Pro-Ject Amp Box RS2 silver

Pro-Ject Amp Box RS2: hybrid tube/transistor amplifier

With the Amp Box RS2, Pro-Ject abandons the pulse-width modulation (PWM) amplification of the Amp Box RS in favor of a fully discrete class AB design. The change is radical: where the previous model relied on the high efficiency of digital modules, the RS2 favors a discrete transistor analog output stage, coupled with a tube input stage. The unit is hand-assembled in Europe and comes in a half-width 206 mm format.

A three-stage hybrid circuit

The RS2 topology is based on a gain chain with three distinct stages. At the input, a differential stage uses a 6922 dual triode (also referenced as 6DJ8), a tube renowned in hi-fi for its linearity and low output impedance. This tube choice, common across the entire Pro-Ject RS range, ensures signal shaping before amplification.

The signal then passes through an oscillation stage (or driver stage) built around 2SA1552T and 2SC4027T bipolar transistors in a complementary PNP/NPN configuration. These references, designed for audio applications, handle the drive voltage required by the final stage.

Finally, the output stage uses a pair of STD03N and STD03P transistors operating in class AB. A DC servo maintains zero DC voltage at the output, eliminating the need for coupling capacitors in the signal path to the speakers.

This fully discrete design, with no operational amplifier in the audio path, lets each component play its precise role in the amplification chain.

Power and bridged mode

In stereo, the Amp Box RS2 delivers 2 × 80 W into 8 Ω and 2 × 130 W into 4 Ω (at 1% THD), with peaks of 100 W and 160 W respectively. These figures are lower than those of the previous Amp Box RS (110 W / 180 W in class D), but a raw watt-for-watt comparison between class D and class AB has its limits: dynamic behavior and instantaneous current delivery differ significantly from one technology to another.

A front-panel switch sets the amplifier to bridged mode. In this configuration, the unit operates in mono and power rises to 280 W into 8 Ω (350 W peak). Two Amp Box RS2 units, one per channel, make it possible to achieve high-power mono amplification, an advantage with low-sensitivity speakers or complex loads.

Connectivity and control

The rear panel offers one pair of unbalanced RCA inputs and one pair of balanced XLR inputs, with input impedances of 47 kΩ and 20 kΩ. Input selection is done on the front panel via push-button or remotely through the RS232 port, accessible via a 3-pin pluggable terminal block. This serial control interface allows integration into a home automation system or a processor-controlled rack.

The gold-plated, insulated speaker terminals accept 4 mm banana plugs, spades, and bare wire. Pro-Ject recommends its Connect it LS cables, but any properly sized speaker cable will do.

Built-in protections

The Amp Box RS2 integrates three monitoring levels, each associated with a status LED on the front panel. A clipping indicator lights up when the instantaneous output voltage approaches the limit imposed by the power supply: this is a useful signal for adjusting the upstream level before audible saturation occurs. A DC voltage detector continuously monitors the output and automatically disconnects the speakers if a DC component appears, protecting the drivers against potential damage. Thermal protection shuts the amplifier down in case of overheating, a welcome safeguard if the unit is installed in a closed or poorly ventilated cabinet.

Power-off works by disabling the internal current sources, which shuts down the transistors and the tube without disconnecting the external power supply. Standby power consumption remains below 0.5 W.

The chassis as a functional element

The all-metal enclosure does more than serve an aesthetic role. The aluminum provides heat dissipation for the class AB output stage, which generates more heat than a class D amplifier. The chassis also acts as shielding against electromagnetic interference and mechanical vibrations. The whole unit weighs 2,660 g (excluding the power supply) and measures 206 × 85 × 200 mm (227 mm including connectors), a compact format that stacks easily with other elements in the Box Design range. Two finishes are available: silver and black.

The external power supply and the Power Box RS2 Amp option

The Amp Box RS2 comes with an external switching power supply providing ± 40 V / 5 A DC, compatible with mains voltages from 100 to 240 V. This standard unit offers solid performance, but Pro-Ject also offers the optional Power Box RS2 Amp, a dedicated linear power supply.

The Power Box RS2 Amp features a large toroidal transformer with copper shielding between the primary and secondary windings. This shielding works like an isolation transformer: it blocks electrical mains noise before it reaches the amplifier. Filtering capacitance reaches 40,000 µF, and a soft-start system prevents inrush current at power-on.

A selector lets you choose between two output voltages: ± 35 V or ± 45 V. The 35 V setting is recommended for operation with 4 Ω speakers or in a confined space, as it reduces power consumption and heat. The 45 V setting unlocks up to 25% more power, but requires adequate ventilation. The power supply alone weighs 4.7 kg, a sign of the size of the onboard transformer.

Technical specifications

Overview

• Fully discrete design, class AB
• Reference-grade tube buffer stage
• Hybrid configuration with dual triode 6922 (6DJ8) differential input stage
• Oscillation stage with 2SA1552T and 2SC4027T transistors
• Output stage with STD03N and STD03P transistors operating in class AB
• Zero DC voltage at the output maintained by DC servo
• Stereo operation or bridged mode (mono)

Power

• Output power: 2 × 130 W / 80 W into 4 Ω / 8 Ω (THD 1%)
  • In bridged mode: 280 W into 8 Ω (THD 1%)
• Peak power: 2 × 160 W / 100 W into 4 Ω / 8 Ω
  • In bridged mode: 350 W into 8 Ω (THD 1%)
• Extreme dynamic power
• Up to 25% more power with the Power Box RS2 Amp

Audio performance

• Background noise: −73 dBV (−76 dBV A-weighted)
• Frequency response 20 Hz - 20 kHz: < −0.2 dB
• Channel separation: −58 dB at 10 kHz into 4 Ω
• Signal-to-noise ratio: 97 dBV (100 dBV A-weighted)
• Total harmonic distortion: < 0.08% at 20 W into 4 Ω
• Gain: 27 dB

Inputs and outputs

• Line-level inputs: 1 unbalanced RCA pair, 1 balanced XLR pair
• RCA / XLR input impedance: 47 kΩ / 20 kΩ
• Speaker terminals: high-end, gold-plated, insulated, compatible with Ø 4 mm banana plugs, spades, or bare wire
• Input selection on the front panel or via RS232
• Compatible with the Connect it LS cable range

Command and control

• RS232 control
• Pluggable 3-pin terminal block for RS232

Power supply

• External power supply: ± 40 V / 5 A DC; 100 - 240 V, 50 / 60 Hz
• Standby power consumption: < 0.5 W
• Power-off by deactivating the current sources, cutting off transistors and tubes to minimize consumption
• Linear Power Box RS2 Amp power supply available separately as an upgrade option
  • Increases output power by up to 25%
  • Large toroidal transformer with shielding between primary and secondary windings
  • Isolation transformer function blocking mains interference
  • Significant power reserve compared with a standard adapter

Protections and indicators

• LED clipping indicator, signaling approach to the output supply voltage threshold
• DC voltage detector and LED indicator, with automatic speaker disconnection in case of DC voltage at the output
• Thermal protection LED with automatic shutdown in case of overheating

Construction and finish

• All-metal chassis, protection against vibrations and interference, optimized heat dissipation
• Available finishes: silver, black

Dimensions and weight

• Dimensions (W × H × D): 206 × 85 × 200 mm (227 mm with connectors)
• Weight: 2660 g without power supply