Now I’ll tell you about the converters installed on the Xonar D2. The Burr-Brown PCM1796 belongs to the Advanced Segment DAC class and is claimed to combine an excellent dynamic range with low sensitivity to jitter. The converter has a dynamic range of 123dB and a THD+noise of 0.0005% for sample rates of 44.4, 48 and 96kHz under normal conditions. The latter parameter increases at 192kHz.
The graphs below are built for the typical application circuit of the DAC. The I/U conversion and filtering cascades of the typical circuit employ NE5534 opamps, which are not exceptional in terms of distortions, so other implementations may bring even better results.
However, Texas Instruments produces an even better model, PCM1792A, which ensures a dynamic range of 127dB and a THD+noise below 0.0004% for sample rates of 44 and 48kHz.
Interestingly, this model is pin-compatible with the PCM1796 and has the same command set. It could be used on the Xonar D2 without any modifications of the PCB if it didn’t cost four times as much as the PCM1796. Frankly speaking, I don’t think it’s reasonable for the sound card’s four outputs to be identical quality. ASUS engineers seem to agree with me, using cheap opamps on every output save for the front one. Following the reasonable sufficiency principle, the rear channels could be based on the PCM1791A (a dynamic range of 113dB, 0.001% TDH+N) that is not only 30% cheaper than the PCM1796 but also doesn’t need an I/U conversion cascade, which would allow to do without six opamps and a number of support elements. There is an even more radical variant of redesign that would yield a low-profile version of the card: the six-channel PCM1602A (a dynamic range of 105dB, 0.002% THD+N) provides three stereo outputs at a lower price than the PCM1796’s one. By the way, the recently released low-profile Xonar DX card is designed in a similar manner: a high-quality dual-channel DAC (Cirrus Logic CS4398, like on the Creative X-Fi Elite Pro) for the front output and a six-channel Cirrus Logic CS4362A (114dB dynamic range, 0.001% THD+N) for the remaining three outputs.
The Xonar D2’s analog-to-digital converter is based on the best model from Cirrus Logic, CS5381, which ensures a SNR of -120dB and a THD+N of 0.0003% at all sample rates up to 192kHz.
This may be the highest-quality mass-produced analog-to-digital converter. Theoretically, it can be used for very precise signal measurements at frequencies up to 50kHz (above that point the signal is suppressed by the digital filter even at a sample rate of 192kHz) but the analog section must be top-class as well. The ADC developer recommends to base the filter on low-noise opamps LT1128 from Linear Technology which have a minimized offset voltage and a superb level of distortions (about -130dB), but ASUS employed NJM5532 opamps which are not so high quality.