Pro-Ject Tube Box S2

The Pro-Ject Tube Box S2 is a compact valve phono preamplifier from Pro-Ject’s S2 line of components. It sits above the solid-state Phono Box S2 and below the Tube Box DS2, offering a tube-based signal path in the same miniature form factor that defines the S2 range. It is the first product from Pro-Ject to feature a fully discrete circuit design without operational amplifiers, and the first to ship with user-replaceable tubes for tube rolling.

Discrete Valve Circuit

The signal path is built around a pair of ECC83 (12AX7A) dual-triode vacuum tubes, mounted in open cages on top of the chassis. The circuit is fully discrete — no operational amplifiers are used at any stage. An ultra-low-noise FET input stage feeds into the valve gain stages, and the layout follows a dual-mono topology to optimise channel separation. Polystyrene capacitors are used throughout where possible, and the high-voltage anode supply is independently filtered to keep the valve operating conditions stable.

Tube Rolling

Both ECC83 tubes are user-replaceable, allowing the sound signature to be tailored by substituting different tube brands or types. This is a deliberate design choice — the Tube Box S2 is the first Pro-Ject product to support tube rolling out of the box. The tube cages protect the valves while leaving them visible, giving the unit a distinctive appearance whether illuminated or not.

MM and MC Cartridge Support

The Tube Box S2 supports both moving magnet and moving coil cartridges. Five gain settings are selectable from the front panel: 40dB and 43dB for MM cartridges, and 50dB, 60dB, and 63dB for MC. Four variable input impedance settings (10Ω, 100Ω, 1kΩ, 2kΩ) accommodate low-output MC cartridges, while a fixed 47kΩ setting suits MM and high-output MC types. Input capacitance is adjustable across six values from 47pF to 587pF, allowing fine-tuning to match a wide range of cartridges.

RIAA Equalisation and Subsonic Filter

RIAA equalisation is implemented in two stages — a passive/active split design that achieves ±0.4dB accuracy across the full 20Hz–20kHz range. A front-panel subsonic filter button engages a 20Hz high-pass filter with an 18dB/octave rolloff, useful for controlling low-frequency rumble from warped records or compliant cartridge/tonearm combinations.

Construction

The chassis is an aluminium and metal sandwich construction, adapted from Pro-Ject’s DS2 line. This provides shielding against both vibration and electromagnetic interference. Gold-plated RCA sockets are fitted for both input and output. Power is supplied by an external 18V/1000mA DC adaptor, with multiple internal power supply stages inside the unit. An optional Accu Box S2 battery power supply is available as an upgrade path for further isolation from mains noise.