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 values (47, 147, 267, 367, 487 and 587 pF) provide further fine-tuning for MM cartridges whose output varies with load capacitance. Gain is adjusted via front-panel rotary controls through six steps: 40, 43, 50, 60 and 63 dB.
Features and Connectivity
A front-panel subsonic filter operates at 20 Hz with an 18 dB per octave slope, providing effective suppression of low-frequency turntable rumble and warped-record artefacts. Connections are made via gold-plated RCA sockets on the rear panel, with one pair of inputs and one pair of line-level outputs. The sandwich aluminium and metal casing shields the circuit from both vibration and electromagnetic interference, an important consideration for a high-gain phono stage operating in proximity to a turntable motor and power supply.
The Tube Box S2 is supplied with an external 18 V DC power adaptor. For those seeking further sound quality improvements, the optional Accu Box S2 battery power supply is fully compatible and eliminates mains-borne noise from the power supply entirely. The replaceable ECC83 tubes also invite experimentation with alternative valve types (tube rolling), allowing owners to tailor the sonic character of the unit to personal preference.
