Just posting an observation I've made with digital transports. I'm sure jitter plays some part in why some sound different from others. But something else I've noticed lately is that the output signal level from a digital transport also seems to make a difference with regards to sound quality. I noticed this recently since I have a few DACs right now in the house, all with varying analog output levels. Ranging from 2- 3.5 vrms through the RCA (single ended) output. Sometimes the output on the high output DACs is just a bit too much with my amps (lacking headroom on the headphone amp's volume pot is annoying), which is what started me to mess around with the volume level of the digital transport.
The output level is adjustable on the M-Audio Transit and the Squeezebox v3. With the Transit the output level is maxed out in Windows and this is not adjustable. But using Foobar2000 you can adjust the output level of the digital signal. And with the Squeezebox v3 it has a built in volume control (adjusts both its analog output and digital output), which is supposedly defeated when the volume is maxed.
What I've noticed is that when lowering the volume on either transport to about 80% volume, and using the headphone amp to increase the volume on the pot results in a degradation of resolution compared to leaving the digital volume at 100% and using a lower volume on the headphone amp's pot. I tried my best to volume match using a Ratshack SPL meter during these comparisons.
This isn't so noticeable on the HD650, but when I switch over to the Omega 2 the difference in resolution is quite obvious; this manifests itself in less micro detail and slightly lesser dynamics. I have no idea if lowering the volume on those transports to 80% is resulting in truncated bits or not.
I'm wondering if this has something to do with the possible differences in CD player (and sound card) digital transports? ie a CDP putting out a lower level digital signal than another competitor.
Any thoughts? Has anyone else noticed this or experimented?