DIY tracking angle and azimuth gage
I got so fed up trying to align my cartridge using protractors, trying to judge when the cartridge was parallel to the little lines, I was ready to shoot someone. Since that's frowned upon, I decided to make a tool to make the job easier.
Specifically, I wanted to be able to very quickly align the cartridge axis to be exactly tangential to the record. In addition, I thought it would be nice to be able to measure the tracking error.
This tool does both these things. In addition, with a slight modification, it can be used to measure azimuth.
Here's the thing in operation,on my experimental turntable, with rega tonearm and Shure M97 cartridge:
It's a strip of 3mm thick aluminum, about 12" long. At the far end I've mounted a laser. Directly in front of that is a section of alu angle, that serves as a screen for the measurements. It has a hole drilled to allow the laser to pass through.
I mounted a small front surface mirror on the cartridge, in such a way that it is parallel to the cartridge longitudinal and vertical axes. Yes, I know this would be very difficult with certain cartridges. But I don't own one of those.
When the cartridge is exactly tangential, the mirror reflects the beam directly back to the laser. If it is not tangential, it will deflect the beam to one side or the other. I place a graduated scale to intercept the return beam, and measure the amount of error. The graduations on the scale are 2mm apart. This picture shows a deviation of 10mm to the left.
When the cartridge is near the outer edge of the record, the graduations represent about 0.2 degress per mm. When 60mm from the platter axis, the graduations represent 0.35 degree per mm. The precision can be increased arbitrarliy by making the optical path longer (i.e. mounting the screen further back). I positioned my screen 100mm behind the platter axis, to simplify the math.
To measure the error, I divide the deviation in mm by the distance from the screen to the mirror, and then take the arctangent of the result.
It's also possible to measure the error by measuring how many degrees the device must be rotated in order to bring the beam into alignment.
The laser mount:
The beam striking the mirror: The spot is larger here, because it's out of focus. It's adjusted to focus to a point somewhere near the measuring screen.
I aligned it so that the beam was exactly parallel to a line between the spindle axis and the stylus, but pointing slightly down so that the return beam would strike the screen just below the hole I drilled for the laser. To measure azimuth. the beam would need to point exactly horizontal, and the measuring scale would be vertical in stead of horizontal.
I mounted the laser in a section of alu channel, with 3 grub screws to facilitate fine lateral alignment. I achieved vertical alignment by placing shims under the laser mount. The alignment was checked by measuring where the center of the beam fell at various distances along the device. It's actually quite easy to get the beam aligned properly.
The results: My attempts at getting the cartridge aligned simply by using protractors were hopeless. When the cartridge looked to be in alignment with the protractor, it was always off by a couple of degrees in either direction. I can use this tool to get the alignment correct with a protractor, and to measure tracking error with any other alignment or with a different arm, etc.
I did notice that the cartridge is extremely sensitive to the overhang length. Changing this slightly causes large changes in the tracking error at different points in the playing area. It's very tricky to get it right.
I'm normally using the Baerwald/Rega alignment portractor. I did notice that I was able, by trial and error, to find an alignment with smaller maximum error than the Baerwald. But I believe that's to be expected, as the Baerwald minimizes the weighted error, not the absolute error, is that correct?
I got so fed up trying to align my cartridge using protractors, trying to judge when the cartridge was parallel to the little lines, I was ready to shoot someone. Since that's frowned upon, I decided to make a tool to make the job easier.
Specifically, I wanted to be able to very quickly align the cartridge axis to be exactly tangential to the record. In addition, I thought it would be nice to be able to measure the tracking error.
This tool does both these things. In addition, with a slight modification, it can be used to measure azimuth.
Here's the thing in operation,on my experimental turntable, with rega tonearm and Shure M97 cartridge:
An externally hosted image should be here but it was not working when we last tested it.
It's a strip of 3mm thick aluminum, about 12" long. At the far end I've mounted a laser. Directly in front of that is a section of alu angle, that serves as a screen for the measurements. It has a hole drilled to allow the laser to pass through.
An externally hosted image should be here but it was not working when we last tested it.
I mounted a small front surface mirror on the cartridge, in such a way that it is parallel to the cartridge longitudinal and vertical axes. Yes, I know this would be very difficult with certain cartridges. But I don't own one of those.
An externally hosted image should be here but it was not working when we last tested it.
When the cartridge is exactly tangential, the mirror reflects the beam directly back to the laser. If it is not tangential, it will deflect the beam to one side or the other. I place a graduated scale to intercept the return beam, and measure the amount of error. The graduations on the scale are 2mm apart. This picture shows a deviation of 10mm to the left.
An externally hosted image should be here but it was not working when we last tested it.
When the cartridge is near the outer edge of the record, the graduations represent about 0.2 degress per mm. When 60mm from the platter axis, the graduations represent 0.35 degree per mm. The precision can be increased arbitrarliy by making the optical path longer (i.e. mounting the screen further back). I positioned my screen 100mm behind the platter axis, to simplify the math.
To measure the error, I divide the deviation in mm by the distance from the screen to the mirror, and then take the arctangent of the result.
It's also possible to measure the error by measuring how many degrees the device must be rotated in order to bring the beam into alignment.
The laser mount:
An externally hosted image should be here but it was not working when we last tested it.
The beam striking the mirror: The spot is larger here, because it's out of focus. It's adjusted to focus to a point somewhere near the measuring screen.
An externally hosted image should be here but it was not working when we last tested it.
I aligned it so that the beam was exactly parallel to a line between the spindle axis and the stylus, but pointing slightly down so that the return beam would strike the screen just below the hole I drilled for the laser. To measure azimuth. the beam would need to point exactly horizontal, and the measuring scale would be vertical in stead of horizontal.
I mounted the laser in a section of alu channel, with 3 grub screws to facilitate fine lateral alignment. I achieved vertical alignment by placing shims under the laser mount. The alignment was checked by measuring where the center of the beam fell at various distances along the device. It's actually quite easy to get the beam aligned properly.
The results: My attempts at getting the cartridge aligned simply by using protractors were hopeless. When the cartridge looked to be in alignment with the protractor, it was always off by a couple of degrees in either direction. I can use this tool to get the alignment correct with a protractor, and to measure tracking error with any other alignment or with a different arm, etc.
I did notice that the cartridge is extremely sensitive to the overhang length. Changing this slightly causes large changes in the tracking error at different points in the playing area. It's very tricky to get it right.
I'm normally using the Baerwald/Rega alignment portractor. I did notice that I was able, by trial and error, to find an alignment with smaller maximum error than the Baerwald. But I believe that's to be expected, as the Baerwald minimizes the weighted error, not the absolute error, is that correct?
Pixpop,
First, let me apologize for the terse response to your original post. I had a busy day and simply wanted to quickly point out that there is much more going on with cartridge alignment than meets the eye (pun intended!).
Your alignment tool is a nice piece of work. You are also correct in assuming that the absolute error of the tracking angle is not as important as the weighted error, which is the angular error divided by the groove radius.
Baerwald's original work was from 1941. Many of the earlier null radius calculations were for records with outer diameters of 10" (and even some for 7").
I've seen suggested R1's as low as 66mm and suggested R2's as high as 126mm. What are you using?
IMHO, the best articles on this are Kessler's and Pisha's (AUDIO, January 1980) and Heidenstrom's (HI-FI NEWS & RECORD REVIEW, July 1981).
My ears tell me that going for mininum distortion on the inner grooves is the most important. Decreasing groove radius and that tremendous crescendo at the end of the Mahler 2nd are a tough combination!!
First, let me apologize for the terse response to your original post. I had a busy day and simply wanted to quickly point out that there is much more going on with cartridge alignment than meets the eye (pun intended!).
Your alignment tool is a nice piece of work. You are also correct in assuming that the absolute error of the tracking angle is not as important as the weighted error, which is the angular error divided by the groove radius.
Baerwald's original work was from 1941. Many of the earlier null radius calculations were for records with outer diameters of 10" (and even some for 7").
I've seen suggested R1's as low as 66mm and suggested R2's as high as 126mm. What are you using?
IMHO, the best articles on this are Kessler's and Pisha's (AUDIO, January 1980) and Heidenstrom's (HI-FI NEWS & RECORD REVIEW, July 1981).
My ears tell me that going for mininum distortion on the inner grooves is the most important. Decreasing groove radius and that tremendous crescendo at the end of the Mahler 2nd are a tough combination!!
I think in general, it's enough to repeatably set the angle of the cartridge body. If there's a difference between that angle and the angle of the cantilever, I think it will become a constant error term, which can easily be added or subtracted once the angle is measured. If this difference is not constant, then the cartridge is broken.
I haven't settled on an alignment yet. I'm still experimenting.
Most often, I'm noticing a sweet spot with the inner null point somewhere around 70mm. Sometimes the outer null is off the record, but the error is not very high. I don't want to make any definitive statement until I can get the overhang adjustment under control as well as the tracking angle.
What I need is a way to adjust the overhang precisely and repeatably. Tweaking it by loosenig the screws and making tiny adjustments is very time consuming and tedious. The Rega cartridge mount is not exactly a marvel of precision engineering, which doesn't help.
Perhaps I can do this with a micrometer head, measuring the distance from the front of the headshell to the cartridge. I could set the micrometer to a particular value, and then butt the cartridge up against it, and rotate it until I get the tracking angle I want. Then, tighten the screws.
Also, I need some LPs that will make it easier to hear the effect of changes that I make. Maybe there's a test record that's good for this.
I haven't settled on an alignment yet. I'm still experimenting.
Most often, I'm noticing a sweet spot with the inner null point somewhere around 70mm. Sometimes the outer null is off the record, but the error is not very high. I don't want to make any definitive statement until I can get the overhang adjustment under control as well as the tracking angle.
What I need is a way to adjust the overhang precisely and repeatably. Tweaking it by loosenig the screws and making tiny adjustments is very time consuming and tedious. The Rega cartridge mount is not exactly a marvel of precision engineering, which doesn't help.
Perhaps I can do this with a micrometer head, measuring the distance from the front of the headshell to the cartridge. I could set the micrometer to a particular value, and then butt the cartridge up against it, and rotate it until I get the tracking angle I want. Then, tighten the screws.
Also, I need some LPs that will make it easier to hear the effect of changes that I make. Maybe there's a test record that's good for this.
Yes, I have room. Actually, I also left room at the back for a linear tracker. But I'm not ready to tackle that until I've managed to make a pivoted arm that works tolerably well (i.e. no worse than my Rega 250).
But I'm actually going to abandon this plinth. I don't like working with the MDF, and it turned out to be much more resonant than I expected. Also, it warped a little, probably because it had been standing leaning against a wall for about a year. I think my next one will be a wood/metal sandwich, with metal (aluminum) on top to make mounting things easier and more precise.
My next effort will be direct drive, rather than belt drive, as I have a nice DD motor and platter to try out. Mounting these cheesy Technics components on this quick and dirty MDF plinth made a huge improvement, and got me motivated to keep experimenting.
I have more plans that time to complete them, though
BTW, when I mention overhang, I'm actually referring to the fore and aft adjustment of the cartridge in the headshell. Perhaps that's not the correct terminology? Anyway, it's very difficult to adjust the tracking angle without inadvertently changing the fore-aft position as well.
But I'm actually going to abandon this plinth. I don't like working with the MDF, and it turned out to be much more resonant than I expected. Also, it warped a little, probably because it had been standing leaning against a wall for about a year. I think my next one will be a wood/metal sandwich, with metal (aluminum) on top to make mounting things easier and more precise.
My next effort will be direct drive, rather than belt drive, as I have a nice DD motor and platter to try out. Mounting these cheesy Technics components on this quick and dirty MDF plinth made a huge improvement, and got me motivated to keep experimenting.
I have more plans that time to complete them, though
BTW, when I mention overhang, I'm actually referring to the fore and aft adjustment of the cartridge in the headshell. Perhaps that's not the correct terminology? Anyway, it's very difficult to adjust the tracking angle without inadvertently changing the fore-aft position as well.
That's the correct terminology - overhang is the amount of distance the center of the stylus is from the center of the record (or spindle). And yes, most of the time it is adjusted by moving the cartridge, fore and aft, in the elongated slots in the headshell. So, what you really need is something that can measure that.
Usually the arm will pivot far enough so that you can measure it, perhaps not very accurately, with a small ruler. Why not make up a small metal disc, the height of the spindle, with concentric rings, maybe spaced 1mm, to help?
Usually the arm will pivot far enough so that you can measure it, perhaps not very accurately, with a small ruler. Why not make up a small metal disc, the height of the spindle, with concentric rings, maybe spaced 1mm, to help?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.