Pass Labs HPA1 Headphone Amplifier

• Works with headphones with impedances of 15 ohms to 600 ohms
• Low feedback wide bandwidth discreet design.
• Designed as a power amplifier rather than a typical headphone amplifier to be able to drive low impedance planar headphones.
• Class-A direct coupled Mosfet output stage
• Custom low noise Torodial power transformer with a Farady shield
• Discreet regulated power supply for audio circuits.
• Locking Headphone jack.

Starting at the power cord, the AC line is first filtered for high-frequency noise and then isolated by a special toroidal transformer shielded by mu-metal for lower magnetic noise and shielded for electrostatic noise by a separate Faraday shield, grounded to analogue ground. This is followed by fast/soft recovery rectifiers, generous CRC passive filtering and sophisticated discrete DC regulators and then more passive RC filters.

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 or power consumption or both.

Pass Labs’ designers knew that in order to stand out from the crowd, and satisfy their demanding customer base, the HPA-1 had to establish higher standards for audio performance. That has been accomplished first, by the conceptual approach of designing the HPA-1 as a real Class-A power amplifier, and not as an accessory offering only incremental performance gains. Second, by cutting no corners in circuit design, while omitting unnecessary frills. Thirdly, by sparing no necessary expense in execution.

The foundation of the HPA-1’s engineering is a custom, low-noise shielded toroidal power transformer with a Faraday shield between windings as well as a mu-metal shield around the circumference, 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 overall performance of the amplifi er. The HPA-1’s amplifier circuits are low-feedback, wide-bandwidth discreet designs employing a J-Fet input stage and a Mosfet output stage biased into Class A-biased direct-coupled output stage. 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 “locking” 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 used as a preamp and it will compete against contemporary preamps. The rear panel holds the power switch and fuse. Volume control is via a hefty rotary knob connected to an ALPS Potentiometer or circuit. 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 aluminium shared with other Pass Laboratories products.