For awhile now, I became aware in an acute sense of how fickle a USB connection and power supply is from many PC's, especially a laptop - even one with an elaborate docking station, etc. The reason is that the pupDAC, designed by cobaltmute, seems to push the limit of what is possible with a USB-powered DAC. Every voltage source needed on the pupDAC is regulated. However, the power consumption and voltage needed to maintain the regulation and data streams may significantly tax a computer's USB power supply.
At work, we've all converted to laptops with docking stations (part of the "teleworking" movement). I regularly have my pupDAC knock off line at work, have to re-start Foobar2000, and unplug and re-plug the DAC numerous times, all depending on what I'm doing at the time I'm listening to music. Besides that, the exquisite detail is lost compared to when I listen to it at home with my ample-power-supplied tower PC. For awhile, I thought perhaps that a powered-USB-hub was an answer. For instance, there are numerous reports of the ODAC performing better with a powered-USB hub compared to a basic USB connection. It didn't seem to work in my case. The sound was still bad on my docking station at work and it didn't prevent it from knocking offline.
Enter Avro_Arrow with a design called the Doodlebug - a USB Isolator.* Avro's design is based on some new chips that Analog Devices have developed for isolating a streaming, USB connection. The Doodlebug is based on the ADuM3160. Besides isolating the USB data streams, it also isolates the downstream power connection from the upstream USB bus.
Accordingly, the Doodlebug also includes an externally-powered, linear-regulated power supply. Further, since the voltage is small with USB, Avro uses several diodes to provide the voltage drop control and reference for the LM317 power supply. The result is a stable, high-quality and low-noise power supply that IMHO, improves the sound quality and reliability of any USB-powered DAC. Even with my [formerly] OK desktop tower at home, the Doodlebug has noticeably improved the sound quality of the pupDAC. More micro-detail is heard, firmer bass, and none of the knock-off-line irritations. Tests at work have yielded similar results: sound quality is equal to at home and there are no more line knock-offs.
Best of all, it's a fairly simple build and inexpensive, to boot. Here is a pic of the first prototype:
The terminal blocks, heat sink, and switch were pre-pre-prototype configurations for simplicity of testing. The next set of prototype PCB's (currently in shipment) use USB connectors on both ends and with the selected walwart, no giant heat sink is needed for the LM317. The switch was also deleted and a trimmer included to account for the variances in USB bus voltages of a source PC.
I think it's an easy statement to make that the Doodlebug will improve the sound quality and reliability of any USB-powered DAC.
EDIT: Full details on the production prototype here:
* Full disclosure, there is a somewhat similar device available on HiFiDIYme called the "USB Isolator," but it does not use a full-blown, linear-regulated power supply.
Edited by tomb - 2/7/14 at 10:12am