Pro-Ject Pre Box RS2 Digital black

Pro-Ject Pre Box RS2 Digital: preamplifier, DAC and headphone amplifier

Three functions coexist in a compact aluminum enclosure: stereo preamplification, digital-to-analog conversion, and headphone amplification. The Pre Box RS2 Digital follows the principles of the more modest Pre Box S2 Digital, with a more extensive spec sheet and a larger chassis. Switchable output stage between tubes and transistors, dual ESS Sabre converters in balanced mono configuration, expanded set of digital inputs including one HDMI I²S: Pro-Ject stacks up technical choices rarely found in a device of this size.

Dual mono around the ESS9038Q2M

The DAC is built around two ESS Sabre ESS9038Q2M chips, one per channel. Each channel has its own converter and its own signal path, an uncommon approach in this category where sharing the same chip between both channels remains the norm. The ESS9038Q2M handles PCM up to 32-bit / 768 kHz and DSD up to native DSD512 (or via DoP) on the USB input.

Eight digital filters can be selected, including a transient-optimized filter designed by the creators of MQA. High-quality upsampling can be enabled for PCM and/or MQA streams, using a proprietary Pro-Ject filtering algorithm.

Analog output: tubes or transistors, your choice

The output stage is fully balanced in dual mono. Two circuits coexist: a discrete transistor stage, and a tube stage equipped with four 6922EH tubes. Switching from one to the other is done from the front panel or the remote control.

The figures reflect two signatures rather than two quality levels. In transistor mode: THD+N below 0.006% at 2 V RMS / 1 kHz, output impedance 180 Ω. In tube mode: 0.017% THD+N under the same conditions, 220 Ω at the output.

Headphone amplification without coupling capacitor

The headphone amplifier uses DC coupling throughout the entire circuit, including at the output: no coupling capacitor in the signal path. A very fast DC servo neutralizes any residual DC component going to the headphones.

The available power covers common headphones: 90 mW into 300 Ω for high-impedance models, 245 mW into 33 Ω for moderate-impedance models, 150 mW into 8 Ω for very low-impedance headphones. The output uses a 6.35 mm jack on the front panel.

Connectivity focused on digital

On the digital side: USB 2.0 Audio (32-bit / 768 kHz, DoP, native DSD up to DSD512), HDMI I²S reserved for the CD Box RS2 T, two optical TOSLINK inputs (24/192), one coaxial S/PDIF (24/192), one balanced AES/EBU (24/192). Bluetooth v4.0 includes SBC, AAC and AptX codecs.

On the analog side: one balanced XLR pair and one RCA pair at line level, with 15 kΩ input impedance. At the output, one XLR pair and one RCA pair for preamplification, plus the front-panel headphone jack.

Digital processing and full MQA decoding

The USB signal passes through an XMOS multicore microcontroller with RISC architecture. An Altera MAX II CPLD handles downstream reclocking, in conjunction with the XMOS. This two-step process is intended to reduce jitter before digital-to-analog conversion.

MQA decoding is performed entirely in hardware (Master Quality Authenticated full unfolding). The device handles MQA streams without relying on the host software, whether they come from USB, from CD via the CD Box RS2 T, or from a network stream sent by an upstream-connected Stream Box S2 Ultra.

Connection with the CD Box RS2 T

The HDMI input on the rear is not a standard audio input. It receives a differential I²S bus and a 16.9344 MHz master clock. This clock is generated by the Pre Box RS2 Digital itself and sent to the CD Box RS2 T: the transport locks to the converter’s clock rather than the other way around. The goal is to minimize jitter by eliminating the clock recovery stage usually required with a conventional S/PDIF connection.

Frequently asked questions

Is the Pre Box RS2 Digital an integrated amplifier? No. It is a preamplifier: the line output (XLR or RCA) must feed an external power amplifier or active speakers. The only circuit that directly drives a transducer is the headphone amplifier, accessible via the 6.35 mm front-panel jack.

What is the benefit of choosing between tubes and transistors? The sonic signature differs between the two modes. The transistor stage achieves lower distortion (0.006% versus 0.017% at 2 V RMS / 1 kHz) and remains more neutral. The tube stage adds the harmonic coloration typical of tube stages, traditionally described as warmer. The choice depends on taste, system, and music program, and can be changed at any time from the front panel or the remote control.

Does MQA decoding work on all inputs? Hardware MQA decoding applies to received MQA streams, mainly via USB (from a computer or network streamer) and via the HDMI I²S link with the CD Box RS2 T for MQA-encoded CDs. The device does not depend on the source software to perform decoding: the entire process takes place inside the Pre Box.

Can the HDMI input accept a device other than the CD Box RS2 T? This input is not standard HDMI audio: it carries a proprietary differential I²S bus and receives the master clock emitted by the Pre Box. It is specifically intended for the CD Box RS2 T. Connecting a standard HDMI source (Blu-ray player, game console) will not provide a usable signal.

Does Bluetooth v4.0 limit quality compared with wired inputs? The built-in Bluetooth v4.0 supports SBC, AAC and AptX codecs, which means lossy compressed transmissions. Convenient for occasional use (phone, laptop, guest), it remains below the quality achievable via USB, AES/EBU or the HDMI I²S link. For regular high-resolution listening, a wired input is preferable.

Is the device compatible with Roon? Yes, the Pre Box RS2 Digital is Roon Tested certified via its USB input. A Roon Core (computer, Nucleus) automatically recognizes it as an audio output endpoint as soon as it is connected. Playback up to 32-bit / 768 kHz and DSD512 remains available in this configuration.

Technical Specifications

Overview

• Pre Box RS2 Digital: high-end preamplifier, DAC, and headphone amplifier
• Recommended retail price: €2,199 incl. VAT
• Available colors: black, silver

Digital-to-Analog Converter

• DAC: 2x ESS9038Q2M (dual mono, 32-bit / 768 kHz / DSD512)
• 8 different digital filter settings
• Optional selectable high-quality upsampling for PCM and / or MQA content with proprietary filtering
• MQA support: full hardware MQA decoding (Master Quality Authenticated)
• USB signal processing via XMOS multi-core microcontroller with advanced Multi-Core RISC architecture
• Advanced re-clocking driven by Altera CPLD MAX II (Complex Programmable Logic Device) combined with the XMOS microcontroller

Digital Inputs

• 1x USB 2.0 Audio 32-bit / 768 kHz + DoP and native DSD up to DSD512
• 1x HDMI I²S for CD Box RS2 T
• 2x optical S / PDIF 24-bit / 192 kHz
• 1x coaxial S / PDIF 24-bit / 192 kHz
• 1x balanced AES / EBU 24-bit / 192 kHz
• Fully integrated Bluetooth v4.0
  • Supported Bluetooth codecs: SBC, AAC, AptX

Analog Input

• Line-level input: 1 pair XLR, 1 pair RCA
• Input impedance: 15 kΩ

Analog Outputs

• Preamplifier output: XLR and RCA
• Headphones: 6.35 mm jack
• Output impedance: 180 Ω in transistor mode, 220 Ω in tube mode
• Headphone output power:
  • 90 mW into 300 Ω
  • 245 mW into 33 Ω
  • 150 mW into 8 Ω
• THD+N (20 Hz - 20 kHz): < 0.006% in transistor mode / 0.017% in tube mode at 2 V RMS, 1 kHz

Output Stage

• Fully balanced dual mono design
• Discrete, balanced output buffer, switchable between 6922 tube and transistor
• Ultra-linear and ultra-low noise
• Tubes: 4x 6922EH

Headphone Amplifier

• Full DC coupling, without coupling capacitors, including at the output
• Very fast DC servo ensuring no DC voltage at the output
• Output power suited to all types of headphones

Synchronization and System Connectivity

• Master clock output: 16.9344 MHz for CD Box RS2 T
• Connection to CD Box RS2 T via reliable differential I²S bus on HDMI connector

Display and Control

• Large high-quality front display
• Responsive and fast user interface
• Clear and structured display of all relevant information

Included Accessories

• Premium full aluminum remote control

Power Supply

• Power supply: 20 V / 3 A DC, 110-240 V / 50, 60 Hz
• Maximum consumption: 1.2 A DC
• Standby consumption: < 0.5 W

Physical Specifications

• High-quality aluminum chassis (silver or black)
• Dimensions W x H x D (with connectors): 206 x 72 x 222 mm
• Weight: 1710 g without power supply unit