Pros: Excellent DAC for the price
Cons: Too high output impedance for many headphones
This review focuses mainly on the performance of the card from a technical point of view. It most likely also applies to the Xonar DX, which is basically the same card, but with PCIe interface instead of PCI.
The strongest point of these cards is the (CS4398 based) DAC, which supports sample rates up to 192 kHz, and 24 bit resolution (I tested mainly at 44.1 to 96 kHz). The specs claim 116 dB A-weighted signal to noise ratio (112 dB for the input) relative to the 2 Vrms full scale output, and 0.00056% harmonic distortion at 1 kHz/-3 dB. As far as it can be determined with loopback tests, the card can come fairly close to those values in an electrically "quiet" PC. The distortion is also very low with a variety of more difficult test signals, and the DAC can handle a 0 dBFS signal at full volume (the volume control is apparently digital) without any problems. It has some additional headroom for signals that need more than 0 dBFS level in continuous time. Standard 44.1 kHz CD sample rate is also handled well, without significant artifacts. The DAC uses a minimum phase lowpass filter. In the 20 Hz to 20 kHz audio range, the frequency response is flat within less than 0.1 dB relative to 1 kHz. The output signal is not inverted (polarity is preserved).
To keep this review short enough, I do not get into further details, but so far I did not find anything to complain about the DAC performance, especially for a <$70 card. Of course, the higher Xonar cards should be even better.
The analog input / ADC is not as good as the DAC, but still quite decent for the low price, and also supports 192 kHz/24 bit. Unlike the DAC, it does not seem to handle a full scale signal perfectly (the distortion increases somewhat already 1-2 dB below clipping level), and on the card I have the left channel has a small amount of signal dependent noise that is not on the right channel. The input has an impedance of about 3.8 kΩ, preserves polarity, and does not capture DC.
The main disadvantage is the headphone output. While it has quite low distortion even when driving low impedance loads at the maximum level, indicating that at least some attempt was made at amplification, the output impedance is about 100.5Ω according to my tests. This is too much for most popular low impedance headphones. With the 2 Vrms maximum output voltage (actually, I measured about 3% less), the power into a 100 Ω headphone load is limited to 9-10 mW, and about half the maximum power into 16 Ω and 600 Ω. The front channel output is coupled by 220 μF capacitors, this slightly rolls off the bass with low impedance loads (about 0.5 dB at 20 Hz/32 Ω).
The limitations related to driving headphones are the main reason why the value rating is not 5.
The other analog outputs are of slightly lower quality than the front channel, and use CS4362A DAC chips. Interestingly, these have much lower output impedance (only a few Ω), but are nevertheless not well suited to driving headphones because of the very small (22 μF) coupling capacitors that audibly roll off the bass even with 250 Ω headphones. Also, the distortion of these channels does increase significantly with low impedance loads at high output levels.
The card supports the use of the front panel, although this is not recommended if the noise level is to be kept as low as possible.
UPDATE: detailed measurements are available here.