Well, PCB's have been ordered and I hope to have things done sometime in April. Keep in mind that this is a team effort with the primary designer, Avro_Arrow.
Those of you that follow the ODAC may have learned to despise RMAA, but if used for direct comparison rather than absolute measurements, I don't think you can fault it:
(note that RMAA uses a 1K carrier signal, hence the spike anomaly)
More details are available on the DIY section of the Head-Fi, but I don't want to risk the accusation of doing nothing but shilling.
The PupDAC is admittedly, a Beezar product. However, it is a quite sophisticated USB-powered DAC, having as many as 5 separate regulators onboard in addition to numerous electrolytic power caps. The USB for the DAC communicates via the TI PCM2706/7 chip, which is limited to only 16-bit/48kHz, maximum. The DAC chip is a TI PCM1794, which is rated to -129dB S/N in stereo configuration. Nevertheless - except for the tiny spikes, one can see that the total DAC implementation is less than -125dB S/N for pretty much everything from 300Hz to 20kHz. (Oops! That was an absolute reference. )
Anyway, whether the DoodleBug provides a benefit depends on the power supply inherent on a USB-powered DAC's PCB. For those DACs with few or no regulators, little performance improvement will be seen. (Low-fi is still low-fi, IOW.) For those DACs that are more sophisticated and include many regulators on the PCB, it will provide an improvement by ensuring sufficient voltage for the regulators to be within their regulating regime, if nothing else. Some regs operate super-close to the bottom tolerance of USB voltage. If there's not enough voltage available on the USB buss, they're not going to regulate as designed.
Other USB isolator products do not provide a completely separate, linear-regulated power supply. The DoodleBug will definitely be superior to those. Similarly, as for USB hubs, I am not aware of any - including the Belkin - that uses a true, linear-regulated power supply ... except for the $200 iFi iUSB product. Except for the iFi, they will all add to the distortion in the USB power supply for an audiophile device.
In the tests that I've conducted through the years, it is absolutely amazing how poor USB power is, relative to audiophile needs. Laptops and docking stations are absolutely atrocious. Desktop/towers are very little better, except that the DAC may not knock offline as much due to irregularities in the USB stream. One might go so far as to say the difference between a low-fi DAC and mid-fi/hi-fi DAC is whether it is USB-powered or not. The DoodleBug can correct that.
Edited by tomb - 3/14/14 at 8:16pm