Anyway, according to this Wikipedia article line level for consumer audio appears to be around 0.447V peak: http://en.wikipedia.org/wiki/Line_level
I was gonna just adjust the volume in software to reduce the voltage, but I decided I'd get better sound quality with a voltage divider and it would also allow me to connect both a speaker and amp to the board at the same time without sacrificing volume on the speaker.
I don't know if I need them on the amps output.
I've found a lot of different designs for them but they all talk about reducing decibels rather than voltage
I'm a bit confused about how one would wire that ground connection with the 47nF capacitor though. I'm pretty sure I can't ground my whole analog circuit through a capacitor. But perhaps that ground it and the other capacitors are going to isn't the mp3 shield's ground but rather a connection to the amp's ground.
Just connect your optional amp to the DAC output (with a blocking capacitor) and you don't need a differential attenuator. Since that signal is single-ended, you can use a regular voltage divider or pot to bring the signal down to "line level". Or if the amp has a volume control, you can just feed-in the "slightly hot" signal (slightly higher than normal line-level) and you'll be fine!It's no problem connecting your home-made amplifier and another one to the DAC at the same time.
If you want to connect another amplifier to the differential output of the amp you build, YES, you need a blocking capacitor. Just use one-side of the differential output and put a capacitor in series. Connect the additional amplifier's (single-ended) input to the capacitor, and connect its input-ground to your amplifier's ground. (One-half of the differential output won't be used.)
I'm not familiar with that device at all. Somewhere, all of the grounds need to be connected directly together. That is, the gound(s) on the DAC circuit need to be connected to the ground(s) on amplifier circuit. On a regular hi-fi amplifier, that common ground is on the RCA input jack, which has one signal wire and one ground/shield wire.