Phono Preamplifiers

The HD Phono is structured so that its dual EF806S pentode-based EQ stage (serving RIAA, Teldec, NAB 100μsec and Victor EUR/USA) is effectively unity‑gain. Ahead of this, the MC inputs are routed via a custom, cobalt‑cored transformer offering +25.8dB gain while, after the EQ section, an E88CC triode‑based output buffer offers a notional ±6dB cut or boost. Nagra’s rated 30.5dB, 36.5dB, 42.5dB (MM) and 56.5dB, 62.5dB, 68.5dB (MC) gains work out as 30.4dB, 36.3dB, 42.4dB and 56.2dB, 62.2dB and 68.3dB, respectively, with a channel‑balance error of 0.1dB. In practice, the +6dB setting will be ideal for most MMs and 0dB or +6dB for medium or lower‑output MCs. Usefully, the HD Phono’s A‑wtd S/N ratio is held at 72.0dB (MM, re. 5mV) and a superb 77.8dB (MC, re. 500μV) regardless of the ±6dB gain setting. It appears that both noise and the input overload margins are set by the unity‑gain EQ stage so, once again, the 8.7mV (+24.8dB) and 172mV (+30.7dB) clipping points for MC and MM, respectively, are both well in excess of what is required and, importantly, not compromised in the +6dB gain settings.

The bass/treble extremes of the corrected‑RIAA responses differ slightly because the MC inputs are transformer‑coupled [red trace, Graph 1] but both show a bass boost amounting to +1.95dB/20Hz and +0.95dB/20Hz, MM/MC, respectively. Be cautious of warped LPs or set-ups with a lively arm/MM resonance! Distortion increases linearly with level above a minimum 0.02% ‘sweetspot’ at 200‑300mV up to 0.06%/1V, 0.13%/2V and 1% at the full 10.2V output (all re. 1kHz). While THD increases with level it also decreases at higher frequencies [see Graph 2] from 0.048% at 300‑400Hz down to 0.023%/20kHz (all re. 0dBV/1V). These figures are comfortably below vinyl replay distortion. PM