Phono Preamplifiers

Tested via its balanced XLR outputs, the UltraPhono Pro achieves a maximum 17.5V output from a reasonably flat 250-265ohm source impedance. This output is determined by both the gain(s) of the phono stage and their respective input overload margins – phono preamps typically 'clip' at their input or midway through the RIAA network, rather than at the final output buffer. In this case, MoFi's specified +40dB, +52dB and +65dB gain settings amount to +46.2dB, +58.7dB and +71.0dB as the balanced stage adds a further 6dB, these figures well judged to accommodate MMs, high output MCs and low output MCs, respectively. Headroom necessarily falls with increased sensitivity at 89mV, 22mV and 5.3mV, respectively, but the relative overload margins are consistent and very generous at +25.2dB, +25.6dB and +25.5dB. Consequently, the UltraPhono Pro will not clip when your chosen cart is tracing the fiercest of grooves.

Assuming the lower two gain settings are for MMs and the final +65dB step is for MCs, the A-wtd S/N ratios amount to 88.8dB for MMs and 75.9dB for lower output MCs, the latter still slightly ahead of the best turntables' rumble and noise figures. Distortion [see Graph 2], too, is significantly lower than that achieved by any MM/MC cartridge at 0.0011-0.033% (+40dB) and 0.0065-0.028% (+65dB, all re. 20Hz-20kHz at 0dBV out). And the icing on the cake – MoFi's passive RIAA network delivers a very flat response (within ±0.04dB, 20Hz-20kHz) that extends out to +0.14dB/100kHz [black trace, Graph 1]. The bass extension amounts to –6dB/1Hz, or –6dB/6Hz with the subsonic filter in tow for set-ups where you have a very excitable arm/cart resonance and/or large ported speakers [red trace, Graph 1]. PM