AnalogMagik Tutorial No. 11:
Zenith Errors & Alignment Angle
(Function available on Verison 2 Only)
If a turntable cartridge can be thought of as an airplane, then the adjustment parameters we do can be interpreted using airplane terminology.
The stylus diamond on a cantilever are all glued on by hand, they are seldom glued onto the cantilever at the perfect position on all 3 axis.
When the diamond is slightly off on the ROLL AXIS, we adjust the Azimuth angle to compensate so that the stylus can sit in the LP groove which yields the lowest crosstalk level between Left and Right channel.
If the diamond is to rake the LP at the optimal angle, we adjust the VTA/SRA to a level which yields the lowest Intermodulation Distortion (IMD%).
The remaining parameter, is the Yaw Axis or Yaw Angle. This is the Alignment Angle we choose based on the initial cantilever alignment process using an alignment tool (such as the SMARTractor). Depending on the geometry we choose (Lofgren, Baerwald, or Uni geometry), we align the Cantilever according to the grid lines so that the cantilever sits perfectly aligned to the corresponding gridlines on the template.
Proper cantilever alignment rest upon the assumption that the stylus is glued onto the cantilever at a perfect position. In reality, this is rarely the case. When the stylus is slighlyy off axis (at the Yaw axis), this is called Zenith error.
Zenith angle, when off axis, basically affects the Alignment Angle at the null point. If there is no zenith error on the stylus, then the tracking error is "assumed" to be the lowest at the null point. If there is a slight zenith error, then point with the lowest tracking error would have shifted to the Left or to the Right versus the null point. In other words, you are no longer adhering "exactly" to the alignment geometry. This will increase the overall tracking error, and it will also shift the zero tracking error location (null point) to a different location on the LP.
Zenith Errors, is no more or no less important than the errors on the other two axis. That is to say, the Yaw (Alignment Angle), Roll (Azimuth), and Pitch (VTA/SRA) angle are all equally important. And just like the Roll and Pitch angle, Zenith Errors is difficult to adjust simply by eyeballing.
The analog setup and cartridge alignment process is basically an adjustment of the stylus position on all 3 axis. We all begin the adjustment process using various tools which allows us to approximately eyeball the stylus in a static position.
As soon as the platter spins, the grove modulation and dragging force will immediately change the static position which you assumed to be perfect into something sub-optimal. The static visual benchmark, essentially turns to an arbitrary position during dynamic playback of the vinyl. Needless to say, the optimal position may not be perfectly horizontal for azimuth, it may not be 92% (or whatever degree) for the SRA, and it may not be perfect even if the cantilever is aligned to gridlines.
If the goal of analog setup is to retrieve recorded signals at the lowest possible distortion, then we must perform measurements on a distortion analyzer to determine the optimal level, rather then going after visual satisfaction. Again, the benchmark is the accurate retrieval of recorded signals at the lowest possible distortion level.
While developing Version 2, we gathered a huge amount data from V1 users from around the world. For reasons which we cannot fully comprehend, we observed a strong correlation between Intermodulation Distortion Levels and the alignment angle of the stylus. In other words, the IMD% analysis can be used to adjust zenith angle imperfections, in additions to VTA/SRA.
In Version 2 of AnalogMagik, we have included an algorithm for Intermodulation Distortion measurements which have been optimized to detect incremental changes of alignment angles.
We observed that "square" diamonds exhibits a stronger correlation to IMD%, than elliptical or round diamonds. Although we cannot find a correlation in 100% of the cases, in cases where a correlation can be established, we were able to fine tune alignment angle, thereby compensating for zenith angle errors, by choosing a Yaw angle (alignment angle) which yields the lowest IMD% at a local close to the null point.
Usually when users observe an Intermodulation Distortion Level (IMD%) above 7% to 10%, then it strongly suggest zenith angle is likely off axis (zenith error).
The Azimuth, VTA/SRA test track (Track 2 on Side A and C) is strategically positioned on a location of the LP which is very near the alignment null point. Users should adjust the Yaw Angle (changing alignment geometry by rotating the cartridge to the Left or Right on the Yaw Axis, until IMD% yields the lowest number.
After accounting for Yaw Angle changes, users can go back to measuring VTA/SRA angle using the same test track. Typically when zenith angle is accounted for, the IMD% measurements for a good setup is usually below 5%, and in some cases near 1 or 2%, which makes detecting incremental changes on VTA measurements much easier.
Track 3 and 4 on Side A and C, are located in a position far from null points, which means the IMD% should carry an inverse relationship with the IMD% using Track 2. If IMD% goes down on Track 2, Track 3 and 4 should have a higher reading and should trend up if track 2 trends down. Therefore these 2 track are confirmation tracks for Alignment Angle adjustments for Track 2.