I would want it less than 10ft.
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USB2.0 is specified for a maximum cable length of 5 meters, or about 16.4 feet. You might get away with 18 feet, if you obtained extension cables that add up to that distance. If not, you could put a powered usb hub somewhere in the middle of the run, and that should work fine.
I would not put the dac near the pc, and run the analog signals through a long cable. My main concern would be that digital noise generated by the pc would couple onto the analog signals. And of course there is also the possibility that the long cable itself could degrade the analog signals, for instance by the cable's capacitance causing a treble rolloff if the dac's output impedance is not low enough to prevent that.
If your DAC is 1st class and your system is very revealing, you will be satisfied with nothing less than the best short USB cable possible. In my experience, longer IC's are preferable. I have tried optical (allows a very long run,) and its OK, but no comparison to my current Ridge Street Audio 1m USB. Difference is night and day!
Thanks for the response guys. I'll play around with some variations. One upgrade I could pursue over time would be to get a new PC that's quiet, or maybe I can put some noise reduction felt or other absorbant material around my current PC and then position it together with my other gear.
I build my own computers, and the quietest cases I am aware of are some of the Antec's, including the P180 and P182 models, and possibly (although I'm not certain) some of their more recently introduced models.
You might find this site of interest:
If you don't build your own computers, there are lots of custom builders out there, findable on the web, who I'm sure could make a very quiet computer, using these or comparable cases. You might also want to consider transferring the innards of your present computer into one of these cases, assuming that:
-- The computer is reasonably contemporary and performs satisfactorily.
-- The computer uses a standard ATX motherboard (Dell's, in particular, often use non-standard motherboards, and possibly power supplies as well, that will not fit into a standard case.
-- The computer's internal noisemakers that are not part of the case (cpu fan, hard drives, power supply, video card) are not unreasonably loud, or would be replaced with quieter counterparts.
The new flat WireWorld USB cable can handle lengths up to 7 meters. I have a 1.5m and 5m in my system and I can't hear a sonic difference between the two cable lengths like some other cable brands. BTW, the sound quality is superb compared to other cables (Kimber, Belkin Gold, Monster) costing the same price.