What innovative, unconventional cartridge designs can you recommend?

Even worse when the Optical Cartridge, to have any real chance of being Optimised for processing the sent signal and later reverse RIAA requires a custom matched energiser for each Cartridges Photosensors used.

@pindac 

I'll address your contribution first!  Exclamation mark, sorry @devinplombier..

Even worse when the Optical Cartridge, to have any real chance of being Optimised for processing the sent signal and later reverse RIAA requires a custom matched energiser for each Cartridges Photosensors used

With all due respect, this is absolute drivel at least as far as DS Audio cartridges are concerned.  To make it crystal clear, any Equalizer can be used with any cartridge.

The need for a 'special' Equalizer arises because the output signal from the cartridge depends on the position of the stylus, not its velocity.  If I remember my schoolboy mathematics, velocity is the first derivative of position.  Fluxions according to Newton.

If you think of a bass note, optical cartridges produce their maximum output where the excursion of the needle is greatest.  Makes sense, right?  But electro-magnetic cartridges produce zero output at that point.

My understanding is that other cartridge types also need 'special' equalisers, for example strain gauges.

The reviews I have read suggest the resultant sound quality depends more on the Equalizer than the cartridge.

All the mainstream Equalizers include RIAA equalisation.  In fact, DS Audio has published a circuit schematic to allow anybody with the skills to build an Equalizer.

I know you are keen on one non-mainstream device which passes its output to an external RIAA equaliser (normal phono-stage or pre-amp) through its MM input but this adds all the noise associated with MM gain.  Sorry, does not do it for me ...

@lewm 

From where I sit, it seems there are more different optical cartridge equalizers on the market than there are optical cartridges.  Is that a fair conclusion?

Absolutely fair from my couch too.

Although, as others have pointed out, there have been previous attempts at making optical cartridges, the only real player in the market at the moment is DS Audio.

For equalisers, my reference is here DS Audio which currently lists 13 manufacturers and 21 models.  The list seems to grow almost by the day.

DS Audio currently makes six stereo cartridges and six (I think) mono variants of the same.  There will be older models as well.

DS Audio only seems to list third party manufacturers with credible world-wide distribution networks.  There are many more makers, especially in Europe, who lament not getting on to the list.

Only time will tell whether the market is oversubscribed.  If @pindac keeps up his diatribe ...

The beauty of DS Audio open-sourcing their phono stage design (that’s what it is after all, a phono stage) is that you or I can build one with $50 worth of parts if we wish. That, and it primes an ecosystem that supports optical cartridges.

If you’ve looked at the schematic, it’s not rocket science. If I recall, the biggest opportunity for differentiation lies in the output stage; you can make it as simple or as elaborate as you like.

I otherwise fail to see why cart and phono stage should be matched one-to-one, if in fact that’s what was being suggested. But maybe doing so does bring about a sonic benefit? I don’t know.

@devinplombier 

I believe it is far more difficult to manufacture the cartridges!  I have a highly magnified photograph of mine, and the precision is phenomenal.  What I originally thought were the shading plates are the optical sensors (the giveaway is a solder joint on each).  The actual shading plates are incredibly close to the sensors, and so thin as to appear translucent, or maybe it is beryllium shine.  They only have to block infrared.

Anyway, I can give some first impressions from my first two days with this cartridge, and they confirm any expectation bias I might have had 

Low bass is phenomenal, especially very low organ notes, which are particularly obvious in quiet passages.

Quiet passages are really, really quiet. The delicacy of the music shines though

Surface imperfections are mitigated, to the extent that records I got my money back on as unplayable have become useable and even pleasurable.  Seems that clicks, pops and scratches cause bigger changes in velocity than in position 

Fierce passages seem fiercer maybe because there is just so much detail.  Love ’em.

I have my SoulNote Equalizer set so I can switch to a MM cartridge on my Garrard with the press of a button, and a 5-second mute.  I have no doubt that the SoulNote is a far more detailed phono stage than my entry-level Krell pre-amp, even for MM.

With the DS Audio cartridge, the combination is stunning to my ears.  I suspect without any proof at all that the SoulNote outshines DS Audio Equalizers at the price point.  They have to, really, or there would be no point competing .  They have either got to be cheaper, or better, or both.

All the usual caveats.  The cartridge is not broken in and neither is the SoulNote.  My balanced cables are cheap 6-m microphone cables.  I am not a dedicated audiophile, I just love a small selection of classical music.  I have not connected my oscilloscope, nor done any other measurements.