Which headphone amplifier model to choose?
Headphone amplifier selection criterion should be primarily sound quality and the type of headphones with which it will cooperate. To drive dynamic headphones with low and medium impedance, and those with a higher impedance retaining high efficiency, sufficient model is the Moonlight. In addition, due to lower gain and moderate maximum power, it will be more convenient in terms of volume adjustment when using headphones with low rated power like IEMs.
People looking for maximum versatility and flexibility in choosing headphones may be interested in  the Sonic Pearl and Black Pearl. Power offered by these models is at least 100 mW at 600 ohm load and more than 2W for less than 20 ohms impedance.
The Sonic Pearl offers a unique design, high mechanical stability and rigidity, low susceptibility to vibrations thanks to 8kg of weight, and advanced solutions in terms of power supply section. It also allows you to shape its sound signature to a decent extent through the selection of the mains power cord fitting into the IEC inlet of the Sonic Pearl.
Black Pearl is dedicated to those who are ready to sacrifice the aesthetic value of the device and put aside some unique solutions dedicated to advanced audiophiles.
The Black Pearl offers flexibility in raising its sound quality by upgrading the power supply unit as the power amplifier is a separate element to be connected through a standard DC connector 5,5 / 2,1 mm. You can choose between a standard power supply and a dedicated NiMH battery supplied together with a low noise linear battery charger.
How much of sound quality improvement brings the battery power supply offered for Black Pearl?
Battery as a voltage source is characterized by very low noise, lack of ripples resulting from the conversion of the mains voltage and a very high current efficiency of at least eight amperes. Due to the simplicity of construction and limited peak output current, the standard power supply cannot fully compete with the dedicated battery. Ina  high-end headphone system you can hear the difference in the form of stronger and fuller bass, richer nuances and smoother midrange.
Amplifier that offers the level of quality standards in every aspect of the sound at least no worse than any variant of the Black Pearl is the Sonic Pearl due to the two-stage active DC supply regulation and high quality toroidal transformer as opposed to the standard EI in the power supply for Black Pearl.
What should I do when the Black Pearl's battery is discharged?
Simply plug the charger into the dedicated socket in the battery and you can continue listening to the music. Just keep in mind that using the amplifier while charging will prolong the charging time.
Why is there no battery power supply offered for the Moonlight headphone amplifier?
Some of the Moonlight headphone amplifier circuitry relies on the principle of the fixed supply voltage as a means of preserving optimal sound quality. Battery voltage depends on the level of discharge. In other words, it changes over time when the amplifier is working. The Black Pearl in opposition to the Moonlight shows independence of the sound quality from the supply voltage level within a range much higher than battery charge dependency.
Why do Ear Stream headphone amplifiers use cheap parts in some places?
The only criterion for the selection of each component was the quality of sound and meeting the electrical requirements. Their choice was not the implication of price but the price was calculated after all parts had been selected. For example, the Alps RK27 potentiometer, very popular in competing products, gave lower sound quality than the smaller RK09 or Vishay Sfernice P9A possible to use as an alternative. We had the same situation when trying several other potentiometers i.e. from Bourns. In addition, before installing, the potentiometers are checked for channel balance, and those which do not meet our requirements are discarded even if they are within the manufacturer's guaranteed electrical parameters.

What sonic signature do Ear Stream headphone amplifiers present?

The idea behind the foundation of Ear Stream is to create affordable products that provide experience resembling the perception of live performance and true sound of instruments played without amplification. Hence Ear Stream headphone amplifiers are characterized by excellent sound speed and transparency, the ability to differentiate and reveal various aspects of sound. We can make a device that will have its own "sound" as many years of experience in the construction of headphone amplifiers gave us enough knowledge for this purpose, however we believe that a more important feature of the amplifier is polymorphism, the ability to play back any mood contained in the recording and not to define certain aspects of music in its own way, as such constant addition to any recording being played back makes the listener bored quickly and stimulates constant changes in one's audio system. Both uniquely warm and sweet sound, as well as purely analytical playback, both are kinds of frames that make it impossible to have choice between synthetic fascination with the whole composition and its presentation, and analytic focus on individual instruments and their tonal qualities, and virtuosity of performers. Just as it is possible for classical music concerts where the listener can choose what is more interesting to him at a moment and move his attention to any part building the spectacle or relax and perceive it as undivided masterpiece.
I would like to have access to the technical advantages of Ear Stream headphone amplifiers but I have specific expectations in terms of sonic presentation. How can this be reconciled?
Overall method of building an audio setup is making choices of each component starting from the one of special interest. The first step should be the selection of headphones as they most clearly define the presentation of the music. Indirectly, they impose the search area of the amplifier posessing  matching sound signature and adequate driving capabilities. In the case of Sonic Pearl or Black Pearl the problem of driving is practically non-existent when considering headphones in the current production equipped with a jack plug. So the question is sound. The easiest way to shape it iwhen deciding on an Ear Stream amplifier is choosing appropriately sounding source, making the last cuts by selection of audio signal cables, power supply cord, or an alternative for the one that comes with headphones. Sometimes, however, this pathway seems to be too tedious, or the client does not want to get rid of their existing audio system components, and would like to get an amplifier exhibiting specific characteristics of sound. In this case, we offer an option for alternative components installed in the headphone amplifier. For example, in each Ear Stream headphone amplifier, you can use other than carbon potentiometer Alps RK09, such as polymer Vishay. Models Sonic Pearl and Black Pearl also enable an alternative for the output resistors which instead of metal film might be carbon, making the midrange sound thicker and add some sweetness and reserve to the trebles. It is also a carefully selected type of resistors as well as in case of the default metal film ones. There are also ways to increase the weight of the bottom end of audio band by selection of capacitors in the power supply or altering the output impedance of the amplifier.
My headphones connected to an Ear Stream amplifier have less bass than when they are connected to the headphone out of a speaker amplifier, CD player or OTL tube amp. Does this mean that Sonic Pearl, Black Pearl and Moonlight have little bass?
Ear Stream headphone amplifiers have enough potential to reproduce the lower end of the audible frequency range with roll-off not greater than 0.3 dB with the lowest applicable impedance of used headphones. The confirmation of the figures are the opinions of the great bass control and articulation in the newer versions of Moonlight, Black Pearl and Sonic Pearl. It results not only from the low impedance and high energy reserve within the power supply section of the amp, but also from low output impedance which varies between 0.2 and 6 ohms depending on the model. This ensures a high damping factor and exact presentation of audio spectrum. However, the headphones connected hi-fi equipment equipped with headphone outputs of high impedance, change their tonal balance due to their impedance versus frequency  not being constant. Resonance of the transducer is usually in the range of 80 ~ 100 Hz and can cause even more than twofold increase in the impedance of the nominal figure at 1kHz or voice coil resistance. For example, if we have the headphones with an impedance of 100 ohms, headphone out with a resistance of 150 ohms, while the impedance of headphones measured in the bass range grows to 200 ohms, the bass boost due to uneven impedance curve will be more than 3dB which is clearly perceptible. So the correct answer to the question is that this is not the Ear Stream amplifier reducing the amount of bass but just not emphasizing it like the equipment one got used to after prolonged exploitation. Fortunately, the adaptability of the sense of hearing is big and fast so you can switch to a slightly different tonal balance over short time, appreciating other benefits delivered by a headphone amplifier keeping good control over acoustic transducers.
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