The Pro-Ject Phono Box S3 B is a dedicated phono preamplifier designed for moving-magnet (MM) and moving-coil (MC) cartridges. Its standout feature is Pro-Ject’s True Balanced Connection design, which provides a balanced input and output path for compatible turntables and audio equipment.
Beyond balanced connectivity, the S3 B offers adjustable gain, input impedance and capacitance, allowing it to be configured for a wide range of cartridges. It is a more specialized component than a basic phono preamp, particularly useful if you want flexible cartridge matching or plan to build a balanced vinyl signal chain.
Pro-Ject Phono Box S3 B: Quick Specs
| Specification | Details |
|---|---|
| Cartridge support | Moving Magnet (MM) and Moving Coil (MC) |
| Gain settings | 40, 45, 60 and 65 dB |
| Additional gain | +6 dB when using balanced XLR outputs |
| Input impedance | 10, 50, 100, 1,000 and 47,000 ohms |
| Input capacitance | 50, 150, 300 and 400 pF |
| Signal-to-noise ratio, MM | 103 dBV / 110 dBV, A-weighted |
| Signal-to-noise ratio, MC | 85 dBV / 90 dBV, A-weighted |
| THD at 1 kHz | Below 0.001% MM; below 0.005% MC |
| Inputs | 1 pair RCA and 1 five-pin mini-XLR |
| Outputs | 1 pair RCA and 1 pair balanced XLR |
| Circuit design | Fully symmetrical, discrete gain stage and dual-mono layout |
| RIAA equalization | Semi-passive |
| Subsonic filter | 20 Hz, 18 dB/octave |
| Dimensions | 206 × 55 × 153 mm |
| Weight | 930 g without power supply |
| Power supply | 18 V DC, 500 mA |
These are manufacturer specifications. The published signal-to-noise figures vary according to cartridge type, gain and output connection.
Who Is the Phono Box S3 B For?
The Phono Box S3 B is aimed at vinyl listeners who want more control over their phono stage than a basic MM preamp provides.
It is particularly relevant for:
- Owners of turntables with a compatible balanced tonearm connection.
- Listeners using moving-coil cartridges that require different gain and impedance settings.
- People who want adjustable loading for cartridge matching.
- Systems with balanced XLR inputs on the following amplifier or audio component.
- Vinyl enthusiasts building a system with separate turntable, phono preamp and amplifier components.
Its balanced connection is the main reason to consider this model over a simpler phono preamp. However, the RCA input and output connections also make it usable in conventional systems.
Who Should Consider Another Phono Preamp?
The S3 B may be excessive for a basic turntable with an entry-level MM cartridge and no plans to use balanced connections.
If you only need a straightforward MM phono stage, a less expensive model may provide everything your system requires. The additional loading controls and balanced circuitry are valuable features, but they do not automatically translate into an audible improvement in every setup.
It is also important to understand that buying the S3 B alone does not make a conventional RCA turntable connection fully balanced. Your tonearm wiring and cable must support the appropriate balanced connection.
For simpler alternatives, see our guide to the best phono preamps.
True Balanced Connection: What Makes the S3 B Different?
The main feature separating the Phono Box S3 B from more conventional phono stages is its balanced signal architecture.
A phono cartridge generates a very low-level signal that is particularly vulnerable to electrical interference. A balanced connection carries the two signal polarities separately, allowing compatible equipment to reject common-mode interference.
Pro-Ject’s True Balanced Connection uses a five-pin mini-XLR input and a symmetrical gain stage. The unit also provides balanced XLR outputs for connecting to compatible downstream equipment.
What You Need for Balanced Playback
To use the complete balanced signal path, your setup needs:
- A cartridge with separate positive and negative connections for each channel.
- A tonearm and internal wiring that preserve the separate signal conductors.
- A compatible five-pin mini-XLR balanced phono cable.
- The Phono Box S3 B.
- An amplifier or other component with balanced XLR inputs if you want to maintain the balanced connection beyond the preamp.
A conventional RCA connection can still be used, but it does not provide the same fully balanced signal path.
Balanced connections are especially useful for rejecting interference in some systems and cable arrangements. They should not be treated as a guarantee of better sound in every installation.
MM and MC Cartridge Support
The Phono Box S3 B supports both major cartridge categories: moving magnet and moving coil.
This flexibility is important because different cartridges can require substantially different gain and loading settings.
Moving-Magnet Cartridges
MM cartridges commonly use a 47 kΩ input impedance. The S3 B offers this setting, along with four selectable input capacitance values.
For example, Pro-Ject’s manual lists the Ortofon 2M Red as an MM cartridge using 40 dB gain, 47 kΩ impedance and 147 pF capacitance.
The S3 B’s available capacitance settings are 50, 150, 300 and 400 pF. The correct choice should account for the cartridge manufacturer’s recommended loading and the capacitance of the tonearm cable and other connections.
Moving-Coil Cartridges
MC cartridges often require higher gain and lower input impedance than MM cartridges.
The S3 B provides selectable impedance settings of 10, 50, 100 and 1,000 ohms, as well as 47 kΩ. Its 60 and 65 dB gain settings are useful starting points for many lower-output MC cartridges, although the correct configuration depends on the cartridge’s output and recommended load.
The ability to adjust these settings makes the S3 B more versatile than a fixed-loading MM-only preamp.
Gain, Impedance and Capacitance Settings
The S3 B places its main adjustment controls on the front panel. This makes it possible to change cartridge settings without opening the enclosure or accessing small switches on the underside.
| Setting | Available options | Purpose |
|---|---|---|
| Gain | 40, 45, 60, 65 dB | Matches amplification to cartridge output |
| Input impedance | 10, 50, 100, 1,000, 47,000 ohms | Sets the electrical load seen by the cartridge |
| Input capacitance | 50, 150, 300, 400 pF | Adjusts capacitive loading, particularly relevant to MM cartridges |
| Subsonic filter | On / Off | Reduces very low-frequency signals below its filter region |
The correct settings should come from the cartridge manufacturer’s recommendations rather than choosing the highest gain or lowest impedance automatically.
The unit also stores settings independently for its two input selections. This is useful if you connect two turntables, one through RCA and one through the balanced mini-XLR input.
Setup and Connections
The Phono Box S3 B can be used in either a conventional RCA system or a compatible balanced setup.
Standard RCA Connection
For a conventional turntable:
- Connect the turntable’s RCA output cables to the S3 B’s RCA input.
- Connect the turntable ground wire to the preamp’s grounding terminal, if provided by your turntable.
- Connect the S3 B’s RCA output to an AUX, LINE or CD input on your amplifier.
- Select the RCA input on the front panel.
- Set gain and cartridge loading according to the cartridge manufacturer’s specifications.
- Connect the supplied 18 V power supply and switch the unit on.
The turntable must send an unamplified PHONO signal to the S3 B. If it has a built-in phono preamp, set it to PHONO or bypass the internal preamp.
Balanced Connection
For a compatible balanced turntable:
- Connect the turntable using a suitable five-pin mini-XLR phono cable.
- Select the XLR input on the S3 B.
- Configure gain, impedance and capacitance for the cartridge.
- Connect the balanced XLR outputs to a compatible amplifier or audio component.
- Confirm that the receiving equipment is set to the correct balanced input.
The S3 B provides an additional 6 dB of gain when using its XLR outputs, so account for this when matching output levels.
What Does the Subsonic Filter Do?
The S3 B includes a switchable subsonic filter operating at 20 Hz with an 18 dB-per-octave slope.
Very low-frequency signals can come from record warps, mechanical vibration and other sources below the normal musical range. These signals can cause unnecessary woofer movement and use amplifier headroom.
The filter is designed to reduce this unwanted low-frequency content. Whether to enable it depends on your records, turntable setup and listening system.
Sound and Performance: What the Specifications Tell Us
Pro-Ject specifies low distortion, adjustable gain and a symmetrical discrete gain stage for the S3 B.
Its published figures include:
- Less than 0.001% THD for MM at 1 kHz.
- Less than 0.005% THD for MC at 1 kHz.
- RIAA equalization accuracy within 0.3 dB from 20 Hz to 20 kHz.
- A-weighted signal-to-noise figures of 103 dBV for MM and 85 dBV for MC at the stated gain settings.
These specifications indicate a carefully designed phono stage, but they do not establish how it will sound in every system. Cartridge matching, turntable wiring, amplifier compatibility and the rest of the audio chain all matter.
The balanced design is most relevant when the rest of the connection supports it and when interference rejection is useful. It is not a reason to assume that every RCA-based phono preamp will sound inferior.
What Does It Pair Well With?
The S3 B can work with a broad range of MM and MC cartridges, provided their electrical requirements fall within its adjustment range.
For an MM cartridge such as the Ortofon 2M Blue, the standard 47 kΩ input setting is relevant, with gain and capacitance configured according to the cartridge manufacturer’s recommendations.
For an MC cartridge, the adjustable gain and lower impedance options provide more flexibility. Always check the cartridge’s recommended load and output voltage before selecting settings.
The preamp can also be used with a conventional RCA turntable, so owning a balanced-compatible turntable is not a requirement for basic operation.
If you are choosing a cartridge for your system, see our guide to the best turntable cartridges and our explanation of moving-magnet versus moving-coil cartridges.
Final Verdict
The Pro-Ject Phono Box S3 B is a feature-rich phono preamp for listeners who want adjustable cartridge matching and the option of a balanced signal path.
Its MM and MC support, selectable gain, impedance and capacitance, dual-mono design and subsonic filter make it adaptable to a wide range of vinyl systems.
The main attraction is True Balanced Connection, but it is only fully useful when the turntable’s tonearm wiring, phono cable and downstream equipment support the required connections.
For a basic turntable setup, the S3 B may offer more functionality than necessary. For a system built around compatible balanced connections or cartridges that benefit from adjustable loading, it is a much more compelling option.
Frequently Asked Questions
Is the Pro-Ject Phono Box S3 B worth buying?
It depends on your cartridge and system. Its balanced input and output, MM and MC support, and adjustable loading make it a versatile phono stage. If you only need a basic MM preamp and will use RCA connections, a less expensive model may be sufficient.
Do I need a balanced tonearm cable to use the S3 B?
No. The RCA input works with conventional turntables. A compatible five-pin mini-XLR cable and suitable tonearm wiring are required to use its balanced input.
Can the Phono Box S3 B be used with a moving-coil cartridge?
Yes. It supports both MM and MC cartridges, with selectable gain and input impedance settings to accommodate different cartridge requirements.
Does the S3 B have a built-in subsonic filter?
Yes. It includes a switchable 20 Hz subsonic filter with an 18 dB-per-octave slope.
Can I connect two turntables to the Phono Box S3 B?
Yes. The unit has one RCA input and one five-pin mini-XLR input. The front-panel input selector switches between them, and the settings for each input are stored independently.
Does the balanced connection automatically improve sound quality?
Not necessarily. A balanced connection can help reject electrical interference when the entire signal path is compatible. The audible result depends on the equipment, wiring and listening environment.
Can I connect the S3 B directly to powered speakers?
Yes, if the powered speakers have a suitable line-level input. Connect the S3 B’s RCA or XLR output to the corresponding speaker input. Do not connect its output to a dedicated PHONO input, because the S3 B has already applied phono amplification and RIAA equalization.