Author: Geoff Husband - TNT France
When I wrote the VTA/SRA article a few weeks back I suppose it would be inevitable that people would write asking me about other aspects of cartridge alignment. The letter below was sent last week, and as it puts the various problems so succinctly I found myself answering with what amounted to another article!
Rather than let it be lost in the "letters" section I decided to .html it up and put it forward as a short article people could refer to. This is not the in-depth investigation of my SRA article because I think the subject rather more straight-forward and I didn't want to repeat too many of my arguments. So here goes, this is what the reader wrote in its entirety...
I was very pleased to read Geoff Husband's recent article in regards to VTA, particularly since I came to much the same conclusion the last time I set up a cartridge - a Denon DL-301 II MC which is still available here in Japan. My conclusion was based on a much more abbreviated investigation than Geoff's: I decided that an eyeball approximation and a very subjective listening test using three or four of my favorite LP's was sufficient. In previous setups, I had obsessed over the question of VTA. After hours of bending over my player with Allen keys in hand, I failed to find a vital point where everything jumped into a new, startling focus. Still, dramatic or eccentric adjustments did make a difference. Yes, and there was a tolerance, but incremental changes seemed pointless.
So while the question of VTA is settled for me at least, the adjustment for LATERAL TRACKING ERROR on a radial arm remains. And it begs for more attention. For me, the most vexing problem occurs when I buy a cartridge, and I find that the cantilever is some degrees out of alignment to the centerline of the cartridge block. Are cantilever variances exhibiting a purposely designed suspension bias, or are they the result of production flaws? If they are the former, how does one statically adjust the cartridge block in the head-shell in accordance with the protracted crossover points without the actual playback's LATERAL FORCE applied? Do we adjust on the bias, or use the block's centerline as a reference? In all the literature I have read on the subject of cartridge setup, I have found nothing to help me with this. This is merely one of a number of outstanding questions relating the adjustment which uses those eccentric holes in the head-shell bracket.
I am thinking that more sophisticated methods may be necessary. Up to now, I have not used a volt meter to monitor Left-Right output. I do not own a scope or a test LP disk. Up to now, all I have used was a DIY protractor I made by scribing lines on an old LP, a downforce gauge, a mirror and a flashlight. Are we all going to need more than that to do the job correctly? I hope Geoff will apply the same wisdom to this subject as he did in the case of VTA. Thanks Geoff and TNT!
Thanks for the kind words, as for your question it really deserves another article so I hope you'll excuse me using it in this format...
So to your first question - if the cantilever is out of alignment it is a manufacturing fault, in theory it should be perpendicular to the front of the cart when viewed from above, both when off the disc and when in use with the correct anti-skate setting. However as often as not it isn't...
In theory (again) the diamond stylus should be aligned correctly in the cantilever, so when you align the cartridge it is the cantilever that should match the parallel lines of your protractor, not the sides of the cartridge. In theory (sic) the stylus should then be correctly aligned in the groove providing the geometry of the arm is correct and matches the protractor. But most manufacturers of quality cartridges can only align the stylus +/- 0.5 degree or worse, so even if it's spot on on the gauge it's quite likely to be misaligned.
Does this matter? Well with any "radial" arm the cartridge and thus stylus
swings through an arc such that it is only perfectly aligned with the
original alignment of the cutter at two points. These points vary with arm
geometry, but most manufacturers use a geometry that puts these 'null'
points at about 1/4 and 3/4 the way across the disc. Doing this means that
the maximum error, i.e. when the stylus is most out of alignment, is
minimised along with the "average" misalignment.
The actual 'null points' and thus arm geometry, is the subject of much controversy and some manufacturers - e.g. Naims ARO, place the inner 'null' point on the last track of the disc because that is where the stylus has the greatest trouble tracking and so needs as much help as it can get. The downside of this approach is that then the maximum error elsewhere on the disc is greater - you pays your money and makes your choice. As with SRA the longer the arm the less the misalignment. However if your cartridge (and inevitably stylus) is misaligned by a couple of degrees (not a lot and difficult to spot) then when added to the maximum misalignment due to the radial arm's geometry it can become significant. What happens is that the stereo signal has a left/right anomaly as the right hand groove modulation (for example) is 'read' by the stylus before the corresponding left hand groove, this you could call "stereo time-smear" if you want to be pretentious...
So as with the SRA problem we are left not with the question "how do we achieve perfection?" ('cos you can't), but with "how close does it have to be before we notice the effect?". And once again we have to be practical. Aligning styluses to a fraction of a degree is tough enough to do simply by undoing a couple of bolts and wiggling a cartridge (which is what nearly all arms expect us to do). To do it repeatably so we can do A/B comparisons is simply impossible unless you make up a series of complex jigs each allowing - say - 1/2 degree difference. SME's have a sliding base which is the neatest solution I've seen, but I've also seen a couple of rotating headshells though here the cure may well be worse than the disease. The trouble is that even with a perfectly aligned cartridge the stylus will be out of alignment by 2 or more degrees at some points of the disc. So there has to be some tolerance built into the system to allow us get away with this - the secret being not to compound the misalignment so that the cartridge is - say - 5 degrees out at the end-of-side.
So getting back to the real world, if you think you can hear distortion and/or a collapsing of the sound stage at some points on the disc rather than others then you've probably exceeded the tolerance of your ears/stylus and need to try adjusting the cartridge angle. Fine tuning has to be done by ear especially given the possible misalignment of stylus in cantilever.
So that's why parallel tracker are so good? Well yes and no - for all those proponents of parallel trackers, it's worth noting that any misalignment, either in angle due to manufacture or adjustment or, just as important, of effective length (i.e. the cartridge is a mm too far down the armtube) will produce an error right across the disc which makes it very difficult to spot.
A radial arm on the other hand is a bit like a stopped clock - even if misaligned (within reason) it will be spot-on twice on the disc:-). So here we have a big advantage over the problem of the 'magic spot' of SRA - we know that that stylus will be in the 'sweet spot', (should it exist) twice on the disc, so what we are looking for is unacceptable deterioration elsewhere, rather than chasing some mythical crock-of-gold. If you can't hear a deterioration then you are OK as the stylus will be operating within its tolerances. If you do, then you can try altering the alignment - simple:-)
So once again be realistic. If you can hear a problem try to sort it - if not, leave it alone and listen to music...(*)
(*) A note to obsessives. To be sure you have the right alignment you need a dual-trace oscilloscope and a mono recording - make sure both traces line up and you will know that for that point (a null) you will be perfectly aligned - and the best of luck...
© Copyright 2004 Geoff Husband - www.tnt-audio.com