Pro-Ject Phono Box RS2 silver

Pro-Ject Phono Box RS2: balanced and discrete phono preamp

Succeeding the Phono Box RS, from which it takes the continuously variable impedance adjustment principle, Pro-Ject’s Phono Box RS2 takes the concept further with an output stage that is now fully discrete (without operational amplifiers) and the addition of a channel balance control. The result is a compact phono preamplifier, compatible with MM and MC, whose feature density exceeds what is usually found at this price level.

An operational-amplifier-free circuit, and why it matters

The Phono Box RS2 is based on a fully discrete design: resistors, capacitors, inductors, and transistors, assembled as hundreds or even thousands of individual components on the printed circuit board. No operational amplifier (op-amp) is involved in the signal path.

The difference compared with a preamp based on integrated circuits is not merely aesthetic. A discrete circuit gives the designer finer control over each amplification stage, at the cost of significantly longer development and higher manufacturing costs. Pro-Ject states that its teams found, after years of comparative listening tests, that even the best op-amps do not reproduce the signal with the same neutrality and naturalness as a well-designed discrete circuit.

Balanced dual-mono design

Signal processing is balanced from input to output. In practical terms, the + and − signals of the left and right channels are amplified independently, which doubles the number of amplification sections on the printed circuit board. This topology makes it possible to subtract noise and interference accumulated along the signal path, a measurable advantage: the signal-to-noise ratio reaches 89 dB (A-weighted) in MM at 40 dB of gain and 79 dB in MC at 60 dB.

Connections are available in XLR (balanced) and RCA (unbalanced), both for input and output. Two inputs are provided, with front-panel switching, allowing two turntables to be connected simultaneously.

A useful clarification: even with RCA input, the Phono Box RS2 separates the + and − signals internally to process them in a balanced way. MM cartridges, which generally connect one channel to chassis ground, do not however allow full end-to-end benefit from a balanced connection.

Separate passive RIAA and DECCA equalization

The Phono Box RS2 equalization is of the separate passive type (split passive). The various corrections of the RIAA curve are distributed across several gain stages instead of being handled in a single block. This approach, which is more expensive to design, reduces deviation from the ideal curve: Pro-Ject claims accuracy of ±0.4 dB between 20 Hz and 20 kHz. Measurements published in the specialist press confirm very tight RIAA conformity.

A second equalization curve is available: DECCA. It is intended for playback of certain old mono pressings cut according to the Decca curve, predating RIAA standardization.

Continuous impedance adjustment, including during playback

This is one of the most striking features of the Phono Box RS2. MC load impedance is adjusted by a rotary potentiometer, continuously from 10 to 1,000 ohms, with no detents or switch. According to the brand, this system was invented by Pro-Ject and allows the load to be adjusted while the record is spinning, with the effect on the sound heard immediately.

In MM mode, impedance is fixed at 47 kohms (standard), and load capacitance is selected in 50 pF steps, from 50 to 400 pF. Eight values are available, covering most MM cartridges on the market. Capacitance adjustment nevertheless remains a parameter that is often overlooked, because few cartridge manufacturers publish a recommended value, and the tonearm cable capacitance must be taken into account in the total calculation.

Adjustable gain over a wide range

Gain is selected in four steps for each cartridge type. In MM, the values range from 40 to 50 dB on RCA output, with an additional 6 dB in XLR. In MC, the range extends from 60 to 70 dB (again +6 dB in XLR). This represents amplification ranging from a factor of 100 (40 dB) to a factor of 3,690 (70 dB).

At such gain levels, the slightest residual noise in the circuit is amplified in the same proportion as the useful signal. The balanced and discrete design of the Phono Box RS2 is especially meaningful here, since it maintains a very low noise floor even at maximum gain. Total harmonic distortion remains below 0.0015% in MM (40 dB, 1 kHz) and 0.0024% in MC (60 dB, 1 kHz).

Channel balance control

Every phono cartridge has a slight imbalance between its left and right channels. Pro-Ject cites the example of the Ortofon 2M Red, whose balance difference reaches 1.5 dB at 1 kHz, and the MC Ortofon Anna Diamond, still at 0.5 dB. The Phono Box RS2 includes a balance potentiometer that allows correction of the center of the soundstage over a range of ±2 dB. This adjustment, rare at this price level, can be disabled to shorten the signal path as much as possible when correction is not needed.

Front-panel controls and subsonic filter

All settings are accessible from the front panel, with LED indicators for each parameter. It is an appreciated ergonomic choice compared with preamps that relegate their DIP micro-switches under the chassis.

A switchable subsonic filter cuts at 20 Hz with a slope of 18 dB per octave. It attenuates infrasonic frequencies caused by record warp or tonearm resonances, which can overload an amplifier or make the speakers move excessively without producing useful sound.

Technical Specifications

Overview

• Phono Box RS2: fully balanced and fully discrete phono preamplifier
• Compatible with MM and MC cartridges
• Fully balanced design, true dual mono
• Fully discrete hardware architecture, without operational amplifiers
• Separate passive RIAA and DECCA equalization (split passive)
• Excellent signal-to-noise ratio and very low distortion
• Metal chassis protecting against vibrations and interference

Inputs and outputs

• Inputs: RCA and XLR
• Outputs: RCA and XLR
• Balanced XLR and unbalanced RCA connections on both input and output

Settings and controls

• Continuously variable input impedance load (potentiometer, system invented by Pro-Ject Audio Systems), adjustable even during playback
  • Fixed impedance: 47 kohms
  • Variable impedance: 10 to 1000 ohms
• Switchable load capacitance: 50, 100, 150, 200, 250, 300, 350, 400 pF
• Channel balance control: precise adjustment of the center of the soundstage over a range of 2 dB to the left or right, can be disabled to shorten the signal path as much as possible
• Allows 100% matching with any MM or MC cartridge

Gain

• MM gain on RCA: 40, 43, 46, 50 dB (+6 dB in XLR)
• MC gain on RCA: 60, 63, 66, 70 dB (+6 dB in XLR)
• Amplification range: from 40 dB (×100) to 70 dB (×3690)

Audio performance

• Signal-to-noise ratio
  • MM (40 dB): 89 dB A-weighted (10 mV)
  • MC (60 dB): 79 dB A-weighted (1 mV)
• Total harmonic distortion (THD)
  • MM 40 dB at 1 kHz: < 0.0015 %
  • MC 60 dB at 1 kHz: < 0.0024 %
• RIAA accuracy: ± 0.4 dB from 20 Hz to 20 kHz
• Subsonic filter: at 20 Hz with 18 dB / octave slope

Electronic design

• Fully discrete audio circuit, without operational amplifiers
  • Passive components: resistors, capacitors, inductors
  • Active components: transistors
  • Several hundred to thousands of components per circuit
• Truly balanced design (true fully balanced)
  • Independent processing of + and − signals for the left and right channels
  • Doubling of amplification sections on the printed circuit board
  • Optimal suppression of noise, errors, and interference
• Separate passive equalization for RIAA and DECCA curves
  • Optimized impedance matching
  • Minimal deviation from the ideal curve

Power supply

• External power supply: 20 V / 3 A DC
• Mains voltage: 110 to 240 V, 50 / 60 Hz
• Maximum consumption: 350 mA DC
• Switching power supply with guaranteed standby consumption of 0.1 W (regulatory compliance)
• Compatible with optional linear power supplies Power Box RS2 Phono and Power Box RS uni 1 & 4 ways for superior performance

Chassis

• Steel structure covered with screwed aluminum panels
• Excellent rigidity and insulation against interference
• Available finishes: silver or black

Dimensions and weight

• Dimensions (W × H × D): 206 × 72 × 200 mm (230 mm with connectors)
• Weight: 1600 g without power supply