Interesting note from EnjoyTheMusic.
Pass Laboratories new HPA-1 Headphone Amplifier ($3500) was designed on a "clean sheet of paper" as though it were to be used as a power amplifier. Pass Labs' HPA-1 is capable of delivering full-range dynamics across the entire frequency range, even when driving headphones that present difficult loads in terms of impedance, power consumption or both. The conceptual approach was to design the unit as a real Class A power amplifier and not as an accessory offering only incremental performance gains. No corners were cut within the circuit design and thus they spared 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. Pass Labs' HPA-1 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 unit has a single high-quality headphone jack on its 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. Dimensions are 4.5" x 11" x 13" (HxWxD) and it weighs 14 lbs.
