The purpose of the iFi iUSBPower is to regenerate the dirty 5V power signal coming from a PC USB port, with clean 5V taken from an external source and passed through the iFi device itself.
There are, in generally, two types of USB DAC (or USB-to-SPDIF converters) in the audio market: those that actually use the 5V signal coming from the USB connection, to power their electronics; those that only use the 5V to detect the flow of an incoming audio signal, but use their own separated power source instead of the incoming 5V signal.
The iFi iUSBPower is mainly aimed to the first type of audio devices, since it cleans the 5V power signal, and also decouples the ground coming from the PC, with its own ground switch.
The iFi iUSBPower keeps has two USB outputs. A twin USB cable (two inputs, one for signal, the other for power, to keep the power separate) would plug each end to a different output: in this case, the iFi Power is effectively splitting a USB signal from the computer in two different paths.
In such case, with a twin USB cable (like the Forza Audioworks, the iFi Gemini, or the Acoustic Revive), the behaviour is going to be similar to plugging the signal branch into the computer, and the power branch in a 5V battery (something like the Trent iCarrier USB battery).
The iFi can also be used with a single USB output: a standard (non-twin) USB cable would ‘just’ enjoy a cleaner 5V power, carried along with the 5V in the same enclosure. The USB converter at the other end would enjoy, if USB powered, a better 5V power as well, for improved clock and electrical stability.
I have used the iFi Power along with the Forza Audioworks twin cable, and a few DAC’s (namely, the Metrum Octave Mk2, the Mytek 192DSD, the Yulong DA8, the AMR DP-777).
The sonic effect I have detected with the iFi Power is generally consistent and gives a better sense of microdetail definition and cleaner background. This is using my twin cable, against using it with the power signal connected to a second USB port in my computer.
With the Metrum Octave Mk2, the increase in treble detail and quantity is more noticeable than with the other DAC’s.
By converse, with AMR DP-777, which I have been testing at a friend’s place, with his Acoustic Revive twin usb cable, the iFi Power did actually degrade the sound, which was perfectly analogue with such cable, and which sounded a bit more fizzy with the iFi iUSBPower.
In fact, it can make sense, since the AMR only takes signal from the USB, and has its own power source for the USB receiver, so the iFi Power was basically just adding itself to the signal path, along with a second, short USB cable that connects the PC to the iFi device.
Basically, the iFi iUSBPower is a device that’s mainly useful with USB-SPDIF converters and USB DAC’s that actually use the 5V power. It improves the signal quality in those situations.
It’s impact over the sound, in my test, is on the same order of magnitude as that of swapping a cable, but can vary from DAC to DAC. I obtained the best results with the Metrum Octave Mk2. The Mytek and Yulong, two Sabre-based DAC’s, are a bit less sensitive to the USB signal quality given the nature of the ESS Sabre chipset, but still sounded more polished.
Compared to slightly cheaper combinations (for example, if using a USB battery with a twin cable), it’s a more engineered solution, better thought out and more user friendly, since it doesn’t need to be plugged / replugged, while a USB battery does, thus needing constant care.