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.
Well, the issue with that is twofold.
1) I don't even know yet if external amps want a single ended or differential output. I suppose, looking at how I wire a single ended input to my amp, that if I do something similar to that, and send one output to ground via a DC blocking capacitor that that might achieve what I want, but I'm not sure about it, nor am I sure whether I should do that to the + or - output or if that even matters either. Hell, I'm not even sure if it matters if I plug the + or - into the + or - on the external amp. With a differential output I assume it does not matter. but with a single ended, it might.
(Reading the rest of your post, it sure sounds like differential vs single ended input to another amp doesn't matter, but I've been reading so many documents, and there's talk of consumer audio using single ended, and how to design your amp to be able to handle both single and differential and I don't know if the amp designer has done that... But looking at the differential vs single ended input on my amp, the schematic does not look so different. The single ended is just referenced to ground I guess. I'm not super confident about how op amps behave yet though so I'm uncomfortable doing something without being sure it's the right way. It's all too easy to make something that seems like it works only to have it blow up in your face later.)
2) If I do that, I lose the benefit of differential output when using twisted pair for noise cancellation. Honestly, I don't even know if I should be worried about this since the wires will only be 6-10' long and I don't often see twisted pairs used for even very long speaker cables, but I've seen twisted pair used for much shorter lengths of servo cable so I really don't know when I should be worried about noise.
Of course prototyping would answer some of these questions, but I've still got a month of work to do assembling some other circuits I have before I'm gonna be able to get to that, and I'd like a solid point to start from.
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.)
Okay. I'm still a little fuzzy on why exactly the DC blocking capacitors are needed though. AC or DC I'd think the same amount of power is going through the circuit, so I don't get why DC is really bad. Also not sure why a simple resistor to limit current isn't 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.
Hm, so you're saying an RCA input is single ended, and the ground/shield wire of it is just connected directly to my board's ground and my amp's output has one side wired to ground via a dc blocking capacitor, and the other side goes to the center pin on the RCA jack.
Seems like that makes sense.
But... What does this all mean for a differential output? Single ended seems convenient with the ground connection right there. But with differential you still need one more connection for the common ground right? That would explain the ground pins in that schematic on page 8 here:http://www.sparkfun.com/datasheets/Widgets/MP3Trigger/vs10XXan_output.pdf
But why the heck does the left side of that have "out" written on it, when the right side is the MP3 shield, and the left side is the external amp? Shouldn't those be labeled as inputs? Also, I have no idea what the gbuf pin is. But the external amp does go to ground and I guess it can be assumed the MP3 shield goes to ground somewhere as well.
I'm just confusing myself now though. Going back to the RCA input, it seems convenient, but it seems like I wouldn't just stick the signal and gnd pins from that into the + and - pins on the external amp... I would have to go to the + and gnd pins on the external amp. And the - pin on the external amp, and the - pin on my internal amp would both need dc blocking capacitors to ground.
So really, the RCA connection is only simpler than the differential setup if you have an RCA jack on both ends already wired up properly, enagaging those DC blocking capacitors on one side of the single ended output when needed (or having them on all the time if you don't need a differential input/output).
So am I understanding this right? If I have a differential output from my board, and I plug the + into this amp, and connect the grounds:http://www.sparkfun.com/products/9612
Do I still need do place a DC blocking capacitor on that amp's - input going to ground?
If that's the case, it seems like it would be less confusing to the end user to just tell them to plug + into + and - into - and connect the grounds of their batteries. And then I just have to worry about ground loops and figure out how to avoid them.
Thanks again for the help btw. It's hard figuring this stuff out when I don't have any way to know if what I think is right is actually right.