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The accurate reproduction of signal information contained in a record groove is wholly dependent upon the cartridge stylus tracing the identical path traveled by the cutting stylus in making the original lacquer. Since the axis of the cutting stylus shaft assembly is aligned to an arc tangent of the groove being cut, the tangential alignment of the playback stylus must duplicate that arc tangent to achieve optimum reproduction of groove information.
For any pivoted tonearm effective length, there exists but one alignment of the playback stylus which will duplicate this tangential relationship established by the cutting stylus. A different alignment is required for each change in effective tonearm length.
From the time that the monumental mathematical treatise on pivoted tonearm geometry was published by H.G. Baerwald in 1941, there appears to have been only two other major papers published on the subject B.B. Bauer's in 1945 and J.D. Seagrave's in 1956/57, each confirming the work of Baerwald. Baerwald conclusively established mathematical proofs for the required tangential relationship and commented on the readily audible effects of tangential misalignment of the playback stylus.
Although the work of Baerwald, as well as Bauer and Seagrave, has been more or less ignored over the years, the development of modern phono playback systems demanded the re-examination of the phono cartridge alignment problem in pivoted tonearms. Dennesen Electrostatics tackled the problem and developed the Geometric Soundtracktor, which has been designed to translate the rather complex lateral tonearm geometry of proper alignment into a simple, easy-to-use tool, provided the tonearm has an obviously marked pivot center. The Geometric Soundtrack tor is made up of a base plus a trammel and pointer. The base is placed over the turntable spindle, and the trammel is set into the base track and positioned so that the trammel pointer is set directly over the pivot point of the tonearm.
When this is accomplished, the trammel is locked to the base plate with a thumb screw, placing the Geopoint in the optimum position. While maintaining the pointer over the pivot point of the tonearm, the cartridge is adjusted in the headshell so that the stylus tip is directly on the Geopoint and the sides of the cartridge are aligned parallel to the grid lines surrounding the Geopoint, at which time the cartridge screws are tightened. The cartridge is now aligned for the proper offset angle and overhang, as well as the two optimum null points. The time spent in performing an accurate lateral alignment is usually less than five minutes, as compared with over an hour with most previously available tools.
Accompanying the Geometric Soundtracktor is a vertical tracking angle (VTA) reference gauge, which looks like a tonearm rest post, plus a bubble level. Although the unit does not determine the actual vertical tracking angle of the stylus, it does establish the reference number for each record in your collection where the sound is most balanced and focused. We found this aspect of the total effort to be rather tedious, but once the VTA is established and marked on the record sleeve, it is a relatively simple matter to set the tonearm to the established VTA number.
We have used the Dennesen Geometric Soundtracktor for several months and highly recommend it to tonearm and cartridge designers and manufacturers as well as anyone involved in setting up a phono cartridge for playback.
Using the Geometric Soundtracktor eliminates the tediousness of the job and delivers a resultant accuracy to better than 0.003 inches.
--B. V. Pisha
(Source: Audio magazine, Mar. 1980)
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