Phono Preamplifiers

There's an interesting coupling here of a 6C45 indirectly-heated triode with a 6922 double-triode in each of the PH 9.0's 'dual mono' L/R channels. This two-stage gain block is wrapped around a passive RIAA network while a Lundahl LL9226 transformer brings another +14.1dB to the top of the MC input. ModWright rates the MM gain as 40dB, 46dB and 52dB (or –12dB, –6dB and 0dB via the toggle on the fascia) and, in practice, this is close enough at +41.7dB, +45.1dB and +51.0dB, respectively. The MC input is rated at 52dB, 58dB and 64dB but the real gain is a little higher at +55.9dB, +59.3dB and +65.1dB, the latter conferring a sensitivity of 0.56mV (re. 0dBV) which, on face value, suggests it's best suited to medium rather than super-low output MCs. Input overload margins are low, however, but the A-wtd S/N ratios are generous at 84.1dB (MM) and 77.4dB (MC) so, in practice, the PH 9.0 may well give off its subjective best with lower output MMs and MCs...

Distortion [see Graph 2] is also 'shaped' by the tube stages, as is both the frequency response and output impedance [see Graph 1]. So THD is highest at high frequencies (1.15%/20kHz re. 0dBV output) while the source impedance peaks at low frequencies (6.2kohm/20Hz), the latter a big swing from the far more agreeable 184ohm achieved through the midband and treble. I'd avoid passive preamps with the PH 9.0. The RIAA equalisation is broadly unaffected by gain but shows a gentle bass lift below 120Hz, reaching +0.55dB/20Hz, before a 'warp' filter kicks in – slightly too late – to deliver an attenuation of 3dB/5Hz and 10dB/3Hz. Big reflex speakers are also still possibly best avoided, but the broad +0.4dB presence/treble uplift might just give some female vocalists a pleasing boost! PM