① (Original box included) Fostex HP-A4 Headphone Amplifier / DAC

Please note that I am listing the same product with and without the original box. The reason for listing two units is that while it's true that you can easily swap op-amps to enjoy different sound qualities, opening the case lid each time was stressful. Therefore, I enjoyed the amp with two units side-by-side to minimize stress. I believe this speaks to how interesting this headphone amplifier is.
I am selling the HP-A4 op-amp headphone amplifier as part of decluttering my PC desktop audio equipment. It was working perfectly until just now when I was listening to it. There are tape marks on the bottom panel of the case (where I had written down the type of op-amp inside, driver size, and screw locations for opening). There are also minor scratches.
The op-amp has been replaced with an OPA627AU. The original OPA2134PA will also be included, so please listen to the difference. Please note that a Phillips head screwdriver (size PH0) is required to remove the case (available at home improvement stores). I will also include five spare flat-head screws.
As this was used daily, please forgive any oversight of minor scratches.

Original box, instruction manual, warranty card, etc. are included.