Phono Preamplifiers

There's an inventive coupling of a high gain, low noise 12AX7 triode (two 6AV6 triodes in one bottle) with a high current, low noise 12DW7 (a combined ECC83 and ECC82) in each of the Andros Deluxe II's 'dual mono' L/R channels. This two-stage gain block is wrapped around a purely passive RIAA network and offers a +5dB boost between 'Low' and 'High' gain settings while the MC transformer brings another +18dB. Zesto rates the MM gain as 45dB/50dB and the MC gain as 63dB/68dB while, in practice, this is closer to +47.4dB/+52.4dB (MM) and +65.3dB/+70.4dB (MC) via the transformer-coupled balanced output. This is a wide range that will accommodate standard MMs with its 4.3mV Low Gain/MM sensitivity all the way up to low/medium output MCs with the High Gain/MC sensitivity of 300µV (all re. 0dBV balanced output).

Nevertheless, with the passive RIAA soaking up a lot of gain, the headroom is not especially generous for any of these four input settings. For example, instead of a typical ~100mV margin for the Low Gain/MM input this is just 18mV, or +12.5dB headroom before output THD reaches 1%. However, this being a low-feedback tube circuit, 'clipping' is progressive rather than abrupt and so the headroom increases to 32mV at 2% THD and 40mV (or +20.4dB) if we relax the limit to 3% THD. For the same reason, THD is a fairly consistent 0.2-0.3% (20Hz-20kHz, re. 0dBV) regardless of gain or MM/MC input [see Graph 2]. Similarly, the RIAA equalisation is unaffected by gain or input and remains flat within ±0.25dB from 8Hz-27kHz combined with a usefully steep sub-bass filter [see Graph 1, below]. Noise is also fairly low – not a given with a high gain tube stage – yielding A-wtd S/N ratios of 74dB and 69dB for the MM and MC inputs, respectively. PM