Nagra HD AMP power amplifier Lab Report

Nagra, not unlike Naim [HFN Jun '15], describes its flagship power amplifier as a 'Statement'. Neither is kidding and both amplifiers – despite being rated very differently at 250W and 746W/8ohm, respectively – achieve pretty much the same maximum power/current at 1.65kW and 1.59kW/1ohm. So Nagra's HD AMP has it by the nose at 40.6A at which point its electronic protection kicks in. Protection also prevents the HD AMP from clipping under continuous conditions, or at least beyond 0.5% THD at 290/8ohm and 475W/4ohm. Both figures are above Nagra's specification but it's the dynamic performance of 330W/8ohm, 621W/4ohm and 1.11kW/2ohm that really tells the story of the HD AMP's unburstable demeanour [see Graph 1].


Dynamic power output versus distortion into 8ohm (black trace), 4ohm (red), 2ohm (blue) and 1ohm (green) speaker loads. Maximum current 40.6A


Distortion vs. extended frequency at 10W/8ohm (grey) vs. response into 8ohm (black trace), 4ohm (red), 2ohm (blue) and 1ohm (green)

There is absolutely no difference in distortion versus level or versus frequency between the 1V and 2V input sensitivity options (33dB vs. 27dB gain), though the former does offer an improved A-wtd S/N ratio of 84.6dB (re. 0dBW). Distortion holds to within 0.002% through bass and 0.005% through midrange frequencies over the first 150W of its range, although this increases at high frequencies to 0.035%/10kHz and 0.045%/20kHz in line with an increase in output impedance (flat at 0.02ohm from 5Hz-20kHz) of 0.045ohm/40kHz and 0.142ohm/100kHz. This distortion trend is revealed in Graph 2 [grey infill] alongside traces of the HD AMP's frequency response into 8, 4, 2 and 1ohm loads. While all Nagra's specifications are met by a wide margin, the '+0/–0.5dB, 0Hz-120kHz bandwidth' is not, but the 'engineered' –0.1dB/5kHz, –0.3dB/10kHz, –0.6dB/20kHz response shelf is both exceedingly mild and ingenious in its subjective impact. PM

Power output (<1% THD, 8/4ohm): 290W / 475W
Dynamic power (<1% THD, 8/4/2/1ohm): 330W / 621W / 1.11kW / 1.65kW
Output imp. (20Hz–20kHz/100kHz): 0.019–0.022ohm / 0.142ohm
Freq. resp. (20Hz–20kHz/100kHz): +0.0 to –0.6dB / –1.38dB
Input sensitivity (for 0dBW/250W): 65mV / 1030mV (balanced)
A-wtd S/N ratio (re. 0dBW/250W): 84.6dB / 108.6dB
Distortion (20Hz-20kHz, 10W/8ohm): 0.0015–0.044%
Power consump. (idle/rated o/p): 161W / 430W
Dimensions (WHD) / Weight: 238x644x542mm / 56kg (each)
Price (pair): £62,500

Audio Technology
Switzerland S.A.
Supplied by: Padood Ltd, Cambridge, UK
01223 653199