Pro-Ject Phono Box S3 B silver

Pro-Ject Phono Box S3 B: balanced and discrete phono preamp

Until recently, balanced connections were reserved for the RS range, the top of the Pro-Ject catalog. With the Phono Box S3 B, the Austrian manufacturer brings this technology down to its S3 series, while adopting a new, larger chassis, front-panel controls, and a fully discrete gain stage. The result is a compact MM/MC phono preamp, made in Slovakia, that combines an uncommon number of features in this price range.

A redesigned steel and aluminum chassis

The Phono Box S3 B adopts a larger enclosure than the Phono Box S3 from which it is derived. This new chassis combines a steel structure with aluminum panels and fully aluminum buttons. Steel provides mechanical rigidity, while aluminum ensures electromagnetic shielding. The whole unit weighs 930 g (without the external power supply), with compact dimensions of 206 × 55 × 153 mm (164 mm including connectors). Two finishes are offered: black or silver.

The power supply remains external (18 V / 500 mA DC), an approach that allows the transformer to be kept away from the audio circuit. Power consumption in operation is 170 mA at 18 V, and drops below 0.5 W in standby. Enthusiasts wishing to go further may consider the Pro-Ject Power Box S3 Phono, a filtered linear power supply compatible with this preamp.

The “B” stands for balanced

The “B” in the name means “balanced.” This is the most visible new feature compared with the original Phono Box S3. The balanced input uses a 5-pin mini XLR connector (a compact format found on Pro-Ject turntables in the B range such as the X1 B or X2 B). The balanced outputs, meanwhile, use a pair of full-size standard XLR connectors.

The principle of a balanced connection is based on transmitting two identical signals in opposite phase (a “hot” signal and a “cold” signal). The receiving stage can then subtract the noise accumulated along the cable path, since this noise affects both conductors in the same way. In recording studios, this technique is the norm for microphones and long-distance connections.

One technical detail is worth noting: moving coil (MC) cartridges are balanced transducers by design, because their coils separate the ground conductors of each channel. Moving magnet (MM) cartridges, by contrast, share a common ground between channels, which prevents full use of the balanced connection. To fully benefit from the mini XLR input, an MC cartridge must therefore be used.

The unbalanced RCA input remains available, and the Phono Box S3 B supports both MM and MC cartridges on this input. It is a preamp that can evolve with the system: start with RCA and an MM cartridge, then move to balanced with an MC cartridge during an upgrade, without changing the phono preamp.

Discrete gain stage and dual mono design

The gain stage of the Phono Box S3 B is built from individual components (resistors, capacitors, transistors), with no integrated circuits or operational amplifiers. This so-called “discrete” design is more expensive to implement and requires longer development time than an op-amp-based circuit, but it gives the designer finer control over circuit behavior.

The design is dual mono: each channel has its own amplification stage. Combined with the balanced structure, this topology doubles the number of amplification sections required (two signals per channel, two channels). The gain stage processes the + and − signals independently, then recombines them before sending them to the semi-passive RIAA equalization stage. For the XLR outputs, a balancing stage regenerates a balanced output signal, with an additional gain of 6 dB. The RCA outputs, unbalanced by nature, receive the signal directly from the RIAA stage.

Published measurements confirm very low distortion levels: below 0.001% THD at 1 kHz for the MM configuration, and below 0.005% in MC. The signal-to-noise ratio reaches 103 dBV (110 dBV A-weighted) in MM at 40 dB gain, and 85 dBV (90 dBV A-weighted) in MC at 60 dB gain. RIAA curve accuracy is specified at ± 0.3 dB between 20 Hz and 20 kHz.

Front-panel settings: rare convenience at this level

On most phono preamps in this category, impedance, capacitance, and gain settings are adjusted using DIP switches placed under the enclosure or on the circuit board. You have to turn the unit over, sometimes open it, and handle tiny switches blindly. The Phono Box S3 B breaks with this habit by placing all settings on the front panel, accessible via simple rotary knobs.

The loading options are extensive. Input impedance can be set to five values: 10, 50, 100, 1,000, and 47,000 ohms. Input capacitance offers four values: 50, 150, 300, and 400 pF. Gain is adjustable across four levels: 40, 45, 60, and 65 dB (plus an additional 6 dB via the XLR outputs). The low values (10 ohms in impedance, 50 pF in capacitance) make it possible to compensate for certain difficult situations, such as the use of high-capacitance phono cables. A 20 Hz subsonic filter with an 18 dB-per-octave slope completes the panel. This filter attenuates infrasonic frequencies produced by record warp, tonearm resonance, or pressing defects, and relieves the downstream amplifier and speakers.

Compatibility and integration

At the input, the Phono Box S3 B pairs with Pro-Ject turntables in the “B” range (X1 B, X2 B, X8 Evolution B, among others) via the Connect it S cable with 5-pin mini XLR, but it accepts any turntable equipped with a standard RCA output. At the output, the two pairs of connectors (RCA and XLR) allow integration into almost any hi-fi system, whether it is an integrated amplifier with RCA inputs or a preamplifier with balanced inputs.

The gain range (40 to 65 dB, or even 71 dB via XLR) covers most cartridges on the market, from high-output MM models to very low-output MC models. The output level can be adjusted to match that of the system’s other sources (CD player, network player), which avoids volume differences when switching from one source to another.

Technical specifications

Design

• Fully balanced, discrete gain stage
• Dual mono architecture
• Amplification section built from individual components, with no integrated circuits
• Compatible with MM and MC cartridges
• Semi-passive RIAA equalization
• Very low noise and very low distortion
• Steel and aluminum chassis with fully aluminum buttons, ensuring excellent rigidity and optimal isolation from interference
• Aluminum enclosure protecting against interference

Balanced design

• 5-pin balanced mini XLR input
• Balanced XLR outputs
• Fully balanced, discrete gain stage
• Balancing stage generating a balanced signal for the XLR outputs
• RCA outputs fed directly by the RIAA equalization stage

Settings and operation

• Easy adjustment of settings from the front panel
• Ability to connect two turntables simultaneously
• Independent memory of settings for each input

Electrical specifications

• Input impedance: 10, 50, 100, 1k, 47k ohms
• Input capacitance: 50, 150, 300, 400 pF
• Gain (+ 6 dB via XLR output): 40, 45, 60, 65 dB
• MM signal-to-noise ratio (40 dB): 103 dBV, 110 dBV A-weighted
• MC signal-to-noise ratio (60 dB): 85 dBV, 90 dBV A-weighted
• THD at 1 kHz: < 0.001% MM, < 0.005% MC
• THD (20 Hz - 20 kHz): < 0.008% MM, < 0.01% MC
• Accuracy of the RIAA equalization curve: < 0.3 dB / 20 Hz - 20 kHz
• Subsonic filter: at 20 Hz with a slope of 18 dB / octave

Connectivity

• Inputs: 1 x 5-pin mini XLR, 1 x pair of RCA sockets
• Line output: 1 x RCA pair, 1 x XLR pair

Power supply

• DC power supply: 18 V / 500 mA
• Consumption: 18 V / 170 mA DC, < 0.5 W in standby

Dimensions and weight

• Dimensions: 206 x 55 x 153 mm (164 mm with connectors)
• Weight: 930 g without power supply