Phono Preamplifiers

This little box is best described as a 'hybrid', combining a J-FET/triode gain stage with passive RIAA eq and an op-amp-based output buffer. The latter brings with it a modest 96ohm source impedance and a substantial 7.4V maximum output but, arguably, a higher distortion than might have been achieved with the solid-state electronics left 'naked'. However, with 0.12-0.5% (20Hz-20kHz, 0dBV) whatever modest colour the two-stage ECC83s might have introduced is still lower than the distortion we typically measure from any MM/MC pick-up under real world conditions [see Graph 2, below]. The freq. response [see Graph 1] is also subtly shaped with a very gentle rise of +0.13dB between 50Hz-300Hz, an equally subtle dip through the presence, followed by a lift of +2dB at an ultrasonic 100kHz. The subsonic filter steepens the LF cut to –11dB/10Hz and –35dB/5Hz.

Pro-Ject claims +40dB, +45dB and +50dB gain (which I took as default MM settings) with +60dB and +65dB notionally for MC, values that were closely matched on test at +40.5dB, +45.1dB and +50.1dB for MM, and +60.0dB and +65.1dB for MC. In practice, the lowest MM gain offers a 9.45mV sensitivity (good for high output MMs) with the highest MC gain equal to 0.56mV (556µV – and with a minimum 10ohm load option, useful for mid-output MCs). The Tube Box DS2 carries enough headroom to maintain input overload limits of 72mV, 51mV and 31mV (MM), and 10.5mV and 6.1mV (MC), which are just sufficient to accommodate the 'hottest' >16dB groove modulations (re. 11.2µm/300Hz). Fortunately the tube RIAA stage clips very gracefully so there's some additional margin with the first three (MM) gain settings. The A-wtd S/N ratios are also superior at 83dB (MM) vs. 63dB for the last two (MC) settings. PM