USB connection to DAC: multi stage improvement

@mitch2 - the Reference 3P is an outstanding improvement over the Reference 3. I owned the Series 1 and 2 USB converters prior to the Reference USB. When I originally upgraded my DAC to the 3P I had the Series 2 USB converter. There was a noticeable improvement in overall space and dimension and clarity. Once I installed the my new Reference USB there was another "step up" in weight, texture and "believability". Because I changed both the DAC and the USB within a short period of time it's hard for me to pinpoint how much of the "change" came from the DAC vs the Converter. The Series 2 is still an excellent piece. I would recommend starting with the Series 2 - it's less than half the price, and seeing what you experience in your system. 

Thanks @jhconnor that is solid advice but everything I read indicates the Alpha USB Ref is a very significant step up from the Series 2 and consistent with your “weight, texture and "believability" comment, so I believe I will go that route and purchase the USB Ref.  It will connect both a Merason DAC 1 MkII (or DAC 2) via coax and an Aries Cerat Helene via AES. I would like to switch to a single solid state DAC some day and the Ref 3P is on my short list to hear, as is the Merason Mountain.

Berkeley Audio Design excludes USB connections from their DACs to protect the delicate digital-to-analog conversion circuits from computer-generated electrical noise and clock phase noise. They believe that connecting a DAC directly to a "dirty" USB source inevitably degrades audio performance.

@jmrrobbie1 - I have read and heard about the detriment of USB connections from others, in addition to Berkeley.  Interestingly, Aries Cerat has just released a $12K interface (Hronos-Project X) (link) that accepts a USB signal and feeds their DACs a customized discrete signal on a board that replaces the USB input board.  They imply it makes the source/streamer quality mostly irrevalant and that it achieves a level of sonic improvement that is comparible to upgrading to the next higher DAC in their line-up.

Because I use two different DACs in my main system, it has been easier for me to route a high quality USB output into a DDC, which can then route two separate (non-USB) signals to the two DACs. 

After reading conflicting reports about which inputs sound best for my DACs, I tried comparing the AES or S/PDIF rca inputs on my DACs (output from the Singxer SU-6 DDC) to a direct USB input (output from my Sonore Sig Rendu SE Deluxe Opt. streamer).  The results I heard were close and I concluded the USB directly from the Sonore sounded very slightly more fluid. 

After researching the SU-6 as well as reading more about the pros/cons of USB vs. AES or S/PDIF, I have decided to try again with the supposedly higher quality Berkeley Alpha USB Reference Series interface, which I now have on order. 

There is so much to this digital source issue, including the network capabilities and isolation, streamer quality, interface choice, and of course the DAC itself and how well it is optimized for a particular interface.  It seems there is clearly no one-size fits all approach.

Just a comment on DDCs: the principal benefit of USB connections is that the DAC‘s clock slaves the server. So unless the DDC‘s clock is as good as the DAC‘s (unlikely unless the DAC is cheap) that benefit is lost. The problem with USB is it‘s proneness to incursiins of noise via the 5V line and picking up noise elsewhere due to its two way protocol (ground, RFI/EMI). Hence the effectiveness of galvanic isolation in decrapifiers. What I am trying to point out is that clocking and noise reduction need different measures

"Highly sensitive to clock phase noise. Direct USB connections can introduce this jitter into the DAC’s master clock, reducing sonic clarity".

@jmrrobie1 Hi, they fed you confusing marketing blurb here: clock phase is one thing (frequency related), jitter is another (time domain related).