The way I understand it (sorry, it is not very precise, but I am quite a beginner...) is something like this:
Up to the SAA7220 it is pretty much the usual Philips architecture. The 7220 performs 4x oversampling.
After that, some shift registers "move things around" so that there are 4 copies of a samples, each one delayed differently and each converted by one of the four TDA1541s. This corresponds to another 4x oversampling, for a total of 16x.
The four DACs are then "superposed" together before the output stage; they just go through a resistor to the same point (this is in the big DAC PCB, not the TDA1541 "daughter-boards").
Thanks to the high oversampling and high current, the output stage can be passive and very simple.
Was forgetting: I/V is the usual OpAmp (5534), one for each channel of each DAC (this is in the TDA1541 "daughter-boards").
Hope this helps "a little bit" (pun?)
It's a player that I really love. Quirky transport, but the DAC gives this large dynamic precise sound.
I did try to find a CD3 on Ebay at afordable cost but there is none.
Since I know that it was a very nice sounding cd player and since I have few TDA1541A , I would like to do a DAC using the digital section and especially the shift registers part of the CD3, and I would do a I-V convertion and low pass filter tube output.
I know the thread "Building the ultimate NOS DAC using the TDA1541A" but I found the schematic post by ecdesigns to be hard to understand and I would only use 4x TDA1541A chips, so I would find easyer to use a simplified version like the CD3 digital section and the shift registers part, but not the 4x oversampling.
I will try to find the bones of this player. Unfortunately I got it before internet existed and after unsuccessful attempts at contacting the maunafacturer I threw out the faulty parts but I definitely have the pcb with the 4 dacs on it.
Sorry for the "low-fi scan" (a picture actually), I don't have time to draw it better right now. And I do not guarantee that the diagram is 100% accurate...
(Above the diagram is a piece of the DAC card of a Cambridge CD2, same structure as the CD3, just implemented much worse...)
Gaetan, in you previous post you say "...only use 4x TDA1541A chips, so I would find easyer to use a simplified version like the CD3 digital section and the shift registers part, but not the 4x oversampling".
I'm not sure I understand you completely, but I think that the very simple output filter of the CD3 (is that a first order?) only works with 16x oversampling. If you have "less" oversampling, you may have to start the lowpass much lower, so a steeper filter.
If it doesn't come earlier from someone else, I may find the time to trace the shift-registers part for you. Can't promise, though...
Edit 1: I noticed that the output resistor value is not marked: it's a 100K.
Edit 2: Saw your latest post that came while I was typing; if you buy the DAC card, I don't need to trace it, right...?
Yes, long after.
This is the output stage, when the signal is analog.
The whole chain is
CDM --> SAA7210 --> SAA7220 --> Shift-regs --> DACS --> I/V --> Out filter
The diagram is from DAC onwards, and it is just one channel.
Notice that "Shift-regs" splits into four parallel streams (per channel), so there are four DACs and I/Vs (per channel, the TDA1541 is stereo). The four streams join again after I/V, and the out filter is just one.
...er, this is for an integrated player.
A DAC would have an SPDIF receiver or something, feeding I2S to the SAA7220.
I have a DAC board but the TDA chips and the output caps removed. I also have a complete functional PSU board. If you are interested, I can sell them for a nominal price, but consider that postage would be from Hungary.