The Pre Box S2 Digital sits at the heart of Pro-Ject's S2 Line from Box Design — a compact chassis series that pairs audiophile-grade engineering with desktop-friendly dimensions. It combines a high-performance DAC, a variable pre-amplifier output and a dedicated headphone amplifier in a single full-metal enclosure measuring just 103 × 37 × 122mm. Above it in the range sits the Pre Box RS2 Digital, which adds balanced XLR outputs and a more elaborate power supply stage; below it, the Pre Box S2 Analogue provides a purely analogue pre-amplifier without digital conversion.
DAC Architecture
At the core of the Pre Box S2 Digital is a dual-mono configuration of ESS Sabre ESS9038Q2M 32-bit DAC converters. Via the rear-panel USB Type-B input, the DAC accepts PCM signals up to 32-bit/768kHz and DSD up to DSD512 in both DoP and native modes. The coaxial (RCA) and optical (TOSLink) inputs handle PCM up to 24-bit/192kHz. Hardware MQA unfolding is built in, and the unit is Roon Ready certified.
Clock Design and Digital Filtering
Pro-Ject's proprietary Optimum Transient digital filter works alongside a redesigned discrete clock circuit to suppress jitter to below 100 femtoseconds. Eight selectable digital filters — seven standard plus the proprietary Optimum Transient filter — allow listeners to fine-tune the sonic character to their preference. The 2023 Edition introduced a further-refined clock layout alongside a new internal power supply design.
Analogue Output Stage and Headphone Amplifier
The analogue output stage drives a single pair of variable RCA outputs, delivering 2.05 Vrms. Measured performance figures include a dynamic range of 124 dBA, THD of 0.0003% at 0 dB/2V RMS/1kHz, and a frequency response of 20 Hz–20 kHz. The front-panel 6.3mm headphone output is powered by an ESS9602 headphone amplifier and output buffer, delivering 100mW into 32Ω and 6.6mW into 600Ω with a dynamic range of up to 122 dB and THD+N of −117 dB at 600Ω. The 2023 Edition redesigned the headphone output stage with increased Class-A operating current for improved drive capability.
Construction
The internal layout uses polymer capacitors and thin-film miniMELF resistors on a gold-plated four-layer PCB. A dual-regulator power architecture separates the analogue and digital supply rails. The unit is powered by an external 5V/1A DC supply via a micro-USB connection, keeping the internal environment free from mains transformer interference. The full-metal chassis weighs just 366g excluding the power supply.
What the Press Says
- "A tiny DAC and headphone amp that's big on features" — What Hi-Fi? ★★★★☆ (February 2019)
| DAC Chip | Dual mono ESS Sabre ESS9038Q2M 32-bit |
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| Digital Inputs | USB Type-B, Coaxial RCA, Optical TOSLink |
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| USB Resolution | Up to 32-bit/768kHz PCM, DSD512 (DoP and native) |
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| Coaxial/Optical Resolution | Up to 24-bit/192kHz PCM |
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| MQA Decoding | Yes (hardware) |
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| Analogue Output | 1 × variable RCA (stereo pair) |
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| Headphone Output | 6.3mm (front panel) |
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| Headphone Amplifier | ESS9602, 100mW/32Ω, 6.6mW/600Ω |
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| Output Voltage | 2.05 Vrms |
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| THD | 0.0003% (0 dB, 2V RMS, 1 kHz) |
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| Dynamic Range | 124 dBA |
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| Jitter | Lower than 100 femtoseconds |
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| Frequency Response | 20 Hz–20 kHz |
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| Digital Filters | 7 selectable + 1 proprietary Optimum Transient |
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| Power Supply | External 5V/1A DC (micro-USB) |
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| Dimensions (W × H × D) | 103 × 37 × 122mm |
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| Weight | 366g (excluding power supply) |
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| Also Available As | Silver |
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