The Tube Box S2 sits at the heart of Pro-Ject's compact electronics range, offering a genuinely valve-based phono preamplifier in a form factor small enough to pair neatly with any Pro-Ject turntable or third-party deck. While many entry-level valve designs use solid-state amplification for most of the signal path, the Tube Box S2 sets itself apart as Pro-Ject's first product to feature a fully discrete circuit design with no operational amplifiers anywhere in the audio path. The result is a phono stage that delivers the warmth and harmonic richness associated with valve amplification without the bulk or expense of a conventional full-sized component.
Valve Design and Circuit Architecture
Two ECC83 (12AX7A) valves sit at the centre of the Tube Box S2's dual-mono circuit, each visible through individually caged housings on the top panel. The use of a dual-mono layout optimises channel separation, while polystyrene capacitors are used wherever possible in the signal path for low distortion and accurate RIAA equalisation. The RIAA correction itself is implemented in two stages, using a passive/active split, which keeps equalisation errors tightly controlled to within ±0.4 dB across the full 20 Hz to 20 kHz bandwidth.
An ultra-low noise FET input stage precedes the valve gain stages, helping to achieve a noise floor of 80 dB(A) at the 40 dB gain setting. For those requiring higher gain for low-output MC cartridges, the noise floor remains a respectable 75 dB(A) at the 50 and 63 dB settings. Total harmonic distortion is kept below 0.02% at 40 dB gain, rising to no more than 0.05% at higher gain positions.
Cartridge Compatibility and Gain Flexibility
Five selectable MC input impedance settings (10, 100, 1k and 2k ohms) allow the Tube Box S2 to be matched to a wide range of moving-coil cartridges, while a fixed 47k ohm impedance setting accommodates high-output MC and standard moving-magnet designs. Five selectable input capacitance settings (47, 147, 267, 367, 487 and 587 pF) are available for MM cartridges, allowing the user to optimise the load for their specific cartridge. Gain is adjustable across five settings spanning 40, 43, 50, 60 and 63 dB, providing enough range to work with low-output MC cartridges producing as little as 0.2 mV.
A subsonic filter operating at 20 Hz with an 18 dB/octave slope reduces low-frequency rumble from the turntable bearing and record warps, while a rear-mounted power switch and single pair of RCA input and output sockets keep the connection arrangement simple. All gain and loading settings are accessed via a bank of DIP switches on the underside of the chassis.
Position in the Pro-Ject Range
The Tube Box S2 sits above the entry-level Tube Box E in Pro-Ject's phono stage lineup, offering the fully discrete, op-amp-free circuit topology and wider cartridge compatibility that step-up buyers will appreciate. It falls below the Tube Box S2 Analog, which adds a switchable analogue output stage. The Black finish version of the Tube Box S2 is complemented by a Silver alternative, and both share the same compact 103 mm wide chassis that pairs well with Pro-Ject's Debut and Xtension turntable families.
