Table of Arm Data

Linear Trackers & Arms equipped on system players are omitted from list.

NOTES: there are many discrepancies in the descriptions/performance of arms due to the following reasons.
1) Manual and model do not always coincide (manual is not enough if there are too many modification:
history of models may be required). Some dimensions (esp. effective length) of same model may differ between early manual and later manual even though the product itself remains unchanged. And vice versa: same model may change dimensions without referring to them. Often we find irregular conversion from inch to mm in the manuals for UK/US products.
2) Incorrect worked angle: the processed tube will tend to change the angle after a while
(someone said that Mr Breuer was watching his arms for more than 3 months).
What is the worked allowance for the offset angle of bent arm?
3) Misprint or dubious description in original documents or repetition from secondary sources. Moreover many documents show unreliable descriptions on error angles irrelevant to the geometry of arm. There is no standard style of description/definition about error angles. Some adopted average error angle divided by groove radius (for example: 0.1degree/cm). Other adopted maximum error angle at optional radius of groove or size of record. Usually maximum error range +/-degrees are mentioned without specifying the range of groove radii. Who knows: the designer is nonchalant about geometry or the manual is wrong from typo or the manufactured arm is not made in accordance with drawing/manual etc?
4) Strained to some fixed ideas such as Baerwald/Loefgren designs. For the honour of them, please note that so-called Stevenson/Baerwald/Loefgren designs are not directly related with them who gave mathematical solution, method and thought on lateral tracking error.
Some fixed ideas of designs will lead us to overlook the actual product as it is produced.
In case of my KMAL M9BA MK3 Serial No. 75187(fixed shell with 2 thread holes for inch screws),
the linear offset measured is 77mm(19.7degree)
- far from 89mm(23degree) as proposed by Audio Magazine 1980 which stated O16.2 A23 NP60/118.
*Keith Monks M9BA MKIII L228.6 O12.7 A19.7 NP60/94 when linear offset is 77mm.
I suppose that the actual geometry of my arm is similar to the above simulation.
Arm Distance (Spindle Centre to Base Centre) Mounting template indicates 8.5inch
Arm Effective Length (from arm pivot to stylus): nominal 9inch
Overhang: 0.5inch as a result and also matching to my arm. There were various headshells so that the offset angles might be differing among these shells (perforated one/black thin plate shell of mine/big and robust one as seen in the picture gallery of The Vinyl Engine). Anyway incredible offset angle 23degrees and overhang 12.7mm were described in documents for early M9BA with robust shell (M9BA MK I) originally produced by Audio & Design around 1967. But alignment under such geometry shall indicate funny null points such as 41/137mm - it might be intended for transcription disks: effective 9inch arm for 12inch records, 12inch arm for 16inch records? See also null points on Decca International/Professional.
In case of my Infinity Black Widow with silicon oil bath and SME type bedplate 60.5x41mm.
Audio Magazine 1980 stated L237 Distance222.641 O14.359 A21.017 NP60/110.
*But original setting template indicated distance 222mm and I think that L237/Offset 21degrees/Overhang 15mm/Distance 222mm shall be more realistic design figures. Then Null points 67/103mm though I could not confirm these figures since I have not original alignment template. Anyway the linear offset length is approx. 85mm as I measure.
5) In case of Thorens, there is much confusion due to the different exchangeable head tube (Endrohr) types such as TP62 & TP63. Actual offset, Overhang & Effective length shall be determined with the combination of head tube/shell and arm tube.
6) Some old & longer arms are designed for transcription or sound track record (16inches). I think there was standard broadcasting equipment recommended by NAB in USA or BTS in Japan though I could not find any specific document but  suggestion in old catalogues. It is interesting to note that the distance (from arm pivot to turntable spindle) of such arms (Audio-Technica AT-1501II, Denon DA-302, Grace G565 & Gray 108 etc) is mostly around 270mm. 

I believe the users know the worked products better than designers or manufacturers. Hence please email to me if you find reliable data confirmed/measured by yourself (hearsay evidence is unwelcome - I've had quite enough of such kind).

Above and following calculations are based on my EXCEL sheets about Overhang adjustment on shell & comparison of arm designs which can be used at your end after saving it if you like.

Please note that "geometrical offset angle" is not same as headshell offset. Headshell offset or German wording for offset "Kroephung"( "bending the pipe") is leading us often to misunderstand the geometrical meaning of "offset angle". See above drawing. HO=Headshell offset angle to the straight pipe measured at the neck of shell or at the centre of slots for mounting cartridge. A=true offset angle is measured at stylus point against the imaginary straight line from stylus tip to arm pivot. Hence it is not easy to measure the offset angle from appearance. Linear offset can be measured with a carpenter's square. The overhang adjustment on shell is not changing linear offset length as far as the cartridge is mounted square within shell (see above right drawing) - that is changing the effective length and the distance between two null points-  see the details on another sheet: Alignment.xls    IMO: The discernible limit on our adjustment of cartridge azimuth on shell is: 0.5mm in length and/or 1 degree only so that I cannot believe too much in any method of alignment. The thickness of grids on protractor must be made very thin and we must discern the difference of 0.1mm order for obtaining exact angle alignment as seen in the following table. Man confident of having such good sight ability might be happy enough. Now I have poor sight. [15inch monitor screen (4:3=12x9inch) having 1024x768pixcels : one pixel thickness is about 0.3mm - Can you find any dead pixel/dot on your screen?] It resembles "the problem of mice who bell the cat" in Aesop's Fables.

SME drawings for series V/IV (& 300) are also misleading or confused by indicating: 

1. headshell offset D instead of true offset angle (=D minus included angle between grey line and wand centre line) 

2. pivot to tip distance (A) projected laterally to horizontal line instead of straight effective line from pivot to tip

The above confusion seems to be arising from shell design. The following drawing of AT1120 for example shows various cartridges with 10±3mm distance from the centre of mounting holes to tip can be used/aligned on this shell. This allowance 10±3mm (no mention of tracking force) should comply with JIS standard(1979) about pick-ups. IEC(1987) recommended cartridge mounting holes centre to tip:  9.5mm +/-1mm at the tracking force recommended by the manufacturer though the latter recommendation is not adopted by many cartridge makers except Audio Technica/Shure etc.

Here headshell offset angle equals actual offset angle because of the following reasons:

Thus there is no fixed alignment in design, make and performance (so that I believe alignment with protractor is effective as compromising method). Please also look into my another page about various alignment gauges in the past.

Alignment Table & Designs of Arms

I calculated offset angle and suitable overhang based on conventional/expedient linear offsets each specified for Stevenson and Baerwald types (these linear offsets were based on disputable/dubious recorded groove radii: inner groove radius 60.325 and outer radius 146.05mm). I assume that Stevenson in type 1B tried to compromise with smallest music radii of record formats: 7inch records for 45rpm & 33 1/3rpm (RIAA 2 1/8inch=53.975mm) and 12 inch LP records (DIN 57.5mm). His method is not differing from Baerwald essentially. It is a matter how to set inmost groove and outmost groove of actual records to be played on. Most Japanese arms seem to be designed similar to Stevenson type for EP/LP and have often constant overhang 15mm for arm length ranging from 230 to 260mm.  See also my reading of Stevenson paper.

Generally for the Japanese arms, the overhang is instructed while the place of Null points is often neglected. "Japanese record-philes" have usually several cartridges ready to use equipped on head shells (there was an accessory named "cartridge keeper or headshell stand") and are exchanging the cartridges along with head shells so frequently to enjoy the respective reproducing sound. Hence simple adjustment method (lengthwise adjustment on shell) is enough for them. Overhang is not measured directly in many cases and usually overhang adjustment is done indirectly by measuring the stylus point on shell lengthwise from the neck of exchangeable headshell when arm is pre-mounted on record player (i.e. arm distance from arm pivot to turntable spindle is already fixed so that proper overhang is obtained by adjusting effective length of arm as designed). On the other hand the method for adjusting null points and offset angle is very clumsy thing: I doubt that  users can adjust their specific arms perfectly in accordance with alignment tools as designed. Moreover the distortion due to lateral tracking angle error is not so great as compared with other distortions (i.e. tracing distortions).  We can calculate an expected distortion due to lateral tracking angle error, but there is no actual measuring data of this distortion respectively because this rather small distortion (less than 2%) is usually masked by other much bigger distortions (often exceeding 10%) in playing a record actually. As far as I know, any authentic measuring method of this distortion due to lateral tracking angle error is not yet established. Also read my recspcs about the comparison of various distortions. Very often we discuss an expected distortion due to lateral tracking angle error while we neglect an expected distortion due to vertical tracking angle error. Maybe it is again not so big problem when we hear monaural recordings (lateral recordings) only. Also read my VTA page.

GEOMETRY OF ARM: LATERAL TRACKING ANGLES

Explanations of drawing: the arc of radius L is tracking with pivoted arm while the arc of radius r is groove . 

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