Koetsu cartridges. They must be something special.

Koetsu cartridges deliver a warm, rich and “saturated” sound without sounding murky or slow sounding which is not an easy trick to pull off.

That’s a perfect description @larryi

I used to think they were stupid, overpriced little boxy rocks. Then I actually heard one, and have been a huge fan ever since. I have 6.

One of the problems was that the North American and Euro distributers would artificially amp up the price of "premium" stones (Coral, Blue Lace) versus "base" stones (Onyx, Jade) to segment the high-end of market. The price disparity was much more modest in other markets (Japan, SE Asia). That said, they were all still expensive cartridges, and the Blue Lace I have does in fact sound better than all other stones. The Coral - eh, maybe not so much. Perhaps my Coral needs more burn in, but the Blue sounded amazing from hour #1. Could be some variation from sample to sample, too.

Supposedly Koetsu bought up the whole run / supply of Platinum magnets, which is why other manufactureres don’t use them (only Lyra, in very limited fashion). They are nice sounding magnets, more smooth and creamy sound versus samarium-cobalt and neodymium. Alnico is pretty neat too, though.

but what is the possible mechanism, other than the above, mass, field strength, and geometry (meaning how the magnet and coil are placed in space)? Can someone ’splain me?

"Dynamic conditions". The magnetic field varies during playback (however slightly), and supposedly the different magnet types have different behaviors in this respect. No that’s not a satisfactory scientific / engineering explanation, but this is sort of analogous to arguing around "all amps measuring below 0.01% THD (or whatever) must sound the same". Now the anecdotes, for which I have a unique one to this topic:

I have 2 Onyxs, one an old 80s/90s short body with samarium-cobalt magnet. I acquired it with original parts, but worn stylus. It sounded absolutely glorious for about 70% or the record, before the stylus showed its age towards inner grooves. I had this rebuilt by Koetsu in 2018, specifically instructing them to keep the original magnet (because I thought this was part of its "magic"). The other is a circa 2000 Onyx Platinum that I’ve heard both in original condition, and after its own 2018 Koetsu rebuild.

I draw the following observations from these examples:

• The Onyx Plantinum (both incarnations) sounds very similar to other platinum models (Coral, Jade, Blue Lace, RSP), with slight differences among them. The Blue Lace is the best one (for some reason), but they are all of similar character.
• The old Onyx in original condition had the most euphonic and least accurate sound. It was glorious, however. Just a pure music machine.
• The new coils & parts (other than SC magnet) put into the Onyx have a brighter, more extended, dynamic and "modern" sound compared to the old coils & parts. Apparently this is "tempered" by the smoother sounding platinum magnets, because that’s its only meaningful difference now from the other Onyx.
• The SC magnet gives about 2 - 3dB higher signal output versus the Platinum magnet of same size. I guess we’d need larger Platinum magnets to compare them at equal-output levels, but that will never happen.
• In hindsight, I would’ve asked for a new platinum magnet with the old Onyx SC rebuild. The platinum magnet and new coils appear to be properly "balanced" for each other.

IIRC people still chase old Lyra Parnassus cartridges to use as a "donor" for its platinum magnet to use in a new Olympus build.

One thing I would advise for folks considering a retip on Koetsu: Koetsu used an "interface" type mount, where the stylus is fitted through a collar of different (softer) metal, then into a groove prepared in the boron. Glue is added mostly on the other side, opposite the collar. It’s generally a very clean looking mount under magnification, with less glue used. 

I believe there’s a Japanese supplier of the whole boron rod cantilever & stylus with this interface mount (Ogura?), which should be very similar to what Koetsu used. Personally, this would be my preference. The old cantilever can be removed from joint pipe, and the new one can be added in - stylus mount and all. 

When you compare to Euro and USA style tip mounts, they use a lot of visible glue - it either looks like a mound of cement (Benz, Van den Hul) or a clear-ish bubble (Ortofon). 

The interface mount will track closely to what was originally intended by Koetsu. It’s totally the owner’s perogative to try something different (and I do quite like Fritz Gygers), but it’s something to keep in mind if you want to keep your Koetsu as close to original as possible. 

Ana Mighty Sound in France has shown retip work that tracks beautifully with that of an original Koetsu tip. Same with suakimlp.com, in Vietnam. And I believe Joeseph Long (groovetickler) is also easily capabale of this (or anything else you might want). Those are the 3 I’d trust for keeping a Koetsu as close as possible to original performance.