Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!csd4.csd.uwm.edu!info-high-audio-request From: eacj@theory.TN.CORNELL.EDU (Julian Vrieslander) Newsgroups: rec.audio.high-end Subject: Re: I want my VTA Message-ID: <10211@uwm.edu> Date: 14 Mar 91 14:01:16 GMT Sender: news@uwm.edu Lines: 85 Approved: tjk@csd4.csd.uwm.edu Originator: tjk@csd4.csd.uwm.edu In article <10137@uwm.edu> bilver!bill@uunet.UU.NET (Bill Vermillion) writes: >In article <10019@uwm.edu> eacj@theory.TN.CORNELL.EDU (Julian Vrieslander) writes: >> Note the line from the stylus' >>contact point on the record to the point where it meets the cantilever. >>There may not be a visible line on the stylus, so you may need to just >>imagine a line through the center of the stylus chip. This line should be >>close to vertical, or tipped forward a few degrees (5 to 10). By "forward" >>I mean that the stylus tip should be behind the stylus top, or the tip >>should be closer to the tonearm pivot than the top. > >Julian - admittedly I know a lot more about setting up mag-tape >recorders than record playing devices, but this seems backwards to >what I have understood. > >The cutting stylii are normally at about a 15 degree angle with the >tip forward of the top. This is to prevent the stylus from digging >in to the lacquer during mastering. I should think this geometry >should be matched in the playback mechanism. The recommendation for setting stylus rake angle excerpted above comes from some Shure engineers, writing in an Audio article of several years ago. I looked for that magazine last night but could not find it. I did find a couple of other articles that seemed to confirm the use of a near zero value for SRA in the playback stylus (J. Gordon Holt, in the 7/90 Stereophile, and A.J. Van den Hul, in the 4/83 Audio Amateur). I'm sure there are more authoritative sources than these. For clarity's sake, I'll recap the geometry and add a few more comments on the VTA/SRA situation, as I currently understand it. A * * * * * B * * |||||||||| \ |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||| C This is a crude diagram of the stylus assembly. The asterisks represent the cantilever and point A is its pivot point inside the cartridge (in some designs the cantilever ends here, in others it extends through the pivot, but this is unimportant for the definitions that follow). The diamond stylus chip is fixed to the cantilever at point B, the top of the chip. C is a point near the tip of the stylus. In this side view, line BC parallels the contact line of the stylus on the groove wall. Vertical tracking angle ("VTA") is usually identified with the angle between horizontal and the cantilever (line AB); more precisely it is the angle between horizontal and line AC. The two definitions are essentially the same, since the length of the stylus (BC) is usually much less than the cantilever length (AB). Stylus rake angle ("SRA") is the angle between vertical and line BC. If the stylus/cantilever design is such that BC is not perpendicular to AB , then SRA will not be the same as VTA. In most cartridges, SRA is designed to be smaller than VTA, with the stylus contact line AB nearly vertical. SRA is close to zero when VTA is set close to the nominally correct value (approx. 20 degrees). This makes sense, because a steeply raked SRA (as shown in the diagram) would cause the moving stylus to dig into the groove. A near zero SRA matches the stylus contact patch to optimally trace the ridges and valleys on the side of the groove. These ridges and valleys are represented by the ||||||| characters in the diagram, and in this side view they are nearly vertical. I don't know very much about LP mastering, but I believe that similar geometrical considerations apply to the stylus assembly and cutting head when making a master lacquer disk. The effective rotation center of the stylus motion in the cutter head is positioned to give the desired VTA value. The orientation of the cutting stylus is probably chosen to avoid excessive digging, under-cutting, and slanted modulation ridges. I imagine that it would be harder to accurately mold a vinyl LP if the ridges were high amplitude and very steeply angled: \\\\\\\. (Think about pulling it from the mold.) In the real world, the folks who operate cutting lathes choose different values for VTA, but I don't know why. And I seem to recall that there are practical reasons for setting a small non-zero SRA for the cutting stylus - this may have to do with helping to cleanly remove the excavated lacquer from the groove. -- Julian Vrieslander Neurobiology & Behavior, W250 Mudd Hall, Cornell University, Ithaca NY 14853 INTERNET: eacj@theory.tn.cornell.edu BITNET: eacj@crnlthry UUCP: ..cornell!batcomputer!eacj