Let’s have a look at the card’s design and components.
Interestingly, the ASUS Xonar Essence STX has no mini-jacks typical of most other sound cards. The line output is implemented by means of consumer RCA connectors. The line-in and headphone outputs are stereo 6.3mm jacks, making the cards similar to both semiprofessional products like the ESI MAYA44 or Infrasonic Quartet and consumer cards like Audiotrak Prodigy 7.1 HiFi and Prodigy HD2. It does not have a digital input but offers a separate headphone output that is enabled by choosing the appropriate mode in the card’s control panel. The line input is combined with the microphone input but you can connect the microphone and headphone interfaces to the connectors on your system case. Besides FP_audio, the card has two useful onboard headers: AUX_in and SPDIF_out. The connectivity options look good to me and I’ve never had any problems with them.
The card needs additional power via a standard Molex connector (like the connector you use for PATA drives). ASUS’ engineers claim that the PCI Express x1 slot cannot ensure enough current to power such high-quality components as are installed on the Xonar Essence STX and is too susceptible to current ripple and electromagnetic interference. While I do not agree with the latter statement because I have faced the need for additional filtering of power for devices connected directly to the computer’s power supply (the external module of the Creative X-Fi Elite Pro or the Scythe KamaBay Amp), there is no denying that a reserve of power is needed for the integrated headphone amplifier.
There are two 270µF SANYO OsCon capacitors near the power plug that make it somewhat difficult to insert the connector into the latter. The other electrolytic capacitors are all marked as Nichicon. Both brands are well known to everyone who is interested in developing audio equipment with high sound quality. Practice suggests that capacitors can influence the subjective reproduction quality of a device very much. Electrolytic capacitors with solid dielectric have earned a good reputation for their acoustic properties as has been proved in the previous generation of sound cards (Auzen X-Fi Prelude and ASUS Xonar D2). However, specialists still prefer aluminum electrolytic capacitors of special types in premium-class equipment; therefore it is good that two kinds of such capacitors are employed in the Xonar Essence STX. I will try to find any subjective difference later on.
The burnished screen you may have seen on the ASUS Xonar D2 covers about one half of the card’s PCB, leaving not only the digital section, which does not need any screening, but also all the electronics of the line/microphone input exposed. Besides, the lack of a slant in the corner of the screen makes it impossible to fix the card screwlessly in system cases where add-in cards are fixed by means of plastic locks.
There is a PCI → PCIe bridge from PLX Technology in the bottom right corner of the card. It acts as an intermediary between the peripheral bus and the card’s main chip marked as ASUS AV100. I don’t know what differentiates it from the ASUS AV200 that was installed on the Xonar D2, but both chips have the same specifications.
Internal signal switching is done by small electromagnetic relays from NEC. Like in the Xonar D2, the line input is based on a CS5381 analog-to-digital converter with NJM5532 operation amplifiers. I can remind you that this ADC is the topmost model in Cirrus Logic’s product series and is somewhat superior to similar products from Asahi Kasei (AKM) in its declared parameters. The digital-to-analog converter is an improved version of the Burr-Brown PCM1796, which did well on the Xonar D2 and some other devices like Audiotrak DrDAC2. The PCM1792A features Advanced Segment architecture and current outputs, differing from the PCM1796 with an improved output filter, lower distortions (especially at a sample rate of 44.1kHz) and a larger dynamic range (127dB and 132dB in stereo and mono modes, respectively). I compared the distortions of these two converters in my Xonar D2 review, so I will just show you two graphs that illustrate the advantage of the PCM1792.
PCM1792A Digital Filter Response:
PCM1796 Digital Filter Response:
I want to dwell upon the implementation of the analog outputs in more detail. The lack of a separate headphone output was a serious drawback of the Xonar D2. In the Xonar Essence STX the outputs are not only separated physically but there is a special headphone amplifier based on a TPA6120A2 chip from Texas Instruments. This is a second sound card I can recall (after the HT Omega CLARO halo) that employs such a high-quality amplifier. The chip is a high-power current-feedback opamp that features distortions of less than 0.001% at a 32Ohm load. When the load is lower, the distortions are lower than 0.0001%, which is comparable to the best opamps of today.
Judging by the specifications, this is the highest-quality serially produced single-die amplifier for headphones. Better characteristics can only be achieved by means of composite circuits with discrete components. On the downside of the high-speed nature of this opamp is that an isolating resistor must be employed, which leads to worse damping and increased low-frequency distortions with low-resistance headphones.
I can note that ASUS used the circuit recommended by Texas Instruments in which the TPA6120A2 sums the balanced output signal of the DAC. Depending on the mode selected in the control panel, the electromagnetic relay switches the signal from the I/U converter either to the line output opamp or directly to the headphone amplifier chip.
Typical circuits for TPA6120A2 and PCM1792A:
The three line output opamps are the same NJM2114 and LM4562 that were used in the Xonar D2’s front output but inserted into sockets for easy replacement (as the text on the product box says, this can be done to adjust the card’s sound to your taste). Frankly speaking, in such a top-end sound card as the Xonar Essence STX I’d prefer to see six soldered-in high-speed single-channel amplifiers rather than DIP sockets which may affect the stability of high-speed opamps.