What innovative, unconventional cartridge designs can you recommend?

@richardbrand 

Are there Moving Iron designs other than Decca and its direct derivatives worth considering?  Serious question.

Absolutely! Four of my six mounted cartridges are MI, and only one is a Decca. Decca (by derivatives you will have to mean London Decca as there are no others) is a special case as it uses a design not in use by any others today. There was an Ikeda MC version, and the Neumann DST MC, but they are long gone. There are three current manufacturers of MI cartridges of non-Decca design, all resemble MM cartridges except that they have a ferromagnetic alloy on the proximal end of the cantilever and fixed magnets and coils in the body of the cartridge. This means the cantilever assembly can be lighter and more responsive. The oldest manufacturer is Grado, but Soundsmith and Nagaoka also make similar designs. Grado cartridges use a tiny disk as the ferrous metal on the cantilever, I'm not sure of the shape of that used by SS or Nagaoka. Both Grado and SS claim to make the lightest moving mass of any cartridge. Goldring do not currently make an MI cartridge and in the past Stanton and Empire among others made similar designs. Decca and Nagaoka are high output (5 and 3mV respectively), Grado are in the middle at 1mV (so you can use an MM input with sufficient gain, or an MC input), and Soundsmith make both low (0.4mV) and high (2.12mV) output cartridges. Peter Ledermann has hinted that he only makes the low output versions so that people can use their fancy MC phono stages, but it is true that his most expensive cartridges are all low output.

The top models from Soundsmith and Grado can give a high end MC cartridge serious competition. Nagaokas compare well with very good MM cartridges and are relatively cheap. Deccas, well, they are in a class of their own. Quirky, but once you have heard one, you may be hooked for life.

"Unfortunately, it is far from the full story because the more capacitance you add for a given final voltage, the lower the ripple."  I think there you run into a law of diminishing returns.  Even if the relationship is linear and defined by a constant (of which I am not certain).  Regardless, you can easily see that no matter how endlessly you add C to reduce ripple, you cannot get it to zero just by increasing capacitance.  It will approach zero asymptotically.  Anyway, if you take a PS under load and look at the PS output with an oscilloscope, and if you can turn up the sensitivity sufficiently, you will eventually see ripple. The amplitude of the ripple V in relation to DCVout will give the S to N ratio of the supply. My earlier point was that there is little to be gained by using amounts of capacitance that yield a SN ratio much lower than the inherent noise of the system, although it makes for good bragging rights. Having said all that, I wonder why there is no proposal that I know about for a battery supply for the optical cartridge.  The current demand cannot be too great, and the battery can be re-charged between listening sessions. You could use a 6V battery and regulate it down to 5V.

You also wrote, "Thinking about reducing ripple, I wondered about using a Class A amplifier being fed a constant signal.  The output should be constant voltage, whatever the load."  Yes, but in TT speed control they use a particular frequency as the constant signal to drive the motor. For your cartridge, you want DC out; there is no need at all for such complexity. Furthermore, the output of the amplifier(s) used to drive the WB TT motor is subject to the same imperfections as those of a linear PS used to power the optical cartridge.

@richardbrand 

The difference in our set-ups is the equalizer. I was using the DS companion piece, and you have the SoulNote.

@lewm 

you run into a law of diminishing returns

Yes, that's what I wrote too.

Mindful that my SoulNote Equalizer has about 2 orders of magnitude less capacitance than some DS Audio ones, I note that SoulNote uses an active Darlington-effect ripple mitigation circuit, plus fast diodes.  This could get three orders of magnitude improvement.

The SoulNote keeps the diodes in my cartridge on, whenever the SoulNote is on, even if its source is switched to MM or MC.  The cartridge is warm to the touch, which will be nice in wintertime 

Like you, I often wonder why devices with low power requirements, of less than say 100-Watts, are not routinely powered by batteries. The house batteries in my motorhome can power my modest domestic set up for a few hours, but cost about A$10-k all up.  It would be cheaper if they directly provided regulated DC