Implement ac-dc converter for low frequency and very low voltage without external powering?
This circuit as it is will NOT work because the forward voltage drop for the 1N5817 is about 0.55 volts at its lowest. Unless you use external power, the suggestion of a step-transformer by @Bruce Abbott is a good idea. You have almost no options at this low of a voltage. Use a tiny audio transformer and see if the speaker output (measures a couple of ohms dc) can be your signal input, the measure the secondary windings (many ohms dc) and see what kind of boost you will get, and is it enough. Increase C6-C9 to double or triple their present values. You are working on the fringes here.The 140NEX transformer should work fine. It could increase the input signal a lot, but your voltage multiplier is going to load it down a lot. The big issue is if this gives you enough to run the circuit. Now it comes down to how strong your raw 0.3vac is (how much current can it supply). If it can maintain 100 mVAC under load I think this circuit may work. It depends on the LTC3105 start-up current.