Gettin' Into the GroovesTHE ART AND SCIENCE OF TURNTABLE OPTIMIZATION 3/01/2006 7:00 AM Eastern
Just as digital audio jump-started the vintage analog craze, club DJs have kept turntable and cartridge manufacturers in production. This month's focus is on turntable optimization.
The most popular (and most often cloned) DJ turntable is the direct-drive Technics SL-1200. Audiophiles would choose something more esoteric for archiving and transfers — preferably a belt drive model — but nearly all respectable models have common design features.
The front end of every turntable is the cartridge, a user option that is dependent on personal taste and playback requirements. The cartridge is typically mounted to a removable headshell at the end of the “tone arm,” a name that dates back to the acoustic phonograph era. (Purists will sacrifice this convenience for reduced mass.) Regardless of cartridge choice, the first goal is mechanical alignment, the five primary parameters of which are overhang, vertical tracking angle, horizontal tilt, stylus pressure and anti-skate.
To realize the importance of the interrelationship of tone arm, cartridge and stylus, you must first understand how a master record (or dub plate for DJs) is created on a lathe. The key component is the cutter head, which comprises a magnet assembly and voice coils that convert electrical energy into motion. The coils connect to a shaft (or cantilever) to which a cutting stylus is mounted. Grooves are cut into an acetate-coated aluminum disc.
The cutter is driven linearly through the radius of the disc by a “feed screw,” the speed (or pitch) of which is computer-determined based on program level and groove depth. Obviously, the goal is to play back the records exactly as they were cut, with the stylus precisely tangential to the groove and at a right angle to the radius. However, playback with a linear arm is not so easy because to follow (rather than create) the varying groove pitch requires precision frictionless mechanics, coupled with a bit of electronic intelligence. Only the most esoteric of turntables are so outfitted and still functional.
Enter the more common and economical pivoting-style “s”-shaped and straight tone arms. Both make an arc across the record, and while neither is a perfect solution, each keeps the cartridge within a few degrees of optimum. The longer the arm, the more gentle the radius. The first mission-critical tweak is the soon-to-be obvious overhang adjustment.
The primary disadvantage of pivoting arms is their tendency to “skate” across a record (either toward or away from the spindle). A stylus “leaning” on either side of the groove wall causes wear, mistracking (distortion) and groove-hopping. The anti-skate adjustment counteracts this.
During the years, manufacturers have created an assortment of templates, jigs and tools (such as those shown in Figs. A and B) to assist the user in optimizing overhang, the phono-equivalent of tape head azimuth. Elongating the cartridge mounting holes in the head shell, for example, allows front-to-back cartridge tweaking.
STRAIGHTEN UP AND FLY RIGHT
The cutting stylus is not perpendicular to the groove; it's some 15 to 20 degrees off-axis, like the claws of a garden rake, so the Vertical Tracking Angle (VTA) is sometimes referred to as Standard Rake Angle (SRA). To get VTA in the ballpark, start by observing the arm while a record is playing. The arm should be parallel to the record. (See Fig. C.)
Both the Technics SL-1200 and the Audio-Technica AT-SL120 allow the arm to be raised or lowered with an outer ring located below the arm's pivot area. The AT-SL120 adjustment did not go low enough, so another slip mat was added to raise the disc.
For the stylus to accurately trace the grooves, the cantilever to which it's attached must be highly flexible in both the lateral and vertical planes, as determined by the rubber-like substance used at the pivot point. User requirements determine the actual tracking force.
Audiophiles and archivists strive for the lowest mass and resonance, taking advantage of space-age cantilever and tone arm materials so that the force exerted by the stylus on the grooves is as low as possible. By contrast, a DJ wants a cantilever that can tolerate scratching without bending. Not only is the cantilever (which is hollow) thicker and more robust, but its suspension is tighter to allow increased tracking force without the cartridge body dragging on the record.
To set stylus pressure, minimize the anti-skate. Calibrate the arm by turning the counterweight until the arm floats. Without turning the counterweight, the black “calibrator” ring is rotated until “0” lines up with the black stripe on the arm so that the arm still floats. Now rotate the counterweight until the calibrator ring indicates the desired force. With the anti-skate set to the same value and the turntable spinning, lower the needle down on a groove-less land (in between two tracks, for example), and the arm should stay in place until a groove catches the needle. Increase anti-skate if the arm wanders toward the spindle or decrease if it wanders toward the disc edge.
For a standard stereo record, the playback stylus contacts the groove wall at 0.7 mils (0.0007 inches). With no modulation, the top of the groove — at the disc surface — is typically no more than 2 mils (0.002 inches) wide, so that 400 grooves can be squeezed into an inch for a standard stereo LP. By contrast, a club track's kick and power requires increased depth to keep the needle from bouncing out of the groove, expanding the pitch to less than 100 grooves per inch.
If, like me, you play 78 rpm records for archiving purposes, you'll need something other than the stereo needle, which typically has a 2.7- to 3-mil tip radius and costs around $50. Different eras and wear patterns require a wider tip radius range (from 2-mil to 5-mil, for example), whereas prices are easily three- to five-times standard.
Last fall, I purchased two Audio-Technica AT-SL120 turntables specifically for their ability to play 78 rpm records. The AT looks like an exact Technics clone, but pressing both the 33 and 45 buttons engages a high-speed mode — although there is no strobe to indicate speed accuracy. Note: While the stock SL120 has two speeds, there is an aftermarket 78 rpm mod available, but it's not for sale in “kit” form.
The next time I tackle this subject, I'll address stylus size optimization, preamps, disc equalization and artifact-removal software. Until then, groove on!
Eddie credits his dad for inspiring his life and career. Listen to his 78 rpm transfers at www.tangible-technology.com.