John Elison

 

I miss my tubes., posted on July 6, 2012 at 12:56:56
You probably won't recall, but I've had a couple different tubed phono stages in the past few years, but I had to sell everything when I moved (sigh). I really liked the Wright AG Phono that I sold here on AA, but no sense crying over spilled milk, as they say....

Sure, give the tubes a try, John. What's the worst that can happen?

And if you ever decide to get out of vinyl completely (and are serious about it), let me know, ok? We'll "talk"...

 

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