millzners said:
For my own education, I'm trying to discover where the weakest links in the chain really are. You read a lot about the analog stage because that's something your average DIY'er can attack.
And probably where there is the least to improve.
But would upgrading the crap crystal to a top of the line TCXO yield more audible difference?
The "crap crystal" is probably good enough for another order of magnitude in performance, unless it is integrated in a lousy design. One has to deploy a lot of efforts to make a crappy crystal clock (perhaps with help from a bean counter). :-[ A much more likely terrain for improvement is the clock recovery from external sync. It has been demonstrated times and again that the PLL circuits used in many cheap converters leave a lot to be desired.
Is there a limit that these cheap old chips can really achieve before you hit the wall, or is the chip really the least important component?
There is definitely a limit to any converter chip, that no external tweak could help transcend. You seemed to agree with that when you compared the M-Audio and the ART.
These "old" chips are not that bad; testimony is in the recordings done with the ADC1 and 8824, they stand the test of time. Improvements in converter chips affect the perceived quality of sound very marginally; what's the difference between 114dB dynamic range today and 108dB yesterday? Nothing to write home about in practical terms. Today's chips have lower consumption, easier interfacing but audio performance is about the same, still hitting on the same old analog limitations (Johnson noise).
I intend to take that ART DIO to the limit and find out, simply because I have the schematic and it's easy to work on.
Please, let us know your findings. There's a whole industry there, trying to convince poor bastards that their doomed cheapo converter can be made to equal a Lavry or a Prism just by changing a few opamps and the PSU, as if putting a Marelli injection in a Pinto would turn it in a Ferrari.