Phono Preamplifiers

Michell’s principal claim for the Apollo is ‘low noise’ and this, as we will discover, is no idle boast although getting the best from this phono stage still requires a little fine-tuning. How so? Well, the indicated +40dB, +50dB, +60dB, +68dB and +73dB gain settings are rather wide of the mark because the Apollo is 2-3dB more sensitive in each position at +42.7dB, +52.2dB, +63.5dB, +71.7dB and +77.4dB, respectively. So, if you are using a typical ~5mV MM (an Ortofon 2M, for example) then you should stick to the +40dB setting where the input headroom is 86mV [black trace, Graph 1]. This represents a useful +25dB margin, but if you switch to the +50dB gain setting [red trace] then the input will clip at 30mV – a margin of just +16dB. You might be tempted by the phenomenal 94.5dB A-wtd S/N offered in the +50dB position but although this drops slightly to 88.8dB in the +40dB setting, the difference lies below the level of diamond-on-vinyl ‘roar’.

Of three notional MC settings, the +68dB gain (a sensitivity of 267μV re. 0dBV output) best matches most of the cartridges recently tested in HFN. The A-wtd S/N is a fine 79dB (re. 500μV) and the headroom is 3.2mV [green trace], or +22dB (re. 267μV). If your MC has an output closer to 500μV/0.5mV then use the +60dB gain setting. In all cases the Apollo’s response shows a slight (+0.15dB) lift from 20-200Hz while the subsonic roll-off of –3dB/2Hz suggests warped LPs or big, reflex-loaded loudspeakers should be avoided. The treble response tips slightly upwards, reaching +0.3dB/20kHz, before lifting to +3.4dB/100kHz. Distortion is fabulously low, reducing with level up to the maximum 11.5V output and typically falling with frequency from 0.0025% (20Hz) to 0.00025% at ~6kHz [see Graph 2]. PM