This afternoon I was listening to a new DAC for review purposes. This DAC was connected via USB right next to a Benchmark DAC1 PRE. I was listening for a while on one DAC then switching to the other and back to the first etc... For some reason the sound was cutting out whenever I listened through the new DAC I was reviewing. I tried many things to solve the problem, but not the one thing that would have fixed it. I contacted the creator of this DAC and he soon reproduced the problem. He also told me how to solve the problem and that it was solely related to a design decision by Apple. Read more for the details that will effect every single USB DAC on the market.
As you can see in the photo the MacBook Pro 17" and the MacBook 13" models have two USB ports side-by-side. I had a DAC plugged into each of the ports and the performance was vastly different. The DAC plugged into the front port sounded the same as it had the previous few months. But, the new DAC I am currently reviewing was plugged into the rear USB port. The sound coming from this DAC was cutting out at least two times per song. After trying several different remedies I contact the DAC's creator. I'll leave the name out for now because I don't want this DAC to be connected with this problem in any way. The DAC's creator told me what was happening. He suggested I connect my Benchmark DAC1 PRE to the rear USB port. When I did, the Benchmark was the one cutting out during songs. With the review DAC in the front USB port it performed just as it was designed.
So, what is the deal? Apple's design of the USB architecture in MacBook/Pro models is flawed in my opinion. Apple routed the keyboard and mouse through the rear USB port/Bus (see photo). The keyboard and mouse are designed to go into sleep mode after short periods of nonuse. When activated again the keyboard and mouse draw too much power from the USB Bus and cause audio dropouts.
Thus, the remedy is to avoid the rear USB port at all costs. On a somewhat related note is the use of USB ports for both external hard drives and DACs. As you can see USB ports are easily effected by devices connected to the same USB Bus. I highly recommend using a USB DAC with a FireWire drive or a network attached disk. This separates the DAC from other possible interference that is easily avoidable. We can't change the design of a MacBook but we can mitigate the problem by being smart about our computer connections.