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Headphone amplifier + power supply unit



Price (when reviewed): 3490 PLN + 1780 PLN

Division of Audio Tuning Vertriebs GmbH  
Margaretenstrasse 98 | A-1050 Wien | AUSTRIA


Provided for test by: VOICE

he Austrian Pro-Ject company, celebrating its 25th anniversary this year, with factories in the Czech Republic (turntables) and Slovakia (electronics), is also known outside our small domain. Thanks to a well-thought approach and focus on high-class products for “ordinary” people who do not want to spend more on their audio equipment than they would pay for a car, it has become the leading manufacturer of audio products from the basic and medium price range. To a large extent, the company owes its status to its small electronic devices sold in the Box Design series.

We have been writing about this series and the RS sub-series for many years. The latter one includes the Head Box RS headphone amplifier and the Power Box RS Uni 1 power supply unit; it is enough to mention the two-piece CD BOX RS + PRE BOX RS DIGITAL CD player and the PRE BOX RS digital-to-analog converter. They all have a very similar housing half the size of a classic device, really well-made using aluminum plates and a steel chassis.

The Head Box RS is a headphone amplifier with XLR and RCA inputs, and a hybrid design with tubes on the input and A-class FET transistors on the output. The amplifier is powered by a small impulse power supply unit, but the company also offers a large linear Power Box RS power supply unit in two versions: for a single device (Uni 1) and for four RS series devices at the same time (Uni 4). We tested the amplifier with the Uni 1 version.

There is a single non-balanced headphone output in the Head Box RS. The manufacturer has equipped us with an important tool to optimize sound – a switch which can change the output impedance of the amplifier. Three positions: 5, 20 and 50 Ω are available. Sound is a bit different with each of them and it is best to select the setting which suits us the most.

The so-called signal “pass through” will also be useful in many situations – apart from inputs, the amp also has RCA and XLR outputs with the same signal as on the input. So, if we cannot connect the headphone amplifier to our system, it is enough to connect the signal source to it and to connect the output to a line input of the amp. Then we will avoid cable switching. Although the company writes about “a fully balanced signal path”, only the input buffer is balanced and the rest of the circuit is not. In my system, the amp worked best with RCA cables.

PRO-JECT in “High Fidelity”
  • TEST: Pro-Ject RPM-5.1 CARBON – turntable
  • TEST: Pro-Ject PRE BOX RS – DAC, read HERE
  • TEST: Pro-Ject BOX DESIGN MaiA – integrated amplifier, read HERE
  • AWARD OF THE YEAR 2014: CD BOX RS + PRE BOX RS DIGITAL – Compact Disc transport + D/A converter/preamplifier, read HERE
  • TEST: CD BOX RS + PRE BOX RS DIGITAL – Compact Disc transport + D/A converter/preamplifier, read HERE
  • AWARD OF THE YEAR 2014: Pro-Ject 1XPRESSION CARBON CLASSIC + Ortofon M SILVER - turntable + cartridge, read HERE
  • TEST: Pro-Ject 1XPRESSION CARBON CLASSIC + Ortofon M SILVER – turntable + cartridge, read HERE
  • TEST: Pro-Ject Box CD SE + DAC Box FL - CD player + D/A converter, read HERE
  • TEST: Pro-Ject ART-1 (+ Denon DL-A100) – turntable (+ cartridge), read HERE
  • TEST: Pro-Ject RPM6 SB + PRO-JECT PHONO BOX SE – turntable + phonostage, read HERE
  • TEST: Pro-Ject 2XPERIENCE – turntable, read HERE
  • TEST: Pro-Ject RPM5 SUPERPACK – turntable, read HERE
  • TEST: Pro-Ject HEAD BOX MkII – headphone amplifier, read HERE
  • TEST: The Π Ject Family Saga vol. 1, read HERE
  • TEST: The Π Ject Family Saga vol. 2, read HERE

  • Recordings used during the session (a selection)

    • Arne Domnérus, Jazz at the Pawnshop. Vol. I, II & III, Proprius/Lasting Impression Music LIM UHD 071 LE, 3 x Ultra HD CD + DVD (1976/2012)
    • David Darling, 8-String Religion, Wind Over The Earth Music/Lasting Impression Music LIM DXD 041, silver-CD (2009);
    • Jean-Michel Jarre, Electronica 2: The Heart of Noise, Columbia/Sony Music Japan SICP-30956, BSCD2 (2016);
    • King Crimson, Live in Toronto, Panegyric/WOWOW Enterteinment IECP-20252/253, „King Crimson Collectors’ Club: Special Edition”, 2 x Ultimate HiQuality CD (2016)
    • Pet Shop Boys, Super, Sony Music Labels (Japan) SICX-41, CD (2016)
    • Shota Osabe Piano Trio, Happy Coat, Lasting Impression Music LIM K2HD 031, K2HD CD (2002)
    • Zbigniew Namysłowski & Modern Jazz Quartet, Lola, Decca/Universal Music Japan UCCM-9173, „From Europe With Jazz. Vol. 2”, CD (1964/2003)
    Japanese issues available at

    The Pro-Ject headphone amplifier is really well-made and nicely designed, and it inspires trust. It is a great advantage in the world of single-use plastic devices. If I can make a reference to architecture, the amp’s design is “international”, so it will be suitable for any place on Earth. The device heats up a lot, so I would not put anything on it. In order to improve ventilation, it is worth placing the amp itself on something – I used Pathe Wings feet made of beautifully finished wood. The PSU also heats up, so let us buy two sets of feet (three pieces each). The Synergistic Research HOT transducer will also be helpful, making sound more open and resonant.

    It is a universal device. It does not emphasize the drawbacks of the source and does not discriminate headphones. However, with time, having tested different headphone models and designs, I have come to the conclusion that Austrian headphones – Sennheiser and AKG suit the amp best. These are just speculations, but I bet that they were used in the design stage and when the final product was assessed.

    The amp does not fit headphones of low sensivity, so I would look for something else for the top HiFiMAN HE-6 model. The Audeze LCD-3 also did not fully show their potential and the amp was unable to break free from their individuality. However, if we choose to use dynamic headphones, these may be both the expensive HD800, as well as the inexpensive AKG Y50, headphones of low or high impedance – the Pro-Ject will produce nice, pleasant and coherent sound each time.

    It is an amp which celebrates the midrange. The treble is clear, undistorted and quite resonant, but lacks high energy and serves only to open the midrange. The low bass is nice but it is not as energetic as from, for example, the headphone input of the Exogen Comet DAC, not to mention the Bakoon HPA-21 amp. There are tubes inside the Pro-Ject amp and, in this case, they define the overall sound, giving it the specific “tube” quality.

    I have already said this is a really pleasant musical message. The pleasure comes from a lack of dominance of attack, soft decay and lack of emphases. We can adjust the tone using the output impedance switch, mainly in the treble. I obtained the best coherence at ‘50’. However, I will not be surprised if someone is going to need a stronger high midrange – ‘5’ is probably going to be THE right setting then.

    The amp is equally democratic for headphones (in the selected range) and recordings. When I had it in my system, I got the new Jean-Michel Jarre’s album – Electronica 2: The Heart of Noise on Blu Spec CD2, King Crimson – Live in Toronto on Ultimate HiQuality CD, the Japanese edition of Super Pet Shop Boys and others.

    The better the recording, the better the sound was, but there were no problems with any of the albums. The warm and soft treble makes even highly compressed recordings, such as the latest Megadeth album, sound sensible, nothing “bawls” or attacks with a hard attack. It is a very egalitarian amp which does not exclude anything or anyone.

    The stereophonic panorama is very well set in the amp. Of course, writing about the “sound stage” is a bit of an exaggeration in the case of headphones, but it can be heard especially clearly with binaural recordings, for example. The Pro-Ject amp lays everything out in a hemisphere in front of us. In this respect, a lot depends on headphones, but the amp did not stand in their way, neither in the case of Sennheiser headphones with their mechanically angled membranes, nor the electric (passive) circuit in the Beyerdynamic DT-990 Pro (600 Ω) headphones.

    What we do not get can be summarized in two words: resolution and selectivity. OK, I am exaggerating a bit, but this is not an amp which assigns the highest priority to these two elements – they are of secondary or, perhaps, tertiary importance. So, the tested device is not to be used for “overexposing” recordings, or master monitoring. When I compared a copy of a Master CD-R made on a workstation in Jacek Gawłowski’s mastering studio and the same recordings from Polish Jazz, the differences were clear, but not critical.


    Summarizing this test is easy. Pro-Ject is a brand in itself and it has its own unique style. It promotes nicely made, solid devices which never go below a good quality level, which is also the case this time. The Head Box RS headphone amp produces even, clear, warm and pleasant sound. It works best with headphones of moderate impedance and effectiveness, and, when it comes to brands, with AKG and Sennheiser devices. It does not mean it will sound worse with other headphones – I am simply unable to check that. However, I would avoid headphones of low effectiveness and, generally speaking, planar ones.

    It is not a transparent device and it leaves its mark on sound, in the form of softness, a layer of warmth and fluidity. However, not everyone wants to have “monitor” sound on their head, as some prefer to enjoy recordings no matter what their origin is. Also a turntable listened to through the Head Box will sound better because crackling will not disturb us too much. Let me repeat myself: it is a solid and good headphone amplifier to be used for many years. It sounds really good with an ordinary impulse amplifier, but I would think the Uni 1 amp is not an option but the target element.

    The RS series belongs to Box Design, part of the Audio Tuning Vertriebs empire, where “small” means “suitable”. Both devices – the amp and the PSU – are small and measure 206 x 72 x 200 mm. They are made really well from aluminum and steel elements – aluminum plates are attached to a steel chassis. Housings are available in two colors – natural aluminum and black anodized aluminum. We used the latter one in our test and it looks especially elegant.

    The amp has few buttons or knobs. In the middle there is a small, manually-controlled aluminum volume control knob. On the left there is a single large headphone jack (6.3 mm in diameter) and a small power toggle switch with a miniature blue LED over it. On the other side of the knob there are two similar toggle switches. One of them lets us adjust the output impedance of the amp and the other one – the active input (RCA or XLR).

    The Head Box RS has two input types – a balanced XLR and an unbalanced RCA input. Although some people will say that this is a balanced dual-mono circuit, it is not true (see below). The sockets, including the output ones, are good quality. They have the same signal as on the input, so we can connect our main sound source directly to the amp and then send output signal to a power amplifier. Alternatively, one can use an output for recording in an integrated amplifier (preamp). However, these are usually preceded by buffers that seriously change sound. So, it is best to deliver signal to the Head Box directly from source outputs.

    Power supply (20 V DC voltage) is delivered from the outside – either from a small impulse power supply unit which is part of the set or the optional external Uni PSU. There are two kinds of the latter – Uni 1 for a single device and Uni 4 for four devices at the same time. Nice low-voltage sockets deserve praise – they are so nice that the thin dull cable that is connected to them seems inappropriate.

    The electronic circuit is placed on one printed circuit board. It is a hybrid circuit – there are tubes (double Electro-Harmonix 6922EH triodes, one per channel) on the input and transistors on the output (class A MOSFET transistors attached to small radiators). Signal has to be desymmetrized right after the input stage, as the nice volume control black Alps potentiometer is a two-channel device. Large polypropylene Wim capacitors (one per channel) deal with coupling the tube and transistor sections – another sign that signal is unsymmetrical already here. What is interesting, the potentiometer is integrated with a small control engine, but its power supply and an infrared receiver are missing.

    The PSU is as heavy as the amp itself, due to both the rigid housing and a large toroidal transformer from the well-known Talema company, with the inside filled with epoxy resin. It works with a large rectifier and two integrated voltage stabilizers attached to a large radiator. Noise is initially removed from voltage on the transformer input in a “double Pi” filter with a choke and condensers.

    Technical specifications (according to the manufacturer)

    Power output: 350 mW/32 Ω | 60 mW/300 Ω
    Crosstalk: -58 dB
    Signal-to-noise ratio: -99 dB
    Frequency response: 30 Hz – 20 kHz/(+0.2 dB; -0.05 dB)
    THD (IMD): 0.17% (XLR in, output impedance setting 5 Ω, load 32 Ω)
    Gain: 9 dB
    Outboard power supply: 20 V/3A DC
    Power consumption: 1.3A DC
    Dimensions (W x H x D): 206 x 72 x 200 (209) mm (D with sockets)
    Weight (without the power supply unit): 2.15 kg



    - Turntable: AVID HIFI Acutus SP [Custom Version]
    - Cartridges: Miyajima Laboratory KANSUI, review HERE | Miyajima Laboratory SHILABE, review HERE | Miyajima Laboratory ZERO (mono) | Denon DL-103SA, review HERE
    - Phono stage: RCM Audio Sensor Prelude IC, review HERE

    - Compact Disc Player: Ancient Audio AIR V-edition, review HERE
    - Multiformat Player: Cambridge Audio Azur 752BD
    - Line Preamplifier: Polaris III [Custom Version] + AC Regenerator, regular version review (in Polish) HERE
    - Power amplifier: Soulution 710
    - Integrated Amplifier: Leben CS300XS Custom Version, review HERE

    - Stand mount Loudspeakers: Harbeth M40.1 Domestic, review HERE
    - Stands for Harbeths: Acoustic Revive Custom Series Loudspeaker Stands
    - Real-Sound Processor: SPEC RSP-101/GL
    - Integrated Amplifier/Headphone amplifier: Leben CS300XS Custom Version, review HERE
    - Headphones: HIFIMAN HE-6, review HERE | HIFIMAN HE-500, review HERE | HIFIMAN HE-300, review HERE | Sennheiser HD800 | AKG K701, review (in Polish) HERE | Ultrasone PROLine 2500, Beyerdynamic DT-990 Pro, version 600 - reviews (in Polish): HERE, HERE, HERE
    - Headphone Stands: Klutz Design CanCans (x 3), review (in Polish) HERE
    - Headphone Cables: Entreq Konstantin 2010/Sennheiser HD800/HIFIMAN HE-500, review HERE

    - Portable Player: HIFIMAN HM-801
    - USB Cables: Acoustic Revive USB-1.0SP (1 m) | Acoustic Revive USB-5.0PL (5 m), review HERE
    - LAN Cables: Acoustic Revive LAN-1.0 PA (kable ) | RLI-1 (filtry), review HERE
    - Router: Liksys WAG320N
    - NAS: Synology DS410j/8 TB
    System I
    - Interconnects: Acrolink Mexcel 7N-DA6300, review HERE | preamplifier-power amplifier: Acrolink 8N-A2080III Evo, review HERE
    - Loudspeaker Cables: Tara Labs Omega Onyx, review (in Polish) HERE
    System II
    - Interconnects: Acoustic Revive RCA-1.0PA | XLR-1.0PA II
    - Loudspeaker Cables: Acoustic Revive SPC-PA

    System I
    - Power Cables: Acrolink Mexcel 7N-PC9300, all system, review HERE
    - Power Distributor: Acoustic Revive RTP-4eu Ultimate, review HERE
    - Power Line: fuse – power cable Oyaide Tunami Nigo (6m) – wall sockets 3 x Furutech FT-SWS (R)
    System II
    - Power Cables: Harmonix X-DC350M2R Improved-Version, review (in Polish) HERE | Oyaide GPX-R (x 4 ), review HERE
    - Power Distributor: Oyaide MTS-4e, review HERE
    - Stolik: SolidBase IV Custom, read HERE/all system
    - Anti-vibration Platforms: Acoustic Revive RAF-48H, review HERE/digital sources | Pro Audio Bono [Custom Version]/headphone amplifier/integrated amplifier, review HERE | Acoustic Revive RST-38H/loudspeakers under review/stands for loudspeakers under review
    - Anti-vibration Feets: Franc Audio Accessories Ceramic Disc/ CD Player/Ayon Polaris II Power Supply /products under review, review HERE | Finite Elemente CeraPuc/ products under review, review HERE | Audio Replas OPT-30HG-SC/PL HR Quartz, review HERE
    - Anti-vibration accsories: Audio Replas CNS-7000SZ/power cable, review HERE
    - Quartz Isolators: Acoustic Revive RIQ-5010/CP-4

    - FM Radio: Tivoli Audio Model One