The Tube Box S3 B sits in the middle of Pro-Ject's phono stage range, above the Phono Box S3 and below the flagship Phono Box RS3. It brings genuine balanced topology to a tube-based design, making it the natural choice for listeners who want the warmth and character of valves alongside the noise rejection and transparency that a fully symmetrical signal path provides.
True Balanced Design
The "B" in Tube Box S3 B stands for Balanced, and that distinction runs through every part of the circuit. The unit accepts a balanced connection via a 5-pin mini XLR input, which is compatible with Pro-Ject's True Balanced Connection tonearm wiring scheme, and also provides balanced XLR outputs alongside the standard RCA sockets. A balanced signal path splits the audio into two equal and opposite phases from cartridge to output, so any noise or interference picked up along the way cancels out when the signal is recombined. In a phono stage, where the cartridge output is extremely small, this advantage is particularly significant.
Tube Gain Stage
Amplification is handled by a pair of ECC83 (12AX7) tubes operating in a fully symmetrical and discrete gain stage. The ECC83 is a well-established choice for phono amplification, valued for its low noise floor and the natural, musical quality it lends to the top end and midrange. Rather than using tubes as a decorative or cosmetic addition to a solid-state circuit, Pro-Ject has built the gain stage entirely around the tube assembly, so the character of the valves shapes the sound throughout. The result is an open, engaging presentation that flatters both modern and vintage cartridges.
Flexible Cartridge Loading
The Tube Box S3 B supports both moving magnet and moving coil cartridges, with loading options controlled directly from the front panel. Input impedance is selectable at 10, 50, 100, 1k, or 47k ohms; input capacitance can be set to 50, 150, 300, or 400pF; and gain is available at 40, 45, 60, or 65dB, with an additional 6dB available when using the balanced XLR outputs. This level of adjustment allows the stage to be matched precisely to a wide range of cartridges without opening the case or fitting resistor plugs.
