The Phono Box RS2 is a phono preamplifier for MM and MC cartridges, balanced from input to output and built from discrete parts only, with passive RIAA and DECCA equalisation and cartridge loading you can adjust while the record plays.
Phono Box RS2: circuit and adjustments
No operational amplifier sits in the signal path: gain comes from individual transistors, resistors and capacitors, an approach that needs far more parts and a longer design process but which Pro-Ject considers more neutral and dynamic. The layout is dual mono and truly balanced: the positive and negative halves of each channel have their own amplifying section, and noise picked up along the connection cancels out by subtraction.
Equalisation is passive and split across two stages, for the RIAA curve and the DECCA curve alike: this improves impedance matching and keeps the deviation from the ideal curve within 0.4 dB between 20 Hz and 20 kHz. A subsonic filter is fitted, working at 20 Hz with a slope of 18 dB per octave.
Gain has four settings for MM, from 40 to 50 dB, and four for MC, from 60 to 70 dB, with a further 6 dB when the XLR connections are used. Load impedance can stay fixed at 47 kohm or be varied continuously between 10 and 1000 ohm with a potentiometer, even during listening, so the effect is heard at once; capacitance is chosen from eight values between 50 and 400 pF. A second potentiometer trims channel balance by 2 dB towards the left or the right to centre the image.
The chassis is steel, clad in screwed-on aluminium panels, a stiff structure that shields against interference. Inputs and outputs are provided on both RCA and XLR. Power comes from an external 20 V unit; owners who want to go further can replace it with a linear supply from Pro-Ject's Power Box range.
Specifications
- Cartridges supported: MM and MC
- Input impedance: 47 kohm fixed or variable from 10 to 1000 ohm
- Load capacitance: selectable from 50 to 400 pF in 50 pF steps
- MM gain: 40, 43, 46 or 50 dB on RCA, 6 dB more on XLR
- MC gain: 60, 63, 66 or 70 dB on RCA, 6 dB more on XLR
- Signal-to-noise: MM at 40 dB 89 dB A-weighted (10 mV); MC at 60 dB 79 dB A-weighted (1 mV)
- Harmonic distortion: below 0.0015% for MM at 40 dB, below 0.0024% for MC at 60 dB, at 1 kHz
- RIAA accuracy: inside 0.4 dB from 20 Hz to 20 kHz
- Subsonic filter: 20 Hz, 18 dB per octave
- Inputs: RCA and XLR
- Outputs: RCA and XLR
- Power: external 20 V / 3 A DC unit, 110-240 V mains
- Current draw: 350 mA DC, under 1 W on standby
- Size: 206 mm wide, 72 mm high, 200 mm deep, sockets included
- Weight: 1.6 kg, power supply excluded
- Finishes: black, silver
