"Here's a brand that's aptly named - its products providing a 'benchmark' technical performance. The HPA4 is no exception with its relay-switched stepwise volume attenuator calibrated in 0.5 dB steps over a 'useable' 105 dB range."
"Distortion and noise are vanishingly low."
"The frequency response is ruler flat (+/- 0.01) dB from 1 Hz - 100 kHz."
"The [line output] source impedance is a sensible 30 Ohms and the maximum output a huge 22 V."
"Via the headphone amp, the maximum voltage is 11.2 V and ... the maximum power output is a substantial 4.4W/25ohm and 7.7W/8ohm."
"So, not only will the HPA4 drive the toughest headphone loads but its ... insignificant -102 dBV (8uV) residual noise also ensures exceptionally quiet backgrounds when listening with the most sensitive headphones/earbuds."
"Distortion is broadly unaffected by loading."
"The source impedance is a low <0.6 Ohms up to 10 kHz, increasing to 0.78 Ohms at 20 kHz."
- Paul Miller, Hi-Fi News
