Fostex HP-A4
Fostex HP-A4 headphone amp/DAC fitted with an asynchronous USB input and a Toslink optical input, for high-resolution headphone listening.
Description
The Fostex HP-A4 is a digital-to-analogue converter paired with a headphone amplifier, designed to serve as an audio interface between a digital source and headphones. It is intended to improve headphone listening from a computer or a source fitted with a digital optical output.
Its USB input operates in asynchronous mode, an architecture that hands clock management to the DAC itself rather than to the computer, which limits synchronisation errors during conversion. This input relies on a proprietary clock circuit and a Burr-Brown PCM1792A conversion chip, and accepts PCM signals up to 24-bit/192 kHz. The HP-A4 also has a second digital input, in Toslink optical format, capable of receiving the same type of PCM signals.
On its USB input, the unit is also advertised as compatible with DSD up to 1-bit/5.6 MHz, via DoP (DSD over PCM) encapsulation, which allows a DSD stream to be carried over a connection originally designed for PCM. This same USB input can also be used to power the HP-A4, a useful alternative if you wish to operate it without a dedicated external power supply.
This headphone amp/DAC is aimed at a user wishing to make the most of a high-resolution digital library or a DSD stream, without investing in a full hi-fi system, by connecting a computer or optical source directly to their headphones.
What to check before buying used
On the asynchronous USB input, test recognition and check for dropouts or digital noise depending on the operating system used, a point that can sometimes be sensitive with this type of interface. Also check that the Toslink optical input works correctly, along with the volume knob, whose potentiometer can develop crackling with use. The DoP mode for DSD is worth confirming with the intended software setup, as its settings are not always recognised on the first attempt.
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