Does the other sound card perform better thanks to its special cascade for headphones?
The ASUS Xonar D2 having an extremely high output resistance, the level of the measured signal for the 32Ohm headphones was reduced considerably which didn’t allow me to perform the measurement at two levels of volume. Therefore the results are rather ambiguous: the card has the lowest coefficient of harmonics but the highest coefficient of intermodulation. However, as I wrote above, it is important to view the signal spectrums rather than the totals to correctly evaluate an amplifier.
The frequency response diagram of the ASUS Xonar D2 sound card shows the dependence of the headphones’ impedance on frequency in detail. The first hump at 100Hz corresponds to the resonant frequency of the dynamic heads employed in the Grado SR 325i. You can also see two small resonant peaks in medium frequencies which are caused by the speakers’ acoustic enclosures. The smooth growth of impedance at high frequencies is due to the inductance of their chokes. As you see, this sound card can be successfully used for purposes other than just listening to music.
So, the extremely low THD delivered by the ASUS Xonar D2 at a load of 32 Ohms coupled with a very short spectrum of harmonics makes this sound card as good as the best headphones amplifiers. No wonder I could not easily decide in my subjective tests what sound I liked more – with or without the amplifier. The high intermodulation distortion coefficient shouldn’t mislead you – the intermodulation proper is not conspicuous in the spectrum of the recorded signal as is also confirmed by measurements made according to another method.
RightMark Audio Analyzer contains an alternative intermodulation distortion test that uses two “sliding” sinusoidal signals with a frequency difference of 1kHz.
Here we can see that the intermodulation distortions of the C.E.C. amplifier are almost always lower than the level of noise whereas the Scythe KamaBay Amp is but slightly inferior to it in comparison with the line output of a regular sound card.
To wind up this section, I will show you the results for all the four devices.
An attentive reader might have noticed that connecting headphones to the line output of the Creative X-Fi Elite Pro is not quite correct because this sound card is equipped with an external unit that has a dedicated headphones output. Theoretically, it should cope better with headphones load, but practice can be different. As a matter of fact, headphones with an impedance of 250 Ohms (Beyerdynamic DT 770 Pro) sounded less detailed when connected to the external unit’s output and my measurement showed that the external unit was far inferior to the line output in terms of SNR. Why? The external unit is powered by one of the standard PSU connectors that are used for powering HDDs besides other components. The increased noise is the consequence, just as when I tried not to use the external power adapter of the Scythe KamaBay Amp. Moreover, the digital-to-analog converter in the external unit is lower quality than the line outputs’ DAC, so the owner of high-quality headphones should not count on the external unit of the X-Fi Elite Pro. The X-Fi Extreme Gamer coupled with Scythe KamaBay Amp will sound no worse at a much lower total price. After all, the X-Fi Elite Pro performs in this test session just as an example of a regular sound card (without integrated amplifier) working with headphones.
The additional buffer in the line output of the ASUS Xonar D2 card coped with the headphones excellently save for the extremely high output resistance. This buffer consists of two RC4580 opamps whose outputs are usually connected with resistors of a few dozen Ohms. A similar solution can be seen in M-Audio Revolution 5.1 and Audiotrak Prodigy HD2 sound cards. For some reason the resistors in the front output of the ASUS Xonar D2 have a very high rating, which is the reason why the impedance is beyond reasonable limits. Perhaps this defect has long been corrected and could only be seen in a small batch of sound cards sent for hardware reviewers to test.
The higher noise of the C.E.C. HD53R-80 in comparison with the other three tested devices is due to a ground loop between the sound card and the amplifier. It is thus the result of certain circumstances. For comparison, you can read our ASUS Xonar D2 review and find that the noise was much better when the measurement was performed through the Xonar D2’s line output (a Creative X-Fi Elite Pro was the sound source then, too).
As you can see, at a certain position of the volume controls and the sensitivity switch at the back panel, this amplifier can yield a dynamic range of over 100dB and a channel separation of almost 90dB at the expense of a minor increase in harmonic distortions.
The table above gives you a notion of the quality of the interblock cable supplied with the Scythe amplifier. It is 50 centimeters long and its electric characteristics are in between the 50cm Monster Interlink 200 and the 1m-long cable included with the ASUS Xonar D2.
The Scythe KamaBay Amp SDA-1000 has confirmed its right for the title of amplifier as it copes with the headphones better than the regular sound cards. But when compared with special sound cards, it can only offer its zero impedance and better separation of the channels.
By the way, RightMark Audio Analyzer helped find out the exact gain setting selected by the KamaBay Amp developers out of the four values available for the YDA138. The headphones amplifier increases the amplitude of the input voltage by 12dB. So, the developers selected the third value with a sensitivity of 0.28V.