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Custom tailored headphones amplifier review

QES Labs HPBA-2 SEditor’s Choice


Over the last few years the world of dynamic and orthodynamic headphones has undergone an enormous evolution. The performance of some top-of-the-line models is comparable to a pair of speakers of a famous brand. The use of more advanced materials has made it possible to push the authenticity of musical reproduction higher and higher, in an era where the liquid music often goes hand in hand with high-resolution audio files.

The industry that follows the evolution of headphones’ performance, is that of headphone amplifiers. Different manufacturers offer models with different technologies: from those working with tubes to solid state ones, from models that allow both unbalanced and balanced connections, to those who work completely in balanced mode. In this regard we would like to tell you about a product that we have used professionally for some years. The amplifier is the HPBA-2 S by QES Labs. QES Labs is a company founded by Valerio Riccelli, a young designer and musician. The aim of QES Labs has always been to build professional audio equipment that cater for high-end consumers. QES Labs is always careful and able to satisfy the customers’ requests and needs, developing and designing their devices from scratch.

The HPBA-2 S system originates from the need to provide the user with a headphone listening device that is completely devoid of sound coloration. Born for professional purposes, such as audio mastering, it can also be used by the most discerning audiophiles looking for utmost sonic fidelity within its range. The amplifier differs from the various models available on the market because it works completely balanced and above all because the customer can choose between various available components: in other words the external chassis, the type of internal wiring, the transformer type, the power supply and more. What we would like to highlight is the fact that the HPBA-2 S only uses current drive techonlogy that we will explain in more detail below.

Design and operation

Front panel

Front panel

The HPBA-2 S system is simple and functional construction. The chassis is in die-cast light grey aluminium with the functions printed on the front panel. It doesn’t weigh that much. On the front panel, in addition to a volume potentiometer with 24 steps, there are two female XLR connectors for balanced connection of headphones. On the rear panel there is the socket for the power cable and the respective XLR female connectors made by Neutrik, as with the input of the balanced line audio signal on the front panel. The customers can tailor their own device. On my HPBA-2 S I asked for a longer chassis, narrower than the stock one which is easier to place on a rack or on a type of fastener installation. All the internal wiring is in pure silver. The power is supplied by a toroidal transformer. The main features are the fully dual-mono construction, fully balanced internal circuitry, the absence of capacitors on the signal path, the absolutely flat frequency response, low harmonic distortion, a fast transient response and finally the current driving. But what is the current driving? According to QES Labs website:

In the dynamic transducers, it’s the current flowing through the voice coil which produces the magnetic field which interacts with the magnetic field of the magnet in the driver that causes the diaphragm to move back and forth. Thus a constant current amplifier has several advantages over the usual constant voltage amps: a current source amplifier delivers a precise current to the voice coil of the loudspeaker driver, ignoring the series impedance elements in the circuit, including the wire, connectors, the inductance of the voice coil, the resistance of the voice coil versus temperature, and so on, delivering higher accuracy and linearity.

Amplifier side

Amplifier side

An advantage of the current driving is the direct control of the circulating current in the coil to avoid distortion related to driving. The current back-EMF vanishes and the very large output impedance prevents the current to circulate. This impediment eliminates the nonlinearity due to the overlap of different recall forces. The current driving of the headphone allows extraordinary musical performance thanks to its behaviour. The current driving has much broader potential than the voltage driving, which is present in most headphone amplifiers, because it allows the impedance to exploit changes and to model the overall response. A series of concepts that cannot immediately be understood but if properly applied, as in the case of HPBA-2 S system, it becomes evident that this listening system allows headphones to express their full sonic potential.

Listening test

The headphones that can be used with the HPBA-2 S system are those that have an impedance equal to or greater than 200 Ohms and that allow the user to remove the stock cable and install a cable with balanced XLR male connectors. The test took place in my little studio, where I carry out audio mastering and critical listening sessions for the evaluation of various appliances. I wanted to use recordings that were not strictly of an audiophile level but were still of a very good quality. The test was conducted with this listening chain:

  • PC DAW with Microsoft Windows XP, Steinberg WaveLab 6 and Foobar2000

  • USB interface Matrix Audio X-SPDIF

  • Violectric V800 DAC

  • Sennheiser HD 800 and HD 600 headphones

  • ALO audio and Double Helix Cables headphone cables

  • Cardas Audio Clear USB cable

  • Cardas Audio Golden Reference XLR analog cable

  • Wireworld Gold Starlight 5 AES/EBU digital cable

  • Custom power cables for all equipment

  • Belkin main filters

These are used recordings:

Back panel

Back panel

What immediately strikes you when first listening is the transient speed at low frequencies, especially on the bass and on the bass drum which the listener can fully appreciate in terms of presence and power. Neutrality and transparency of the HPBA-2 S could make it a Wire with gain. The main purpose of the designer and builder of the machine is to return the source without the slightest alteration or sonic coloration. The sound spectrum is returned in all its breadth without any particular predominance for some portion of the same. The tone is neutral and balanced. The state of the art recordings played impeccably, lesser ones are mercilessly invested of all their faults. The tails of the reverbs, the compressor pumping, any problems of phase inversion, are perceived in a clear and precise way, as any click or wrong fade-out. The picture of what comes from the source is accurate. Stereo image is notable. You can clearly distinguish the location of all instruments inside the song. The image is almost three-dimensional with a clear separation between front and back.

The dynamic is spot on for transient response and it’s fast, accurate and clear. Much faster than our previous amplifier Lake People G100. The advantage of the current driving and balanced internal circuitry makes you feel its weight especially with a demanding headphonen such as the Sennheiser HD 800. The HD 800 is driven by adeguate power. The transient articulation is excellent as well as the frequency response. Even with the Sennheiser HD 600, that was my reference headphone for a long time, the performance almost reaches the headphone dynamic limits. We find ourselves faced with an excellent product under many aspects. The assumption must be to buy this amp to listen to and to return the sound source in its entirety. It’s a very demanding product that requires an adequate listening chain to express its full potential. Therefore to avoid any problems with its performance you need to make a smart choice as regards all of components that will form part of the reproductive chain.

The DAC must have an analog output really based on balanced topology. Input signal cables to the amplifier must have a low capacity value. No less important is the amplifier’s power cord equipped with adequate section and components. I admit that our listening chain is a high-end one. On the HD 600 both foams that recover the drivers were removed. This has made it possible to extend the frequency response enormously at the extreme top and bottom, making the headphone’s sound more neutral and balanced overall. The HD 600 was also equipped with one of the best balanced aftermarket cables developed by Alo audio. The same HD 800 was wired with a pure cryoed OCC copper cable produced by Double Helix Cables. The use of the Violectric DAC, neutral in tone and often used in some mastering studios and the use of high-performance power and signal cables, are all factors that have allowed the HPBA-2 S to perfectly unleash its full potential.

Final thoughts



The QES Labs HPBA-2 S provides noteworthy headphone listening. Of course this is not a product for the occasional listener or for the audiophile looking for the colorful and emotional listening of that of a tube amp, but caters mostly for the professional market or the most demanding users. It is an amplifier which highlights the flaws of today’s recordings. This product can not be used with all types of dynamic and ortodynamic headphones on the market. It needs a headphone that has an impedance equal to or greater than 200 Ohms and one that can have its cable replaced with a balanced one. It is a product of high quality that returns an excellent reproduction, allowing the user to listen as faithfully as possible to the source without any particular sound coloration. A product that can provide very good performance as long as the rest of the reproductive chain is up to scratch. At present the HPBA-2 S is, both for professional and amateur markets and one of the top solid-state models of headphone listening using technology that is not widely used but is really valid such as the current driving. Therefore we believe that the system developed by QES Labs can be considered one of the new reference points for all lovers of listening through headphones.


  • Scoring110
  • Sound quality [1]10
  • Bang for the buck [2]10
  • Measures [3]10
  • Build quality [4]7
  • Versatility [5]7
  • Overall rating9
  1. 1. Sound quality of the product puts it on such levels as to become a reference
  2. 2. Price allows you to make a purchase from value
  3. 3. Measures stood at very high levels on a par with much more famous products
  4. 4. Build quality is a bit thin but basic and functional
  5. 5. Versatility of the product narrows a little the number of usable headphones

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