Усилители для наушников — Pass Labs HPA-1c

The HPA-1c is a headphone amplifier and line preamplifier from Pass Laboratories of Auburn, California, and an updated version of the HPA-1, the company's only headphone amplifier. It keeps the core amplifier circuit of the original but adds an upgraded power supply, improved grounding on the board and local regulation for the servo circuits, changes prompted by parts reaching end of life, and Pass states that noise improved by about 10 dB.

Key specs that matter

Gain is 8 dB. Output power is 3,500 mW into 20 ohm and 200 mW into 300 ohm, and Pass also quotes up to 2 W into 32 ohm. Output impedance is below 2 ohm, and the amplifier drives headphones from 15 to 600 ohm, including low-impedance planar models. Frequency response is 10 Hz to 100 kHz within 1 dB. THD+N is below 0.005% at 1 V output, and input impedance is 50 kOhm. Power consumption is 23 W. The case measures about 279 x 102 x 343 mm and weighs about 6.4 kg. On the front is a Neutrik locking 6.35 mm headphone jack with silver contacts rated at 10 A and an isolated signal ground, and on the rear are two RCA input pairs and one RCA preamplifier output. Only one output, headphone or preamplifier, is active at a time.

Engineering and use context

Pass designed the HPA-1 as a power amplifier rather than as a typical headphone amplifier. The discrete circuit has low feedback and wide bandwidth, with a JFET input stage and a direct-coupled Class A MOSFET output stage. In Class A the MOSFETs never switch off during the waveform, and direct coupling means no capacitor sits between the output stage and the headphones. The custom low-noise toroidal transformer has a Faraday shield and a mu-metal band to contain its electric and magnetic fields, and a discrete regulated supply feeds the audio circuits. Volume is set by an ALPS potentiometer. At power-on the output is muted for 20 seconds, and Pass designs the unit to be left switched on, reaching its best performance after about an hour. The preamplifier output drives power amplifier loads of 10 kOhm or more, so the HPA-1c can also serve as a simple line preamplifier with two inputs. The case is brushed aluminium.

Pairing

Its high current output suits low-impedance planar magnetic headphones, and the low output impedance keeps the response of dynamic headphones unaffected by the amplifier. Through the preamplifier output it can drive a Pass power amplifier directly.

Alternatives

The Pass Labs XP-12 is the brand's line preamplifier for systems built around loudspeakers rather than headphones.