Tweaking your Record-Player Page 7

Playback VTA was standardized some years ago at 23 degrees, and most cartridges today are designed so that, with their top surface parallel to the disc, the cantilever will assume that 23 degres angle when tracked at the recommended force. It turns out, though, that VTA is too critical to be optimized by a standardized measurement; a fraction of a degree can produce an audible change. The adjustment can only be done by ear, using the nominal 23 degrees as the starting point.

Generally, when the VTA is too great (that is, when the back of the arm is too high), the sound will tend to be slightly hard and steely, excessively "detailed," and lacking in body. When VTA is too small (with the arm base too low), the sound will be somewhat dull and lifeless and lacking in detail. When you get close to the ideal VTA, though, the last small adjustment has an effect that is out of all proportion to what one would expect; almost abruptly, everything just seems to "snap in." The sound becomes sweeter, smoother, better focused, and more transparent, and the soundstage seems to open up in all directions. This is hard to relate to VTA.

A disparity between vertical cutter motion and playback-stylus motion should only cause a small and linear increase in harmonic distortion; there should not be a sharp "focus" point. But for another VTA-related phenomenon—scanning rake angle (or SRA)—there should.

Groove modulations have a height dimension. They extend from the bottom of the groove to the top, and at shorter wavelengths, the undulations become much narrower than they are high, resembling the corrugations of a tin roof (fig.5). Playback styli shaped to simulate a cutting stylus—which includes all non-conical types—have a narrow vertical ridge on each side which is supposed to fit snugly between the corrugations of these short-wavelength modulations. When the fit is perfect (it never is, quite), the playback stylus will be able to follow precisely every subtle corrugation of the groove wall. But when the fit isn't perfect, as when the stylus's ridges are not quite lined up with the groove-walls' corrugations, it can no longer penetrate the folds, and will start bouncing over the tops of them instead. This would explain the abruptness of "VTA" optimization.

Fig.5 Treble disc-groove modulations (left) are high and narrow; the side ridges on the cartridge must line up with them (right) for proper groove tracing.

What this means to the tweaker is that VTA adjustment is far less of a judgment call than any other tweak. There is one, and only one, unequivocally correct setting for it, and that's what to look for. A small caution, though: If your VTA is badly off to begin with, you may notice a small "false focus" point before you get to the one you're looking for. Continue past that until you hit pay dirt. You'll know when you find it. (Hint: If you go beyond 2 degrees of your starting point, you've probably passed it or are going in the wrong direction. For an arm with a 10" pivot-to-stylus length, 2 degrees is about a 7/16" height change.)

If you use several different discs for setting VTA/SRA, you will be dismayed to find that they differ slightly, requiring a small readjustment from disc to disc. If you are a compulsive tweaker, you may elect to readjust the tonearm height for each disc you play, but saner audiophiles (who value their sanity) will simply go for a compromise setting for a few favorite record brands, and not think about VTA/SRA thereafter. Here's how to hit that happy medium.

Obtain a paper clip, and bend its outermost straight section at a right angle to the rest of the clip, then use cellulose sticky tape to attach this to the adjustable-height part of the tonearm base so that the bent length sticks out horizontally. Cut about an inch from one end of a file card, fold this along its length, and tape it to the motor board so it stands upright next to the end of the paper-clip pointer. Then, each time you optimize arm height for a given disc, mark the pointer's height on the folded-card column. After half a dozen or so discs (the more, the merrier), you'll see by the marks where most of them had to be set. Readjust arm height for the average of these, and don't even think about it again unless you commit a tweak at a later date that changes the platter height.

You can minimize readjustment time in future if you identify one disc as your "standard" for VTA/SRA setup. Buy a set of four fine-tip colored markers (such as Pilot Razor-Points), and draw a column of different lines (an inch or so long) down the left side of a sheet of paper. For example, one line might be red, one might be black, one might be half red and half black, one might be half black and half red, and so on. Each time you optimize for a different record, identify it on the line sheet, and use its corresponding "color code" to draw its marker line on the folded-card column. Make a note of the disc that came closest to your average point on the column, and use that disc for future VTA/SRA adjustments.

Raising and lowering the base of a pivoted arm may change the azimuth tilt angle of the cartridge by enough to impair its channel separation. After you've optimized VTA, recheck the tilt angle and readjust if necessary. (This, of course, will never be necessary with a straight-line-tracking arm.)

In pivoted arms, there is one kind of vertical-pivot configuration that makes cartridge tilt independent of arm height. This is the type where the axis of the vertical pivot is perpendicular to the groove tangent line (fig.6), causing the arm to swing vertically as though it has no cartridge offset at all. This allows you to change VTA without having to readjust cartridge tilt.

Fig.6 When the tonearm's vertical pivots are at right angles to the groove-tangency line, changes in arm height have no effect on cartridge tilt.

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