Search for the DAC...
My idea was to look for an old CD player with the desired DACs and see how I could modify it to connect with the RPi. In theory a DAC is interfaced with a pretty common synchronous serial connection similar to I2S (no not I2C). It is a 4 wire bus, it's signals can be seen in any DACs data sheet - take a look at http://www.ti.com/product/pcm56
So lets g"gle a bit... maybe "CD player DAC list"? Yeah there is a link to http://wzone.vegalab.ru/faq/cd_transport_list/y
a list of CD players from ventor Y.. and what DAC they use, remove the y and you find more lists with other vendors/players. I looked for my PCM56 or better two of them and then search for that particular player on ebay.
A bit later I got a Kenwood DP 2010, who's drive did not work.
Next step is reverse engineering the PCB and schematics. It helps very very very much to have a service manual and/or the data sheets of all the chips found in the player. I stumbled upon a service manual
for a Kenwood DP-3010 on the net, it is more or less the same as the DP-2010 (apart from the head phone level poti :) as far as i can tell, well the 3010 seems to use a different oversampling chip as well, anyway it will be extremely helpful.
The most interesting chips found in the player are the PCM56Ps of course, next to them is a bit of logic (7474 (flipflops) and 7402 (nor, inverter)) and most interesting what's this.. a SM5807 ...searching g... reveals it's an oversampling circuit, great, i just interface this one then. The oversampling circuit does all the cumbersome signal conversion for the pcm56 and it pushes the 44.1kHz up to 176.4kHz. Some talk about NOS DACs meaning Non-OverSampling is the greatest, but I don't get what advantage this should have. The oversampler moves the requirements for the past DAC low pass filter to a simple 12bB filter and smooths the signal by interpolating. That makes perfect sense to me, I go for the oversampling DAC (feel free to convince me about NOS advantages).