Best jitter free USB DAC

Can anyone tell me which of the DACs with USB support have delt with the jitter problem described below?

I cannot decide between the Transporter, SB/DAC1/USB, SB/Brick/USB, SB/Lavry etc. I want to stay in the 2K range.

That is correct. Some of the USB-interface DACs I have measured have ridiculously high jitter. There is also the problem of ground-loops, that affects the performance.

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Could someone explain to me why this is the case? My understanding is that the USB interface protocol contains error correction and clocking mechanism just like the old serial interface did, whereas the sPDIF does not. So, theoretically, there should be NO jitter at all when going through the USB. What am I missing here?
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You're missing the fact that as almost always implemented, the USB interface lacks a high-precision,local clock to control the remote DAC. This is because, again as almost always implemented, the DAC does not communicate back to the USB data source to control the flow of data.

As I understand it, the USB spec does include the possibility of implementing bidirectional communication so that the DAC clock can become the system master clock, but no-one has yet done that. If they do, the DAC clock can now be a high-precision VCXO rather than the usually sloppy PLL, with a significant reduction in jitter.

I'll try to find some references for you to study.

John Atkinson
Editor, Stereophile