Phono Preamplifiers

Tested here with the optional Compact PSU [see boxout] the Compact Phono’s two gain settings are accurately specified by Nagra at +47.35dB (Low) and +62.0dB (High), implying sensitivities of 4.29mV and 794μV, respectively, for a 1V (0dBV) output. In practice, the Compact Phono’s maximum output level is 1.63V and the maximum input is just 7.15mV (Low) and 1.28mV (High) before clipping – so this little unit is not as accommodating of all MCs as the Classic Phono [HFN Aug ’22] with its 18mV input threshold. It is, however, more realistically engineered than the old BPS model [HFN Aug ’10] with its meagre 0.93mV input overload limit.

In practice this means MCs rated above 150-200μV (re. 1kHz/5cm/sec) should be used in the ‘Low’ gain mode to ensure an adequate ~18dB headroom for the boldest groove excursions. Only super-low output MCs should be used in ‘High’ gain mode, although both settings enjoy a very ‘useable’ 78dB A-wtd S/N ratio (re. 500μV in/0dBV out). MCs with rated outputs above ~1.2mV are not especially compatible with the Compact Phono. As input headroom is pushed this is reflected in distortion on the output, the Compact Phono dipping to 0.002-0.012% at 100mV output before rising to 0.0078-0.04% at 500mV and 0.019-0.1% at 1V output [all re. 20Hz-20kHz, see Graph 2]. Even the latter is still lower than the THD encountered with moving-coil carts. The equalised response [Graph 1] shows a subtle –3dB/5Hz infra-bass roll-off that’s also reflected in an increase in output impedance from 4.5ohm to 50ohm/20Hz [orange trace]. There’s a broad –0.45dB dip through the presence and very high treble, but the response is flat to within ±1dB from 8Hz-90kHz. PM