The HPA-1 BOM on the HPA-1 Headphone Amplifier has been linked to the X-Altra Mini II BOM (HPA-1 page) since they are both the same board. Additionally, the presentation on the HPA-1 webpage has been updated to reflect the latest PCB’s.
Note that the latest iteration of the HPA-1 AC couples the inputs and includes facilities for winding a coil around the 3.3 Ohm output resistors to lower the amplifier output impedance to a few milli-Ohms. This offers improved bass performance on large diaphragm open back headphones and better headphone damping factor (about 3000 at 1 kHz into 32 Ohms).
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