Asus Xonar Essence STX Reviewhardnrg -
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As you probably already noticed, the card itself sports the Chime of Tiger design on an EMI (electro-magnetic interference) shield. This metal shield acts as a Faraday cage, to block any EMI from other components inside the computer. It's not a new idea, and previous cards in the line of soundcards from Asus, such as the D2, have a similar EMI shield. Creative has put EMI shields on some of the more recent Soundblaster soundcards, but the Asus EMI shields are more substantial and I find the brushed metal finish more aesthetically pleasing. Unlike the Xonar DX and D2X soundcards, the Essence STX does not have a PCI counterpart, and is only avaiable in PCI-E x1 form. The reverse side of the card is unsurprisingly plain, although each of the headers and sockets are labeled on the PCB.
On the back of the card, from top to bottom, you have right and left line-level RCA/phono outputs. These are intended to be connected to stereo computer speakers or a stereo amplifier. If you have computer speakers with a 3.5mm stereo jack plug, you can use the included adapter lead to convert these two RCA/phono sockets to a 3.5mm stereo jack socket. Next, there's a 1/4" stereo jack socket for driving headphones directly from the onboard amplifier. Similarly, if your headphones have a 3.5mm stereo jack plug, you can use the 1/4" to 3.5mm jack adapter to enable connectivity. The second 1/4" socket can be used either as a microphone or as a line-level stereo input. The last connector is a combination RCA/phono (coaxial) and optical digital output. This lets you have multi-channel (e.g. 5.1 surround) audio via a digital interconnect cable. Again, you can use the included optical adapter if required. I like the range of inputs and outputs on the back of the card, the use of more substantial connectors than 3.5mm stereo jack sockets, and the fact that the digital output is separate to the microphone and line input, unlike the combination jack of the Xonar D1, and those of the X-Fi series.
Along the top of the EMI shield there is a cutout for the Auxiliary and Front Panel connectors. After using Soundblaster cards without a standard front panel audio header for many years, it's somewhat of a novel idea to have high quality audio on the front panel of a computer case. The Asus logos on the EMI shield are in raised lettering; I like how Asus chose to place the logo along the top edge of the EMI shield, as you can see it when the soundcard is installed, especially if you have a panel window on your computer case.
The last two connectors on the card are a 4-pin "Molex" power connector and a S/PDIF header. The "Molex" input supplies power to the amplification sections of the card rather than relying on the 12V and 3.3V pins of the PCI-E slot. The S/PDIF header allows you to connect the soundcard to a graphics card with an HDMI output. The J14 header is undocumented.
The AV100 is the same C-Media OxygenHD (CMI8788) sound processor used on the rest of the Xonar line and on HDA/Bluegears/Auzentech cards. It provides a mixture of hardware and software support for DirectSound/DirectSound3D, OpenAL, and A3D.
To the left of the AV100, you can see the Cirrus Logic CS5381 ADC used for the analog inputs. The DJ100 is apparently an AC'97 codec DAC (C-Media CMI9780), and therefore the microphone section of the AC'97 feature set is used as a pre-amp for the microphone (and front panel microphone). AC'97 is not synonymous with the highest quality, but as this soundcard is tailored toward high-end audio playback, it's not really an issue. You could always get a proper microphone pre-amp if you wanted to do some production-level recording from a microphone. The microphone input cannot be configured as balanced anyway.
The New Japan Radio NJM5532 and Texas Instruments R4850 op-amps are used for the line-in/microphone input. Asus chose to use miniature relays to do the switching between line-in and microphone modes. My theory is that the relays route the signal through the NJM5532s for the line-in, or the R4850s for the mic mode. The use of relays to switch the routing eliminates the signal loss that you would get if you used solid-state switching.
I had to take the EMI shield off to see what was hiding underneath. You can see it's fairly thick metal and not like some "Coke can" thickness shields.
Underneath, you can see the analog output stages, and the internal analog input stages. These areas are shielded from each other with a small shield "wall."
Sanyo OS-CON capacitors are used as smoothing capacitors, to ensure clean power delivery from the "Molex" connector. Another Sanyo OS-CON is used for the S/PDIF output. The remaining electrolytic capacitors on the PCB are Nichicon FG "Fine Gold," and are specifically designed and manufactured for high-end audio equipment. It's very impressive to see high-end capacitors used throughout.
The Burr-Brown PCM1792 DAC is used to feed the analog output stages. This DAC is very high end, and is reason enough for audiophiles to use this soundcard simply because most other soundcards use inferior DACs and therefore the analog outputs on other soundcards don't perform as well.
Probably the most important and unique feature of the Essence STX is the Texas Instruments 6120A2 headphone amplifier chip. This is a very capable dedicated amplifier chip for headphones, and can handle impedance up to 600 ohms! This chip is the basis of some standalone high-end headphone amplifiers.
Two New Japan Radio NJM2114 op-amps are used for the left and right channels, these either pass the signal to the 6120A2 headphone amplifier, or to the RCA/phono line-level output via the National Semiconductor LM4562 op-amp. Again, the switching between outputs is implemented with the NEC relays to minimize signal loss. If you want to change the sound of the headphone output, you would replace the NJM2114 op-amps. Changing the line-out sound is achieved by replacing the NJM2114 and/or LM4562 op-amps.
The op-amps are all DIP8 packages, and can be pulled out and installed without any tools. You might need to use a tool if you have large fingers and thumbs, but it is still a very simple, quick and painless process.
It's nice to see metal film capacitors used for the LM4562 op-amp. These are a step above electolytics for most audio applications, but their physical size versus capacitance prohibits usage all over the soundcard.