Phono Preamplifiers

‘Specifications subject to change because they just might’ is the jovial disclaimer on the Oasis webpage but, on the whole, this Chinook-derived phono stage absolutely lives up to its billing. Gain is expanded from the Chinook’s two settings to +45dB (+45.2dB, measured) and +50dB (+50.2dB) for MM with +60dB (+60dB) and +65dB (+64.9dB) for MC. This is executed in a FET ‘headamp’ with a pair of 6922EH tubes wrapped around a passive RIAA eq network and a further pair driving the output to a maximum of 45V (yes, forty-five volts) from a 100ohm source impedance. The latter increases slightly to 120ohm/20Hz in line with the Oasis’s built in subsonic filter that rolls away from –0.3dB/20Hz to –1.1dB/10Hz and –6dB/4Hz. The top-end response shows a slight +0.3dB boost to the presence with an ultrasonic lift to +1dB/50kHz and +3.8dB/100kHz [see Graph 1].

I mentioned in the Chinook Lab Report [HFN Dec ’21] that its maximum +60dB gain (966µV input sensitivity, re. 0dBV output) wouldn’t suit the lowest output MCs but that is rectified here with the Oasis’s extra +65dB/ 569µV setting. Moreover, Manley has broadened the Oasis’s compatibility without compromising either noise or input headroom. The low noise yields A-wtd S/N ratios of 83.2dB (+50dB re. 5mV MM input) and a whopping 88.3dB (+65dB re. 500µV MC input). Input headroom extends to 248mV, 105mV, 46mV and 22mV for each gain setting, respectively, which is very generous – amounting to +33.1dB for the lowest 5.5mV sensitivity and +31.8dB for the highest 569µV sensitivity (+20dB would suffice). Distortion increases with level from ~0.01% up to 1V output, to 0.05%/4V and 0.1%/9V, and also increases at low (0.022%/20Hz) and high (0.065%/20kHz) frequencies [see Graph 2], but remains lower than from any MM/MC pick-up. PM