The Pass Labs HPA-1 is a Class A headphone amp that was designed to amplify headphones with impedances of 15 ohms to 600 ohms. It can deliver full-range dynamics across the entire frequency range. The HPA-1 can even drive headphones that present difficult loads in terms of impedance, power consumption or both.
In order to stand out from the crowd, Pass Labs conceptually designed the HPA-1 as a real Class-A power amplifier, not as an accessory offering only incremental performance gains. The HPA-1 cuts no corners in the circuit design and spares no necessary expense in execution. The foundation of the HPA-1's engineering is a custom, low-noise shielded toroidal power transformer feeding a discreet low noise regulated power supply for the audio circuits. The importance of the power supply is often overlooked and plays a large part in the overall performance of the amplifier.
The HPA-1's amplifier circuits are low-feedback, wide-bandwidth discreet designs employing J-Fet input stages and Class A-biased direct-coupled MOSFET output stages. The HPA-1 easily drives headphones presenting loads from 15 to 600 ohms, particularly excelling on planar headphone designs. The sound is rich and detailed.
The HPA-1 has a single high-quality headphone jack on the front panel, two sets of single-ended analog inputs via RCA jacks on the rear panel, and also a set of switchable "Preamp" line-level output jacks on the rear panel. There are no compromises with the HPA-1 when used as a stereo preamp, and it will compete against contemporary preamplifiers. The rear panel holds the power switch and fuse. Volume control is via a hefty rotary knob connected to an ALPS Potentiometer. The other front-panel controls are three pushbuttons to select inputs or to engage the Preamp output.
The substantial casework is the customary and well-recognized brushed aluminum shared with other Pass Laboratories products.