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Sound section of the AOpen AX4B-533 Tube Motherboard

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.

Well, we managed to get this motherboard and I intend to test it with a great pleasure.

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 SOVTEK vacuum tube is made in Russia. Tubes of this manufacturer are often used in professional and audiophile equipment by various brand-name companies (for example, Behringer). However, a special guide from AOpen which sheds light upon all details of the vacuum tube suggests making experiments with some other similar tubes from other manufacturers - from Philips to Telefunken. The Russian's product has probably the best price/quality ratio.

The general idea of using the tube in the preamplifier is to soften sound and add the audiophile's peculiar warmth.

The sound subsystem of the motherboard consists of:

Quality is, thus, ensured by the tube, as well as by the whole circuitry. There is just one flaw in the device which generates sound: the DAC of the Realtek ALC650 AC'97 codec can't be called a Hi-End converter. An expensive DAC on the I2S bus from Burr-Brown or Crystal would be a better choice. Or at least an AC'97 codec from Analog Devices.

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.

Triode is a primitive tube, an analog of a transistor. It consists of an anode (1), a cathode (3) and a controlling electrode - grid (2). However, there is a principle Hi-End equipment should follow: the less there are links in a chain and the higher their quality, the better the section. From this standpoint a twin triode is an excellent alternative for a stereo preamplifier on tubes.

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 capacitors are really gigantic because of the voltage the tube needs to work.

The tube stage gets enabled with a jumper onboard and a special option in the BIOS.

The engineers flashed in a CD player into the BIOS which appears on the screen right after the start-up if you press and hold the Insert button.

It's interesting that music comes via an analog cable enabling the DAC in the CD-ROM drive. However, a bonus is a bonus: it's an unusual and original gimmick.

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.

The first tab allows choosing you a reverberation type. 

Its quality is average, but sufficient for integrated sound and multimedia speakers.

The second tab offers us a digital equalizer.

The third tab of the codec's software shows an incredibly flexible configuring approach when any input can work as an output.

The next tab deals with testing speakers.

Finally we can see versions of DirectX, codec and drivers.

Test listening

The 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:

General impressions. It looks like a landscape through turbid glass: on the one hand, the sound is less realistic, but on the other hand, some can like it more.

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 tests

As 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.


The 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 (maxim@ixbt.com)

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