Audiophile's output stage on the Sovtek 6922 vacuum tube (Made in Russia)When AOpen released a motherboard with a tube output stage the computer society split into three parts. Some considered that step a long-awaited revolution in the computer audio world, others thought it was idiocy; there also was an opinion that it was a good marketing step attracting a great deal of attention.
Here is an advertising composition from the AOpen's site.
The box is divided into two parts, one containing the board and the other - the Sovtek 6922 tube and a bracket with CARDAS cables.
The sound subsystem of the motherboard consists of:
The Realtek ALC650 features such advantages as a low price, 6-channel support and easy integration into a motherboard with modern chipsets. And according to AOpen, the tube can add something that will smooth away imperfection of the cheap converter and make sound enchanting.
Here is the circuitry of the tube stage on the triode.
The tube doesn't look so romantic in operation as it looks on the ads. It doesn't give much light and doesn't winkle with time in music. :)
The tube stage gets enabled with a jumper onboard and a special option in the BIOS.
The tests were carried out in the Windows using the DAC of the codec on the motherboard.
The software is ordinary. An icon emerged in the tray enables a software sound manager.
Test listeningThe tests were carried out with the professional sound card Lynx Two (~$1400) used as a reference and the active computer acoustic system Event 20/20 bas (~$1000). The multimedia wooden speakers Microlab SOLO-1 ($50) and the same codec but on a usual mainboard for an Athlon XP came in handy as well.
The tests revealed:
The motherboard with the tube hasn't passed the tests with the Sennheiser HD600 headphones. In spite of the dedicated 1/4" connector for the headphones the output stage is probably unable to playback without reloading. The sound cracked at any volume level.
Objective testsAs usual, we used the RMAA4.0 program to take measurements.
The AFCs for 50% and 100% volume levels coincide,
the frequency response is normal
The noise level doesn't reveal noticeable pickups from high-voltage circuits of the section
The harmonic distortion level is 1..2%
- it's typical of tube equipment
The spectrograms show that the sound is rich in warm even harmonics, as well as in high uneven components. At the 50% volume level there are no more distortions after the 4th harmonic as it is exactly the tube sound sound we were promised. At the maximum level the output stage is overloaded. The signal level is quite low (-10 dB as compared to the standard level of -10 dBV) and the overload capacity of the section is low as well.
ConclusionThe AOpen AX4B-533 Tube motherboard is a wise marketing step of the manufacturer the primary purpose of which is to attract attention. It's probably also an interesting solution for audiophiles and everybody who miss good old tube-vinyl times. One can excellently feel the tube sound on high-quality equipment though it doesn't improve it much. The cheap AC'97 codec bounds the might of the sound. If they used a higher-class DAC, usage of the tube could be justified.
Maxim Liadov (firstname.lastname@example.org)
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