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This is a review and detailed measurements of Asus STX II PCI Express sound card. It is on kind loan from a member and retails for USD $219 on Amazon including Prime shipping. Full list price is $299.This is a medium sized PC card which oddly requires external power. You can see it here together with the recently reviewed USB DAC, the Khadas Tone Board So inch for inch, it better measure a lot better. OK, some of that real estate is dedicated to headphone output and analog input which the Khadas board lacks. Still, with all the space, extra shielding, etc. it better do well. Asus specification shows near state of the art performance specs:If it lives up to these specs, it would rate at tier just below state-of-the-art desktop DACs. Will it get there? Let's measure it and find out.Turns out the ASIO interface with my AP software uses is stuck in 16 bit mode. Changing to 24 bit using external software, does remedy this and allows a few more dBs of performance. See later in the thread or this post: https://www.audiosciencereview.com/...f-asus-stx-ii-pci-sound-card.4915/post-111244 WASAPI interface is not impacted so if you use that, you get 24 bit support.The ASUS STX II allows "rolling" (swapping) of the current to voltage and output buffer op-amps. For this testing, I left the common buffer op-amp as shipped but swapped out one of the I/V op-amps to an LME49720. This way we kill two birds, testing the stock unit and effect of op-amp rolling to some extent. Here is our dashboard view:Well, this is disappointing. Distortion specs are much worse than specified with SINAD being missed by whopping 13 dB!Both output voltage and SINAD are better than my Gigabyte B8 motherboard though:On the other side of the coin, the Khadas Tone Board completely cleans its clock though with 110 dB of SINAD.Let's test the claim of 124 dB for dynamic range:Another massive miss. Here the Gigabyte B8 performs substantially better with 11 dB higher dynamic range.The picture stays just as ugly with Jitter and noise:The J-test signal toggles all the sample bits to the tune of 250 Hz. This should be invisible since it is being applied to last bit of 24 bit words. Yet, we easily see its spectrum here indicating bleeding of digital circuits into analog.Once again, the Gigabyte B8 motherboard did much better (inset).Let's look at intermodulation distortion versus level:The higher noise level of Asus STX II hurts it here in the initial parts of the graph. Fortunately performance stays well behaved without the "hump" that the Gigabyte B8 demonstrates in mid-levels.The Topping D50 also has a hump but it easily beats the Asus STX II in the rest of the spectrum.Last but not least, let's look at linearity:This is remarkably bad. And in both channels so differing op-amps in I/V state makes no difference. The error is off the charts by the time we get to the limit of 16 bit audio (96 dB). You lose accuracy in as low as 11 bits. Once gain, the Gigabyte B8 motherboard performs better and by large margin.I did not test the headphone output. It has a 10 ohm output impedance which is a bit problematic.My experience with add-on sound card has uniformly been poor. Despite claims of high measured performance, I would routinely hear my computer activity through them. Alas, I had not tested them for a while so went into this review thinking the situation may be much improved. That was not meant to be.At best we can say it is susceptible to differences in motherboards. But isn't that what they were supposed to do right for nearly $300? Make a card that would sound good regardless of the machine activity? Measured performance is far, far below specification and in a number of cases, worse than my motherboard.Ultimately,USB interface is plenty fast, is plug-and play, and doesn't require opening the machine to use them. The ASUS STX II by the way, required custom drivers to work. Heaven forbid Microsoft changes its driver interface again and Asus doesn't provide an update for that future OS.If you want a bare board solution, take a Khadas Tone Board, stick it inside your PC with a bit of sheet metal work and be golden for $99.As always, questions, comments, corrections, etc. are welcome.-----If you like this review,for these reviews using Patreon ( https://www.patreon.com/audiosciencereview ), or upgrading your membership here though Paypal ( https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054 ).