Phono Preamplifiers

At first sight the row of 12 DIP switches governing input loading and gain may be enough to make all but the most ardent of audiophile's hearts sink. However in practice only the two outermost switches are required to cycle through MoFi's selection of +40dB/+46dB (MM) and +60dB/+66dB (MC) gain options – figures very close to the +40.3dB/+46.3dB and +60.7dB/+66.7dB measured on the bench. These are sensibly chosen steps, the lowest MM gain offering a 9.7mV sensitivity with the highest MC gain representing 0.46mV (460µV), the latter suitable for standard rather than very low output MCs. The +40dB setting is best used for all high-output (>5mV/5cm/sec) MMs where the 82mV input overload yields a margin that's just sufficient to accommodate the 'hottest' +18dB groove modulations (re. 11.2µm/300Hz).

The active RIAA equalisation is extended out to +0.8dB/100kHz and is very flat through mid and treble even though there's a 'warming bump' of +0.43dB between 70-200Hz [solid trace, Graph 1]. The subsonic filter is very effective [dashed trace] with a –3dB point of 28Hz and a 2nd-order roll-off thereafter. It offers a useful 20-30dB suppression in the typical LP warp/arm/cartridge resonance spectrum. The RIAA-eq'd distortion [Graph 2] is very low, with a minimum of 0.0006% through the midrange and a maximum of just 0.018%/20kHz (re. 0dBV). This is about three orders of magnitude lower than the best MM/MCs, and also rather better than MoFi's own specification for the UltraPhono. The only slight fly in an otherwise very fine ointment is noise – the 81dB (5mV/0dBV) figure for both MM settings is good but 61dB for MC is slightly poor. But that's where costlier phono preamps come into their own... PM