Mnyb wrote: > The trend I was referring to with my blanket dismissal of NOS DAC was > the ones completely without any filter, this was very popular as DIY for > while so popular that it seems to be the norm :-/
That's it in a nutshell. Most recent work with NOS DACs has centered around the Philips TDA 1541 as a more-or-less stand alone DAC with no or minimal to vestigial low pass filtering. If you study its history it was never really intended by its developers to be used that way. Information about how the TDA 1541 was supposed to used was discussed in the early days of digital audio (1981-1983) by professional magazines like as EDN and EE Times (if they existed them and if they didn't their forebearers), but issues of those periodicals that old seem to be lost to posterity. There may be relevant Philips Application notes, but I can't find them either. The TDA 1541 chip was originally designed to be a component of an oversampled DAC, most significantly to be followed by a digital filter chip such as the SAA7030 and its sequels. One fact that I haven't seen is references to what used to be called Aperture Effect which is the reason why the output of many DAC chips have an unexpected (to many) droop at high frequencies. This issue is discussed in the context of data acquisition here: http://www.cypress.com/?docID=45630. When used all by itself the output of a TDA 1541 chip is going to have that HF-drooping frequency response that is shown in the Cyprus Semiconductor reference in figure 1. The digital filter chips such as the SAA7030 that it was supposed to be used with such as the contained corrections for this that were in the day called . It is as simple as that - the TDA 1541 was never intended to be used without some kind of Aperture Correction, and if you don't provide it, its going to sound a little soft compared to an accurate DAC. There are other potential bad consequences to not having an appropriate reconstruction filter following the DAC such as IM in following stages of amplification due to the fairly large amounts of HF noise that is not filtered out like it should be. The noise itself is > 22 KHz and likely to not be heard by many if anybody. The IM doesn't always happen to an audible degree, but I'm sure given the relatively high amplitude of this noise it does happen some times. I suspect that there are even people who like their music with a little aharmonic nonlinear distortion spurious responses tossed in. ------------------------------------------------------------------------ arnyk's Profile: http://forums.slimdevices.com/member.php?userid=64365 View this thread: http://forums.slimdevices.com/showthread.php?t=93483 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/audiophiles
