Benchmark HPA4 headphone amplifier Lab Report

Here’s a brand that’s aptly named – its products typically providing a ‘benchmark’ technical performance. The HPA4 is no exception with its relay-switched stepwise volume attenuator calibrated in 0.5dB steps over a ‘useable’ 105dB range with the top +15dB to –15dB sequence offering a ±0.03dB accuracy and –16dB to –48dB within ±0.09dB. At –60dB the error is just –0.12dB. Gain is precise too, so 0.0dB on the display really is unity, +6.0dB is +5.98dB (x2) and +15.0dB is +14.98dB (x5.61) via its balanced line outputs. Distortion and noise are vanishing low, amounting to 0.00008% through bass and midrange at 0dBV, increasing to 0.00018%/20kHz and 0.0005%/40kHz [black trace, Graph 2]. The frequency response is ruler flat (±0.01dB) from 1Hz-100kHz, the source impedance is a sensible 30ohm and the maximum output a huge 22V – sufficient to drive any power amplifier beyond clipping and into, protection notwithstanding, a charred pit of smoke.


Continuous power output versus distortion (black, into 25ohm load; red, low imp 8ohm load)


Distortion versus frequency from 5Hz-40kHz (black, 1V unloaded; red, 10mW into 25ohm load)

Via the headphone amp, the maximum voltage is 11.2V (or 209mW/600ohm) and, with protection kicking in, the maximum power output is a substantial 4.4W/25ohm and 7.7W/8ohm [black/red traces, Graph 1]. So, not only will the HPA4 drive the toughest headphone loads but its wide 103.3dB A-wtd S/N ratio and insignificant –102dBV (8µV) residual noise also ensures exceptionally quiet backgrounds when listening with the most sensitive headphones/earbuds. Distortion is broadly unaffected by loading, the values as vanishingly low as via the line output [Graph 2] but there’s some very slight adjustment in HF response. The source impedance is a low <0.6ohm up to 10kHz, increasing to 0.78ohm/20kHz and 1.5ohm/100kHz, so the response tips very slightly from 0.0dB/20kHz and –0.38dB/100kHz (unloaded) to –0.12dB/20kHz and –0.7dB/100kHz (25ohm). PM

Maximum output (<1% THD into 47kohm): 22100mV (22.1V, XLR line out)
Maximum power output (<1% THD): 4.37W/25ohm / 7.65W/8ohm
Output Imp. (20Hz-20kHz, line/head): 30ohm / 0.46-0.78ohm
A-wtd S/N ratio (re. 10mW/0dBV): 103.3dB / 103.3dB
Distortion (20Hz-20kHz, re. 10mW/0dBV): 0.00005-0.0002% (both o/ps)
Frequency resp. (20Hz-20kHz/100kHz): +0.0dB to –0.0dB/+0.01dB
Stereo separation (20Hz-20kHz): 80-120dB
Power consumption: 17W
Dimensions (WHD) / Weight: 220x99x237mm / 3.6kg
Price: £3295

Benchmark Media Systems, Inc.
SCV Distribution
03301 222500