Pre/Power Amplifiers

Tested without its optional input/output transformers, the PREAMP II-S still bears comparison with the far costlier HD Preamp [HFN Nov ’18]. Nagra does not specify the absolute gain of either model, though both are effectively buffered unity-gain amplifiers when used in their fully balanced, XLR in/out modes – the HD Preamp offers –0.3dB, the PREAMP II-S offers +0.4dB (in ‘+6dB’ gain mode). The Modulometer is accurately calibrated in this +6dB setting, the lower scale indicating 1V and the upper scale 0dB when the preamp has 1V/0dBV on its balanced XLR output. The scale shows a maximum 3V – depicted ‘in the red’ from 1V – although the true maximum balanced output is significantly higher at 10.5V from a usefully low/cable agnostic 8ohm source impedance, the latter result courtesy of that ECC81 plus feedback. The A-wtd S/N ratio is generous at 96.7dB (re. 0dBV), a figure very close to the 98dB achieved by the HD Preamp – both results are very good for tube line stages.

The PREAMP II-S’s frequency response does not show the volume dependence we saw with the HD Preamp’s transformer-coupled output, but the general trend showing a gentle lift to +0.35dB at an ultrasonic 100kHz is the same. The subsonic response is also extreme, extending down to +0.4dB/1Hz [see Graph 1]. Distortion is determined more by the input than output level, as we saw in the HD Preamp. In this case the lowest 0.0018%/1kHz is achieved at full volume with 800mV in and 840mV out, the distortion increasing steeply to 0.25% with 2V in and ~2.1V out. Versus frequency, distortion is low at low bass frequencies – again, the PREAMP II-S lacks the HD Preamp’s transformer-coupled output – but otherwise shows the same gentle increase to 0.01%/20kHz (re. 0dBV) [see Graph 2]. PM