This unassuming little box, just 12cm wide and 3cm high, can have a profound effect on your hi-fi sound. It’s an offboard phono stage, an amplifier dedicated to the output of a turntable, raising the tiny signals generated by the stylus and cartridge up to the levels required for the line-level inputs of an amplifier.

The Phono Box S2 is part of Pro-Ject’s S2 line, and is described as the company’s ‘Best Buy’ phono stage; there is one below it in Pro-Ject’s range but several larger and more complex-looking phono stages above it in the company’s DS2 range, including models which use valves. Compared with these, the little S2 looks exceedingly simple, with just one button on the front to power it up and down, while at the rear (pictured below) it has the input sockets for the turntable, including the grounding screw, plus the output sockets to your amplifier. Add the input from the supplied 18V power transformer, and a single button which is a subsonic filter (or ‘rumble filter’ as they used to be called), and that’s your lot.

Except — well, there should be one more button. The Phono Box S2 is listed as suitable for both moving magnet and moving coil cartridges, and these require very different gain/EQ circuits applied. There must surely be a switch between these two options?

The answer lies underneath — not one switch, but 16 of them, in two ranks of eight little DIP switches. Each set should be set the same way, as they affect left and right channels individually in this dual-mono design. Labels next to the switches (see below right) indicate that the first three of each row are used to set the gain to four possible levels, between 40dB and 63dB. The next three set the input impedance (10/100/1000 ohms or a combination), and the final two set the input capacitance (this, however, becomes an irrelevance for low-output moving coil cartridges).

Before you panic as to cartridge complexities, these are just specifications that you should be able to find easily by looking up your cartridge online, or in the information that came with it. Then refer to Pro-Ject’s S2 manual, which explains the combinations achievable for capacitance in particular (it’s not as simple as adding up the switch values) and also lists a few helpful examples for particular Ortofon cartridges.

So the Pro-Ject Phono Box S2 looks simple, but those DIP switches actually make it impressively configurable for specific cartridges.

And of course Pro-Ject knows a thing or two about cartridges and turntables. Its founder Heinz Lichtenegger invested in Czech turntable factories when vinyl was on the way out. Since its revival, Pro-Ject has been in a unique position to lead the market in turntables, arms and phono electronics.

So why might you need such a phono stage? Some amplifiers have a dedicated turntable input with a phono stage built in, but many amplifiers don’t, especially those made during that time when vinyl seemed to be a thing of the past, before it became cool again. Solution — put a standalone phono stage between turntable and amplifier.

But it may also be worth upgrading a phono stage even if you already have one. The quality of built-in phono stages in amplifiers is variable — all products are built to their price, and lesser circuits like the headphone output and phono stage can often get minimal funds. Similarly some turntables today have their own phono stages built in, so that they can work with any amplifier — but you may achieve better sound by bypassing that and using a dedicated phono amp instead.

The reason? This is the most delicate signal in hi-fi, and handling it is one of the greatest challenges in audio engineering. The signal which comes from a turntable is produced only by the tiny movements of a magnet attached to the needle, inducing current in a surrounding coil of wire. Or, instead of moving the magnet, you can attach the coil to the top of the needle and surround it with a magnet instead — moving coil, instead of moving magnet. With no power supply, no internal amplification for this process, the result is a tiny electrical signal, its potential down in the single millivolts, compared to the two full volts that come from a digital source. Moving-coil cartridges are the lowest of all, often below a single millivolt.

Such a low signal will be a thousand times more prone to interference than your normal line-level signal — and that’s a very good argument for keeping the initial treatment of your phono signal physically apart from the higher-powered functions going on inside an integrated amplifier, especially one with digital processing. Pro-Ject’s S2 further protects against external interference by having its housing made of an aluminium/metal sandwich construction, with the external finish available in either silver or black.

Secondly the quality of that initial treatment can have an enormous effect on the final sound. The application of thousand-fold amplification prior to that of your main amplifier means that any added noise or distortion is magnified, as will be other characteristics such as the quality of channel separation.

And the phono stage does not only amplify. It also applies EQ — indeed a massive dose of EQ. Vinyl fans often go on about purity, but the whole vinyl path to playback is squeezed through two curves. The first is applied prior to cutting the disc, tilting the frequencies up to reduce bass frequencies and raise the treble. Then in the phono stage a reverse curve is applied, boosting the bass and curbing all that extra treble.

Why do this? Because the reduced bass content qat the initial stage makes LPs easier to cut and safer to play, since large bass fluctuations create such deviations in the groove that the needle’s tracking ability can be compromised. Meanwhile the downward treble adjustment on replay allows a reduction of hiss and clicks while bringing the music content back to its original level.

So the use of emphasis and then de-emphasis allows safer and easier mastering, and some advantages to playback. The cost is the jiggery-pokery of phono EQ curves.

Making things still more complicated, there were originally many different curves used by different record companies; early amplifiers could switch between the more common ones. But thankfully RCA Victor’s ‘New Orthophonic’ curve has been a de facto global standard for records since 1954, becoming known as RIAA equalisation, so phono stages today just adhere to this.

It’s not a simple curve — there are three transition points where the curve is redefined. Pro-Ject quotes a maximum 0.4dB deviation from the RIAA curve for the Phono Box S2.

All of this is why phono stages can make such a difference to the sound of a turntable, and why they can sound so different to each other. They mould the signal at a very early and delicate stage, and even small differences become amplified into large ones. So a good phono stage can easily outperform those within amplifiers by merits of signal isolation and more dedicated design.

And the Phono Box S2 proves a good phono stage. We used it first with a visiting turntable equipped with a relatively modest AT-VM95E phono cartridge, under $80 if bought separately. We checked the specs for this straightforward moving-magnet cartridge — load impedance 47kohms, load capacitance 100 to 200pF, and the gain we left at 40dB as recommended for other moving magnets of 4mV output. So that meant clicking up three of each eight DIP switches — numbers one, three and seven. And we made direct comparisons with three other phono stages — the one available in the turntable itself, the stage in our resident Musical Fidelity preamplifier, and (rather rudely, given its price) the impeccable phono input of Yamaha’s 5000 Series preamplifier, which has its own set of tuning options.

One clear conclusion is the confirmation of how much difference phono stages can make to the overall sound. On Pink Floyd’s ‘Animals’ closer Pigs on the Wing pt2, the turntable’s built-in phono stage sounded clear enough, but switch to the Pro-Ject, or indeed any of the other three, and the panned acoustic guitars were tonally filled, from a thinner jangle to a richer, more real acoustic guitar tone. The vocal was also more solid, even more central, which you’d think a tracking trait rather than being subject to the phono stage’s variation, but it sounded as if the left-right treatment from the turntable’s own internal circuit was not as well matched as Pro-Ject’s dual mono circuit design, which delivered a firm non-slipping central image.

At the start of Keith Jarrett’s trio’s Flying pt 1 there didn’t seem much to pick between the renditions of Jarrett’s solo piano, but once Gary Peacock’s drums and Jack DeJohnette’s drums entered, the differences were far clearer in the depth and richness of the bass tone, its ability to deliver the microdynamics of a firm pluck over a light one. The ride and China boy cymbals were held more smoothly in check by the Pro-Ject’s balance, whereas the turntable’s in-built phono stage added a little too much ‘sting’; the Pro-Ject was simply more musical. We loaded a recently-acquired LP of Tears For Fears’ ‘Seeds of Love’, hearing it on vinyl for the first time through the S2; it took a full 40 minutes out of our reviewing time, simply enjoying this richer version of the album — and unmasking a vocoder vocal track doubled behind verses and bridges of Sowing The Seeds of Love — never before noted, and heck, we’ve played that album a lot.

The synth pad and electric vocal on Jeff Lynne’s ELO’s Sun’s Gonna Shine was given a tonally more thrumsome spectrum by the Pro-Ject in comparison to the built-in phono stage, again keeping the highs in check atop a full and natural sound. Only the Yamaha phono stage (this on a $13,000 amplifier) achieved a more refined result, and the S2’s delivery was hardly less enjoyable.

With our resident Thorens TD 203 turntable, twice the price of the first, we repeated this listening for three of the four phono stages — the Thorens not having its own. The Pro-Ject (no DIP changes required) again served the Thorens’ higher detail and soundstaging with a natural, full and controlled sound. The Kate Bush… oh, we were going to make notes, but we clean forgot. We enjoyed it too much. Good sign.

What of that subsonic filter button? The RIAA EQ curve, boosting up previously reduced bass frequencies, also emphasises any low frequency mechanical noise, such as rumbling from the bearings, or from warped vinyl, which can be further emphasised by resonance between the arm and cartridge; they can affect speakers even if you can’t hear it. So some phono stages — including this Phono Box S2 — include a rumble filter to sharply attenuate very low frequencies. Pro-Ject describes its subsonic filter as applying a -12dB/octave filter below 20Hz. The turntables we used in this test didn’t suffer from audible rumble or visible speaker cone subsonics, but for those that do it’s an essential function — we once sat through a vinyl session watching some very expensive speakers making frightening physical distortions due to the want of a subsonic filter in the phono stage. Some don’t include one at all, or have one switched in permanently, which can then adversely affect low frequency performance.

We’ll mention an initial glitch with the S2 only in case this happens to someone else — the Australian clip-on plug for the power transformer didn’t work; the S2 wouldn’t power up. We thought to try the UK clip-on plug instead (into a UK powerblock left over from a former life), and the S2’s front LED immediately illuminated, so we used it like this throughout the review.

In matching the phono stage of our Musical Fidelity preamp for quality, the Phono Box S2 performed above its price and showed the quality of its componentry as well as its design. It also had the harder task of competing with the phono stage of Yamaha’s 5000 Series preamplifier, and it was impressive that it was no distant backrunner there either. This performance supports two welcome surprises — firstly its hidden ability to be tuned to specific cartridge requirements by those DIP switches underneath, and secondly its effective and musical performance as the vital link between turntable and amplifier, treating those tiny vinyl signals with the respect they deserve. You won’t find a better standalone phono stage at the price.

Pro-Ject Audio Phono Box S2 phono stage
Price: $280

+ mc/mm stage with four gain selections
+ Selectable input impedance/capacitance
+ Switchable subsonic filter

- No negatives here

Input: 1 x pair RCA sockets plus grounding post
Output: 1 pair line-level RCA sockets
Input gain: 40dB, 43dB, 60dB, 63dB
Input impedance: 10ohms, 100ohms, 1kohm, 47kohm
Input capacitance: 100pF, 200pF, 320pF and 420pF at 47kohms
Noise floor: 94dBA (40dB/43dB input gain); 75dBA (60dB/63dB input gain)
THD: <0.01% (40dB/43dB input gain); <0.05% (60dB/63dB input gain)
RIAA accuracy: <0.4dB 20-20kHz
Subsonic filter: at 20Hz with 12dB/octave
Dimensions (whd): 119 x 37 x 103mm
Weight: 340g