When I think of Pass Labs electronics, the last word that crosses my mind is little. Nelson Pass and his team think big, literally, as their line of Class A and high-power Class AB amplifiers attests. They don’t build amps for the bookshelf, the desktop, or the faint of heart. Even their INT-250 integrated amplifier—a component in a supposedly modest, smaller-footprint category—tipped the scales northward of a hundred pounds. But that’s not to say that Pass Labs can’t think small or isn’t paying attention to trends, such as the surge in popularity of personal audio. The HPA-1 headphone amplifier is not only proof that the company is paying attention but also that it is strongly committed to competing at the highest levels in the headphone arena.
The HPA-1 is an all-analog amp that marks Pass Labs’ rookie effort in the headphone market. Outwardly, its brushed aluminum cosmetics are tasteful, and bear a strong resemblance to Pass Labs’ stocky, full-sized components. The front panel is dominated by the single quarter-inch headphone jack and a large volume control (a potentiometer sourced from Alps with excellent tracking and good feel to the touch). Tiny blue LEDs indicate which source has been selected or whether the signal is being directed to an outboard amplifier or powered loudspeaker. The HPA-1’s aluminum plate casework is classic Pass in that it’s built like a vault, albeit one the size of a cigar humidor. There are two unbalanced RCA source inputs and a set of preamp outs, also unbalanced. This leaves open to conjecture whether Pass Labs will one day consider a fully balanced version of the HPA-1.
Did someone just say preamp outputs? Yep. Lest you think that you’re stuck with just a fancy desktop-dedicated, headphone amp, the HPA-1 is also a very capable dual-source line preamp that that can be conscripted for general system duty as well. Of course its feature set is minimalist, with two single-ended inputs and no remote control, but for many end users it offers all the flexibility needed.
Internally, the HPA-1 is designed more like a power amp than a headphone amp. The Pass team specified high bias to keep the transistors in Class A operation, and the circuit is capable of handling large voltage swings. (Pass amps always sound as if there are a couple output tubes hidden away deep in the chassis, although there aren’t.)
The low-feedback, wide-band discrete design employs a JFET input stage and a direct-coupled MOSFET output stage. The custom toroidal transformer is shielded by mu-metal for lower magnetic noise and safeguarded against electrostatic noise by a separate Faraday shield. It feeds a regulated power supply for the audio circuits. The circuits are fully discrete—no ICs carry the signal at any point. Of course, signal paths are kept as short as possible. There’s a significant amount of internal heat sinking to dissipate the heat generated by the heavily biased audio circuit.
Critical to its mission, the HPA-1 covers a wide range of headphone impedances—from 15–600 ohms. Why no internal DAC or at least the option to add one? The answer lies in Pass’ philosophy to design amplifiers that stand the test of time—perhaps twenty years or more. DACs, on the other hand, continue to be a fickle segment, formats changing with the wind. Many will likely be obsolete or unsupported in five years or less. Maintaining this separation is a valid argument in my view.
Because the headphone category offers relative affordability and ease of storage (compared with loudspeakers), ownership is usually not limited to a single pair. Mixing and matching headphones according to taste and musical genre is an acknowledged category pastime. So, I had three at the ready for the HPA-1 to dig into. My trusty old AKG K501 (120 ohms), and a pair of planar-magnetic designs in the form of the HiFiMan Edition X (25 ohms), and the Audeze LCD-X (20 ohms). I used each headphone’s original equipment cabling plus Audience Au24 SX interconnects and power cables into the HPA-1.