And now I'm going to show you the new tests. Below you can see square wave oscillograms illustrating the operation of the playback section’s digital and analog filters. The contribution of the E-MU’s line input filters is negligibly small because the recording was performed at a sampling rate of 192 kHz while the analog input filter is set at a rather high cutoff frequency.
You can see the smooth increase in the signal amplitude on the Auzen Bravura’s line output which is not observed with the other sound cards. The Mute feature is usually implemented with field transistors or a relay but the extra components worsen the sound quality, therefore Auzentech's engineers selected some nontrivial method like switching the DAC into sleep mode. The operation of that mechanism is audible when the Bravura is set at a sampling rate of 44.1 kHz: when an application (a video game, for example) uses the sound card, there appears an audible noise. Otherwise, the Bravura's outputs are absolutely silent.
PCIe Xtreme Audio
The Auzen Prelude’s graph looks rather unusual, too. Judging by the article from Ayre, the maker of disc players and DACs, the developers of the Auzen Prelude implemented a minimum-phase filter in it. This is not the normal operation mode for the AK4396 converter because the other sound cards with this chip have the characteristic pre-ringing typical of a line-phase filter. The Prelude doesn’t have it at all but its post-ringing is far larger than usual in amplitude and duration. It is no wonder then that the Prelude sounds differently from other cards.
The Bravura’s line output filter is even more unusual: the pre-ringing is shortened to two cycles rather than eliminated altogether. That's what the phrase “we accurately designed the DAC filter” means! Interestingly, this optimization didn't concern the headphone output filter but you can enable a special filtering mode for it.
Bravura headphones out in Sharp filter and Smooth filter modes
Bravura headphones out in Sharp filter mode
Bravura headphones out in Smooth filter mode
In the Smooth filter mode the extra ringing is eliminated completely. Many people consider it to be an inherent downside of delta-sigma converters but you can see that the converter’s architecture has nothing to do with it. The “pretty” switching response is achieved by means of the digital filter with slow decay. However, this filter has one serious drawback. Take a look at the filtering quality.
The sonograms below show you the spectrum of the signal: the X-axis shows time and the Y-axis, frequency. The brightness of a dot indicates the intensity of the signal at that frequency.
The lilac cloud at the top is ultrasound noise which is largely determined by the characteristics of the E-MU 1820’s ADC, but we can see quite clearly that it is brighter in the Smooth filter mode. That’s not the whole problem, though. The noise might be lowered but it is absolutely impossible to get rid of the aliasing which is supposed to be suppressed by the filter. The aliasing is harmful as it provokes intermodulation with the true signal in the subsequent analog section, making different sounds less distinct. The parasitic lines are perfectly visible in the sonogram of the line output and barely noticeable in the sonogram of the headphone output.
The purple lines deflecting from the yellow line in the sonogram of the Bravura’s line output represent harmonics of the main signal. Judging by the sonogram, the third and fifth harmonics are especially high, which agrees with the results I obtained with RMAA. We can also see a less distinct line of the seventh harmonic and a mesh of intermodulation products which is not visible with the Creative card but can be seen with the Auzen Prelude.
The nonstandard implementation of the Auzen Prelude’s filter is good at suppressing aliasing but provokes unexpected artifacts that occur in the analog section of the sound card itself or in the E-MU 1820. When the level of the output signal is lowered, the problematic frequency band gets smaller and the artifacts from the audio range move up to at least 100 kHz. These data explain the oddities of the Auzen Prelude such as its genre predilections and its fastidiousness about connecting cables. It doesn't like recordings with constituents higher than 19 kHz whereas most of today's sound editors do not care to filter their recordings before digitizing them. When reproducing such materials, this sound card has wide-spectrum and rather intensive noise in its output.
The rest of the sound cards are not sensitive to the ultrasound constituents, including the Auzen Bravura which has the same AK4396 DAC in its headphone output and whose line output filter is based on a CS4382A converter that has a similar switching response. By the way, the Cirrus Logic converter is better at suppressing aliasing than either of the Asahi Kasei converters.
And finally, I can show you the test results for the opamps you can buy from Auzentech.
As you can see, the Bravura’s headphone amplifier reacts eagerly to the change of opamps, the difference being obvious in the results. However, I won’t comment on them because I did not check the results out for repeatability.