Pre/Power Amplifiers

As with the original Momentum preamp [HFN Nov '13], the C2Z's display meter arm indicates gain rather than output level. Full gain is +17.3dB (volume 100, with a maximum 30V balanced output from a very low sub-1ohm source impedance) with 0dB gain realised at volume '61'. The steps are within ±0.5dB over the top 90-100 of the volume range, offering a 4dB span that increases to 4.5dB from 80-90 and 70-80, and 5dB from 60-70, 50-60, 40-50 and 30-40. The volume steps from 20-30 occupy a total 10dB, with 12dB from 10-20 and 2dB per division from 6-10. With steps varying between 2-4dB over the lowest 1-5 volume settings, the C2Z's effective working range is 73.5dB with '0' as a mute position (–91.5dB relative to full output).

The C2Z preamplifier's response is flat to within ±0.01dB from 2Hz-20kHz out to –0.07dB/100kHz [black trace, Graph 1] while stereo separation is better than 90dB (re. 20Hz-20kHz). The profiles of the bass/treble tone settings are also revealed on Graph 1 [±2dB, green; ±4dB, cyan and ±6dB, red], the traces normalised to 0dB at 1kHz. Note that there's an ultrasonic boost beyond 20kHz with the +ve treble settings, and only a 1dB difference between the –4dB and –6dB bass settings. A subtle but broad adjustment is evidently possible from the ±1dB and ±2dB settings. Distortion, meanwhile, is low and impressively consistent with frequency at 0.00025-0.0004% in the full gain position [red trace, Graph 2] but increases with decreasing volume position to 0.003-0.004% at –6dB gain [black trace, Graph 2; volume setting '49']. Finally, thanks to the exceptionally well-screened and filtered DC supply, and new JFET gain stages, the C2Z's A-wtd S/N ratio is very wide at 99.6dB (re. 0dBV). PM