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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
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.
Realtek ALC650 AC'97 codec;
Vacuum tube (twin triode) Sovtek 6922 (Made in Russia) which can be replaced
with any similar one;
ELNA capacitors in the power circuitry of the tube;
CARDAS copper cables between the amplifier and connectors;
REL MultiCap capacitors in the sound section;
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
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
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
The first tab allows choosing you a reverberation type.
Its quality is average, but sufficient for integrated sound and multimedia
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.
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
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.
In spite of my worst expectations the sound wasn't the worst of its kind!
As compared with the original, the codec coupled with the tube stage gives
soft sound with a smoothed "attack" and a little blurry details, and noticeable
distortions at a high volume level;
The sound is more interesting than in a pure codec but Hi-End is still
Acoustic compositions with live recording sound best of all; the tube doesn't
cope well with electronic music where aggressive attack and details are
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.
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.
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