No more tweaking necessary, this thing will get a proper box now (Farnell #939766). Implementation details:
PSU
I settled with a Panasonic LC type valve regulated lead-acid battery; the 'blackness' and 'naturalness' it introduces have to be heard to be believed. My guess is that this level can only be achieved with SRegs when using normal (mains based) PSUs.
There is some amount of decoupling capacity near the DAC board (2*1.000uF Pana FC, 1*4.7uF Pana FC, 1*0.01 uF Wima MKP10), which delivers enough current for demanding dynamics, and filters some HF the battery produces (which it does, but obviously not as bad as a mains based PSU).
Board and Parts
The board itself can be seen
here [dddac.de]. I settled with 1% MF resistors and 22uF OS-CON SPs, good quality trim pots, WIMA caps, etc.; there is no real hifi boutique stuff to be found here. That comes in the next section.
Coupling cap & internal cabling
Lucky me I still had a couple of metres of DNM shielded SC cable, which seems to work well in this context. I decided to go for a really good coupling cap and tried Mundorf MCaps (0.47uF, as recommended by the board's designer), but found both the sonic signature and the amount of capacity lacking in this context. So I went for
MCapZNs, as these are said to have a very low loss and 'quick' and lively sound. I tried 4.7uF and was immediately pleased. They inserted a certain crispness into the sound that made rock music fun again. At the same time the bass gained a bouncy, well-defined character. They fit so well that I cannot be bothered to try other caps now.
Comparison to internal DAC
This is the fun part. My NAD CD540 had its share of critical acclaim by the hifi press, yet I was never really happy with it. It sounds bland, has a weak bass and is overall too bright. When you open the box to look for the possible problem, you get to see a comparatively large toroid, lots of regulators, nice caps and all; yet the DAC chip is a PCM1716E, which is an "all-in-one"-DAC. I've heard several opinions about this chip, all of them pretty negative.
The new DAC sounds a tad quieter (passive I/V, 1543 running at 8V), so I have to turn up the volume slightly. It is still louder than the other sources (tuner, deck) though.
I got hooked on its 'naturalness' immediately; the total absence of any audible digital artefacts makes this the most analogue-sounding digital unit I've ever come across. In direct comparison the NAD produces lots of background sizzle, which makes me feel uncomfortable.
Then comes... the bass. The NOS1543 produces oodles of it, well-defined and well-timed. It sinks NAD here (honestly, the NAD is no match); it will reproduce everything, no matter what you throw at it. I discovered groovy bass lines in grungy music I had not heard before. Amazing.
Another aspect is the sheer presence of music. With the 1543 the music is _very_ present, images are stunningly solid.
On the negative side, the NOS1543 with passive I/V puts out an inverted signal. Some claim to be able to hear absolute phase, other don't. I don't mind really, as I've been told that CDs vary in terms of absolute phase, too.
The treble resolution could be somewhat better, but can successfully tackled by using Reson/DNM interconnects. This may not be enough for some, I guess, but in the context of my system it is sufficient.
Does it make music?
Yessir
On the whole, a DIY project worth doing. Thanks to everybody contributing his knowledge to this topic.
Best regards,
Oliver, happy camper