I was curios about some old school R2R DACs namely the BurrBrown PCM56P. They where used in the late 80's in some of the better CD players like SonyES222/DenonDCD1500/MaranzXY whatsoever. There is some discussion in certain forums about their more or less superior sound quality, compared to this and that chip bla bla... I simply needed to experience one myself.
So why not use one of these oldies with modern equipment ? Read on...

That is probably not exactly a step by step "instructable" you can just follow. Parts used are somewhat special and it's pretty advanced stuff I suppose. On the other hand all components used are big and handy through hole parts, making it easy to solder.
I would call this instructable "experience sharing". Hope you can find some ideas for your own projects or at least have fun reading.
Cheers, Mario :)

Warning !!!
Opening Hifi equipment might expose you to dangerous high voltage ! You should know what you do !
This kind of hacking can damage your superduper hifi equipment, especially speakers then used with that thing, Better run tests with cheap old stuff or old headphones !

What you need...
An old CD player, a RaspberryPi (with accessible P5 header), soldering iron, breadboard, handfull of logic chips, some wires, patience, an idea how TTL/CMOS logic works, and an oscilloscope could help to track down signals.

Results anticipated...
It plays sound nicely. For an internet radio this sounds really good. I have heard over my headphones so far, there the hights seem a little sharp, on the other side it plays a nice bass and clear mid. Not bad for a ~25 year old device.
Sound quality might have a bit of potential when using a modern and Elko free output stage, see last page for improvements.

Update 16.2.2014:
Added Pictures and schematic for I2S converter and how to hook up everything.
Added Pictures and schematic of the 6x clock multiplier.
Added annotated picture of CD player.
Slight changes in text.

Update 20.2.2014:
Fixed Bug in I2Sconv schematic, LRout was connected to V+ instead of Q2

Step 1: ...searching a CD Player...

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).

<p>Hi</p><p>I want to do the same with Thomson dpl950 + RPI + MPD, but I do not know where to start;-)</p>

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