Phono Preamplifiers

The PW1’s default (MM) gain is just +37.1dB, requiring a pick-up output of 13.95mV (re. 1kHz/5cm/sec) to raise 0dBV (1V) from the balanced XLR outputs. In practice, adding the +10dB output gain will be advantageous for most MM users because there is no significant penalty in noise or S/N ratio – 86.2dB versus 87.3dB (re. 5mV input) – and no practical difference in input overload margin where a 5mV input realises ~0.35% distortion and a 30mV input produces ~2% distortion in both settings. This steady increase in distortion from 0.06% at 50mV output, 0.35%/500mV, 0.75%/1V, 2%/2.1V and 13.4% at the maximum 12V balanced output certainly favours lower output MMs used with the +10dB output buffer [see boxout]. Distortion also increases with frequency from, for example, 0.75%/1kHz to 5.75%/20kHz [re. 0dBV out; red trace, Graph 1]. These are very high figures but not too different from the THD measured from the MM/MC pickups tested every month in [HFN].

Hawk-eyed readers might already have seen that two of the PW1’s MC settings yield the same overall gain of +67.5dB – that is +20dB input gain with +10dB output boost and +30dB input gain with no final boost. However, the latter is by far the preferable option. Why? Because while the 430µV sensitivity is unaffected and the 10dB input headroom (re. 3% THD) is unchanged, the +30dB setting realises a superior 80.8dB A-wtd S/N ratio (re. 500µV input), or some 3dB in advance of the +20dB/+10dB mode. Grimm specifies an RIAA eq accuracy of ±0.5dB which is broadly matched from 20Hz-20Hz, though there is a +0.6dB ‘bump’ at 30-40Hz before a subsonic roll-off of –3dB/7Hz and –9dB/4Hz [see Graph 2, below]. PM