I finally got around to doing some more tests (incredibly busy these days). I still can't get decent repeatable results ADM so I tried a different approach.
I have a very good HP spectrum analyzer which is very good at looking at clock spectra. Its quite sensitive at showing clock jitter as sidebands around the main clock peak. So I took the Touch apart and hooked the analyzer up to the clock going into the DAC chip. I was stunned, this is the best looking clock spectrum I've seen in a commercial product. Almost a perfectly flat line right at the measurement limit. Not quite perfect, there are a few little squiggles, but they just barely peak above the floor. So I tried sending flac and PCM to the Touch, no discernible difference. There might be but I can't see it with this test equipment. Then I remembered that I was hearing the difference over headphones, so I tried plugging the phones in. Wowa! That made a big difference. Now there were significant sidebands on the clock spectrum. Interestingly enough mostly at multiples of 120Hz. Which is interesting because the PS is a switcher! It certainly looks like the current drawn by the headphone amp actually driving phones is somehow causing some increase in the clock jitter. I did try flac and PCM with the phones plugged in and again I could not see any difference. I also tried connecting to the analog outs and got the same results as no connections: almost perfectly flat line. So the spectrum analyzer could see no difference in jitter between flac or PCM. That doesn't mean its not there, it still could be lower than what this instrument can detect. BTW the theoretical limit of this instrument for measuring jitter comes out to somewhere around 10-20ps, so if its right at the edge, the Touch is phenomenal. Jitter was one of the possible mechanisms by which sound could be changed by different streaming formats. Another is power supply noise directly into the DAC supply or other analog devices, since the Touch has no devices between the DAC and the analog outputs, that pretty much limits it. But the headphone jack does have an amp that does seem to affect things internally so next is to look at the PS rails at the DAC chip and especially the headphone amp and see what can be seen. BTW I did do some simple blind listening tests (simple to do because someone else can switch the stream format remotely and there is no indication on the Touch's screen which is in effect). I had a very good rate at hearing the difference in these tests. My wife didn't want to sit still long enough to do the tests so I don't have someone elses ears as well. I know it was not scientific at all, etc etc. But for me at least its lending a little bit of evidence that I'm not completely imagining this. John S. -- JohnSwenson ------------------------------------------------------------------------ JohnSwenson's Profile: http://forums.slimdevices.com/member.php?userid=5974 View this thread: http://forums.slimdevices.com/showthread.php?t=71321 _______________________________________________ audiophiles mailing list audiophiles@lists.slimdevices.com http://lists.slimdevices.com/mailman/listinfo/audiophiles
