Fostex claims this DAC is compatible with Windows 10 however the software they provide on their website "USB Driver for Windows (V.2.1.0)" does not have a digitally signed driver that works with Windows 10 Anniversary Update (Build 1607) and Windows 10 will block the installation. You can temporarily override this behavior and allow the driver to install, but it will be disabled every time Windows 10 is restarted. Other DAC vendors such as Oppo Digital have updated their drivers to be compliant with Windows 10 driver signing requirements. What gives Fostex?

The HP-A4 is a great little USB DAC/Amp combo. Fostex should have addressed the Mac OSX Sierra firmware update here on Amazon and other major online sellers. There is a firmware update for Sierra. It must be applied via Micro SD card before you can use the HP-A4 with Sierra, but it's easy to do and only takes a few minutes.

I use the HP-A4 with 150 Ohm impedance headphones and it supplies more than enough power on low gain. To give you an idea of how much power the amp supplies, I typically have the volume dial set around 12 o'clock.

The only reasons I wouldn't give the HP-A4 five stars are: (1) The information from Fostex doesn't provide sufficient detail about what headphones the HP-A4 can power. I've heard it works fine for 300 Ohm impedance headphones like the Senn HD600's, but isn't sufficient for some 600 Ohm impedance headphones. (2) The HP-A4 is noticeably noisier on high gain, though high gain selection isn't necessary for the headphones I use. (3) I had one occasion where the HP-A4 would not play properly until it was unplugged and reattached from the computer's USB port.

There is a lot of discussion online about pairing amps with different headphones. You should probably read some of this literature before making a purchase. I can only relate my own experience. While most people seem to think a solid state amp like the HP-A4 sounds too bright with the Senn HD700's, to my ear it's a very good pairing.

I love this DAC but **Warning** - The new MacOS Sierra will not recognize this as an audio output device. Really hoping Fostex addresses this soon with a driver update. I updated my iMac, and my HP-A4 is currently a paperweight.

Sounds great, looks beautiful on my deak

Great DAC/AMP. Solid built. Output selector between headphone or sending to my speakers via a receiver is a simple button press. Overall happy with these product. First ever Fostex product I own, no regrets.
Keep in mind; on windows pc, you need to download the driver software from Fostex website before connecting.
Only downside, I wish it had a power button to power if off when not in use.