Hi Peter

I take it for granted, that you are going to make this device available to the public - e.g for sale in the local musicians shop or similar, and that you plan to make a lot of them.

Then read and understand the advices given by several forum members in this thread. Your customers won't be back for the next device you market, if the first one was crap. Sorry for the language, but what you have to sell, is descent engineering! Period. Your midi device might be the "next best thing to sliced bread", but if the implementation - in terms of engineering - is below par - it doesn't matter!

So, do as Per W suggests, and let your device accept inputs from 8 to 24V or so. Ad a bridge rectifier at the input, so polarity isn't an issue. Since this device almost certain includes a (8051) micro controller, you might ad a common mode filter at the PSU input, and don't forget to use a four layer pcb, with a SOLID groundplane.

Don't cut any corners! Most electronic components is dirt cheap, compared to engineering costs, pcb layout and manufacturing and EMC testing!

Doing things by the book, won't solve all your problems, but it's far simpler to debug problems, if you know that you what have done is proper engineering!

Per