Dear Theo: Your idea to get as close as possible with the ready-made 0.5mm incremented shims to validate the SRA trend, then follow up with a custom job, sounds like an excellent plan to me. Good thinking!
I still recommend that when you have the custom shims made, you make a number of them in varying thickness, since not all LPs are of the same thickness.
>what is your advice on de-magging Lyra cartridges? I see small metal filings, I have no idea how they get there playing vinyl, but I see them in my magnifing glass?
"Demagging" or fluxbusting a ferrous-core MC cartridge is a low-level process intended to "reset" the magnetic alignment of the ferrous core (over which the signal coils are wound) to the magnetic field. Doing so will not exert any significant effect on the magnets or magnet yokes (unless the magnet is vintage Alnico or a similarly weak material), and will therefore not be effective in removing ferrous particles that have collected on the magnets or magnet yokes.
The ferrous metal particles that you saw with your magnifier exist in the environment (including the LPs and their sleeves), and are attracted to the very powerful magnetic field that energizes the signal coils. Any cartridge with powerful magnets (not only Lyras) will suffer from this to a greater or lesser extent.
The existence of these tiny ferrous particles is why we cover our private parts (aka gap and coils) with a pressed-paper "washi" dustcover. Other companies use a thin rubber skin (white, gray, black are typical colors), and so have we on occasion. But in our experience, the washi does a better job in allowing pressure waves inside the gap to dissipate, and sounds better.
That said, the cantilever moves in a big way when VTF is applied and removed, requiring that the hole in the front magnet (front yoke for other cartridge manufacturers) is clearly larger than the cantilever diameter. This leaves a ring-shaped space between the two. Over time, the aforementioned ferrous particles will find their way into this space and collect there, also within the gap.
So the washi (or rubber) dust cover slows down the ingress of the tiny ferrous particles, but does not prevent it completely.
When the amount of collected particles becomes excessive, it can hamper the movement of the cantilever, thereby impairing performance. We see these all the time on cartridges that are returned to us for servicing. Since the particles' ferrous composition causes them to adhere securely to the magnet (or yoke), they won't always come off easily. The best way to get rid of them is to clean them off carefully and thoroughly by hand.
We clean off this ferrous buildup, along with other grime that may have collected where it shouldn't be, as an integral part of our inspection and servicing program for any of our cartridges that is returned to us (caveat - the cartridge must not have been retipped or rebuilt by any other company). We don't charge for doing this.
OTOH, given the painstaking nature of the particle removal process, I can easily understand why companies such as Soundsmith or Andy Kim would perhaps want to ask payment for this.
In many cases, a thorough cleaning of the stylus, and / or the magnetic gap surrounding the cantilever, is all that a cartridge needs to sound pristine again. Of the cartridges are returned to us nominally because the owner feels that the stylus or suspension is worn or damaged, frequently the problem is stylus buildup that prevents the stylus from touching the LP groove properly, or accumulated ferrous particles that clog up the cantilever's freedom of movement.
BTW, if you have the opportunity, please read the following thread, from page 3 to the end.
http://www.whatsbestforum.com/showthread.php?14082-Sme-30-2/page3It talks about the factors that affect cartridge loading, but unlike similar threads here on Audiogon, or the Kleos' user manual, there are frequency charts and test schematics which should give you a better visualization of what happens.
kind regards, jonathan carr
