Thanks for the kind words. Also, thanks for the link to the MagPi article. I am still learning and it is good to understand this kind of thing from different angles.
My electronics design skills and knowlege is far from professional, but while building mine, I did have the thought of designing in something for having the voltage turn on automatically once 5.0v is reached. I believe to do that properly, you would have to put a in a Zener diode across the output that would break at someplace below, but close to, 5.0 volts. The reverse current through the zener could then trigger a relay switch that would close the load circuit. For audio applications, I think it must be a physical relay to close the circuit. This is because transistor-based switching would have a slight voltage drop between the collector and emitter which would be a function of load current.
I did not think of some sort of timed switch. If that were possible and easy, that would be cool to add.
The first attempt I took was also using a transformer, but the one I had was 12V, and that would imply a rather large voltage drop causing some heat (12V * sqrt(2) == 17.37V once the capacitors are charged up, which means ~12 volts drop to 5V). I think for the Pi, one shoulld be designing for at least 500mA of current, so that means about 6 watts of heat to dissipate (P = Vdrop * I).
The other thing that I found uncomfortable about using a transformer and bridge rectifier to make DC from wall AC was that I don’t have a good grasp on the math for determining what the ripple current would actually be with a particular load. To be certain, one would probably need to do a circuit simulation. I started to poke around this LTSPICE tool linear.com/designtools/software/, but about three minutes in, I chickened out up and bought the 7.5V DC wall adapter.