The most important thing in ANY digital audio system, whether CD or computer is the source jitter. Make sure you have the lowest jitter source first. This means put your money into the best clock you can (inside the device), whether its a CD player or USB converter or USB DAC. The jitter of this clock is a function of the clock itsef as well as the power system powering the clock.
Second, DAC's can sound very different. I recommend avoiding hardware upsamplers so that you can hear the benefits of the low-jitter source you have painstakingly selected. Otherwise, all you are hearing is the jitter from the internal upsampling clock in the DAC. Few of these are decent IME, and I have modded a lot of DACs over the last 10 years, more than any other modder in the business.
Try to get a DAC with a discrete analog output stage with good drive and a volume control that allows you to drive amps directly. Avoiding a preamp can be a huge win in SQ.
Dont necessarily bite on the latest technology, such as 32/184 etc.. There are not tracks available, and even 10 years from now when they are, your favorite music will not be in this format. Also, the lastest DAC chip is not necessarily the best sounding. They may have slightly more detail rendered, but they also have the fatigue factor to go with it. some of the new technologies for D/A are not musicl IMO. Better to go with known great sounding chips, such as PCM1704, PCM179X, TDA1543, Analog devices and Some Wolfson chips.
Steve N.
Empirical Audio
