Quoting your manufacturer's FAQ page at http://schiit.com/products/gungnir:
In other words, your AC-powered Gungnir relies on an external source for 5V DC power to operate its USB receiver. This means that the power for the Gungnir's USB input will have to come from the USB Regen and this, in turn, means that you do not want to exceed 9V, because there are two voltage regulator inside the USB Regen, one that supplies 5V power to DACs that need it and another that supplies 3.3V to the USB Regen's own circuits. The higher the supply voltage the hotter these two voltage regulators will get as they convert the excess voltage into heat.
9V is is high as you should go when using a DAC that pulls 5V power from the USB Regen, and I can tell you that your Regen will get very warm even at 9V - hot enough that it really appreciates my use of the heat sink I pictured in the post I referenced above.
It is possible to supply the USB Regen with as little as 5V, even when the DAC pulls power from the USB Regen - it will run cooler and thus, the components will last a few more years, perhaps, than they would at 9V - who knows for sure (?), but heat is the enemy of longevity. The ideal voltage in my mind is something around 6V or 6.5V when using a USB-powered DAC.
Given that UpTone Audio will be offering (in October?) an accessory for the USB Regen's included 7.5V SMPS, an accessory that will provide extremely clean, noise-free power to the USB Regen, I would avoid spending a lot of money on a LPS right now, unless you know you'll have some other use for it later. In the interim, if you just want to make a quick improvement over using the SMPS, I would recommend this, affordable EI-core LPS, which I've had for several years, for use with DACs that will pull power from the Regen, because you can adjust the output voltage yourself by turning an adjustment screw inside. I've got mine adjusted to 6.5V: http://www.ebay.com/itm/TeraDak-TeraLink-X1-X2-DC8-5V-1A-USB-DC5V-port-external-Linear-Power-Supply-/301694857782
As mscot58 has posted, the Hugo is self-powered, so you can feed the USB Regen with as much as 12V, but keep in mind that, the Regen's 3.3V regulator will have to create more heat with the higher voltage. A 5V LPS would allow the Regen to run much cooler than a 12V LPS, even when the DAC is self-powered. mscott58's use of a 0.5 Amp (500 mA) tap on his 12V LPS4, is fine because the USB Regen itself only draws something like 40 mA, but if he was also trying to power a 500mA USB DAC, that tap might not supply enough current for both the DAC and the Regen itself, in addition to overheating the Regen due to running at 12V.
Again, 12V is the maximum voltage permissible for the Regen when using a DAC that is self-powered - no harm will come to the Regen at 12V, according to UpTone Audio, but a lower voltage would allow it to run cooler and thus, last longer.
A few months ago, there was a brief rally of posts in the USB Regen thread at computeraudiophile about whether or not a higher voltage improves sound quality, but the consensus quickly squelched that notion. The vast majority of users would agree with my interpretation of mscott58's conservative position that running at 12V does not, in and of itself, improve sound quality. Noisy power is your enemy.
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I would again recommend that everyone who doesn't already have an LPS for some other purpose, just use the included 7.5V SMPS, while waiting for the October (?) release of UpTone Audio's new accessory - that promises to provide ultra-clean power to the Regen.
For more on the voltage requirements, minimums and maximums, here is the UpTone Audio FAQ page for the USB Regen:
http://uptoneaudio.com/pages/usb-regen-questions-and-answers
Mike
