I have a lot to say on this subject, but its too much for one post so I'll start with the topic of NOS DACs since its being discussed right now.
First off a bit of background, I have been building DACs for quite a few years, I have built at least 30 different DACs with many different DAC chips, filters, output stages etc. I have tried hard to come up with at least some correlations between differences in what I have been building and what I have heard, this hasn't been easy, there are so many variables it's hard to narrow things down. And yes I found there ARE differences in sound, they do not all sound the same. I have done quite a few blind tests with other people listening, they were at least single blind, sometimes double blind. The blind tests do prove that there are differences, but preferences as to what people preferred went all over the place. So the following is going to be my own personal preferences, which are certain to be different than other people. Before going into details of my NOS experience let me say that I am an electronics engineer, I am well versed in sampling theory and have a moderate acquaintance with DSP, but by no means the worlds greatest living expert. When I first started Building DACs I was firmly in the oversampling is a great thing camp, it radically improves upon the old style brickwall analog filter etc. Then I heard about NOS DACs and decided to build a couple and see what they sound like. I was sure it was going to be terrible. But low and behold that is not what happened. I found the sound was significantly improved in some areas and significantly degraded in others. This was a big surprise, I was not expecting the improvement at all. At this point I had no clue WHY it sounded better. I tried several NOS DAC designs of my own and several from other people. What I found was that a lot of the NOS DAC designs "out there" were seriously flawed in many ways, it was obvious the people designing them really didn't know what they were doing. Frequently I could make them sound much better by some simple changes. Even with some of these bad design choices they all definitely have a "NOS sound". To me the improvement part is an increase in subtle detail, being able to hear subtle details of the acoustic environment, being able to hear subtle nuances in performance that purvey emotional content better. At the same time, the sound is "dirty", its rough around the edges. When you go back to oversampling it sounds much cleaner, but also flat and boring in comparison. My supposition is that the people that like NOS DACs are willing to ignore the bad parts in order to gain the good parts. It might also have to do with age, young people with good hearing may be bothered much more by the "dirty sound" (presumably the infamous aliasing). I then spent a couple years trying to find out why the NOS DACs sound better. I won't go into the full journey here, but what I eventually concluded was that it was the digital filters themselves getting in the way. I could build my own digital filters that kept the good qualities of the NOS sound but without the "dirt". I wasn't doing anything special with my filters, just a good proper implementation in an FPGA. The only thing I can think of as to why the commercial ones do not sound so good is that they are NOT properly implemented. In order to properly do a brickwall filter for 44.1 takes a fair amount of hardware resources, my guess is that the manufacturers are cutting corners to save money. They are doing just barely enough to get decent numbers in the data sheets. In particular I'm leaning towards the practice of cascading several small filters rather than using one properly implemented large filter. Looking at data sheet plots of filter performance you can frequently see this cascading of filters. An interesting occurrence happened early in my DAC quest (before my first NOS DAC) I had an inexpensive DVD player and decided to see if I could get it to sound better. I did a number of analog and PS improvements which improved things significantly, but I also noticed that the DAC chip had a "slow rolloff" filter setting as well as the default brickwall filter. So I built a little board that let me reprogram the registers in the DAC chip so I could set the filter type. I found that I liked the slow rolloff much better. In blind tests most people liked the slow better, but several hated it. Looking at the data sheet plots I found that the slow filter was implemented as a single filter but the brickwall was three cascaded filters. I have done similar tests with many other DAC chips and with my own digital filters and its turning out to be a fairly decent correlation that the biggest impact is not the filter function itself but if its implemented as cascaded filters. Filter functions DO make a difference, but if they are all implemented as cascaded filters they all don't sound so great. With filters implemented as single proper filters (enough internal bits for the number of taps and enough bits for the coefficients) differences in filter functions CAN be heard but they are not very large. Just getting the filters implemented properly is the biggie. This brings up the issue of software upsampling and NOS DACs. First off NOS does not mean no filter, just not a digital filter, you can still put an analog filter on a NOS DAC. If the builtin digital filters are the problem, it seems that a good NOS DAC playing upsampled files that were generated with a properly implemented software filter should provide good sound. And my experience is that indeed it does, especially if you put a 2 or 3 pole analog filter after the NOS DAC to get rid of residual high frequency noise. Note this has to be a GOOD NOS DAC, not one of those cheap 16 bit ones from people that have no clue what they are doing. There are a lot of people that are doing software upsampling and feeding the results into soundcards and external DACs that I think are trying to do the same sort of thing, but the data is still going through the compromised digital filter in the DAC chip. It would be much better if they fed it through a good NOS DAC. An interesting side bar on this is an early experiment I did. I had been reading about people that stacked 8 1543 DAC chips, I tried this in two different ways. One group of people spread the chips out on a board (see the picture somewhere up in this thread), but others actually stacked the DAC chips on top of each other. I tried both and found the stacked on top of each other approach sounded much better. Note this was EXACTLY the same circuit, just a different physical layout of the chips. The difference was that the stacked chips got HOT. Doing so cut off the airflow so they got to much higher temperatures. I hypothesized that it was this higher temperature rather better linearity or lower noise that made the improvement. I decided to test this by gluing a power resistor to a single chip and pumping DC through it to raise the temperature. I added a thermocouple so I could check the temperature of the chip (well the temperature of the case, not the actual chip). I then very slowly raised the temperature of the chip and low and behold it sounded way better as the temperature went up high. (still not all that great, the single chip by itself is a pretty bad sounding DAC chip) So all that theory that it sounded better because of the increased bit depth because of stacked chips was hooey, it sounded better because it got HOT. BTW the one with the 8 chips spread out on the board sounded worse than a single chip, that was just a bad idea. Things HAVE been getting better. The latest crop of chips seem to not have as bad digital implementations as previous ones (with the decrease in cost of compute hardware, its probably cheaper to just do it right than spend the money trying to develop creative corner cutting). That doesn't mean they are perfect. Every one I have tried I have been able to make sound better by disabling the internal digital filter and using a properly implemented external filter. The only ones I have found that seem to do a pretty good job of their internal filters are the Sabre chips. Well there you have it, some of my exploration of NOS DACs. John S. -- JohnSwenson ------------------------------------------------------------------------ JohnSwenson's Profile: http://forums.slimdevices.com/member.php?userid=5974 View this thread: http://forums.slimdevices.com/showthread.php?t=93483 _______________________________________________ audiophiles mailing list audiophiles@lists.slimdevices.com http://lists.slimdevices.com/mailman/listinfo/audiophiles
