[simcop2387]

I'd bet it'd be possible to make this much smaller and about half the cost. But I think it'd also end up a lot more difficult to adjust as needed since you'd end up with either tiny pots for adjustment or having to fix the values of everything and use single resistors. Going full DSP would allow you to make this fairly small and cheap (maybe the size of a 9 volt battery) but much more annoying to build and work on if you don't have the skill set. I'd bet this would be a good spot for someone to design a nice open hardware project that'd benefit a lot of people, and if it's setup as kits for headphones that just happen to work well as hearing aids you can probably avoid some of the regulatory issues even though you couldn't ever call them what they are.