Audio T Verdict
The Phono Box S3 B is where Pro-Ject's compact phono stages become significantly more interesting. It will happily handle a conventional Moving Magnet turntable through RCA, but beneath the small aluminium and steel enclosure is a fully discrete symmetrical gain stage, extensive cartridge loading and a genuine balanced signal path for systems built around Pro-Ject's True Balanced connection.
It is exceptionally flexible too. You can leave two turntables connected at once, one through RCA and another through the 5-pin mini-XLR input, then switch between them from the front panel. The S3 B remembers the settings for each input, while impedance, capacitance and gain can all be changed without reaching around the back of the system.
For us, that makes it more than simply a stepping stone to a more expensive phono stage. With MM and low-output MC support, balanced and single-ended connectivity, switchable subsonic filtering and enough adjustment to accommodate a very wide range of cartridges, it can remain useful through several generations of turntable and cartridge upgrades.
Pro-Ject Phono Box S3 B Balanced MM and MC Phono Stage
The Pro-Ject Phono Box S3 B is a compact Moving Magnet and Moving Coil phono preamplifier built around a fully discrete, symmetrical gain architecture.
Alongside conventional RCA phono connectivity, it introduces Pro-Ject's True Balanced system to the S Line with a 5-pin mini-XLR phono input and full-size balanced XLR line outputs.
Extensive front-panel loading and gain controls allow the stage to be tailored to a wide range of cartridges, while two turntables can remain connected simultaneously and their individual settings are retained in memory.
- Moving Magnet and Moving Coil phono stage
- Fully discrete gain architecture
- Fully symmetrical internal gain stage
- Pro-Ject True Balanced compatible
- 1 x balanced 5-pin mini-XLR phono input
- 1 x conventional stereo RCA phono input
- Two turntables can remain connected simultaneously
- Independent input settings stored in memory
- 1 x stereo RCA line output
- 1 x stereo balanced XLR line output
- RCA and XLR outputs active simultaneously
- 10 / 50 / 100 / 1k / 47kOhm impedance settings
- 50 / 150 / 300 / 400pF capacitance settings
- 40 / 45 / 60 / 65dB gain settings
- Additional 6dB level through balanced XLR output
- Switchable 20Hz subsonic filter
- 18dB-per-octave subsonic slope
- Front-panel cartridge adjustment
- Semi-passive RIAA equalisation
- Steel and aluminium casework
- External 18V power supply
- Less than 0.5W standby consumption
- Black or Silver finish
- Made in Europe
What Does a Phono Stage Do?
The signal produced by a phono cartridge is dramatically smaller than the output from a CD player, streamer or DAC.
A phono stage therefore performs two fundamental tasks:
- It amplifies the tiny cartridge signal to a conventional line level.
- It applies RIAA equalisation to restore the correct tonal balance of the recording.
The Phono Box S3 B goes further by allowing the electrical load and gain to be matched to the cartridge rather than assuming one fixed setting will suit everything.
Moving Magnet and Moving Coil
The S3 B is designed for both Moving Magnet and Moving Coil cartridges.
The broad range of available impedance and gain settings means it can accommodate high-output MM designs, conventional high-output MC cartridges and many low-output MC cartridges.
This makes it particularly useful if you want a phono stage that can remain in the system while the turntable or cartridge is upgraded later.
Five Input-Impedance Settings
The front panel provides the following load-resistance options:
- 10Ohm
- 50Ohm
- 100Ohm
- 1kOhm
- 47kOhm
47kOhm is the conventional setting for most Moving Magnet cartridges and many high-output Moving Coil designs.
The lower values are provided primarily for low-output Moving Coil cartridges, where the electrical loading recommended by the cartridge manufacturer can be very different.
Start With the Cartridge Manufacturer's Recommendation
There is no universal best impedance setting. Begin with the load specified by the cartridge manufacturer and then, where appropriate, experiment around that recommendation by listening.
Four Capacitance Settings
Input capacitance can be selected from:
Capacitance is particularly relevant to Moving Magnet cartridges, where the combined capacitance of the tonearm wiring, phono cable and phono-stage input can influence high-frequency response.
The unusually low 50pF option is particularly useful where the turntable cable and internal wiring already contribute a significant proportion of the cartridge manufacturer's recommended total.
Does Capacitance Matter With Moving Coil?
It is generally far less significant with a conventional low-output Moving Coil cartridge than with a Moving Magnet design.
Pro-Ject's own setup example for a low-output Ortofon Quintet Red therefore states that the capacitance setting is not relevant in the same way and concentrates instead on selecting the appropriate impedance and gain.
Four Gain Settings
The selectable gain settings are:
Lower gain is generally appropriate for higher-output Moving Magnet and high-output Moving Coil cartridges, while low-output Moving Coil cartridges usually require considerably more amplification.
Why Gain Matching Matters
If gain is set too low, the phono stage may sound noticeably quieter than the other sources in your system and you may need excessive amplifier volume.
If gain is unnecessarily high, the signal can overload part of the phono stage or following equipment and increase noise.
The aim is to provide enough gain for the cartridge's output while retaining suitable headroom.
Balanced XLR Output Adds 6dB
Pro-Ject specifies the four gain positions as 40, 45, 60 and 65dB, with the balanced XLR output providing an additional 6dB output level relative to the RCA connection.
This is normal behaviour for a balanced line output and should be taken into account when matching playback level against other sources.
Don't Choose Gain by the Number Alone
If you move from RCA to XLR output, remember that the balanced output is 6dB higher. Reassess the gain setting rather than assuming the same position will always be ideal.
Two Turntables Can Stay Connected
The Phono Box S3 B has two independent phono inputs:
- 1 x stereo RCA
- 1 x 5-pin mini-XLR
Both turntables can remain physically connected at the same time.
The front-panel Input button then switches between RCA and balanced mini-XLR.
Settings Stored for Each Input
The S3 B stores the settings associated with each input.
This means you could, for example, use:
- A Moving Magnet cartridge on the RCA input
- A low-output Moving Coil cartridge on the balanced input
Switch between the two turntables and the corresponding cartridge settings are recalled rather than having to reconfigure the stage every time.
Ideal for Two Turntables or Two Arms
The two-input arrangement is particularly useful if you keep separate analogue sources for different purposes, such as an everyday MM turntable alongside a second deck or arm fitted with a Moving Coil cartridge.
True Balanced Phono Input
The standout connection is the 5-pin mini-XLR balanced phono input.
This forms part of Pro-Ject's True Balanced analogue system and is designed to preserve the differential signal generated by a suitably wired phono cartridge from the turntable into the phono preamplifier.
Why Balanced Matters More at Phono Level
A phono cartridge produces one of the smallest electrical signals anywhere in a Hi-Fi system.
That tiny signal then has to travel through the tonearm and phono cable before receiving substantial amplification.
Any noise or interference introduced before the phono stage can therefore be amplified along with the music.
A balanced connection allows a compatible differential input to reject interference appearing equally on the two signal conductors.
A Phono Cartridge Is Different From a CD Player
Most phono cartridges generate a floating differential signal and do not intrinsically reference either audio conductor to ground in the same way as a conventional single-ended line source.
This is why a properly wired cartridge can lend itself naturally to balanced transmission.
Which Cartridges Can Use True Balanced?
Moving Coil cartridges are naturally well suited to a fully balanced connection when the turntable wiring preserves both sides of the cartridge generator separately.
Pro-Ject also now offers the Pick it PRO Balanced, a specially configured Moving Magnet cartridge designed specifically to work within its True Balanced system.
The important factor is therefore not simply whether the cartridge is labelled MM or MC, but whether the cartridge and turntable wiring provide the required balanced signal path.
Balanced Input Requires a Compatible Cartridge and Cable
A mini-XLR socket on its own does not make every cartridge connection True Balanced. Use a suitably wired cartridge together with the correct Pro-Ject True Balanced phono cable and compatible turntable output.
Works With Pro-Ject Balanced Turntables
The Phono Box S3 B is an obvious partner for Pro-Ject turntables equipped for the company's balanced phono system.
One current example is the Pro-Ject Debut PRO S Balanced, which provides the appropriate balanced turntable output architecture and can be upgraded with a suitable cartridge for full True Balanced operation.
Can an RCA Turntable Be Used?
Yes.
The conventional stereo RCA phono input means the S3 B works perfectly well as a high-quality single-ended phono stage with an ordinary turntable.
You do not have to use the True Balanced connection to benefit from its cartridge adjustment, discrete amplification or balanced XLR line outputs.
Does an XLR Output Make an RCA Turntable Fully Balanced?
No.
The S3 B can provide a balanced XLR line output regardless of which input is selected, but an RCA connection from the turntable remains a single-ended cartridge-to-phono-stage connection.
For a complete True Balanced phono signal path, the cartridge, turntable wiring and phono connection into the S3 B must also be appropriately configured.
Fully Discrete Gain Stage
The amplification stage uses individual discrete electronic components rather than relying on a conventional integrated operational-amplifier circuit for the main gain stage.
Pro-Ject also builds the circuit symmetrically, with separate signal paths for the two phases required by balanced operation.
Fully Symmetrical Architecture
A fully balanced circuit requires more than simply fitting XLR sockets to the rear panel.
The positive and negative halves of the audio signal must remain separate through the amplification architecture.
This effectively requires two corresponding signal paths, increasing component count and circuit complexity compared with a conventional single-ended phono preamplifier.
Semi-Passive RIAA Equalisation
After the discrete gain stage, Pro-Ject uses a semi-passive RIAA equalisation network.
The manufacturer specifies RIAA accuracy better than 0.3dB from 20Hz to 20kHz.
That figure indicates how closely the stage follows the standard equalisation curve required to restore the tonal balance of a vinyl recording.
Very Low Distortion
Pro-Ject publishes the following distortion figures:
- MM THD at 1kHz: less than 0.001%
- MC THD at 1kHz: less than 0.005%
- MM THD from 20Hz to 20kHz: less than 0.008%
- MC THD from 20Hz to 20kHz: less than 0.01%
Signal-to-Noise Performance
Pro-Ject's current specification gives:
- MM at 40dB gain: 103dBV / 110dBV A-weighted
- MC at 60dB gain: 85dBV / 90dBV A-weighted
The lower signal level produced by a typical Moving Coil cartridge inevitably demands more gain, which makes noise performance especially important in an MC phono stage.
Switchable Subsonic Filter
A dedicated front-panel button engages the subsonic filter.
The filter operates at 20Hz with an 18dB-per-octave slope.
What Is a Subsonic Filter For?
Very-low-frequency energy can be generated by warped records, tonearm/cartridge resonance and other mechanical movement which is not part of the recorded music.
Although much of this energy may sit below the normal audible range, amplifiers and loudspeakers can still attempt to reproduce it.
The subsonic filter reduces this unwanted low-frequency content before it reaches the rest of the system.
When Might You Use It?
It can be particularly useful if you notice:
- Woofer cones moving significantly on warped records
- Very-low-frequency rumble
- A turntable setup exciting unwanted subsonic energy
If your records and turntable are behaving properly and there is no obvious low-frequency problem, you may prefer to leave the filter disengaged.
Simultaneous RCA and XLR Outputs
The rear RCA and balanced XLR line outputs are both active at the same time.
Pro-Ject's manual specifically allows two receiving components to be connected simultaneously.
This could be useful, for example, where the same turntable system needs to feed two suitable line-level devices.
The Outputs Are Line Level
Connect the RCA or XLR outputs to a normal line-level input on your amplifier or preamplifier. Do not connect the output of the Phono Box S3 B into another phono input.
Balanced XLR Output
The full-size XLR output provides a balanced line-level connection to compatible amplifiers and preamplifiers.
This is separate from the 5-pin mini-XLR phono input used on the turntable side.
Mini-XLR In, Full-Size XLR Out
The balanced turntable connection uses one 5-pin mini-XLR input. The amplified line-level balanced output to your Hi-Fi uses a conventional pair of full-size XLR sockets.
Conventional RCA Output
A stereo RCA output is also provided, allowing the S3 B to connect to integrated amplifiers or preamplifiers without balanced line inputs.
This means the phono stage remains useful even if the rest of the system is entirely single-ended.
Front-Panel Adjustment
Cartridge configuration is handled using dedicated front-panel buttons rather than tiny DIP switches hidden underneath the case.
The front panel provides direct control over:
- Input selection
- Subsonic filter
- Input impedance
- Input capacitance
- Gain
LED indicators show the currently selected setting.
No Need to Open the Case
Being able to change cartridge settings from the listening position is particularly useful when comparing different impedance or capacitance values.
You can make a change, play the same piece of music again and assess the result without unplugging the unit or removing a cover.
Steel and Aluminium Casework
The S3 B uses a substantial metal enclosure combining steel and aluminium.
Pro-Ject designed the larger S3 chassis to provide greater mechanical rigidity and additional shielding from external interference while also creating the internal space required for the fully symmetrical circuit.
Compact 206mm Width
Despite the amount of circuitry inside, the S3 B remains considerably smaller than a conventional full-width Hi-Fi component.
Dimensions are:
- Width: 206mm
- Height: 55mm
- Depth: 153mm
- Depth including rear sockets: 164mm
930g Without the Power Supply
The main unit weighs 930g excluding its external power supply.
External 18V Power Supply
Power is supplied by an external 18V / 500mA DC supply.
Keeping the mains transformer outside the phono-stage enclosure is particularly useful with an audio circuit amplifying signals measured in millivolts or fractions of a millivolt.
Less Than 0.5W Standby
The S3 B draws approximately 170mA from its 18V supply during operation and less than 0.5W in standby.
A front-panel button switches between operation and standby.
Battery-Power Upgrade Option
Pro-Ject currently lists the Phono Box S3 B as compatible with its Accu Box S2 rechargeable battery power supply.
This gives customers who want to take the power supply further an official route to operate the phono stage from battery power rather than directly from a mains-powered DC adaptor.
Black or Silver
The Phono Box S3 B is available in:
Made in Europe
Pro-Ject specifies the Phono Box S3 B as manufactured in Europe.
Phono Box S3 B With Debut PRO S Balanced
The Pro-Ject Debut PRO S Balanced is an especially logical partner because the turntable is already equipped with the appropriate balanced-output architecture.
With the standard cartridge, conventional RCA operation remains available. Fit a cartridge configured for Pro-Ject True Balanced use, connect the appropriate balanced phono cable and the S3 B can then preserve the balanced cartridge signal into its symmetrical gain stage.
Can I Use It With the Debut EVO 2?
Yes.
The Pro-Ject Debut EVO 2 can connect conventionally through RCA and its supplied Pick it MM EVO can be matched using the S3 B's MM loading and gain controls.
You do not need a balanced turntable to use the Phono Box S3 B.
S3 B or a Fixed MM Phono Stage?
A simpler fixed-loading MM stage may be all you need if you know you will always use one conventional Moving Magnet cartridge.
The S3 B becomes considerably more attractive if you want:
- More precise MM capacitance matching
- Low-output Moving Coil support
- Two turntables connected simultaneously
- Front-panel loading controls
- Balanced turntable connectivity
- Balanced line output
- A future True Balanced upgrade
Who Is the Phono Box S3 B For?
The Phono Box S3 B makes particular sense if you:
- Want one phono stage for both MM and MC cartridges
- Want adjustable gain and loading
- Own two turntables or two separate analogue sources
- Want settings remembered separately for each input
- Own a Pro-Ject balanced-ready turntable
- Plan to upgrade to a True Balanced cartridge setup
- Want balanced XLR output to your amplifier
- Need a switchable subsonic filter
- Want adjustment controls accessible from the front panel
- Need a compact phono stage with extensive flexibility
Key Features
- Pro-Ject Phono Box S3 B
- MM phono stage
- MC phono stage
- Fully discrete gain stage
- Fully symmetrical circuit architecture
- Semi-passive RIAA equalisation
- True Balanced compatible
- 5-pin mini-XLR balanced phono input
- Stereo RCA phono input
- Two turntables connected simultaneously
- Input-specific setting memory
- Full-size balanced XLR outputs
- Stereo RCA outputs
- RCA and XLR outputs simultaneously active
- 10Ohm impedance
- 50Ohm impedance
- 100Ohm impedance
- 1kOhm impedance
- 47kOhm impedance
- 50pF capacitance
- 150pF capacitance
- 300pF capacitance
- 400pF capacitance
- 40dB gain
- 45dB gain
- 60dB gain
- 65dB gain
- Additional 6dB through XLR output
- Switchable subsonic filter
- 20Hz filter frequency
- 18dB-per-octave filter slope
- Front-panel cartridge controls
- LED setting indication
- Steel and aluminium enclosure
- 18V / 500mA external supply
- Less than 0.5W standby
- Accu Box S2 compatible
- 206mm compact chassis
- 930g weight
- Black finish
- Silver finish
- Made in Europe
Frequently Asked Questions
Is the Pro-Ject Phono Box S3 B current?
Yes. It remains part of Pro-Ject's current Hi-Fi electronics range and True Balanced phono ecosystem.
What does the B stand for?
It identifies the model's balanced architecture and True Balanced connectivity.
Does it support Moving Magnet cartridges?
Yes.
Does it support Moving Coil cartridges?
Yes, including many low-output MC designs thanks to its higher gain and low-impedance loading options.
How many turntables can I connect?
Two can remain connected simultaneously: one through the stereo RCA input and one through the 5-pin mini-XLR input.
How do I switch between the two turntables?
Using the Input button on the front panel.
Does it remember the settings for each turntable?
Yes. Pro-Ject states that the settings for each input are stored in memory.
What impedance settings are available?
10, 50, 100, 1k and 47kOhm.
Which impedance should I use for an MM cartridge?
Most MM cartridges are designed for 47kOhm, but always check the cartridge manufacturer's specification.
Which impedance should I use for an MC cartridge?
It depends on the cartridge. Low-output MC designs vary significantly, which is why the S3 B provides 10, 50, 100 and 1kOhm options as well as 47kOhm.
What capacitance settings are available?
50, 150, 300 and 400pF.
Why does capacitance matter?
It can significantly affect the high-frequency response of Moving Magnet cartridges. The tonearm wiring and phono cable also contribute to the total capacitance seen by the cartridge.
What gain settings are available?
40, 45, 60 and 65dB.
Which gain should I use?
Choose according to cartridge output. Higher-output MM cartridges normally require substantially less gain than low-output MC cartridges.
Does the XLR output have the same level as RCA?
Pro-Ject specifies an additional 6dB when using the balanced XLR outputs.
Does it have a subsonic filter?
Yes.
What frequency does the subsonic filter use?
20Hz.
What is the filter slope?
18dB per octave.
Why would I use the subsonic filter?
It can reduce unwanted very-low-frequency energy caused by warped records, mechanical resonance or rumble.
Does True Balanced only work with Moving Coil cartridges?
No. Conventional MC cartridges are naturally well suited to balanced operation, but Pro-Ject also offers the specially configured Pick it PRO Balanced Moving Magnet cartridge for its True Balanced system.
Can I use an ordinary MM cartridge through RCA?
Yes. Conventional single-ended RCA operation is fully supported.
What is the balanced turntable input?
A 5-pin mini-XLR connector.
Is the balanced output also mini-XLR?
No. The line-level balanced outputs are a conventional pair of full-size XLR sockets.
Can I use XLR output when the turntable is connected through RCA?
Yes. The RCA and XLR outputs are both active. However, an RCA turntable input does not become a complete True Balanced cartridge-to-phono-stage connection simply because XLR is used after the phono stage.
Are the RCA and XLR outputs active together?
Yes. Pro-Ject's manual states that both outputs operate simultaneously.
Can I connect two amplifiers?
Pro-Ject specifically states that the simultaneous RCA and XLR outputs allow two suitable receiving devices to be connected.
Can I connect the S3 B output to another phono input?
No. Its outputs are line level and should connect to AUX, CD, Tuner, Tape or another conventional line input.
Is the gain stage discrete?
Yes. Pro-Ject uses individual discrete components rather than integrated circuits for the main gain stage.
Is the internal design actually balanced?
Yes. Pro-Ject describes the gain architecture as fully symmetrical rather than simply fitting XLR sockets to a conventional single-ended circuit.
What is the RIAA accuracy?
Better than 0.3dB from 20Hz to 20kHz.
What is the MM THD at 1kHz?
Less than 0.001%.
What is the MC THD at 1kHz?
Less than 0.005%.
Does the unit have a power switch?
Yes. A front-panel button switches between operation and standby.
What power supply does it use?
An external 18V / 500mA DC supply.
What is the operating consumption?
Pro-Ject specifies 18V / 170mA DC.
What is the standby consumption?
Less than 0.5W.
Can I upgrade the power supply?
Pro-Ject currently lists the Phono Box S3 B as compatible with the Accu Box S2 battery supply.
How large is it?
206mm wide, 55mm high and 153mm deep, or 164mm deep including the rear sockets.
How much does it weigh?
930g excluding the external power supply.
What finishes are available?
Black and Silver.
Where is it made?
Pro-Ject specifies the Phono Box S3 B as Made in Europe.
Related Pro-Ject Products
Why Buy the Pro-Ject Phono Box S3 B From Audio T?
A highly adjustable phono stage only really becomes useful when those settings are matched properly to the cartridge. Gain, resistance and capacitance can all change the way a cartridge behaves, while a True Balanced system also depends on the turntable, cartridge and cable being wired correctly from the outset.
Audio T can match the Phono Box S3 B to your existing MM or MC cartridge, demonstrate balanced and conventional RCA operation and help configure a future cartridge upgrade without turning the front-panel adjustment into guesswork.
- In-store demonstrations
- Moving Magnet cartridge matching
- Moving Coil cartridge matching
- Input-impedance setup
- Capacitance setup
- Gain matching
- Subsonic-filter guidance
- True Balanced setup
- Mini-XLR cable guidance
- Balanced XLR system matching
- Dual-turntable configuration
- Debut PRO S Balanced setup
- Pick it PRO Balanced guidance
- Phono cable advice
- Accu Box S2 power-supply advice
- Future cartridge upgrade planning
- Ongoing after-sales support
Contact your local Audio T store to arrange a demonstration or discuss the correct Phono Box S3 B loading and gain settings for your cartridge.