Nagra HD PREAMP preamplifier Lab Report

Nagra’s two synchronised rotaries do not offer continuous volume adjustment but govern relays that switch numerous secondaries on its output transformer. These offer 0.5dB steps over the top 45dB of its range followed by 1-5dB steps over the next 20dB (the full volume range is 65dB). Channel balance is good to within ±0.01dB over this span. Maximum gain is +5.6dB (RCA in/XLR out) or –0.4dB (XLR in/out) and is unaffected by the ‘Studio’, ‘High’ and ‘Low’ input settings on its rear. Nagra’s spec. for its HD PREAMP is very selective, but our independent testing reveals distortion is typically <0.008% from 50Hz-10kHz (re. 0dBV out), increasing marginally at higher frequencies and more obviously so at very low frequencies to 0.014%/20Hz and 0.28%/5Hz. This is more likely a function of the transformers than the dual-mono E88CC NOS tube line stage that’s very precisely ‘voiced’ across left/right channels [see Graph 2]. The triode tube stage is also very quiet, revealing an A-wtd S/N ratio of 98dB (re. 0dBV) and a residual noise of just -101dBV (8.9µV).


Output impedance (+6dB volume position, grey infill; 0dB, pink infill) versus response (+6dB, black; 0dB, red; –5dB, green). Note 3Hz peak


Distortion versus extended frequency at 0dBV (left channel, black; right, red infill). Note excellent symmetry between dual mono channels

The HD PREAMP’s output impedance, and ultrasonic response, is subject to some variation across the top 6dB of its volume range. Source impedance is highest at ~170ohm (+6dB) and falls to a lower (more useful) ~71ohm at 0dB [grey vs. pink infills, Graph 1]. The top-end response also tips slightly from +0.2dB/100kHz (–15dB volume) to +0.25dB/100kHz (–5dB), +0.5dB/100kHz (0.0dB) and +1.7dB/100kHz (+6dB). More intriguing is the huge +8.8dB subsonic peak at 3Hz which, I’m bound to suggest, militates against its use with DC-coupled power amps, phono stages and reflex-loaded loudspeakers. PM

Maximum output (<1% THD): 13V (balanced)
Maximum input (<1% THD): 13V (balanced)
Output imp. (20Hz–20kHz/100kHz): 71–73ohm / 90ohm
Freq. resp. (20Hz–20kHz/100kHz): +0.15 to –0.0dB / +0.5dB
Input sensitivity (for 0dBV): 525mV (RCA in/XLR out)
A-wtd S/N ratio (re. 0dBV): 98.1dB
Distortion (20Hz-20kHz, 0dBV): 0.0056–0.0135%
Power consumption: 38W
Dimensions (WHD) / Weight: 438x121x396mm / 30kg (total)
Price: £49,500

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