As BrianHG points out this little module is a pre-cursor prototype towards a larger project with multiple audio sources and pre-amps, mixer and headphone amp. It's just that when I started to lay that schematic out I realised it was a fairly large undertaking with over 100 components embedded PSU and 4 layer board. I haven't even got to the bluetooth yet, although as that includes RF I will be sticking to a daughter card there.
One miss understanding though is the pre-amp/mixer board will be using an embedded +-15V switching supply, which I am still trying to filter the spikes and ringing off. The ripple isn't that bad, but the ringing spikes are. Oddly it's not signal noise on the amps, they are not amplifying it, but it is present on the output of the amps due to it being present on the power rails. I have stuff for building RC and LC filters to try and clean that supply up.
So, in short I am dividing and conquering through making prototypes of each module to gain experience and find the problems in smaller circuits before I try and build them all into one. With PCBs being so cheap to make these days and I find them fun, why not.
This prototype will be dual layer and USB powered, however the PCM2904 has the option of powering from 3.3V (actually 3.6 to 3.8 something), so I be using a linear regulator to filter the USB power some. My USB is incredible noisy. We are talking about 500mVpp HF noise at times. This is due to it being a gaming PC with a huge graphics card and 650W switching PSU that pre-dates the energy efficiency initiatives. It is most definitely not a pro-audio PC!
For this prototype I will start with an all-in-one TI PCM USB chip. If it sounds grand, I'll probably keep it that way.
I might prototype a 2 stereo channel mixer PCB as well, interconnect them with jack leads, possible prototype the bluetooth audio board. Continue trying to filter the switch mode power supply.
Eventually I'll build it all together into one board/case.
Note I need the switch mode power supply as the intended upstream power source will be a 9-14V DC supply from a solar power battery. I could use a virtual ground, but I have come to hate them, not least because when I connect a USB device into the mix the Earth referenced ground kills the amplifier output.