Audio T Verdict
The Tube Box S2 is one of those wonderfully Pro-Ject products that squeezes rather more engineering into a tiny box than its dimensions suggest. Behind the two exposed ECC83 valves is a fully discrete phono circuit with no op-amps, a low-noise FET input stage, dual-mono architecture and enough loading adjustment to accommodate both Moving Magnet and Moving Coil cartridges.
The valves aren't there simply for decoration either. They are replaceable, giving the Tube Box S2 a future maintenance route as well as the possibility of experimenting with suitable replacement valves if that's something you want to explore.
What makes the S2 particularly easy to live with is that gain can be changed from the front panel, while cartridge resistance and capacitance are configured underneath the case. Add a switchable subsonic filter, proper RCA connections and a compact metal chassis and this is a very appealing way of bringing a valve phono stage into a serious vinyl system without giving over half the equipment rack to it.
Pro-Ject Tube Box S2 Valve Phono Stage
The Pro-Ject Tube Box S2 is a compact phono preamplifier for Moving Magnet and Moving Coil cartridges, combining valve amplification with a fully discrete circuit design and extensive cartridge-matching options.
Two replaceable ECC83 valves sit visibly above the enclosure, while the internal circuit uses a low-noise FET input stage, separate left and right channel architecture and carefully implemented RIAA equalisation.
Gain can be changed directly from the front panel, while resistance and capacitance settings underneath the unit allow the electrical load to be matched to the cartridge.
Valve Phono Amplification
The Tube Box S2 uses two ECC83 valves, also known as 12AX7, within its amplification circuit.
Unlike a cosmetic valve display added to an otherwise conventional circuit, the Tube Box S2 has been designed around replaceable valves as an integral part of the product.
Pro-Ject specifically designed the S2 to allow suitable replacement valves to be fitted, giving owners the possibility of maintaining the product over the long term and, if desired, experimenting with compatible valves as part of the system setup.
Valves Become Hot During Operation
Give the Tube Box S2 adequate ventilation and avoid touching the valves while the unit is operating or immediately after use. Any valve replacement should only be undertaken with the unit switched off, disconnected from its power supply and fully cooled.
Two Replaceable ECC83 Valves
The standard valve complement is:
- 2 x ECC83
- 12AX7 family designation
- Replaceable design
The two-valve layout contributes to the Tube Box S2's dual-channel architecture while retaining the compact dimensions associated with Pro-Ject's S2 electronics.
Fully Discrete Circuit Design
The Tube Box S2 was Pro-Ject's first phono stage of this size to use a fully discrete circuit without operational amplifiers.
Rather than relying on a conventional integrated op-amp for the main amplification circuitry, Pro-Ject builds the signal path from individual electronic components.
This also gives the design a fundamentally different architecture from the simpler solid-state phono stages further down the range.
Low-Noise FET Input Stage
The first stage uses low-noise FET devices.
This is particularly relevant to phono amplification because a cartridge produces a tiny electrical signal. A Moving Coil cartridge can output considerably less than one millivolt, meaning any unwanted noise introduced at the input will subsequently be amplified along with the music.
Dual-Mono Circuitry
The Tube Box S2 uses a dual-mono circuit arrangement to help maintain separation between the left and right channels.
The two channels also have corresponding cartridge-loading switches underneath the enclosure, which is why both sides must be configured identically when changing cartridge settings.
Precise RIAA Equalisation
Records are not cut with a flat frequency response. Bass is reduced and treble increased according to the standard RIAA curve during mastering.
A phono preamplifier reverses this process during playback while simultaneously increasing the cartridge signal to line level.
Pro-Ject divides the Tube Box S2's RIAA equalisation between passive and active circuit sections and specifies accuracy within approximately ±0.4dB from 20Hz to 20kHz.
Suitable for Moving Magnet and Moving Coil
The Tube Box S2 is considerably more flexible than a fixed Moving Magnet phono stage.
It is designed for:
- Moving Magnet cartridges
- High-output Moving Coil cartridges
- Suitable low-output Moving Coil cartridges
The correct gain, resistance and capacitance depend on the cartridge being used.
Front-Panel Gain Adjustment
Gain is selected using the arrow buttons on the front panel, with LEDs showing the selected level.
The available adjustment covers a broad 40 to 63dB range, allowing the Tube Box S2 to accommodate the much higher output of a typical MM cartridge as well as the very small signal produced by many low-output MC designs.
How Much Gain Do You Need?
Higher gain is not automatically better. A typical Moving Magnet cartridge normally requires considerably less amplification than a low-output Moving Coil cartridge. Start with the cartridge manufacturer's output specification and use enough gain to provide a sensible listening level while retaining good headroom.
Why Gain Matters
A cartridge producing 5mV does not need the same amount of amplification as one producing 0.3mV.
If gain is too low, the turntable may sound much quieter than other sources and require excessive amplifier volume. If gain is unnecessarily high, available headroom can be reduced and noise or overload can become more apparent.
Moving Magnet Loading
Most Moving Magnet cartridges are designed around a conventional 47kOhm input impedance.
For MM and suitable high-output MC cartridges, the Tube Box S2 therefore provides a fixed 47kOhm resistance while allowing input capacitance to be adjusted.
Six Capacitance Options
Current Pro-Ject product information publishes the following nominal capacitance options:
- 50pF
- 150pF
- 270pF
- 370pF
- 490pF
- 590pF
This allows the Tube Box S2 to be matched to Moving Magnet cartridges with different recommended capacitive loads.
Why MM Capacitance Matters
The total capacitance seen by a Moving Magnet cartridge includes:
- Tonearm wiring
- Turntable phono cable
- Phono-stage input capacitance
That combined load can affect the cartridge's high-frequency response, so the cartridge manufacturer's recommended range is the logical starting point.
Moving Coil Loading
Low-output Moving Coil cartridges generally require much lower resistance than a typical MM design.
The Tube Box S2 therefore provides nominal loading options of:
- 10Ohm
- 100Ohm
- 470Ohm
- 1kOhm
- 2kOhm
The normal 47kOhm setting also remains available for suitable high-output cartridges.
There Is No Universal MC Load
Moving Coil cartridge requirements vary enormously. Some are designed around approximately 100 ohms, others prefer substantially higher or lower resistance. Start with the cartridge manufacturer's recommendation rather than assuming one setting is automatically correct for every MC design.
Capacitance and Low-Output MC
Pro-Ject states that input capacitance is not relevant to conventional low-output Moving Coil cartridges in the same way that it is with Moving Magnet designs.
For a low-output MC cartridge, concentrate primarily on selecting suitable gain and resistance.
Cartridge Settings Underneath the Case
Resistance and capacitance are configured using DIP switches on the underside of the Tube Box S2.
This keeps the front panel uncluttered while allowing much greater cartridge matching than a phono stage with one fixed input.
Disconnect the Tube Box Before Changing Loading
Pro-Ject instructs that the unit should be disconnected from both its power supply and amplifier before the underside DIP-switch settings are changed.
Set Both Channels the Same
The left and right channels have corresponding loading switches.
Both channels should always be configured identically unless there is a very specific technical reason not to do so.
For normal cartridge setup, the resistance and capacitance seen by the left and right cartridge channels should match.
Switchable 20Hz Subsonic Filter
A dedicated button on the front panel engages the Tube Box S2's 20Hz subsonic filter.
The blue indicator above the button illuminates when the filter is active.
What Is a Subsonic Filter?
A turntable can produce very-low-frequency energy that is not part of the musical recording.
Common causes include:
- Warped records
- Tonearm and cartridge resonance
- Mechanical vibration
- Turntable rumble
Much of this energy may sit at or below the normal audible range, but your amplifier and loudspeakers can still attempt to reproduce it.
When Should You Use It?
If you see bass drivers making large, slow movements while playing a warped record, the subsonic filter is worth trying.
Where the turntable and records are behaving properly and unwanted low-frequency energy is not a problem, you can simply leave it switched off.
Gold-Plated RCA Input
The Tube Box S2 accepts one turntable through a conventional stereo pair of RCA phono inputs.
A grounding terminal is also provided for the turntable earth lead where required.
One RCA Line-Level Output
After phono amplification and RIAA equalisation, the Tube Box S2 sends a conventional line-level signal through its stereo RCA output.
This should connect to an amplifier input such as:
Do Not Connect the Output to Another Phono Input
The Tube Box S2 has already amplified and equalised the cartridge signal. Its output should therefore feed a normal line-level input, not another MM or MC phono stage.
One Turntable at a Time
The Tube Box S2 provides one stereo phono input, so it is designed for one connected turntable or tonearm at a time.
If you need two simultaneously connected turntables, Pro-Ject offers other phono stages with multiple inputs.
Single-Ended RCA Architecture
The Tube Box S2 predates Pro-Ject's current True Balanced phono range and uses conventional RCA connectivity.
It does not provide:
- Balanced mini-XLR phono input
- Balanced XLR phono input
- Balanced XLR line output
- Pro-Ject True Balanced connection
If balanced phono connectivity is required, the Pro-Ject Phono Box S3 B is one current alternative.
Polystyrene Capacitors
Pro-Ject uses polystyrene capacitors where appropriate within the signal circuitry.
These form part of the component choices made around the S2's discrete, valve-based design and precision RIAA network.
Multiple Internal Power Supplies
The Tube Box S2 uses an external DC power adaptor but generates the different supply requirements needed by its internal circuitry inside the unit.
Pro-Ject specifically highlights its separately filtered high-voltage anode supply as part of the valve circuit design.
External 18V Power Supply
The supplied power adaptor provides 18V DC at 1A.
The Tube Box S2 itself draws approximately 470mA DC during operation, with Pro-Ject specifying less than 1W in standby.
Accu Box S2 Upgrade
Pro-Ject currently identifies the Accu Box S2 rechargeable battery supply as an official compatible power upgrade for the Tube Box S2.
This provides an alternative to powering the unit directly from the supplied mains-powered adaptor.
Aluminium and Metal Sandwich Casework
The Tube Box S2 uses Pro-Ject's compact aluminium and metal sandwich enclosure.
The metal housing provides mechanical rigidity while helping protect the sensitive high-gain phono circuitry from external electromagnetic interference.
Give the Valves Some Space
The two valves extend above the main enclosure, so the Tube Box S2 should not be squeezed underneath another component with no ventilation space.
Allow enough clearance above the unit for air to circulate freely around the valves.
Compact S2 Dimensions
The main chassis measures approximately:
- Width: 103mm
- Height: 73mm
- Depth: 119mm
- Depth including rear sockets: approximately 131mm
Despite the exposed valves, it occupies a fraction of the shelf space required by a conventional full-width valve phono stage.
410g Without Power Supply
The Tube Box S2 weighs approximately 410g excluding the external power adaptor.
Black or Silver
The current Tube Box S2 is available in:
Tube Box S2 or Phono Box S2?
Both models support MM and MC cartridges, but their circuit philosophy and adjustment differ considerably.
Tube Box S2
- Two replaceable ECC83 valves
- Fully discrete circuit
- FET input stage
- Dual-mono architecture
- Front-panel gain control
- Adjustable MM capacitance
- Wide MC resistance selection
- 20Hz subsonic filter
- Valve replacement and tube-rolling potential
Phono Box S2
- Solid-state design
- MM and MC support
- Adjustable gain
- Adjustable impedance
- Adjustable capacitance
- 20Hz subsonic filter
- Lower chassis height
- Simpler route if valves are not a priority
The Pro-Ject Phono Box S2 makes sense if you want a flexible conventional solid-state phono preamplifier. The Tube Box S2 is the more distinctive choice if its discrete valve architecture is part of what you want from the analogue front end.
Can It Work With Non-Pro-Ject Turntables?
Yes.
The Tube Box S2 uses conventional RCA phono connections and is not restricted to Pro-Ject turntables.
Compatibility depends primarily on whether the cartridge's output, recommended gain and electrical loading fall within the Tube Box S2's available adjustment range.
Can I Upgrade From MM to MC Later?
Yes, provided the chosen Moving Coil cartridge can be matched successfully to the available resistance and gain settings.
This makes the Tube Box S2 useful in a system where you begin with a conventional MM cartridge but expect the turntable to develop later.
Who Is the Tube Box S2 For?
The Tube Box S2 makes particular sense if you:
- Want a compact valve phono stage
- Use a Moving Magnet cartridge
- Use a high-output Moving Coil cartridge
- Use a suitable low-output Moving Coil cartridge
- Want selectable gain
- Want adjustable MM capacitance
- Want adjustable MC resistance
- Want a switchable subsonic filter
- Prefer a fully discrete circuit rather than an op-amp-based gain stage
- Want replaceable valves
- May want to experiment with compatible valves later
- Only need one turntable input
- Only require conventional RCA connectivity
- Want a compact alternative to a full-width valve phono preamplifier
Key Features
- Pro-Ject Tube Box S2
- Valve phono preamplifier
- Moving Magnet compatible
- Moving Coil compatible
- 2 x ECC83 valves
- 12AX7 valve family
- Replaceable valves
- Tube-rolling potential
- Fully discrete circuit
- No op-amps in main circuit design
- Low-noise FET input stage
- Dual-mono circuitry
- Passive / active RIAA equalisation
- RIAA accuracy approximately ±0.4dB
- Polystyrene capacitors
- Front-panel gain adjustment
- 40 to 63dB gain range
- 47kOhm MM loading
- 10Ohm MC loading
- 100Ohm MC loading
- 470Ohm MC loading
- 1kOhm MC loading
- 2kOhm MC loading
- Multiple MM capacitance settings
- 50 to 590pF nominal capacitance range
- 20Hz subsonic filter
- Front-panel subsonic control
- Gold-plated RCA input
- Gold-plated RCA output
- Turntable grounding terminal
- Aluminium and metal enclosure
- Multiple internal power supplies
- Filtered high-voltage anode supply
- 18V / 1A external power adaptor
- Less than 1W standby consumption
- Accu Box S2 compatible
- 103mm-wide chassis
- Black finish
- Silver finish
Frequently Asked Questions
Is the Pro-Ject Tube Box S2 still current?
Yes. It remains part of Pro-Ject's current S Line and Hi-Fi electronics range.
Is the Tube Box S2 actually a valve phono stage?
Yes. It uses two replaceable ECC83 valves as part of its fully discrete phono preamplifier circuit.
Which valves does it use?
Two ECC83 valves, also identified as part of the 12AX7 family.
Can the valves be replaced?
Yes. Pro-Ject specifically designs the Tube Box S2 with replaceable valves.
Can I experiment with different valves?
Pro-Ject promotes the replaceable design for tube rolling and sound shaping. Only suitably compatible replacements should be considered, and the unit should always be disconnected and allowed to cool before any valve is handled.
Does it support Moving Magnet cartridges?
Yes.
Does it support Moving Coil cartridges?
Yes, including suitable low-output MC cartridges.
Does it support high-output Moving Coil?
Yes. High-output MC cartridges can often use the conventional 47kOhm loading arrangement, subject to the cartridge manufacturer's own specification.
How much gain does it provide?
Pro-Ject provides front-panel selectable gain across a range from 40 to 63dB.
How is the gain changed?
Using the arrow buttons on the front panel. LEDs indicate the selected gain position.
What input impedance settings are available?
Current Pro-Ject product information specifies 10, 100, 470, 1k and 2kOhm variable loading alongside the fixed 47kOhm setting.
What setting is normally used for Moving Magnet?
Most Moving Magnet cartridges are designed around a 47kOhm load, but always follow the cartridge manufacturer's specification.
What setting should I use for Moving Coil?
It depends on the cartridge. Start with the manufacturer's recommended load and select the closest suitable Tube Box S2 setting.
What capacitance settings are available?
Current Pro-Ject product information gives nominal settings of 50, 150, 270, 370, 490 and 590pF.
Why is capacitance adjustable?
Moving Magnet and suitable high-output MC cartridges can be sensitive to the total capacitance created by the tonearm wiring, phono cable and phono-stage input.
Does capacitance matter for low-output Moving Coil?
Pro-Ject states that capacitance is not relevant in the same way for conventional low-output MC cartridges.
How are impedance and capacitance changed?
Using DIP switches underneath the Tube Box S2.
Should both channels use the same switch settings?
Yes. Pro-Ject instructs that both channels should be configured identically.
Can I change the DIP switches while the Tube Box is switched on?
No. Pro-Ject instructs that the unit should be disconnected from its power supply and amplifier before the loading settings are changed.
Does it have a subsonic filter?
Yes. A switchable 20Hz filter is controlled from the front panel.
Why would I use the subsonic filter?
It can reduce unwanted very-low-frequency energy caused by warped records, tonearm resonance, mechanical vibration or turntable rumble.
Does the Tube Box S2 have a digital output?
No. It is an analogue phono preamplifier.
Does it have USB?
No.
Does it have Bluetooth?
No.
Does it have balanced XLR connections?
No. The Tube Box S2 uses conventional single-ended RCA connections.
Does it support Pro-Ject True Balanced?
No. For Pro-Ject True Balanced phono connectivity, consider a current balanced model such as the Phono Box S3 B.
How many turntables can I connect?
One stereo turntable input is provided.
Does it have a turntable earth connection?
Yes. A grounding terminal is provided where required to control hum.
Where does its output connect?
To a normal line-level input such as AUX, CD, Tuner, Tape or Line.
Can I connect the output to my amplifier's phono input?
No. The Tube Box S2 has already provided phono amplification and RIAA equalisation.
Is the circuit discrete?
Yes. Pro-Ject describes the Tube Box S2 as fully discrete and specifically states that it does not use operational amplifiers in the main circuit design.
Does it use an FET input stage?
Yes. Pro-Ject specifies an ultra-low-noise FET input stage.
Is it dual mono?
Yes.
What is the RIAA accuracy?
Pro-Ject specifies approximately ±0.4dB from 20Hz to 20kHz.
What power supply does it use?
An external 18V DC / 1A supply.
What is its operating consumption?
Pro-Ject specifies approximately 470mA DC.
What is the standby consumption?
Less than 1W.
Can the power supply be upgraded?
Yes. Pro-Ject currently lists the Accu Box S2 as a compatible power-supply upgrade.
How large is it?
The main chassis measures approximately 103mm wide, 73mm high and 119mm deep, extending to around 131mm including the rear sockets.
How much does it weigh?
Approximately 410g excluding the external power supply.
Does it need ventilation?
Yes. The exposed valves generate heat during normal use, so avoid covering the unit or positioning another component immediately above the tubes.
What finishes are available?
Black and Silver.
What is the difference between Tube Box S2 and Phono Box S2?
Both accommodate MM and MC cartridges, but the Tube Box S2 uses two replaceable ECC83 valves, a fully discrete FET-based input architecture and front-panel gain control. The Phono Box S2 is a compact solid-state alternative.
Related Pro-Ject Products
Why Buy the Pro-Ject Tube Box S2 From Audio T?
The Tube Box S2 provides considerably more cartridge adjustment than a fixed phono input, but those settings need to be chosen correctly. Gain, resistance and capacitance should all be matched to the cartridge rather than simply adjusted until something sounds louder.
Audio T can demonstrate the Tube Box S2, configure it for your existing MM or MC cartridge and help you compare it with a conventional solid-state phono stage. If your cartridge changes later, we can also reset the loading and gain to suit the new setup.
- In-store demonstrations
- Moving Magnet cartridge matching
- Moving Coil cartridge matching
- High-output MC guidance
- Low-output MC guidance
- Gain setup
- Input-resistance matching
- Input-capacitance matching
- DIP-switch configuration
- Subsonic-filter advice
- Turntable grounding guidance
- Valve care advice
- Compatible replacement-valve guidance
- Accu Box S2 power-supply advice
- Phono cable guidance
- Future cartridge upgrade planning
- Ongoing after-sales support
Contact your local Audio T store to arrange a demonstration or discuss the correct Tube Box S2 gain and loading settings for your cartridge.