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Output Filter Topology

Besides the useful sound signal, a digital-to-analog converter produces wide-range ultrasound noise that puts an additional load on the subsequent amplification cascades and can worsen the quality of sound. Therefore, the analog section of each sound-reproducing electronic device must contain a low-pass output filter. This filter is often referred to as smoothing. It is usually based on an operation amplifier and largely determines the resulting sound of the device. A lot of unexpected nuances have to be accounted for while designing such a filter.

In my Auzen Forte review I quoted a mysterious phrase about the improved design of the output filters. It goes like this: “We have also accurately designed the DAC filter to minimize phase changes.” The following phrase (“Metalized Film Capacitors take full advantage of the superiority of a Film capacitor while minimizing inductance, and are used for the Front channel’s DAC filter.”) indicates that the engineers indeed focused on improving the characteristics of the filter. But what exactly did they improve? And what is this filter like? Let’s read through the documentation on the digital-to-analog converter CS4382A.

Take note of the odd discrepancy between the ratings of the passive components (resistors and capacitors). The documentation (page 9 here and page 12 here, for example) says that this design levels out the AC load on the positive and negative differential outputs of the DAC. In the more traditional design with identical resistor ratings the AC load proves to be asymmetrical.

So, I suppose that the nonsymmetrical load on the DAC outputs may affect the resulting sound quality, but this solution is not good for an operation amplifier.

There are other variants of such filters. One of them is shown in my ASUS Xonar Essence STX review and below is the recommendation of AKM engineers.

Auzentech developers may have not used the recommended design. At least, there are only two electrolytic capacitors near each output connector on the PCB rather than four or six as described in the recommended designs. That is, the constant voltage from the converters’ outputs is applied to the inputs of the opamps, which may produce sudden variations in sound, too.

Thus, there may be lots of reasons for opamps to behave differently in different sound cards. And now I will tell you what is best for the Auzen HomeTheater.

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