Recently got my hands on a scope.
My circuit as is has a problem with parasitic oscillation on the virtual ground. To fix this I swapped out the LM324 quad op-amp with a TL074cn. The LM324 cannot sink enough current at it's output to provide a stable virtual ground, the TL074cn seems ok. TL074cn might not be the best chip for the job - it was just what I had in my box - I guess any that can handle 40mA at the output will do.
FWIW, this fix didn't have any noticable effect on the functionality! It worked fine with the oscillations (they were at about 1Mhz). Might improve the efficiency a little, but I doubt it as most of the juice is used by the LM386 which doesn't use the virtual ground.
cheers
Col
Thanks for the update. Hmm sustainers, I arrived here thinking there'd be a coherent detailed tutorial for a complete system (nope - just the driver apparently), & it seems the whole genre is still beset with issuettes/quirks, niggles and 'oh so nears'!
I'm used to the luxury of confirmed builds (often built by many before I venture that way), so being time 'light' wrt my electronics hobby, I think I'll park this one up & look back in a few years ...there might even be a schematic to accompany the ubiquitous DIY driver coil by then.
Cheers.